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


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


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


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


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


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


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


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?


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


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?


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


Current due to heating by filaments

Use one gauge to measure the

additional current generated by

other hot filament


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?


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


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


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.


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?


Continued Work toward XHV for the Jefferson Lab … AVS 2011.pdf– Compare gauge with electron...

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Transcript of Continued Work toward XHV for the Jefferson Lab … AVS 2011.pdf– Compare gauge with electron...