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････････.. ･･=･･ :ji: ~ ~ i! ;'::"~~.:;...!~= .!:･･･････~~~~ nru

Tangential C02 LaSer InterferOmeter fOr Large TOkamaks

KAWANO Yasunori, NAGASHIMA Akira, TSUCHIYA Katsuhiko, GUNJI So-ichi,

CHIBA Shin-ichi and HATAE Takaki

Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Ibaraki 311-01, Japan

(Received 21 June 1996/Revised manuscript : received 3 June 1997)

Abstract A C02 Iaser interferometer system has been developed in JT-60U. The system has a toroidally tan-

gential chord for electron density measurement on large tokamaks. There are four maj or topics as fol-

10ws ; ( I ) the investigation of conditions for the tangential interferometry in large tokamaks, (2) the de-

velopment of a C02 (10.6 um)/IR-HeNe (3.39 um) Iaser interferometer, (3) the development of a dual

C02 Iaser interferometer with a wavelength combination of C02 (10.6 um)lC02 (9'27 um), and (4) the

development of a very high resolution phase comparator which is designed to have a phase resolution of

2~/12,800 rad for 2 MHZ signals. The results achieved show that the novel dual C02 System is feasible

for not only JT-60U but also future large devices like ITER.

Keyu,o rds:

electron density measurement, tokamak, interferometer,

JT-60U, ITER, high fip H-mode, disruption

CO Iaser, IR-HeNe laser, tangential chord,

1. Introduction

This paper describes the novel C02 Iaser interfe-

rometer developed in JT-60U. There are two major

scopes of the interferometer such as (1) the measure-

ment of the central electron density in JT-60U, (2) the

demonstration of a feasible interferometer for future

large devices like the international thermonuclear ex-

perimental reactor (ITER).

The measurement of electron density of plasmas is

an important topic for tokamak fusion research. Laser

interferometry has been playing an important role in

this field. In order to achieve enough reliability and ac-

curacy of the interferometer in large tokamaks, there

are basic key points such as; a laser used for the light

source, an optical geometry, an accessibility to the

plasma, a system stability, an easiness of handling and

operation, safety, commercial availability, and so on.

Regarding the laser device, it is required to use appro-

priate wavelength to target plasmas. Historically, as the

progress of plasma sizes and performances of tokamaks

(the increase in plasma volume, the magnetic field,

electron density, etc.), Iaser wavelength had been

getting shorter from microwave to far infrared (FIR)

and infrared (IR) regions [ I J . For example in present

large tokamaks, there are the DCN Iaser (195 um) for

JET [2] , the alcohol (CH30H, 118 um) Iaser for TFTR

[3] and JT-60 [4] and the C02 Iaser for DIII-D [5].

Subsequent to these current machines, the laser inter-

ferometry is further expected to be essential to future

large devices like ITER. In fact in ITER diagnostics

[6], a C02 Iaser interferometer is proposed for electron

density monitor and for cross calibration of LIDAR. A

choice of laser wavelength for ITER must be deter-

mined to avoid the refraction effect by the dense large

plasma and Faraday rotation effect due to the large

magnetic field, these can be emphasized especially in

the case of a tangential chord. A wavelength 10.6 um

of the C02 Iaser is not as long as FIR region where

laser beams are much suffered from above effects and is

not as short as visible region where laser beams are less

sensitive to a density change. Therefore, the C02 Iaser

is a suitable light source of the interferometer of ITER.

On the other hand, for the electron density

measurement on large tokamaks by interferometry, it is

870

~~f~~~t~~~~~'1~'~'~~~0~~ Tangential C02 Laser Interferometer for Large Tokamaks 7~~f, ~::~4~;

required that a simultaneous measurement by a differ-

ent wavelength interferometer along the same optical

path for compensation of changes in the optical path

length due to mirror vibration and displacement. By

this reason, the two-color interferometer scheme must

be introduced [7]. For the light source of the second in-

terferometer, an IR-HeNe laser (3.39 um) was pro-

posed [8,9] as being preferable to a conventional visible

HeNe laser (0.633 um) from the point of view of a

measurable vibration speed of mirrors and a reduced

optical transmission of mirrors and windows at shorter

wavelength region.

For density measurement by the interferometry in

JT-60U, an alcohol-laser interferometer system has

been routinely used from the beginning of the JT-60

operation [4]･ This interferometer has two vertical

viewing chords, however, which can not pass through

the plasma central region when the plasma major radius

is small. In order to achieve the central line density

measurement for various plasma shapes, we have de-

veloped a C02 Iaser interferometer in addition to the

alcohol laser interferometer [8,10] . The new features of

the C02 Iaser interferometer are as follows; the toroi-

dally tangent line of sight which can view the plasma

central region for almost all the plasma configurations,

the wavelength combination of C02 (10.6 um) and IR-

HeNe (3.39 um) Iasers, the long distance interfero-

metry of up to about 100 m, the development of an op-

tical technique of the common path mode matching for

propagation of different wavelength lasers. This C021

IR-HeNe laser interferometer was used mainly for the

density measurement of the "high fip H-mode" plasmas

produced in JT-60U which made the highest fusion

triple product in 1993 [11-13] .

The C02/IR-HeNe system in JT-60U is very simi-

lar to that proposed for ITER because the design con-

dition was basically same to that of the ITER. There-

fore it was a good examination of the feasibility of the

C02/IR-HeNe combination of the ITER proposal. As

the results, the proof of principle of a tangential C021

IR-HeNe interferometer was presented, however, we

found several problems in the operation of the C021

IR-HeNe combination in JT-60U as followings; (1) A

darkening of windows and mirrors substantially de-

creased the returned probing IR-HeNe laser intensity

even its wavelength is in the IR region. The signal am-

plitude of the IR-HeNe laser interferometer was so re-

duced that stable phase measurement became difficult.

(2) "Fringe jump" or "fringe counting loss" was often

detected in the IR-HeNe laser interferometer when the

plasma current was large and/or the plasma position

was changed rapidly. (3) A calculated density trace had

often drifted in a time scale of minutes. This indicates

that there was a slight frequency difference between

both beat signals and/or an unbalanced changing in

path length between two interferometers. (4) The opti-

cal alignment of the IR-HeNe laser interferometer was

difficult due to the weak intensity of the IR-HeNe oscil-

lator. The problems above can be in common in large

tokamaks because most of the problems are originated

from the large scale of the interferometer.

In order to resolve the problems of the C0211R-

HeNe combination and to achieve more stable interfe-

rometer for large tokamaks, we have developed a dual

C02 Iaser interferometer based on the C02/IR-HeNe

laser interferometer [ 14]. The two wavelengths of C02

laser 10.6 and 9.27 um are utilized for the dual C02

system. Though this close wavelength combination

causes a substantial reduction of the density resolution

compared with that of the C02/IR-HeNe combination,

the new system can provide a lot of advantages such as ;

( 1) a better robustness to the darkening of vacuum win-

dows and reflection mirrors, (2) a good capability for

large mechanical vibration and displacement of mirrors,

(3) an easiness of laser beam monitoring, and (4) a sim-

plified layout of optical components by using close

wavelengths of C02 Iaser. Additionally, a new tech-

nique of using a single frequency shifter; AOM (Acoust

~ptic Modulator) in common for different wavelength

of C02 Iaser is also developed. This technique con-

tributes to suppress the drift of density trace by com-

plete matching of both interference beat frequencies.

By these advantages, we have succeeded to measure

line electron density reliably for the central region of

JT-60U plasmas using the dual C02 Iaser interfe-

rometer. It also enabled us to investigate density beha-

vior even during fast maj or disruptions which were dif-

ficult to be measured by a conventional interferometer

system using HR Iasers.

Nevertheless that the density resolution of the dual

C02 combination is relatively poor without data

smoothing, it is good enough for the density monitor

with appropriate data smoothing. However, the im-

provement of the density resolution is needed when

more precise density behavior is required. For this pur-

pose, a very-high resolution phase comparator has been

developed [14,15] to improve the effective density res-

olution up to the same order of magnitude to that of

the alcohol laser interferometer in JT-60U. The new

comparator is designed to have a phase resolution of

201;/12,800 rad for 2 MHZ beat signals. This resolution

is more than a hundred times better than that of our

871

j~ )~~ ･ ~~~~i~~'~~~~ ~~*#* 1997~~ 8 ~l

standard phase comparator which has a resolution of

2~/100 rad.

In this paper, we describe the development of the

tangential C02 Iaser interferometer for large tokamaks.

The basic principles of the tangential interferometer is

depicted in section 2. The results of the C02/IR-HeNe

laser interferometer is presented in section 3 . The re-

sults of the dual C02 Iaser interferometer is presented

in section 4. The results of the very-high resolution

phase comparator is presented in section 5 . A compari-

son of the achieved performance of the dual C02 Iaser

interferometer with the requirements for ITER is dis-

cussed in section 6. Section 7 is devoted to discussion

and section 8 is for summary.

2. Basics of C02 Iaser interferometer

Laser interferometry for electron density measure-

ment is based upon the measurement of a refractive

index of a plasma. To realize a C02 Iaser interfe-

rometer, firstly, we discuss the refractive index for a

C02 Iaser wave which is propagating in magnetized

dense plasmas. Secondly, electron density measurement

by a C02 Iaser interferometer is presented. Thirdly, the

effective phase resolution is discussed. Finally, other

important issues with respect to a selection of laser

wavelength are presented.

tion modes can be approximately classified to the

quasi-transverse (QT) mode and the quasi-longitudinal

(QL) mode according to the relative size of the two

terms in A as follows;

Q~sin40 >> 4co~2(co2- co~e)2cos20, QT , (3)

Q~sin4e << 4(0~2(co2 - a)~e)2cos20, QL . (4)

In the QT mode, there are two conditions according to

the :!: sign in eq. (1), i.e. the QT-ordinary (QT-O)

mode and the QT-extraordinary (QT-X) mode,

co2 - co2

n2 ~: Pe co2 - co2 cos2 e pe

n2 ~:

QT-O (E // B) , (5)

2.1 Refractive index

2.1.1 Alter-Appelton-Hartree dispersion rela-

tion

The refractive index for cold plasma electromag-

netic electron modes is presented in Ref. [16] as fol-

lows. It is started from the Alter-Appelton-Hartree dis-

persion relation written by,

2cope(co2 - co~e)/co2 n2 = I - 2(co2 - cope) ~ Q~Sin2 6~ QeA ' (1)

(co2 ~ co~e)2 (02Q2Sln26

co2(co2 - (o~e) ~ co2Q~sin2 e '

QT-X (E i B) , (6)

here E and B are vectors of the electric field of the

wave and the magnetic field of the plasma, respectively.

For a special condition of 6 = ol;/2 in eq. (5) gives the

ordinary (O) mode propagation,

2 1 - co2pe no e: co2 '

(7)

which is the same as the well known refractive index

for the unmagnetized plasma.

The expression of the QL mode is simplified

under a condition in addition to eq. (4) as,

where l l Q~Sin20 << 2(co2- co~.) (8)

A = [Q~Sin40+ 4co~2(co2 - co~e)2cos2e]1/2 (2)

Here, n is the refractive index kc/co for the electromag-

netic wave with the frequency (o and the wave number

k, c is the speed of light in vacuum, O is the propaga-

tion angle against the direction of magnetic field B, cop.

is the plasma frequency (e2ne /80m. )1/2, Q. is the elec-

tron cyclotron frequency eB/m* and n* is the plasma

electron density. According to the Ref. [ 16], propaga-

The refractive index of the QL mode in the range of O

which satisfies both of eq. (4) and eq. (8) is given by

n2 = I - cop. QL-R/L , (9) co(co :!: Q.cos O) '

where R denotes the right-hand circularly wave for the

upper sign of ~ and L denotes the left-hand circularly

872

Tangential C02 Laser Interferometer for Large Tokamaks ~ry ~~" , d~~4~

wave for the lower sign of ~ in eq. (9). If Q,cosO is

negligible (QecosO << co), eq. (9) can be the same with

eq. (7).

2.1.2 Refractive index for C02 Iaser wave in

the plasma ( I ) Propagation mode

It is needed to determine the regime of the propa-

gation mode of a C02 Iaser wave in a dense-magnetized

plasma to know the appropriate expression of the re-

fractive index. Plasma parameters used here are chosen

to mostly cover operational regimes both of JT-60U

and ITER. The ratio of two sides in eq. (3) and (4) as a func-

tion of 6 is shown in Fig. I in cases of n* = 1020 m~3

and B = I T, 5 T, and 10 T, where a, for the C02 Iaser

wavelength (10.6 um) is 1.78 x 1014 rad/s, cop, is 5.64

X 1011 rad/s, and Q* is the 1.76 x 1011 X B rad/s. It is

to note that the propagation mode is in the QL mode at

the most range of O except of a narrow region around

~/2. For example, for B = 5 T, if the critical ratio is

chosen to be 10-3, the propagation of the C02 Iaser

wave is in the QL mode for O' ~ e ~ 85'. Due to the

large discrepancy between co and (~'p. , Q* , there is little

n. dependence, and equation (8) is satisfied at all range

of O. Therefore, for given plasma parameters, the pro-

pagation mode of a C02 Iaser wave in the plasma is the

QT mode approximately written by eq. (5) and (6) for

85' ~ o ~ 90' otherwise the QL mode written by eq.

(9) for O' ~ o ~ 85'.

Fig. 1

C:) aJcl)

O c¥]O

Q)~

31

3 ~ 3 ~r '¥ C:) ~rc:

c¥'(1)

~a)

ll

T-OC

1 02

1 Oo

1 0~2

1 0~

1 0~~

1 0~8

1 0~10

1 0~12

1 0~14

i n20 -3 ne = tu m

ll

'l ll

.. . B = 10T._.-'

/ /'- B ::: IT // ~

'l ll

IL, l' B = 5T ~v ~h I' tt

t'

l¥ ,

Ll I~

(2) Refractive index for interferometry

It is useful if eq. (7) can be used for interferometry

because of its simple form. For this purpose, a valida-

tion of eq. (7) is discussed. Considering that the linear

polarized wave consists of two characteristic waves of

the right-hand circularly wave and the left-hand circu-

larly wave, the refractive index of the linear polarized

C02 Iaser wave in the QL mode regime is given by the

average of refractive indexes for the right-hand and the

left-hand circularly waves [ 1 6]. The nearly cancel out

effect of the ~ term in eq. (9) is expected by the avera-

ging, and the obtained refractive index becomes closer

to eq. (7). From this point of view, Figure 2 shows a

comparison between the refractive index of the ordi-

nary mode no given by eq. (7) and the averaged refrac-

tive index from the Alter-Appleton-Hartree dispersion

relation nAAH_AVE = (nAAH+ + nAAH_ )/2 in the case of

n 1020 m and B 5 T where nAAH+ and nAAH_ are the refractive indexes from eq. ( 1) with different ~

sign. The figure of (no ~ 1)/(nAAH_AVE -1) -1 means

Fig. 2

O 20 40 60 80 100 e [ deg J

Ratio of the right-hand side to the left-hand side in

eqs. (3) and (4) for the C02 Iaser wave as a function

of a propagation angle to the magnetic field (B) e,

where the electron density; n* = I x 1020 m~3 and

the magnetic field B = I T, 5 T, 10 T. The frequen-

cies co, cop. and ~~* are the e[ectromagnetic fre-

quency of C02 Iaser, the plasma frequency (e2n*l 80m*)1/2 and the electron cyclotron frequency; eBl

m*, respective]y. The ratio becomes infinity at 6 =

90 deg and O at 6 = O deg.

xl06

4

T-l 2 , :~

< < c: O ¥'

l -2 o c

-4

20 3 ne=10 m~ , B =5T xl0~5 O

4- (+) - , .

¥

H> .l /

/ /

/ '/ ' <~ (~) .l

>.

/

-0,5 t 1-

1--1 'LU

> < I < -1 ,5 < c .~

-2 I O C

-2,5 ~

-6 3 O 20 40 60 80 100 O [ deg J

Comparison between the refractive index of the or-

dinary mode no shown in eq. (7) and the averaged

refractive index from the Alter-Appleton-Hartree

dispersion relation nALH_AVE = (nALH. + nALH_)/2.

Where nA~H+ is for the plus sign of eq. (1) and nALH_

is for the minus sign of eq. (1). Plasma parameters

are ne = I x 1020 m~3 and B = 5 T. The ratio of (no ~

1)/(n - 1) -1 means the error in a phase A~H-AVE measurement by interferometry in the case of using no instead of (nALH_AVE ~1) because (n - 1)

corresponds to a phase difference by a plasma. Curves of (+) and (-) denote the ratio of ((no ~ 1)/

(nALH. -1) -1) and ((no ~ 1)/(nALH_ -1) -1), respec-

tively. The maximum error of -2.5 x 10-5 (-0.00250/0)

is observed at e = O deg.

873

f~ ~7 ･ ~~~~~:~~~~~---~L* 1997~p 8 ~l

the error in the phase measurement by interferometry

in the case of using no instead of nAAH_AVE because

(n - 1) corresponds to a phase difference due to a

plasma as shown in eq. (10) in the next subsection. In

Fig. 2, the difference between cases of using no and

nAAH AVE rs less than 2 5 X 10-5 (-0.0025'/*) at any

O. The maximum error is observed at e = O', i. e. for the

parallel wave propagation. The B dependence at e = O'

shown in Fig. 3 indicates that the difference is less than

1 x 10-4 (0.01*/.) at the range below 10 T. The n, de-

pendence at O = O' is little on the difference between

no and nAAH_AVE due to the large discrepancy between

co and cop, in the density range in the figure. Hence, for

given plasma parameters and for the linear polarized

C02 Iaser wave for the condition of E // B, eq. (7) is

valid to use as the refractive index for any propagation

angle against the magnetic field of the plasma accepting

a small error. This indicates that a interferometer with a

C02 Iaser wave propagating in toroidally tangential

chord can be realized using eq. (7).

f --A~= (kp k)dl

f = (n-1)-'-coc dl

:x cf , co 2cn ne dl (10)

where kp and kr is the wave number of laser beams of

the probing arm and the reference arm, n is the refrac-

tive index of the plasma, and nc is the cutoff density

which gives n = O, i.e. nc = co2me80/e2. Hence the line

(integrated) electron density is given by,

nedl 2cncA~' f = co

(11)

2.2 Two-color laser interferometer In general, a laser interferometer is realized by two

laser arms; a probing laser arm and a reference laser

arm [ 17]. The probing arm is propagated in a plasma

and its electromagnetic wave includes phase shift due to

the presence of the plasma. The phase difference be-

tween two arms A ip is written by,

Here, the line density which corresponds to a phase

shift of I fringe (2~ rad) for C02 Iaser 10.6 um is 2.11

X 1020 m~2.

On the other hand, in actual interferometers, a

measured phase signal includes not only the phase shift

by the plasma (plasma component) but also the phase

shift by changes of the beam path length due to mech-

anical vibrations and displacement of mirrors (path

component) as,

x I 0~2

1 .5

T-

1-

+ ::

< < C ,¥

1-

O C

1

0.5

o

-o .5

~1

-1 .5

e o deg n 1020m~3 xl0~ 0.5

<H (+) _

H> <- (-) *~, .

1-

~ O l ,-UJ

> < I < -0.5 < c

'¥

lr

o 1 c:

-1 .5

O 2 4 6 8 10 12 B[T]

Magnetic field dependence of the error in a phase

measurement in the case of using no, (no ~ 1)/ l -1) I where O = O deg and n. = I x 1020 InALH_AVE m~3. The absolute value of the error increases with

the increase in the magnetic field.

_ e2 f A~ = ne dl+ kALb , 280mec2k (12)

where ALb is the change in the beam path length. In

order to compensate the path component from the

phase signal, two-color laser interferometer scheme [7]

must be introduced as,

_ e2 f A~l = ne dl+ kIALb , 280mec2kl

Fig. 3

_ e22 f A~2 = 280mec k~ ne dl+k~ALb ,

(13)

(14)

where subscript I and 2 denotes the difference of laser

wavelength. The line electron density is directly given

by eq. (13) and (14),

874

~f ~~5r~;*~~1~~~~~~~ ~~ Tangential C02 Laser Interferometer for Large Tokamaks j~~~, ~~;4~2

280mec2 1 1 ~l f ne dl ( - 1'~~' -,~ ) e2 2 k21 k 2

( 2) A~1 Ac X kl ~ k2

(15)

denominator in the right hand side in eq.

by,

6 kl ~ k2 ) ( 2= A~1 A~ 1 k~1 2JTrf +

~

(17) is given

2 ,Trf

2.3 Conditions forwavelength selection

2.3.1 Resolution of two-color laser interfe-

rometer Considering the resolution of the line electron den-

sity, it is determined by a phase resolution of a phase

comparator (2~ rf ) which is used for an interferometer,

where rf is a electronic resolution of a phase compara-

tor in unit of fringe. An ideal signal to noise ratio is

evaluated by the ratio of A~ to 2~rf as,

Therefore eq.

= ) ( 2JTrf 1+1

kl x

( 17) becomes

r e2 J ne dl := (-) 1 1 1 S/N 280mec2kl 2JTrf x

(19)

(20)

S/N = A~ 2 JTrf

co f n' dl

1

2 cnc 2~Trf (16)

In a case of two-color laser interferometer, the res-

olution of line density is evaluated by an accuracy of

the contents of the last parenthesis in eq. (15) with the

same manner shown in eq. (16) [8],

( 2 ) ) 2 = 1_ ( A~ Ac1 A~ A~ 16 S/N kl kl ~ k2 k2

The numerator is represented by

(17)

By comparison of eq. (20) to eq. (16), it is to note that

S/N is reduced by factor of (1 - 1/x), which means

that the effective phase resolution becomes worse as

2JTrf/(1 - 1/x). To find a wavelength combination

which has good resolution, the effective resolution and

S/N of two-color laser interferometers with a different

wavelength combination are shown in Table I , where rf

is given as I / 100 which is the resolution of the stan-

dard phase comparator presently used. Naturally, Iarger

x is preferable for better S/N.

2.3.2 Refraction effect and Faraday rotation

The axis of laser beam which is propagating in a

plasma is usually bent due to the refraction effect by a

gradient of electron density profile. The refraction of

the probing beam causes not only a reduction of the de-

tectable signal power but also phase shift errors. For in-

stance, in the case of a probing beam inj ected perpen-

dicular to the plasma column with a parabolic density

profile, the refraction angle has a maximum value 6* at

r/a - 0.7 [1] as,

A~1 A~ e2fnedl 1 1 )

( ~ ) := ( -' 2

'r~ '(!1 k 2e m c2k2 x2 2

oe 1 (18)

e* = n (O)

nc

= 8.97 X 10-6ne(O) ~2 , (21)

where x = k~lkl'

determined by rf '

using a formula

Uncertainties of A~l and A~2 are

i.e. 8(Ac1) = ~(A~2) = 2JTrf' By

for the total differentiation, the

where r is the probing distance from plasma center, a is

the plasma minor radius, and n. (O) is the central elec-

875

~~ ~:7 ･ ~~~~"~~"~{~i~･"~~~~~~-~*~ ~~73~~~~ 8 ~~ 1997~~ 8 ~I

tron density. The maximum refraction angle in the case

of n. (O) = 1020 m~3 is shown in Table 1.

The Faraday rotation effect should be considered

in the case of a toroidally tangent line of sight where

the parallel component of the magnetic field B[ is

strong enough. When the Faraday rotation is large, the

stable phase measurement may become difficult. The

Faraday rotation angle a along the beam path is given

byfor n. >> n. [17],

2.3.3 Vibration limit and transparency of

vacuum window A measurable motion speed of a reflection mirror

is limited by the electronic response of a phase com-

parator. It is necessary that the Doppler frequency

modulation due to the mirror motion does not exceed

the response of a phase comparator as [ 1],

2V~/~ < Af* , (23)

a = e f 2mecnc ne Bn dl

e3 ~2 f ne Bu dl 8,T280m2ec3

(22)

In the case of constant B of 5 T along the entire beam

path, the Faraday rotation angles of different wave-

length lasers are shown in Table I .

where V* is the motion velocity of the mirror, ~ is the

wavelength of the laser, and Af* is the band width of

the phase comparator response. Based on the band

width of the standard phase comparator in JT-60, Af. =

1.5 MHz, the measurable mirror speeds are shown in

Table 1.

The transparency coefficient of a vacuum window

reduces chronically due to plasma operations and first

wall conditionings. For example, Figure 4 shows the re-

duction of a spectral transparencies of a quartz window

for JT-60. It is to note that the transmission coefficient

significantly decreased at shorter wavelength region. In

Table 1 Typical conditions of waveiength selection for a C02 Iaser interferometer for large tokamaks: x is the wavelength

ratio of two lasers, rf is the phase reso[ution of the phase comparator, S/Nis the ideal signal to noise ratio for line

electron densities given, a is the Faraday rotation angle for line density and magnetic field given, V~ is the measur-

able vibration speed of the ref]ection mirror and 6~ is the maximum refraction angle of a laser wave which is verti-

cally passing through a plasma with parabolic density profile and the central density given.

876

~~f ~~5f~l "'~i-~~~~o ~C

Tangential C02 Laser Interferometer for Large Tokamaks ~~ ~~･ , J~~;4ti~

this case, the typical distance between the window and

the plasma edge was about 2.5 m. This kind of window

darkening would be more serious when a window is in-

stalled at more closer to the plasma. The darkening of

windows is strongly depend on materials of the first

wall and windows, the location of windows, the geo-

metry of windows, and the type of plasma operations.

2.3.4 Seiection of wavelength combination For a second wavelength of a C02 Iaser interfe-

rometer in large tokamak devices, each combination

shown in Table I has its advantages and disadvantages.

The basic C02/Vis.HeNe combination can be used

when the darkening of windows and the vibration of

mirrors are not serious. Otherwise, the C02/IR-HeNe

combination and the C021C02 combination are favor-

able. The C02/CO combination is rejected because of

the risk of CO gas.

3. C02/lR-HeNe laser interferometer For the central line-density measurement for vari-

ous shape plasmas in JT-60U, there are two possible

viewing chords, i. e. a toroidally tangential chord and a

horizontal chord. On the other hand, a vertical chord is

not suitable because it can not pass through the plasma

central region in the case of plasmas which major radii

are small. Here, the toroidally tangential chord has ad-

vantages with respect to a longer chord length in a

plasma and installation possibility of a reflection mirror

outside the vacuum vessel. Cohtrast to the tangential

chord, a reflection mirror must be installed in the in-

board tiles of the first wall in the case of the horizontal

chord. The mirrors in first wall tiles must face to a

plasma and can be damaged by plasma operations.

Consequently, the tangential chord is selected prefer-

able to the horizontal chord. The tangential chord re-

quires considerations of the darkening of windows and

the mirror vibrations because vacuum windows are in-

stalled close to the plasma and mirrors are installed

near the vacuum vessel. For the first step of the C02

laser interferometer in JT-60U, the C02/IR-HeNe

laser interferometer had been developed [8,10] because

this wavelength combination was expected to well sat-

isfy the conditions of the transparency of windows, the

vibration limit, and the density resolution.

Fig. 4

H

1

JT-60 quartz window

0.8

0.6

0.4

0.2

O

,

/

after -1 920shots

¥

before use

200 300 400 500 600 700 ~ [ nm J

The spectral transparency of a vacuum quartz win-

dow used for a spectrometer in JT-60. The window was made from fused silica. It was instal]ed at 2.5 m

apart from the plasma edge. The transparency rapidly decreased at the shorter wave]ength region

after plasma operations.

Rout =434

Rp

CCR ¥a

f

Window

3~25

¥

¥

¥

Rin= 2370

Ch 1

~ Ch 2

JT 60U Plasmo

/ ~Laser path

3.1 Description of C0211R-HeNe laser interfe-

rometer The C02/IR-HeNe laser interferometer has a Mi-

chelson geometry [8] which consists of six major parts;

laser oscillators, frequency shifters, detectors, relay op-

~

Window

Fig. 5 Laser beam line of the probing arm in the JT-60U

vacuum vessel. Top view of the vacuum vessel with the !aser path in the case of a iarge plasma

configuration. Vertical chords of Ch. I and Ch. 2 are

used for the alcoho[ Iaser interferometer.

tics, vacuum windows, and data acquisition system.

3.1.1 Line ofsight

Figure 5 shows the laser beam line in the JT-60U

vacuum vessel. Laser beams are launched into the va-

cuum vessel tangentially through a vacuum window at a

mid-plane port. A corner cube reflector (CCR) is in-

stalled outside an another window which is installed at

the end of the laser path. The CCR reflects laser beams

providing a vertical shift of 15 mm. The tangent major

radius of the laser path is 3 . 1 1 m. A Iaser path length in

the plasma depends on the plasma shape. In the case of

877

f~~7 ' ~;~~~A:~~"~~~~A~~F~#u 1997~~ 8 ~l

CExp. Room I F]

Cooter

Powe r Sup ply

CB I FJ

JT- 60 V Vacuum Vessei

p csma

*fr~-- --/~'~:~="~~~~.*~}~""~~~~~' ~~ MB9

*-;s~~~~"~i~~~-"'~~~~~~~~~:_-'s~ i:~

::~~ i' . ~~"~,~~i'"~~~i~~~'~~"'__'_'~'~~'~;~:"s~' P~l¥J)llT~ ' : .,~ :~~ ~.~"~3]-~i'~1"'~:'~~!_~;~1~4~:'~~__-' _ i MB8

CCR

Window 2; <:.~

/ SUPPort

~ ~liMB6 MB3

!;~~~ ~; MB5

MB4

MB ?

stage

Vibration isolation bench (3xl .5m)

vis-HeNel

C02 (1 0.6um) BE1

TM6

TM15

7M2

OM4 DM1

TM5 BE2 DM5

IR-HeNe(3.39um)

TM4

is-HeNe2 DM2

TM 1 7 8 E3

Vis-HeNe3 TM16

TM3 DM6

TM7

__{lL---D----

DET2 i!nAs;

TMI DET, L4 fHgCdTe, aoml aom2 , TM14 {}--O~L2~ ~ z .~:;~ 40MHZ ~2MH , L3

AOMI AOM2 HM2 HM1

40MHZ ~S2MHz L1

TM TM12

DM3

TMIO

TM9

TM13

tO Plasma

eas Su pply

C02 Oscillators

TM11 from Plasma

l Laser

MB2 Path

Stage

' MB I ," / Ret**d**,.,/ "p',* vib,*ti.~ FLG

Isol'ti.~ B*nch

¥ Fig. 6 Simplified sketch of the JT-60 torus hali and the ar-

rangement in the case of the dual C02 Iaser interfe-

rometer. Major components of the system are in-stalled at the basement of the building. The probing

beams are guided to the p[asma using the relay op-

tics. which consists of a f[exible light guide (FLG).

seven folding mirrors (MB2-MB5. MB7-MB9). two telescope optics (MBI and MB6) and a CCR. The CCR is insta]]ed outside the vacuum vessel.

Fig. 5, the laser path length is about 6 m (for one way).

3.1 .2 Laser oscillators. frequency shifters. de-

tectors Figure 6 shows the simplified sketch of the ar-

rangement of the tangential interferometer in the JT-

60U torus hall in the case of the dual C02 system

which will be presented in section 4. A Iocation of one

of the C02 oscillators on the vibration-isolated optical

bench was occupied by an IR-HeNe oscillator in the

case of a C02/IR-HeNe system. The retardation optics

was not used for the C02/IR-HeNe interferometer.

Some major instruments of the system are in the base-

ment of the JT-60U torus building. In the basement, a

C02 and an IR-HeNe laser oscillators, frequency shif-

ters, and detectors are installed on the optical bench.

Figure 7 shows the optical arrangement on the optical

bench. The C02 Iaser oscillator is model GLD2042

from NEC corporation which output power is 10 W

and beam diameter of 6 mm. Its wavelength is tuned to

10.6 um (10P(20)) by use of a grating at the one-end

of the oscillator. Though the lasing frequency can be

stabilized by an active feedback control of the cavity

length of the oscillator, usually the laser is operated

Fig. 7 Optical arrangement on the vibration isolation bench of the C02/lR-HeNe laser interferometer. Ba-

sica]]y, there are two interferometers with the C02

laser (10.6 um) and with the IR-HeNe laser (3.39 um)

according to the two color interferometer scheme

for vibration compensation. BE; beam expanders to

modify laser beam parameters so as to propagate in relay optics. TM; total reflection mirror. HM; half

refiection mirror. DM; dichroic mirror working as a

tota[/partial ref]ector for a selected wavelength

beam. L; Iens. AOM; acoust-optic-modulator head

used as the frequency shifter for the heterodyne

detection. The C02 Iaser beam and the IR-HeNe laser beam are superimposed coaxially at the DM1.

The IR-HeNe laser beam is divided into the probing

(80'/･) and the LO beam (20"/.) at DM1. The LO beam

goes into AOMS for 2 MHZ frequency shift for the

IR-HeNe wavelength. The C02 Iaser beam is also divided into the probing (80'/.) and the LO beam

(20'/･) at DM2. The LO beam of the C02 Iaser goes

into AOMS for 2 MHZ frequency shift. Probing beams of the C02 and the IR-HeNe are propagated

to the plasma and back. The DM3 mirror can sepa-

rate the return probing beams in each wavelength again. Interference beat signals of the C02 and the

IR-HeNe are detected by a mercury-cadmium-tel]u-

rium (HgCdTe) detector and an Indium-Arsenide (InAs) detector. Visib]e HeNe lasers (0.633 um) are

used for the alignment of optical axes.

without the feedback control to avoid harmful influence

from varying lasing frequency. A Spectra Physics model

127 HeNe laser is modified to the IR-HeNe operation

of 3.39 um and is used as a light source of the second

interferometer. The IR-HeNe laser has an output

power of 8 mW and beam diameter of 2 mm. Both the

C02 and the IR-HeNe laser beams are divided into two

parts. One part is the probing beams which are propa-

gated to the plasma and another is local oscillator (LO)

beams for the heterodyne signal detection.

The intermediate frequency (IF) of 2 MHZ is chosen for the beat signal in order to satisfy the electri-

cal specification of the standard phase comparators

878

~~f~~5f~:~"~~~"~~~~~,~E~~

Tangential C02 Laser Interferometer for Large Tokamaks j~~p, ~~~,4ti;

which are used for this system. To produce the 2 MHZ

beat signal, a couple of AOMS (for 40 MHZ frequency

up shift and for 42 MHZ down shift) are used as a fre-

quency shifter for LO beams.

The probing beams returned from the plasma are

re-separated into two wavelength components at DM3

dichroic mirror. 2 MHZ beat signals of the C02 and the

IR-HeNe interferometers are detected by a room tem-

perature HgCdTe (mercury-cadmium-tellurium) detec-

tor and a electronically cooled InAs (Indium Arsenide)

detector, respectively.

3.1.3 RelaY Optics

The probing beams has to be propagated from the

optical bench to the CCR and back with a total path

length of up to about 100 m. This is also a long path-

difference interferometry up to about 100 m because

there was no retardation optics for path length match-

ing between the probing path and the reference (LO)

path. The relay optics is used for this long distance pro-

pagation of the probing beams. The relay optics con-

sists of a FLG (flexible light guide), two sets of tele-

scope optics, nine flat mirrors and a CCR. All mirrors

are installed on concrete bases or stable structures in

boxes called mirror box (MB). Protected silver coating

is used for all mirrors because its high reflectivity not

only in the infrared region but also at the wavelength of

a visible HeNe laser which is employed to align the op-

tics.

It is necessary to convert the diameter of the C02

laser beam to an appropriate diameter for its propaga-

tion of about 50 m to the CCR because the original

divergence of the laser beam is too large ( - 2.9 mrad)

to reach to the plasma. Figure 8 shows the diameter of

the C02 Iaser beam designed and measured for propa-

gation in the relay optics and the plasma. The propaga-

tion parameters of the beam are determined so as to

locate the beam waist is at the CCR with the diameter

of ~)ccR (= 1 1 mm) and to minimize the beam diameter at the window, i.e. ~ X ~)ccR (= 15.6 mm).

E E -~ (D

~ (D

E CIS

:~5

E C~S

(D L:

Fig.

! e : C02 ( I0.6um )

JL : IR-HeNe (3.39um )

x i Vis-HeNe (O. 633um)

MBI MB6 30 ~ ~ 20

lO

8

JT-60U VV

r~~l l~~~E~l

O

O I O 20 30 40 50 diStance [m]

Diameters of three different wavelength iaser beams of the C02, the IR-HeNe and the visible HeNe propagating in the relay optics [10]. The dis-

tance is measured from the TM9 mirror shown in Fig. 7. The technique of common path mode match-ing is used for this long distance propagation. Lines

and symbols show designed diameters and measured diameters, respectively. There are three

propagation regions; Region l; [aser beams propa-

gate with their original divergences, Region ll; Iaser

beams are highly collimated, Region lll; beam diameters are optimized to launch into the vacuum

vessel. Diameters of the C02 and the IR-HeNe in the

plasma are about 14 mm and 8 mm, respectively.

=z [1+( ' Rz kr ) J 22 o

Z (25)

a magnification factor for the diameter of the IR-HeNe

is determined to satisfy the following relationship be-

tween the IR-HeNe laser beam and the C02 Iaser

beam,

3.1.4 Common path mode matching Because the relay optics presented in section 3 . I . 3

are designed for the C02 Iaser propagation, the IR-

HeNe laser needs a diameter conversion so as to propa-

gate in a similar shape with that of the C02 Iaser in the

relay optics. For Gaussian beam propagation given by,

rz~ "ti rO

(24)

ro'c02 := ~ k r2 ~ const Or ro'HeNe

~ ox ' (26)

where kx = 291~/~ is the wave number for a wavelength

~, ro is the beam radius at the waist, r, is the radius at a

distance z from the waist, and R, is the radius of the

curvature of a wave front at z. In this manner, r./ro and

R. can be determined only by z, enabling that the C02 ,

IR-HeNe, and other wavelength lasers are propagated

879

~~ ~7 ･ ~~~=~~~~A" ~~*~~~"~-"L~

in a similar shape in a common relay optics. Diameters

designed and measured for the IR-HeNe and the visible-HeNe lasers are also shown in Fig. 8. It is to

note that the common path mode matching works well

in long distance beam propagation for different wave-

length lasers.

3.1.5 Vacuum windows Material of vacuum windows are ZnSe (zinc se-

lenide) plates (110 mm dia. X 10 mm thick). The va-

cuum windows are located at two equatorial diagnostic

ports which are apart toroidally with a toroidal angle of

100 deg. Two plates are used at each vacuum port for

the safety against a leak of radioactive materials. The

effective diameter of a window is 90 mm where a

special coating for high transmissions are applied.

Transparencies for wavelengths of the C02 Iaser, the

IR-HeNe laser, and the visible HeNe laser are 92"/~ for

10.6 um, 95*/. for 3.39 um, and 80"/. for 0.633 um, re-

spectively. All windows are tilted by 3 deg against the

incident axis of the laser beams to avoid stray reflec-

tions from window surfaces to oscillators.

3.1.6 Signal detection and data acquisition

Figure 9 shows a schematic of the signal detection

and data acquisition. A probing laser beam and an LO

laser beam are superimposed on a detector surface to

produce an interference beat signal with the IF fre-

quency of 2 MHz. The beat signal is fed to electrical fil-

ters and pre-amplifiers. After that, it is converted to an

optical signal to be transmitted to a phase comparator

using an optical fiber cable. A reference signal is gener-

ated from AOM driver signals by an electrical fre-

quency mixer. This signal is also transmitted to the

phase comparator and is used as a 2 MHZ reference sig-

nal. The phase comparator can measure the phase shift

signal with a resolution rf = 1/100 of a fringe by 200

MHZ digital scale clock. The phase shift data of the

C02 and the IR-HeNe laser interferometers can be

sampled by every 5 us at the maximum and are pro-

cessed by the main frame computer (Fujitsu M780/10)

to a line electron density f n. dl using eq. (15) as,

j FIR _ HeNe 2- ) f ne dl= - 2.34 X 1020 ( Fco (27) 3'12215 '

where Fco and FIR-H*N* denote measured phase shift

data of the C02 and the IR-HeNe laser interferometers

in unit of fringe, respectively.

~i~73~~~~ 8 7~= 1997~~ 8 ~l

r~~11T~~T~i~TI~~ Torus BUld. B1 F C02 Laser

Af 10~um 2MHZ Beat 40MHZ rJ~

-- Ft!1~t----･---O FC v HgcdTe 42MHZ~:=~~~L~~' '

Detector fo 100HZ fc 4MHZ

IR-HeNe Laser Monrtor out 339 m ~~~:.m 2MHZ 8eat Af '

40MHZ~si;~ nl~~~^~__ . I~~ Am . L.~~ Am E- OFC __ 'Detector fc=100Hz ~~)~ fC=4MHZ Vg * LL:~JO '--'---42MHZ L ' v

Monitor out

AOM Dr'ver

40MHZ

AOM Driver

42MHz

BPF 40MHZ f0=40MHZ

M'xer

2MHZ BPF Beat f0=42MHz 42MHz

Amp LPF OFC Am E<) ----f c=4MHZ

C02 AOM Modulators

BPF 40MHZ f0=40MHZ

Mlxer

Monitor Out

AOM Drlver

40MHZ

AOM Driver

42MHz

LPF Amp fc=4MHZ Am~ E-O

OFC :

2MHZ BPF Beat f0=42MHz 42MHz

IR-HeNe AOM Modulators

Mon'tor Out

Fig. 9

Main Frame

Fujitsu

M 78 O/i O

Data Storage

Schematic of the signal detection and the data ac-

quisition. The signa] detection part consists of two

probing branches and two reference branches. In-

terference beat signals detected by detectors are

filtered and amplified prior each phase counting.

Reference beat signa[s are generated from AOM driver signals, 40 MHZ and 42 MHz, by e]ectrical fre-

quency mixing. The data acquisition part consists

of phase comparators with fringe resolution of 2~/

100, the data storage system and the main frame

computer. Maximum rate of the data sampling is 200 kHZ (5 us intervais).

3.2 Operational results

3.2.1 Typical waveforms Typical interferometer data for a JT-60U plasma

(plasma current lp = I .7 MA) with the neutral beam in-

jection (NBI) is shown in Fig. 10. Figure 10 (a) and (b)

are phase shift signals of the C02 and the IR-HeNe

laser interferometers, respectively. Vibrations with a

frequency of about 10 Hz and an amplitude of 0.05

mm - 0.1 mm are observed in both phase shift signals.

This is caused by the vibration of mirrors installed near

the vacuum vessel. The drift like change in phase shift

signals seen in Fig. 10 (a) and (b) is caused by a mech-

880

Tangential C02 Laser Interferometer for Large Tokamaks i~~~p, d~~4Lh

70 60

50

40 50

20 10

o

SN 1 6348

(L)

o J (¥,

O (,

c:

J :~

~~

a:

'l'

q) c7,

LL

o q) ~~ E ::,

Z

(L)

,~

LL

o (L,

J:)

E

Z

200

l 50

l OO

50

o

fl~

(a)

l

(b)

o 2 4 6 8 lO 12 14

c¥J 2 IE

o c¥'

O L-,

':~~ 1

~~

C ~1

o

8

rl ,¥'

IE 6 'g'

~O 4

O:

LL ~t 1:

02 LS~

o

i 16

(c)

t

(d)

ch.2

ch.i

~ ~;

2

l

l

(e) ~ tirrri

(J-

20

Fig. 10

10 '*a:z~*::

'~-: -

O ' O O 2 4 6 8 10 12 14 t [S]

Typical waveforms of the C02/lR-HeNe laser inter-

ferometer: (a) phase signal of the C02 interfe-rometer, (b) phase signa[ of the IR-HeNe interfe-

rometer, (c) Iine e]ectron density by the C0211R-

HeNe laser interferometer, (d) Iine e]ectron density

measured by the two channel alcohol laser inter-

ferometer, (e) plasma current /p and neutral beam

injection power PNB . In this case, a phase jump is

observed at 12 s in the IR-HeNe laser interfe-

rometer.

anical displacement of mirror mount structures due to

magnetic fields during the tokamak operation. Figure

10 (c) shows the line electron density calculated from

Fig. 10 (a) and (b) for one way path length of 6 m in

the plasma, where the phase shift due to the mirror mo-

tion is compensated. A peak value is about 2 X 1020

m~2 at t = 6.5 s, which corresponds to about one fringe

for one way path of the C02 Iaser in the plasma.

Hence, the line averaged electron density of the tangen-

tial chord is 3.3 X 1019 m~3. The channel 2 of the alco-

hol laser interferometer views vicinity of the plasma

center. The line averaged electron density by the chan-

nel 2 of the alcohol laser interferometer (see Fig. 10

(d)) is 2.9 X 1019 m~3, which is about 150/. smaller than

that of the C02/IR-HeNe interferometer. This is prob-

ably explained by the enhancement effect of the central

density for the tangential chord of the C02/IR-HeNe

system.

An oscillation with a frequency of about 200 Hz

and an amplitude of 2 - 3 X 1019 m~2 is superimposed

on the density trace as shown in Fig. 1 1 . In this case,

the density resolution (width of the density trace) is

determined by this type of oscillation and it is about ten

times worse than the expected value of 3.1 X 1018 m~2

as shown in Table I . In the case of the dual C02 Iaser

interferometer, which will be presented in section 4.1.4,

this 200 Hz oscillation is substantially suppressed when

retardation optics are used, but is still observed without

the retardation optics. Therefore, the origin of this os-

cillation is concluded to as a frequency fluctuation of

the C02 Iaser.

3.2.2 Measurement of electron density beha-vior of high pp H mode plasmas

The C02/IR-HeNe interferometer contributes to

study the high pp H-mode plasmas founded in JT-60U.

The high fip H-mode plasma is characterized by peaked

profiles of electron density and ion temperature related

to the formation of the internal transport barrier and

the edge transport barrier [ 1 8]. Typical temporal evol-

utions of electron density profile and ion temperature

profile are shown in Fig. 1 2, where lp = 2 MA and the

toroidal magnetic field at the plasma center Bt = 4.4 T.

The 28 MW NBI heating is started at 5.05 s. The elec-

tron density profile is evaluated by a functional fitting

for data from the C02/IR-HeNe laser interferometer

(for information of the plasma central region), the two-

channel alcohol laser interferometer (for information of

of plasma half radius) and the visible bremsstrahlung

emission rate [ 19] (for information of plasma edge re-

gion). The ion temperature profile is measured by the

881

j~ ~~7 ･ ~~~:i~~~~:A~S~#*

SN 1 6348

2

1

o

2.1

(a)

2.0

r-1 1 .9 (¥i

IE

o 18 c¥l

O LJ

8 2.0 ~;l

~' c(L)

¥~ 1.9

1 .8

2.0

1 .9

1 .8

Fig. 1 1

o 15

e.o

i

6.0

t [s]

6.0 1

Line electron density of the C02/IR-HeNe laser in-

terferometer. The data are identical to that in Fig.

10. The line density resolution is about 2 - 3 x 1019

m~2, which is determined by the fluctuation like

noise with the frequency of 200 Hz.

charge exchange recombination spectroscopy [ 1 3 ,20].

As shown in Fig. 12, after the NBI starts, electron den-

sity and ion temperature profile become peaking due to

intense NBI deposition at the plasma central region.

Further profile peaking is observed both in the electron

density and ion temperature during clear internal trans-

port barrier is formed at 5.56 s - 5.81 s (high pp mode).

After the edge transport barrier is formed at 5.81 s (H

transition), profiles are broadened by the edge pedestal

with keeping central parameters (high pp H-mode).

1997~1; 8 ~I

8

~~6 E

04 c2

(a )

/

/

/

/

/ /

o

SNI 71 1 a

55

60

t (s)

/ /

40

3a

> ~~ 20

H Io

o

5.0

o 0.2 04 0~ 08 1.0 r (m)

(b)

Fig. 12

Edge transport barrier

nternal transport barrier

High-~ p H-mode

High-~ p mode

5.5

t (s)

Y 5.0 Soundary O 0.2 0.4 O.6 O.8 1.0

r (m)

Temporal evolutions of spatial profiles of eiectron

density and ion temperature of a high pp H-mode

plasma, where r is the p]asma minor radius. The electron density profile is evaluated by a parabolic

function fitting for data of the C02/lR-HeNe inter-

ferometer, the alcohol laser interferometers and a

profile of visible bremsstrah]ung emission. The ion

temperature profile is measured by the charge ex-

change recombination spectroscopy. The internal

transport barrier is observed from 5.56 s to 5.81 s

(high pp mode) and the edge transport barrier is

observed from 5.81 s (high pp H-mode).

Such a combined characteristic of the high fip mode and

the H-mode resulted in the enhanced improvement of

confinement, and the highest fusion triple product in

1993 of 1.1 X 1021 m~3 ' s ' keV is achieved at 6.27 s

[11-13]. Figure 13 shows the plasma stored energy

W . the neutron emission rate S the central electron

d** , ~ , density n. (O) and the volume averaged electron density

< n* > as a function of the line averaged electron den-

sity of the tangential C02 chord i~*co,. Data are taken

from the experiment campaign in 1993. It is found that

the Wdi* increases lineally with increase in the ;~*co, and

the S~ increases parabolically with increase in the ~*c02

For the high pp H-mode case, the < n* > is larger than

that of the high pp mode for the same ~*co,

882

~~~~7~;*~'~~~~FE~~ Tangential C02 Laser Interferometer for Large Tokamaks ~~~~, ~~4t~

This means the profile broadening which results in the

increase in the Wdi* . Furthermore, the fip collapse due

to a highly-peaked pressure profile can be avoided or

softened by the profile broadening. Thus, it is con-

cluded that the performance of the high pp mode and

high fip H-mode plasma are strongly depend on elec-

tron density behavior.

4. Dual C02 Iaser interferometer

In order to resolve the problems of the C02/IR-

HeNe laser interferometer mentioned in section I , the

significant modification has been applied to develop a

dual C02 Iaser interferometer [ 14]. The dual C02 sys-

tem has advantages such as; a good capability for large

mechanical vibration and displacement of reflection

mirrors, a better transparency of the vacuum windows,

an easy laser beam monitoring, a simplified optical

layout, an easiness to get enough lasing power, and so

on. Two closer C02 Wavelengths, however, are disad-

vantageous to extract a density component from two

phase shift signals of interferometers in given phase res-

olution. Therefore, to develop the dual C02 system,

there are three important issues which should be over-

come. They are related to the reliability of the phase

difference measurement in two interferometers, i. e. (a)

the minimization of the uncoaxial path part between

two interferometers, (b) the minimization of the path

difference between the probing arm and the reference

arm (LO beam), (c) the minimization of the difference

of IF frequencies between two interferometers. We

have succeeded to overcome those issues mainly using

techniques of (a) the setting up two C02 interferome-

ters in similar arrangement of optics, (b) the utilizing of

retardation optics, and (c) the simultaneous frequency

shift of two wavelengths lasers using a single AOM, re-

spectively. The results of the techniques are described

in following subsections.

4.1 Description of dual C02 interferometer

The dual C02 Iaser interferometer cbnsists of

seven major parts; C02 Iaser oscillators, frequency shif-

ters, detectors, relay optics, retardation optics, vacuum

windows and data acquisition system. Relay optics and

vacuum windows are not changed from the C02/IR-

HeNe interferometer.

10

o O

H i g h - ~p

H i g h - pp H mode mode

~ 1 :~

~ c5 15

~

8

6

4

2

O

6

~)o

JP lg;e~o

c'

IP=1MA-2.2MA B =4.4T

l-1-

Cl' 4 'JD

1-

O r ¥,2

C:

(D

l~ CO IE

O 1-

O 1-,~,

A a)

C:

V

~ O ~ ,:) =

o

8

5~;..

~;:~~'.

4.1.1 Optical arrangement Figure 14 shows the optical arrangement on the

optical bench. The IR-HeNe laser is removed and an-

other C02 Iaser oscillator is installed on the bench.

Wavelengths of two C02 oscillators are tuned to differ-

Fig. 13

6

4

2

o

ne(O) -~ o

AkA ~~At A <n e >

O 1 2 5 6 3 4 n C02 19 -3 e (10 m ) Electron density dependence on plasma stored energy and neutron emission rate for the high pp

mode (open circles) and the high pp H-mode (c!osed circles) plasmas: (a) plasma stored energy

Wdj., (b) emission rate of D-D neutron S~, (c) cen-

tral electron density n*(O) and volume averaged

electron density <n*>. The Wd., increases li-nearly with increase in the iT*co, and the S~ in-

creases parabolically with increase in the iT*c0=

ent branches as 10P(20) with wavelength of 10.6 um

(10.588149 um) and 9R(20) of 9.27 um (9.268831 um), respectively. Usually, both lasing frequencies are

stabilized by the active feedback control of the cavity

length of the oscillator even during a tokamak dis-

charge. It is to note that there are two sets of C02 Iaser

interferometer, which are aligned in similar form in

each other as possible for the minimization of the un-

coaxial beam path.

883

~~ ;~7 ' ~~~~;~~IA~l~~~--"~t' 1997~# 8 ~]

Vibration isolation bench (3xl .5m)

TM2 - --~:)~:~T:;2 1~~~ ~ HgCd Te Detectors

TM11 TM13

FL2

HM4

Fig. 14 Layout of optical components of the dua[ C02 laser interferometer on the vibration isolated bench. Two C02 oscillators are tuned to ditferent

branches of 10P(20) (10.6 um) and 9R(20) (9.27 um). A visible=HeNe laser is used for the align-

ment of optical components. BE; beam expanders to modify laser beam parameters so as to be pro-pagated in reiay optics. TM; total reflection mirror.

HM; ha]f reflection mirror. DM; dichroic mirror working as a reflector for only 9.27 um. FIL1; fiiter

of transparent tor a 9.27 um iaser beam and cut-ting a 10.6 um beam. FIL2; filter of tranparent for a

10.6 um laser beam and cutting a 9.27 um beam. L; Iens. AOM; acoust-optic-modu[ator head used as the frequency shifter for the heterodyne detec-

tion. After emerging from HMI and HM2, the so[id

line denotes probing beams and the dotted line

denotes LO beams.

To produce the 2 MHZ beat signal, a couple of

AOMS are commonly used as a frequency shifter for

both C02 Iaser beams. In order to match two beat fre-

quencies, a new technique is developed, i. e. the simul-

taneous frequency shift of different wavelength lasers

by using a single AOM. Note that to apply an identical

drive signal to two individual modulators in common is

not sufficient because independent characteristics of the

modulators are not removed. Here, incident beam

angles against a AOM input aperture are 38.55 mrad

for 10.6 um and 28.89 mrad for 9.27 um. By this align-

ment of incident angles, two beams are emerged coax-

ially from the AOM output aperture. The probing

beams of 10.6 and 9.27 um are superimposed each

other at the DMI dichroic mirror and they are 40 MHZ

up shifted at AOMI in common. The LO beams are

also superimposed at the DM2 dichroic mirror and are

42 MHZ up shifted at AOM2 in common. After that,

probing beams are led into the relay optics to the

plasma and LO beams are led into the retardation op-

tics.

The returning laser beams from the vacuum vessel

and the retardation optics are re-separated into two

wavelength components by DM5, DM6 and infrared filters (FIL1, FIL2). Each 2 MHZ beat signal is detected

by a room temperature HgCdTe detector.

4.1.2 Relay optics, retardation optics, and va-

cuum window The relay optics and the vacuum window are al-

most same as that of the C02/IR-HeNe system. The

common path mode matching enables the long distance

propagation for beams of three wavelengths of 10.6,

9.27, and 0.633 um. Mirror mountings installed at the

beam launching port are replaced to be one made of

FRP (~iber ~einforced ~lastics) to avoid an electro-

magnetically induced force. For vacuum windows, it

was not necessary to modify the transparency of the

ZnSe plates for a new wavelength of 9.27 um.

A total path length of probing laser beams in the

relay optics is up to about 100 m to go to the CCR and

back. If there is a certain fluctuations of the lasing fre-

quency, the large path difference between probing

beams and LO beams can produce a harmful phase shift. To suppress this effect, it is useful to match the

both path lengths in each other. In this case, a path

length of the LO beams should be retarded using the

retardation optics. For this purpose, compact retarda-

tion optics which has a three dimensional geometry is

developed. A retarded path difference is 39.61 m,

which corresponds to a path length from the MB2 to

the CCR of the relay optics.

4.1.3 Signal detection and data acquisition

The signal detection and the data acquisition are

modified from that of the previous C02/IR-HeNe sys-

tem. The detector of the IR-HeNe is replaced to that of

the second C02 interferometer. The reference signal is

commonly used as a 2 MHZ reference signal for both of

10.6 and 9.27 um interferometers. The line integrated

electron density is given by,

f ( F9.27 ) ne dl=:: - 9'01356 x 1020 ~Fro'6 ~ 1'14234

where Fro.6 and F9.27 denote

measured by the 10.6 and 9.27

unit of fringe, respectively.

,

(28)

phase shift signals

um interferometers in

4.2 Operational results

4.2.1 Typical waveforms Figure 15 shows a typical waveforms of line elec-

tron density measured by the dual C02 interferometer.

884

I~f ~~5f~~"'~~~~~~n~ ~~ Tangential C02 Laser Interferometer for Large Tokamaks j~ ~~ , ~~~4L~

A plasma is initiated at t = 3.1 s and the lp reaches its

flat top of 2 MA at t = 6 s as shown in Fig. 15 (a). The

ICRF injection is applied in this discharge. Figure 15

(b) - (d) show traces of a line electron density for dif-

ferent time resolution of the data; i.e. (a) 10 us (raw

data), (b) I ms (averaged for 100 samples), and (c) 10

ms (averaged for 1,000 samples). The density resolu-

tion of raw data is - 2 x 1019 m~2, which agrees with

the expected density resolution as shown in Table I . By

an appropriate data averaging or smoothing, however,

better density resolution can be easily obtained as

shown in Fig. 15 (c) and (d). For instance, the density

resolution of - 0.5 x 1019 m~2 is obtained for a time

resolution of I ms.

4.2.2 Measurement of density behavior during fast major disruption

During a fast major disruption, it is often difficult

to measure electron density by using a conventional

FIR alcohol laser interferometer. A rapid change in

electron density and/or a large density gradient in

space prevent a reliable phase detection. The C02/IR-

HeNe system also used to suffer from fringe counting

loss in the IR-HeNe interferometer due to the large

mechanical vibration and displacement of mirrors dur-

ing the disruption. These problems become more seri-

ous with increase in the disruption speed. In contrast to

above systems, the dual C02 Iaser interferometer is

more robust and it succeeded in measuring electron

density during fast disruptions.

Figure 16 and 17 show waveforms of disruptions

which have different current quench speed. The current

quench speed is characterized by the current decay time

Tlp-d"ay defined by lp (O)/(dlp /dt)m" [21], where lp (O)

is the plasma current just before thermal quench, (dlp /

dt)~ax is the maximum current quench rate during the

early phase of the current quench as shown in Fig. 1 8 .

With respect to the increase in electron density, the in-

crement ratio R.e ~ A J n. dl/f n. dl(O) is introduced,

where A J n. d I is the increment in line electron density

from just before thermal quench to just before the cur-

rent decay begins, and f n, dl (O) is the line electron

density just before the thermal quench as shown in Fig.

18. Figure 16 shows a relatively slow density-limit dis-

ruption which Tlp-de'ay rs - 63 ms. In this case, Rne is

relatively small of - 0.3. Figure 17 shows a relatively

fast disruption caused by high plasma internal induct-

ance li , which lr is - 1 1 ms. It is to note that R is

lp-d*'*y "* larger value of - 3.4. It has been found that the disrup-

tion speed becomes faster with increase in R** . This re-

lationship is clearly shown in Fig. 19 for various type

I~< t-':~

':t' fE

l~o ~~c c¥lO_

2.5

o 2

f

o

2

c~' IE i ~'o (L)c¥'

~~O ~O

qt'~:

~)E ':'~D

C ~~t-L~

Fig. 15

2

l

o

SN 28262

(d)

10

-1~:~ 5c~~~_

o

2 4 14 6 8 tO 1 2 t (s)

Typical waveforms of line e]ectron density measured by the dual C02 interferometer. Col-umns (b)-(d) show traces oi a line eiectron den-sity for different time resolution of the data; (a) 10

us (raw data), (b) I ms (averaged for 100 samples),

and (c) 10 ms (averaged for 1,000 samples). The density resoiutions of the 10 us trace and the I ms

trace are -2 x 1019 m~2 and -0.5 x 1019 m~2, re-

spectivel y.

disruptions. The reason of the relationship above can

be explained by followings; considering that the density

increment attributes to influx of the carbon impurity

into the plasma from divertor tiles during thermal

quench, Iarger Rne causes lower electron temperature of

the plasma by intense radiation loss from impurities.

Hence, Iower electron temperature causes larger resist-

ance of the plasma which results in faster disruptions,

i e smaller T As a result lr -. , is strongly af ' ' Ip-decay lp-d*,ay

fected by Rn* and less affected by the type of disrup-

tions.

In Fig. 19, the fastest disruptions which 1~ are lp-de'ay

several millisecond are observed when R** exceeds 10.

This observation might support the impurity influx

model [22] proposed for a mechanism of a fast disrup-

tion. However, there is a difference between the model

and the observation with respect to the timing of in-

crease in electron density, i. e. the model predicts that

885

~~~7 ･ ~~~~~~:AO~~:A7~~~~"~.*. ~~73~~~~8~~ 1997~~ 8 ~l

:~

1.5

o 6

r-I

tE

o (¥10

L~' O

SN20730 15

O

1. 5

h :~~ < :~

r-1 eJ IE o CY

O LJ

3

o

10

o

3

SN21483 1

O

1

h)

~

r-~ <C

~

rl ,c¥lE

o QJQ

Fig.

15

o 6

O

FT)

~

O

9.7 8 9. S 3 t [s]

16 Typical waveforms of a relatively slow density-limit disruption which Tllp-decay is - 63 ms. The

1,Ip-dec'y is the characteristic current decay time

defined by /p (O)/(d/p/dt)~ax' where /p (O) is the

plasma current just before thermal quench, (d/p/

dt)~ax is the maximum current quench rate during

the early phase of the current quench. In column (c) and (d), tl denotes the start time of the positive

current spike.

~

r=1

'E

o (¥JO

o

10

o

---~-------'¥. Jlp

( c) Wdlo ' l

(d) fnedL ~

TTTT

O

h :~

the density increment required for a fast disruption

should be observed before current spike arise, on the

other hand, in the observation, electron density begins

to increase at the mostly same time of the appearance

of current spike and the required density increment is

observed before current decay begins. Though more

precise data analysis is needed, the impurity influx

model should be examined carefully by the observa-

tions.

Consequently, it is expected that the disruption

study is further progressed by using the dual C02 Iaser

interf erometer.

Fig.

1 2.3 8 i 2.4 3 t [s]

17 Typical waveforms of a relatively fast disruption

caused by high p[asma internal inductance /.,. which 1,;lp-d**.y is - 11 ms. In column (c) and (d). tl

denotes the start time of the positive current spike.

xlp-decay ~:Ip(O)

Rne ~Afnedl /

/ (dl p/dt)

f n * dl(O)

5. Very high resolution phase comparator The phase resolution of the standard phase com-

parator which have been used so far is 2~/100 rad,

which corresponds to line electron density of about

2.11 x 1018 m~2. The effective density resolution,

Fig. 18

t

Definition of the characteristic current decay time

Tllp-dec'y and the increment ratio of electron density

R~* during a disruption.

886

Tangential C02 Laser Interferometer for Large Tokamaks ~~~~, ~~~~4~2

1 oO

~ CO

E ~ > cuo 10

~ c~

P

1

o density limit

A Iocked mode

R high lj

v ~p coliapse

Olep I

OA A A

AA

v

l V

I

1;lp-decay= 1 8.3xRne O 38

ll

l

0.1 1 O 1

Rne Fig. 19 Relationship between 1~1lp-decay and R for various

"e type disruptions. It is shown that the T1 lp-d*oay becomes smaller with increase in R~*.

however, is reduced in the dual C02 system due to two

closer wavelengths of C02 Iasers. The effective phase

resolution for the density measurement is down to 201;/

1 2.5 rad and the density resolution is relatively poor of

about 1.7 x 1019 m~2 (see Table 1), which agrees with

the observed resolution presented in section 4.2. This

resolution, however, is still good enough if the density

trace is smoothed or in the case of large density plas-

mas. If measurement for small and fast density change

is required, the density resolution should be improved.

In order to improve the effective density resolution

of the dual C02 Iaser interferometer and to have the

same order of the resolution of that of the FIR alcohol/

Vis.HeNe laser interferometer, the phase resolution of

- 2~/104 rad must be achieved. For this purpose, a

very high resolution phase comparator (VRPC) has

been developed [ 14,15].

5.1 Principle and specifications ofVRPC A very high phase resolution is achieved using the

two-stage time-difference measurement in the VRPC.

For the first stage, a time difference between a ref-

erence signal and a probing signal of the interferometer

tD is measured by a scale clock of 100 MHz. Secondly,

a residual time interval between the last scale clock

pulse and the probing signal A t is measured by the pre-

cise analog-voltage measurement which has 11256

resolution for a interval of two clock pulses. Therefore,

the expected time discrimination becomes 39.0625 ps

(= (1/100 MHz)/256). Actually, the results of an indi-

vidual module test show that the time discrimination

accuracy of the VRPC is about 30 ps [ 14]. According

to these results, it is concluded that the accuracy of the

time discrimination satisfies the designed value. Here,

based on the designed time discrimination, the phase

resolution is 291/12,800 rad for the usual 2 MHZ beat

signal of the dual C02 Iaser interferometer. In this case,

the effective phase and density resolutions of the dual

C02 system are ideally expected to be 2~/1,572 rad

and 1.34 X 1017 m~2, respectively. This density resolu-

tion is in the same order of magnitude of that of the al-

cohol/Vis.HeNe interferometer (see Table 1). Further-

more, the acceptable input frequency ranges widely

from 10 kHZ to 50 MHZ in contrast to the standard

comparator's 2 MHZ only. As a result, the phase resol-

ution ranges from 2~/512 to 2~/(2.56 x 106). Thus, a

wide range of selections of the time response and the

phase resolution are available.

5.2 Operational results

5.2.1 Phase resolution In the previous paper [15], the first operational re-

sults of the VRPC was presented, where 3.3 kHZ noise

component on the 10.6 um interferometer signal was

observed. This noise component was revealed due to

the improved resolution using the VRPC. We found

that the 3.3 kHZ noise was originated from an electrical

power line. Figure 20 shows phase shift signals

measured by the VRPC after the 3.3 kHZ noise source

was eliminated. A phase shift signal of the 10.6 um in-

terferometer is shown in Fig. 20 (a), (c), (e), (g) and a

phase shift signal of the 9.27 um interferometer is

shown in Fig. 20 (b), (d), (D, (h). The signal sampling

rate is 100 kHZ (10 us intervals). Here, plasma current

was started at 3.1 s, its flat top of 1.8 MA was sus-

tained from 5.6 s to 8.5 s and was ended at 10.8 s.

Mechanical vibrations are observed after the beginning

and the ending of the discharge as shown in Fig. 20 (a)

and (b). The vibration with a frequency of - 30 Hz can

be seen in Fig. 20 (c) and (d), which mainly originates

from the vibration of the mirror mounting structure

(this structure is further stabilized by a significant modi-

fication to one mentioned in section 4. 1.2).

A comparison of phase resolutions of the VRPC

with that of the standard comparator is shown in Fig.

2 1 . It is to note that very small changes of the phase

signal are well detected in the case of the VRPC (Fig.

21 (a)). Thus, the phase resolution of the dual C02

laser interferometer is significantly improved by the

VRP C .

887

j~;~7 ･ ~^~~~j~~~~~~-.-"#* ~~73~~~1~87~= 1997~~ 8 ~I

5.2.2 Density resolution

Figure 22 shows the line electron density calcu-

lated from data shown in Fig. 20. It is found that the

density resolution is improved. The line density resolu-

tion is - I x 1019 m~2 which is about a half of that of

the standard comparator case as shown in section 4.2. I .

Considering the path length of about 5 m in the plasma,

the density resolution is - 2 X 1018 m~3 in this case. A

main source of the width of the trace are low frequency

( - 300 Hz) fluctuations with amplitudes of 0.5 - I X

(L)

C~,

C ~-

(L)

O _C Q.

1 20

40 1 20

40

1 Oo

90

80

100

90

100

90 110

l OO

9 9. 1

SN 2e382

t

(L)

~)

*H

(:)

(:)

J= CL

98.94

98.90

98.2e

T 1 /1 O~ LO

Fig. 21

SN2e382

SN26372

98.22

9.0015 9.0020 t (S)

Comparison of phase resolutions between (a) the

VRPC (2~/12,800 rad) and (b) the standard phase

comparator (2~/100 rad). Precise phase measure-

ment is possible using the VRPC in compared with

using the standard phase comparator.

2

1

o

~ c¥J

'E 1.0 o (¥'

O

~~ 10 C'D

~*

0.7

1.01

SN 2e382

98.7 1 04.2

1 03.8

(g)

(h)

9 9.005 t (S)

Phase data of the dual C02 Iaser interferometer

using the VRPC. Columns (a), (c), (e) and (g) are

data of the 10.6 um interferometer and (b), (d), (f)

and (h) are data of the 9.27 um interferometer. P]asma current is built up at 3.1 s, its flat top of 1.8

MA is sustained from 5.6 s to 8.5 s, and is ended at

1 0.8 s.

0.7

1,01

Fig. 20

Fig. 22

1

0.7

9 9.005 t (S)

Line electron density calcuiated from data shown in Fig. 20. The line density resolution is - I x 1019

m~2 (the density resolution of -2 x 1018 m~3)

which is determined by noise components with a

frequency of - 300 Hz. Thickness of the density trace shown in (d) is I - 2 x 1018 m~2.

888

~f'i~~~~C 7u pml Tangential C02 Laser Interferometer for Large Tokamaks ~~~~', J~~~4L~

1019 m~2 as shown in Fig. 22 (c). This fluctuation is the

noise because it exists even during no plasma period.

Origins of the noise are not clear yet but are supposed

to be related to the vibration of mirrors or a fluctuation

of the DC ground level of signals. In detail, the thick-

ness of the density trace is about I - 2 X 1018 m~2

which is seen in Fig. 22 (d). Though this resolution is

an order of larger than the ideal value, it is an order of

better than that of the standard comparator. Hence, the

density resolution of I - 2 X 1018 m~2 is expected by a

further reduction of noise components.

6. Comparison with ITER requirements According to the results shown in section 4 and 5 ,

it is investigated that the dual C02 Iaser interferometer

has enough performances for electron density measure-

ment in JT-60U. Additionally, this system can be also

used on future devices like ITER. Table 2 shows a

comparison between the requirements for a tangential

interferometer in ITER [ 23] and the achieved perfor-

mance of the dual C02 System in JT-60U. In ITER, an

accuracy of lo/. ( - 5.4 X 1019 m~2) is required for a

time resolution of I ms. Regardless of data averaging,

these requirements are satisfied in the dual C02 Iaser

interferometer as shown in section 4.2.1. Further im-

provement of the dual C02 System is also expected by

using the VRPC as shown in section 5.2.2. Conse-

quently, the basic feasibility of the dual C02 Iaser inter-

ferometer on ITER is well demonstrated.

7. Discussions The IR-HeNe laser interferometer as a second in-

terferometer was disadvantageous for JT-60U because

of the relatively weak output laser power and the trans-

parency issue at vacuum windows. It seems difficult to

obtain more higher power of IR-HeNe laser because its

low oscillation gain. To keep windows and mirrors

clean is also difficult in JT-60U and future large de-

vices. Though the CO Iaser interferometer has a poten-

tial to be used as a second interferometer, the safety in

handling of the CO gas must be established. Recently, a

CW YAG oscillator (1.06 um) which has an output

power of several hundreds mW or more is getting avail-

able. Its wavelength is shorter than that of the IR-HeNe

laser, however, the higher output power and the exist-

ence of highly sensitive APD detector are preferable to

the weak power of an IR-HeNe oscillator and an InAs

detector.

In the dual C02 System, it is very important to

align two probing beams in coaxial strictly. Otherwise, a

slight drift appears on the calculated density trace due

to an error in the path length compensation. There are

two ways to suppress this kind of drift. The first one is

to use a firm structure made by FRP to mount the mir-

ror near the vacuum vessel, which is already installed.

The second is an active stabilization of the optical axis

during the plasma discharge by using an feedback con-

trol of the beam inj ection mirror near the vacuum

vessel. This feedback control system is now under fabri-

cated for the JT-60U.

The small number of AOMS makes the interfe-

Table 2 Comparison between the requirements for a tangential interferometer in ITER and the achieved performance of the

duai C02 system in JT-60U as shown in section 4.2.1. The line e]ectron density is calcuiated using a laser path

length in a plasma of - 13.6 m which is evaluated by the plasma geometry of ITER. The laser beam is assumed to

tangentially pass through the center of a plasma in ITER.

889

~~ ;~7 ' ~~~~li~~~;~~~~~#* ~~73~~~~ 8 ~~- 1997~p 8 ~l

rometer optics simple. In the case of the dual C02 sys-

tem, a couple of AOMs, 40 MHZ and 42 MHZ fre-quency shifts, are used to produce a 2 MHZ IF signal.

The VRPC can measure the signal with a frequency

range from 10 kHZ to 50 MHz. Therefore, 40 MHZ beat signals are directly acceptable and only one com-

mercial AOM is sufficient for the heterodyne detection.

An electrical frequency converter from 40 MHZ to 2

MHZ can also reduce the number of AOMs. It is ex-

pected that the interferometer will become more stable

by these ways of simplification of the AOM part.

A design of a multi-chordal interferometer for fu-

ture large devices must face a port limitation issue. Ver-

tical or tangential ports which have a pair of windows at

each side of the vacuum v~ssel are favorable to mini-

mize the influence of darkening and vibration of reflec-

tion mirrors. If the reflection mirrors must be installed

inside of the vacuum vessel to ensure sufficient number

of diagnostic chords, the dual C02 combination is one

of the possible candidates. Its unique capability of the

vibration compensation at the mid-infrared region is an

advantage. The relatively high lasing power, the simple

hardware layout and the easiness of purchasing hard-

wares are also attractive.

The C02 oscillators and other electronic compo-

nents should be installed apart from tokamak devices to

avoid influences of strong magnetic field and high en-

ergy radiations. Therefore, the laser beams have to be

propagated for long distance from the oscillators to the

plasma. The relay optics is used to propagate the prob-

ing laser beams in this study. The beam path guided by

the relay optics, however, becomes complicate because

it must take a roundabout course due to the presence of

obstacles between oscillators and the plasma. Addition-

ally, the accessibility to the vacuum window should be

considered. On the other hand, if an optical fiber cable

is available for the C02 Iaser interferometer for some or

major part of the beam guiding, the probing beam path

has a flexibility to be designed. The retardation optics is

also simplified by using fiber cables. Unfortunately,

there is no fiber cables which fully satisfies the specifi-

cations for the C02 interferometer yet. It is hopeful that

the development of the optical fiber cable for the C02

laser is further progressed.

Laser interferometry can not completely avoid a

risk of "fringe jump" which arises when the interference

condition is broken by some reasons. Once "fringe

jump" occurs, reliability of the density signal would be

reduced afterwards. Though the dual C02 Iaser interfe-

rometer is robust system, a stable diagnostic system

should be developed to improve the reliability in future

large tokamaks like ITER. For this purpose, C02 Iaser

polarimetry based on the mid-plane Faraday rotation

concept is considered to be one of the possible tool for

a reliable density measurement [24]. The most attrac-

tive advantages is that the polarimeter can realize the

density measurement without the interference proce-

dure. In the interferometer, usually a time history of

phase shift signals up to the time of interest is required

to obtain temporal data. Cor~trary to this, the Faraday

rotation angle at any sampling time can provide data

individually. Additionally, Iaser qualities not enough for

interferometry might be still enough for polarimetry,

and a path length change by a vibration along the beam

direction dose not affect the Faraday rotation. Accord-

ing to reasons above, a tangential C02 Iaser polarimeter

has been developing in JT-60U to demonstrate a proof

of principle of the the mid-plane Faraday rotation con-

cept [25].

8. Summary The novel system of the C02 Iaser interferometer

which has a toroidally tangential chord has been de-

veloped for large tokamak devices. The results are sum-

marized as follows;

(1) The propagation mode is in the QT mode for 85'

~ o ~ 90' otherwise the QL mode for O' ~ o ~ 85' for

a C02 Iaser wave propagating in large tokamak plasmas

with an arbitrary angle to the magnetic filed e. The re-

fractive index for the ordinary mode propagation is

valid to use for the C02 Iaser wave with any O accept-

ing a maximum error of -2.5 X 10-5 (-0.0025*/o) from

the Alter-Appelton-Hartree dispersion relation for a

plasma of n* = I x 1020 m~3 and B = 5 T.

(2) The effective density resolution and the selection

conditions of a wavelength combination are discussed

for a two-color C02 Iaser interferometer for tokamak

plasmas. It is found that favorable wavelength combina-

tions for large tokamaks are the C02 (10.6 um)/IR-

HeNe (3.39 um) and the C02 (10.6 um)lC02 (9'27

um) in the case of considering darkened windows and

large vibration of mirrors.

(3) The C02/IR-HeNe laser interferometer has been

developed. A Iong distance interferometry is achieved

with a path difference of about 100 m between the

probing and the LO beams. The technique of the com-

mon path mode matching enables different wavelength

laser to be propagated in a similar Gaussian shape. The

electron density behavior of the high pp H-mode plas-

mas in JT-60U is investigated using the C02/IR-HeNe

laser interferometer.

(4) The dual C02 Iaser interferometer with a wave-

890

~ ~ ~7~) *~~i~~'~~-~~~ Tangential C02 Laser Interferometer for Large Tokamaks ~~T~p, j~~4til

length combination of 10.6 and 9.27 um has been de-

veloped. There are three major issues for the reliable

density measurement by close wavelengths interferome-

ters. Namely, the optics of two C02 Iaser interferome-

ters are arranged in a similar form of optics. The retar-

dation optics is utilized to match the path difference

between the probing and the LO beams. The technique

of the simultaneous frequency shift for different C02

wavelengths is developed using a single AOM. Conse-

quently, the line electron density of the JT-60U is suc-

cessfully measured even during fast maj or disruptions.

The effective line density resolution observed is - 2 X

1019 m~2 for a time resolution of 10 us.

(5) The phase comparator with a very high resolution

has been developed. The designed resolution is 291;/

12,800 rad for 2 MHZ Signals, which is more than hun-

dred times better than that of the standard phase com-

parator. For the precise time discrimination, the two-

stage time measurement technique is developed using

the combination of the digital scale clock and the

analog voltage measurement. The line density resolu-

tion observed is - I x 1019 m~2 so far, which is two

times better than that of the case using the standard

phase comparator. The resolution of I - 2 X 1018 m~2 is

expected by further noise reductions.

(6) The performances of the dual C02 Iaser interfe-

rometer achieved in JT-60U satisfy the requirements

for an interferometer in ITER (accuracy; Io/* ( - 5.4 x

1019 m~2), time resolution; I ms). The basic feasibility

of the dual C02 Iaser interferometer for ITER is dem-

onstrated.

Acknowledgments The authors acknowledge Y. Endo for his special

technical cooperations. The authors appreciate Dr. T.

Matoba. Dr. S. Ishida, Dr. T. Fukuda, Dr. H. Shirai,

Dr. M. Kikuchi and Dr. H.K. Park for their fruitful dis-

cussions. The authors thank NEC corporation, Nippon

Advanced Technology Co. Ltd., Hitachi Plant Con-

struction & Engineering Co. Ltd., Kiyohara Optics Inc.

for their engineering cooperation. The authors

appreciate Dr. M. Mori, Dr. R. Yoshino, Dr. H.

Ninomiya, Dr. M. Shimada, Dr. M. Nagami, Dr. H.

Takeuchi, Dr. A. Funahashi, Dr. M. Azumi, and Dr. H.

Kishimoto for their continuous support and encourage-

ment.

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