Continued Work toward XHV for the Jefferson Lab … AVS 2011.pdf– Compare gauge with electron...
Transcript of Continued Work toward XHV for the Jefferson Lab … AVS 2011.pdf– Compare gauge with electron...
Marcy Stutzman, Philip Adderley, Matt Poelker
Thomas Jefferson National Accelerator Facility
Newport News, VA 23601
Continued Work toward XHV
for the
Jefferson Lab Polarized Electron Source
Thomas Jefferson National Accelerator Facility
• 6 (12) GeV Electron accelerator
for Nuclear Physics
• 85-90% polarization, up to
~250μA beam (CW) to 3 (4)
experimental halls
• DC photoemission electron
source
• Strained superlattice
GaAs/GaAsP photocathode
• NEG and Ion pumps
• Residual gasses are ionized,
back-accelerated and degrade
the photocathode
• Future accelerators (CLIC, EIC,
ILC) require higher currents
Marcy Stutzman AVS 2011
Cryopumped gun project
• Investigate adding bakable cryopump into system
– Leybold Coolvac 2000 BL, special order
– Cryosorber panel can be chilled with LN2 during bakeout
– Isolation valve for regeneration
– Chamber: currently in heat treatment
• Can we measure improvement in vacuum due to
cryopump?
Characterize UHV/XHV gauges
Marcy Stutzman AVS 2011
Ionization gauge current contributions
Ireal: pressure dependent gas phase ions – species sensitive
Ix-ray: x-ray induced electron desorption from collector
– reduce by geometry
IESD: ions arriving at collector from electron stimulated
desorption (ESD) of molecules on the grid
– reduce by degassing grid
Iheating: pressure rise due to filament heating – species sensitive
– reduce by material selection, geometry, long duration
.. neutESDrayxinvESDheatingrayxrealmeasured IIIIIII
Marcy Stutzman AVS 2011
Deep UHV/XHV gauges
• Extractor gauge – available for decades
– x-ray limit reduced through geometry
– x-ray limit quote: 7.5x10-13 Torr
• Axtran gauge – Bessel box energy discrimination
– electron multiplier to assist in low current measurements
– Purchased, not yet installed
– Measurement limit quote: <7.5e-15 Torr
• Watanabe BBB (Bent Belt Beam) gauge – Newly designed (JVSTA 28 (2010) p. 486)
– Operates with Leybold IE540 controller
– 230° degree deflector (similar to Helmer)
– BeCu housing to reduce Iheating
– Manufacturer’s lower limit: 4x10-14 Torr
extractor
BBB
Axtran
Marcy Stutzman AVS 2011
Gauge characterization chamber
• Heat treated twice
– 400°C 10 days
• Outgassing (Q)
– 3x10-14 Torr·L/s·cm2
• Q following 250°C bake
– 6.3x10-14 Torr·L/s·cm2
• 3800 cm2, 12L
• Pumping
– 4 WP1250 NEGs, 60%
1300 L/s
– 40 L/s ion pump
(behind right angle
valve)
• Extractor Gauge
• BBB Gauge
• 2 Leybold IE540
controllers
• 2 Keithley
electrometers
• UHV ion pump
power supply
• Diagnostic cross
with RGA and ion
pump
• NEG activation
flange Predicted pressure
2x10-13 Torr
Marcy Stutzman AVS 2011
Linearity between gauges
1.E-12
1.E-11
1.E-10
1.E-09
1E-12 1E-11 1E-10 1E-09
BB
B (
To
rr)
Extractor (Torr)
cooling runs heating runs
BBB and Extractor compared
vs. pressure
– Leybold IE 540 control
– Keithley Electrometer
Pressure varied in chamber by
heating NEGs or chilling
Conversion to Torr using
manufacturer calibration
factor / sensitivity
– Depends on species
– Ionization energy
– Ionization current
– Geometry
Gauge responses linear response over decades,
possible deviation at lowest pressures
Marcy Stutzman AVS 2011
Sensitivity
e
i
IP
IS
Ion current
Emission current
0.E+00
2.E-08
4.E-08
6.E-08
8.E-08
1.E-07
0.E+00 1.E-06 2.E-06 3.E-06 4.E-06 5.E-06 6.E-06
BB
B c
urr
ent
(Am
ps)
SRG (Torr, Hydrogen)
hydrogen pressure rise
0
2
4
6
8
10
0.E+00 1.E-06 2.E-06 3.E-06 4.E-06 5.E-06 6.E-06
Sen
siti
vit
y (
1/T
orr
)
SRG measured hydrogen pressure
BBB sensitivity vs. SRG
BBB and Extractor
– 120V electron energy
– 1.6 mA emission current
– geometry, collection
efficiency vary
BBB vs. SRG data from previous setup
Marcy Stutzman AVS 2011
BBB sensitivity calculated using extractor
0
2
4
6
8
10
12
14
1E-12 1E-11 1E-10 1E-09
BB
B S
ensi
tiv
ity (
1/T
orr
)
Extractor Pressure (Torr)
SRG Data: pressure 105 higher than our area of concern
Calculate BBB sensitivity from Extractor gauge pressure?
Calculated Extractor
pressure relies on
sensitivity of extractor
gauge.
e
i
IS
IP
Gauge sensitivity depends on
• Ionization energy
• Gas species
• Geometry
• Collection efficiency
same
constant
Marcy Stutzman AVS 2011
relative sensitivity
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.E-14 1.E-13 1.E-12 1.E-11
S(B
BB
) /
S(E
xt.)
Extractor current (Amps)
ratio of manufacturer
quoted sensitivities:
1.21=8/6.6
Define sensitivity ratio
- should be constant EXT
BBB
EXT
BBB
I
I
S
S
Can the deviation from constant behavior be explained by gauge backgrounds?
Marcy Stutzman AVS 2011
Background current measurements
0
10
20
30
0 100 200 300 400 500 600
Co
llec
tor
curr
ent
(fA
)
Reflector voltage (V)
Extractor x-ray current
accounts for
3/5 of total measured signal
BBB signal of 35 fA with
background of -1 fA
-10
0
10
20
30
40
50
0 50 100 150 200 250 300 350
Co
llec
tor
Cu
rren
t (f
A)
Deflector Voltage (V)
Gauge backgrounds
measured at different
times, different pressures
Marcy Stutzman AVS 2011
Sensitivity ratio: x-ray limit correction
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
1.E-15 1.E-14 1.E-13 1.E-12 1.E-11
Sen
siti
vit
y B
BB
/ S
ensi
tiv
ity
Ex
tra
cto
r
Extractor current (Amps)
• Subtraction of x-ray
background for
extractor gauge
overcorrects
• What else?
– ESD limits
– Load due to gauges
– Small current
measurement errors
– nonlinearity in
gauge response?
Marcy Stutzman AVS 2011
Electron stimulated desorption
• Electrons can liberate elements
adsorbed on the grid
• If grid - filament potential equal to
electron energy, ESD difficult to
separate
• Methods to reduce ESD
– high energy electron bombardment
(degas mode)
– operate grid at elevated temperature
– grid material optimization (BBB)
– stabilize for months
– Axtran: energy analysis since
electron energy ≠ grid-filament
potential
... ESDheatingrayxrealmeasured IIIII
Marcy Stutzman AVS 2011
Current due to heating by filaments
Use one gauge to measure the
additional current generated by
other hot filament
... ESDheatingrayxrealmeasured IIIII
0.E+00
1.E-14
2.E-14
3.E-14
4.E-14
5.E-14
0 10 20 30 40 50 60 70 80 90 100
Ga
ug
e cu
rren
t (A
mp
s)
Time (hours)
BBB turned off
BBB
Extractor
ΔI (BBB) = 4.8 fA
ΔI (Extractor) = 5.6 fA
BeCu BBB housing should reduce effect
Difference minimized after 6 months?
Marcy Stutzman AVS 2011
So what is our pressure?
0.0E+00
5.0E-13
1.0E-12
1.5E-12
2.0E-12
2.5E-12
0 5 10 15 20 25 30
Ca
lcu
late
d p
ress
ure
(T
orr
)
Current (fA)
extractor
BBB
x-ray background
BBB heating Ext. heating
... ESDheatingrayxrealmeasured IIIII
predicted BBB (Torr) Extractor
Measured 1.4 x10-12 1.9 x 10-12
Corrected 1.1 x 10-12 5x10-14
Predicted 6x10-13
* pressures nitrogen
equivalent, Svendor
Marcy Stutzman AVS 2011
Conclusions
• Pressure in our systems (nitrogen equivalent) corrected for
gauge effects is near 1x10-12 Torr
• BBB signal to noise good: Noise < 10% signal
• Extractor gauge: measurements at lowest pressures
dominated by background
• BBB and extractor agree very well above 1x10-11 Torr
• The BBB gauge should be able to quantify pressure
improvements in the bakable cryopump system.
Marcy Stutzman AVS 2011
Future work
• Verify outgassing rate of chamber
– Is our predicted pressure correct? Does measured pressure agree?
• Add Axtran gauge to the system
– Compare gauge with electron multiplier to avoid some small signal
measurement issues
– ESD ion discrimination capability
– Determine which of the BBB and extractor is deviating from linear
response
• Gauge calibration to verify / determine sensitivity
– Repeat comparison with SRG (hydrogen vs. nitrogen)
– UHV calibration at NIST, PTB…
– XHV calibration (its own research project!)
• Add cryopump to NEG / ion pumping system
– Can we use the cryopump alone to achieve better pressures?
– Can we better activate NEGs?
– Is NEG / Ion pumping system limiting our pressure?
Marcy Stutzman AVS 2011