Development of Mach probe for the ion flow measurement in VEST

S.M. Yang, N.K. Kim, H.Y. Lee, D.H. Na, J.G. Jo, J.W. Lee, M.G Yoo, G.H. Kim, Y.S. Hwang and Yong-Su Na*

Department of Nuclear Engineering, Seoul National University, Seoul, Korea

The Second A3 Foresight Workshop on Spherical Torus (ST)Tsinghua University, Beijing, China, Jan. 6-8, 2013

*Corresponding author’s E-mail: ysna@snu.ac.kr

Contents

2 Seoul National UniversityFusion and Plasma Application Laboratory

1. Motivation

2. Introduction

3. Basic test for the diagnosis- Circuit test- Area calibration

4. Flow measurement at the linear device with B field curvature

5. Future design

6. Summary

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Motivation– Toroidal rotation at the effect of the edge

• Toroidal rotation is found to have an impact on global energy or particle confinement.

• Toroidal rotation is beneficial on transport and MHD instability in tokamak plasmas.ex) suppression of turbulence, stabilization of the resistive wall mode

• The core momentum transport is not fully understood and it is known that the core rotation is heavily influenced from the flux at the edge.

• Rotation at the edge can be one of the important factors to understand the toroidal rotation

Measure ion velocity at the edge of the VEST

• There are mainly 3 diagnostics to measure the ion flow.

Among these, Mach probe can be the best choice for the ion flow measurement at the edge of the VEST, which has relatively low temperature and density.

Motivation- Diagnosis for the ion flow

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CXRS XICS Mach probe

Principle Doppler shift (line emission)

Doppler shift (X-ray)

Ratio of ion saturation currents

Pros No perturbation to the plasma Simpler to makeCheaper than other diagnosticsAppropriate for low temp. & dens.

cons NBI is needed Difficult and takes long time to make it.

Theoretical models is neededGive perturbation to the plasma Probe cannot endure high temperature

Introduction - What is the Mach Probe?

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• Measures the ion saturation current with 2 tips.

• If there’s any ion flow, ion saturation current at each tip has to be identical.

• In most theoretical models, flow is estimated by taking the ratio ofion saturation currents :

= : Calibration factor

Plasma Flow Direction

[E Ko at el., PPCF ’06]

BNC cable

BNC cablegnd

Introduction - Mach probe and DAQ circuit part

Mach probe part DAQ circuit part• Get the ion saturation current from a

plasma and transmit the signal to the circuit.

- Vacuum seal - Area calibration

• Convert the current signal to the voltage and send it to the oscilloscope.

- Circuit test

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Tip: Receive signal

DAQ circuit

Experimental Setup for Circuit Test

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1. Check the circuit with increasing the voltage level.2. Compare the voltage level between the 2 circuits

Square wave1kHzVsig= 0 - 2 V

Input Signal

Basic test for the diagnosis - Circuit test & result

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• Since Mach probe compares the ion saturation current between two tips, the voltage measured by a circuit using reference voltage has to be identical.

Both circuits gave almost same voltage.Circuit 1 and 2 are identical

-2 -1 0 1 2-2

-1

0

1

2

Meas

ured v

oltag

e(V)

Applied voltage(V)

Reference Circuit 1 Circuit 2

• Cylindrical tungsten tips are chosen since the geometrical effect of probe tips are known to be small in weakly magnetized plasma.

• Probe lengths are smaller than ion-neutral collision length and larger than De-bye length. (Probe area = , expected ion saturation current = 30mA)

• To measure the parallel flow(which supposed to have higher velocity) tip direc-tion is fixed as follows

Basic test for the diagnosis - Design of Mach probe

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tip

Probe 2(upstream)

Probe 1(downstream)

B field

Ceramic insulator(ceramic paste)

[In plasma] [Top view]

Ceramic tubeCeramic insulator(ceramic paste)

[T. Shikama at el., JNM ’06]

Experimental setup- Area calibration (DC plasma)

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• Hot filaments generate electrons in random direction, consider this doesn’t have any ion flow.

• Ion saturation current in the two tips need to be same if the area of those are same.

NS

N

Permanent magnet arraysforming cusp

W-ThFilaments

- 60 VField-free &Uniform DensityPlasma Region

Mach Probe

Basic test for the diagnosis - Area Calibration

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-60 V (ion dominant region) Source characteristic: 60 eV electron

Ion dominant region (‘Area effect on ion current’ only)

Perturbation of ceramic paste on tip (not uniformly distributed)+ Response of the fast electrons

• The current ratio is not unity.• Area ratio could be determined: = 1.13:1• Biasing voltage in this source << - 60 V (negative bias)

-90 -80 -70 -60 -50 -40 -30 -200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Cur

rent

Rat

io S

1/S2

Biasing Voltage

Area ratio S1/S2 : 1.13

Measurement at linear device with B field curvature

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• B field in VEST and linear device with B field curvature are similar.

• Parameters at the edge of VEST and linear device with B field curvature are not significantly different.

• As a preliminary diagnosis of the Mach probe at the edge of the VEST,measurement at the linear device with a B field curvature is done.

VEST(typical value)

Linear device with B field curvature

B field 0.1 T 0.0875 TElectron density

1 x 1018 m-3 (edge) 2 x 1017 m-3

Electron temperature

20 eV (edge) 8 eV

Experimental setup- for linear device (ECH plasma)

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Probe location Te ~ 8 eVne ~ 2 x 1017 m-3

B ~ 0.0875 T Operation pressure ~ 4 x 10-4 Torr, H2

(base 4 x 10-6 Torr)

Flow di-rection

[ECH Resonance region]

[11 cm]B-field di-rection

Magnetic field coils forming resonance B field with curvature

Mach Probe

ECH MicrowaveSource 300W

Measurement at linear device with B field curvature – Result

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• Linear device generally has M ~ 0.3 (, Ar)

• Considering the area difference =1.13:1, • According to Chung’s theory, K ~ 1.6, M ~ 0.2

[Chung, Hutchinson et al., PRA ’88]

[K. Nagaoka, et al J. Phys. Soc.Jpn. 70, 131, 2001]

Ion dominant region (Area effect only)

Ceramic paste perturbed the tip. (not uniformly distributed)Electron response is applied

-70 -60 -50 -40 -30 -20 -10 0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Cur

rent

Rat

io S

1/S2

Biasing Voltage(V)

Current Ratio ~ 0.8

** Area ratio was considered

• Since M ~ 0.2 is measured at linear device with magnetic field curvature,we expect to have similar ion flow at the edge of VEST.

• For the measurement with a better precision, new ceramic design (Alumina) is considered as in the below figure.- Less area difference for each tip- Uniform ceramic thickness will preserve uniform sheath for each the probe.- Two probes can be located in parallel (angle accuracy).

Discussion for the future design

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[ceramic for the tips]

Future design- Probe design for VEST

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• A Wilson type vacuum seal is used for the radial scan.• Length of the probe is determined for the measurement at the core for the

future.

[30cm]Ceramic tube

[40cm]¼” sus tube

Wilson typeVacuum seal

[Edge measurement] [Core measurement]

Summary

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• For the measurement of ion flow at the edge of the VEST, the Mach probe is se-lected for the low density & temperature plasma.

• Basic test for the Mach probe is done.- Circuit test- Area calibration

• As a preliminary measurement, the flow at the linear device with B field curva-ture is measured which expect to have similar parameters as in the edge of the VEST.

• The future design for ceramics for the tip is made.- Less area difference for each tip- Uniform ceramic thickness will preserve uniform sheath for each probe.- Two probes can be located in parallel (angle accuracy).

• To do a radial scan, vacuum type(Wilson seal) is decided with a sufficient length.

Back up- Mach probe models

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[Chung., PSST ’12]

Model by Mott-Smith(1926) and Hudis(1970) is older than other models Other unmagnetized model ranged 1.0~1.34

K can be lower than this model since strong magnetic field condition is not achieved.

But, tendency can be seen since exponential form is still conserved.

Back up- Design of the probe tips

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• The length between two probes considered to be have a small effect by experimentally comparing double sided Langmuir probe and Mach probe.

• Tungsten tip can endure high temperature.

• Debye length~ , ion neutral collision length >> mm (too low ).

• Geometrical effect is explained with comparing MP, DLP and SP with dif -ferent ion collection angle. ( respectively)

• Expected ion saturation current is estimated by Bohm flux, = 40mA

(Assuming

[L Okuz et al., PSST ’04]