Marcy Stutzman, Philip Adderley,
Veronica Over, Matt Poelker
Thomas Jefferson National Accelerator Facility
Newport News, VA 23601, USA
Investigations of cryopumping for
extreme high vacuum systems
Marcy Stutzman IUVSTA Cryo Workshop 2016
Thomas Jefferson National Accelerator Facility
Marcy Stutzman IUVSTA Cryo Workshop 2016
Outline
• Overview of JLab SRF beamline vacuum, operation
• Jefferson Lab polarized electron source
• Cryopumping for XHV
– Conventional charcoal cryopump
– Novel nanomaterial tests
• Summary
Marcy Stutzman IUVSTA Cryo Workshop 2016
JLEIC
Thomas Jefferson National Accelerator Facility
US Department of Energy, 12 GeV electron accelerator
Up to 90% polarization from DC photoemission source
Electron currents to 200μA beam (CW) to four experimental halls
Halls A, B, C
Hall DSource
Polarized source
Marcy Stutzman IUVSTA Cryo Workshop 2016
JLab SRF Accelerator
Original CEBAF
20 cryomodules,
4 five cell cavities in each
CEBAF 12 GeV upgrade
Add 5 cryomodules/Linac,
4 seven cell cavities in each
Charlie Reece, Superconductor Science and Technology Special Issue
Marcy Stutzman IUVSTA Cryo Workshop 2016
SRF Cryomodule Performance
A. Freyberger, IPAC2015
Marcy Stutzman IUVSTA Cryo Workshop 2016
SRF system vacuum: operational issues
Primary concerns at CEBAF
• Gradient / Q achieved in each cryomodule
• Particulate transport mitigation
• Particle / Field emitter elimination during processing
Cryomodule Cryomodule
Warm girder
Ion pumps Beamline valves
Marcy Stutzman IUVSTA Cryo Workshop 2016
(out of 1 GeV/pass)
Marcy Stutzman IUVSTA Cryo Workshop 2016
SRF particulate hunting efforts
JLab, Rongli Geng
Marcy Stutzman IUVSTA Cryo Workshop 2016
Beamline pumping
4x Differential pumping (NEG and ion)
beginning and end of each Linac
35 L/s ion pumps
Warm girder between CM
Pump drop for CM vacuum space
Adding NEGs to one Cryomodule, Girder
Goal: mitigate Ti, Ta
particulate from ion
pump, improve H2
pump speed
Marcy Stutzman IUVSTA Cryo Workshop 2016
Outline
• Overview of JLab SRF beamline vacuum, operation
• Jefferson Lab polarized electron source
• Cryopumping for XHV
– Conventional charcoal cryopump
– Novel nanomaterial tests
• Summary
Polarized source
Marcy Stutzman IUVSTA Cryo Workshop 2016
DC Photoemission Source
• Strained superlattice GaAs/GaAsP photocathode
• Residual gasses ionized, limit operational lifetime
• SAES WP NEG modules
• Gamma Vacuum XHV/SEM style ion pump
• Base pressure approaching XHV ≡ P < 1x10-10 Pa
-130kV
light inelectrons out
0.
36
GaAs
GaAsP
GaAs
Strained
Superlattice
Photocathode
14 layers
Marcy Stutzman IUVSTA Cryo Workshop 2016
Existing Electron Source Pumping
• Heat treat to reduce outgassing
– 400°C, 100 hours
– Q=1.33x10-10 Pa·m·s-1
• Installed pumping
– SAES WP1250 modules x 10 (5600 L/s total)
– Gamma SEM/XHV ion pump
(45 L/s)
• Measured pressure ~1.3x10-10 Pa
(extractor gauge, x-ray limit measured and subtracted)
• CEBAF Lifetime: ~100 Coulombs = 10 days at 100 uA
• Extrapolating e-RHIC to 10 mA: 100 Coulomb lifetime < 3 hours
Marcy Stutzman IUVSTA Cryo Workshop 2016
XHV Cryopumping
Prior research toward XHV
cryopumping
– Double shield walls
• Shiokawa 1996
– Reduced cryosorber temperatures
• Iwasa 1996
• Our initial path toward XHV
cryopumping
– Leybold bakable XHV cryopump
• LN2 chill circuit
– 10K cryosorber
– 30K single shield wall• Presented at AVS 55 by Dieter Mueller
Marcy Stutzman IUVSTA Cryo Workshop 2016
Commercially available XHV gauges
extractor
3BG
AxTran
Backgrounds
• x-ray limits (measured, corrected)
• Heating (measured, corrected)
• ESD effect (degas, long stabilization)
Advertised x-ray limit (Pa)
Extractor 1 ×10-10
AxTran 5 ×10-11
3BG1 5 ×10-12
1) F. Watanabe, JVSTA 28, 486 (2010)
Marcy Stutzman IUVSTA Cryo Workshop 2016
Leybold Cryopump test system
• Three XHV Gauges
• Heat treated chamber for lower
outgassing
• NEG / Ion pumps
– Overboard bake ion pump
• 10” all metal gate valve
• Bellows (12” to 10” adapter,
vibration isolation)
• Cryopump
– Overboard bake ion pump
– Compressor
– ColdHead
– LN2 chill line / chill line evacuation
system
Marcy Stutzman IUVSTA Cryo Workshop 2016
• Getters
– 4 x WP950 modules
– 1 GP500 pump
• Ion pump
– 45 l/s SEM style Ti/Tan
• Outgassing rate
– 1.3x10-10 Pa·m·s-1
• Volume: 40 l
• Area: 8000 cm2
Simple calucuations:
expected pressure
5 x 10-11 Pa
Chamber with NEG/Ion pumps
S=2200 l/s60% NEG activation
Marcy Stutzman IUVSTA Cryo Workshop 2016
NEG, Ion Pumps only NEG, Ion and Cryopump
• Getters 2150 l/s
• Ion pump 45 l/s
• Cryopump 800-1300 l/s
(conductance dependent)
• Area: 25,000 cm2
– 8,000 cm2 Chamber
– 3,400 cm2 Bellows
– 13,700 cm2 Valve
• Outgassing rate valve (not treated)
≥ 6x10-9 Pa m s-1
Calculated pressure
~ 5 x 10-11 Pa
• Getters 2150 l/s
• Ion pump 45 l/s
• Area: 8000 cm2
• Outgassing rate (heat treated)
– 1.3x10-10 Pa m s-1
Calculated pressure
5 x 10-11 Pa
Do we gain no benefit
from cryopumping,
or are our calculations
too simple??
Marcy Stutzman IUVSTA Cryo Workshop 2016
Molflow+ simulations
• Molflow+ software: Roberto
Kersevan and Márton Ady
• 3D CAD model cryopump
system, including vibration
isolation bellows and gate
valve
Chamber with
ion and NEG
pumps
Gate valve
representation
Bellows
Cryopump
Marcy Stutzman IUVSTA Cryo Workshop 2016
Test Particle Monte Carlo
• Tracks simulated
particles
– Pump speeds defined by
sticking coefficients
– Outgassing rates as
measured previously
– Compare simulation with
measured results
Ion pump
Cryopump
Valve thickness,
effective
outgassing rate
XHV gauges
NEG
GP500
pump
Bellows
NEG Sorb-AC
WP950 pumps
Marcy Stutzman IUVSTA Cryo Workshop 2016
Molflow+ cryopump simulations
P(simulated)=
2.7x10-12 mbar
P(simulated)=
1.6x10-12 mbar
gauge
position
Valve Closed Valve OpenSimulations show
outgassing from
valve, conductance
severely limit
cryo-pumping on
chamber
Charcoal sticking
coefficient α=0.6
mbar
Marcy Stutzman IUVSTA Cryo Workshop 2016
Molflow+ Cryopump Only
P(simulated)=
7x10-12 mbar
No NEG pumping
• Simulations predict
pressures a factor of 2.5
higher if NEGs removed:
• Cryopumping largely
ineffective in this
geometry
• High conductance
configuration with NEGs
in chamber far superior
for achieving XHV
mbar
Marcy Stutzman IUVSTA Cryo Workshop 2016
Cryopump Testing
• Full system bakeout: Chamber, Cryopump with LN2 chill
• NEGs partially activated during bakeout (60%)
• Gauges on and degassed
• Record pressure vs. temperature until gauges stable >1 month
Marcy Stutzman IUVSTA Cryo Workshop 2016
Chamber pressures
0
1
2
3
4
5
6
25 30 35 40
Pre
ssu
re (
x10
-12 T
orr
≈ 1
0-1
0P
a)
days
Ext (keithley)
Axtran (keithley)
3BG controller
NEG, ion
pumps only
Open Valve
to cryopump
Corrections for gauge heating, x-ray limits
Pressure drop due to
turning off Axtran
filament
Marcy Stutzman IUVSTA Cryo Workshop 2016
Performance of Leybold cryopump
Pressure ~4x10-10 Pa
◦Small discrepancy in
manufacture gauge
calibrations
Simulations either1. Overestimate
Outgassing
2. Underestimate
conductance to
cryopump
3. Sticking coefficients
(from literature)
incorrect for this
system
0
1E-10
2E-10
3E-10
4E-10
5E-10
6E-10
7E-10
Extractor 3BG AxTran
Pre
ssu
re (
Pa)
Equilibrium pressures
NEG, Ion only
NEG, Ion and Cryopumps
Marcy Stutzman IUVSTA Cryo Workshop 2016
• Cryopump during chamber bakeout?
• Cryopump during NEG activation?
– Heat load in addition to gasses
How best to incorporate Cryopump?
• Cryopump during beam operation?
– Vibration issues due to compressor
Cryopump stage 1
and 2 temperatures
Pressure (Torr)
0
10
20
30
40
50
1
10
100
1000
10000
0 1 2 3 4
Tem
per
atu
re (
K)
Pre
ssu
re (
x1
0-1
2 T
orr
)
Time (hours)
NEG activation
Marcy Stutzman IUVSTA Cryo Workshop 2016
Cryopump as process pumping
1E-10
1E-09
1E-08
Initial bake, CP and
chamber
NEG activation into CP chamber bake into CP
Pre
ssu
re (
Pa
)
NEG, ion only
NEG, Ion, Cryo
Marcy Stutzman IUVSTA Cryo Workshop 2016
Leybold Residual Gas Species Analysis: 9K
XHV test chamber with NEG, ion pumps, gauges, VQM
Not fully baked for this gas analysis
(vented, moved, reconfigured, reassembled)
Cryopump cooled down, 4.5 days elapsed time
Pressure: 1x10-9 Torr
VQM spectrum shows partial pressures at 100 AMU
1.E-14
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-071 5 9
13
17
21
25
29
33
37
41
45
49
53
57
61
65
69
73
77
81
85
89
93
97
10
1
10
5
10
9Part
ial
pre
ssu
res
(Torr
)
AMU
1e-9
Marcy Stutzman IUVSTA Cryo Workshop 2016
Cryopump with BN Nanomaterial Cryosorber?
• Boron Nitride Nanomaterial (BNNT) as
potential cryosorber
– Freestanding boron nitride nanotubes
• Mechanical attachment possible
– High thermal conductivity
– Chemically inert
– Very porous structure
– Local JLab affiliated company
• Potential benefits over Leybold Cryopump
– Simpler bakes
– No adhesives
– hydrocarbon free
– EUV lithography applications?
1 g BNNT
10 g CNT (similar scale)
US Patent pending
Marcy Stutzman IUVSTA Cryo Workshop 2016
First Try: BNNT installed as cryosorber
• Nanomaterial mechanically
attached (no adhesive)
• Copper mesh for retention
• Glass tube / ceramic beads
insulating diodes
BNNT material:
Stays in place through static electricity
Marcy Stutzman IUVSTA Cryo Workshop 2016
Ion
pump
H2
Stabilion
VQMExtractor
Test
chamber
orifice
motor
Cryopump
BNNT cryosorber test setup
• Removable cryopump
motor, displacer: fully
bakable
• Gas inlet system, orifice
for pump speed and
capacity measurements
Marcy Stutzman IUVSTA Cryo Workshop 2016
BNNT cryopump: first cooldown
• Baked 48 hours ~150°C
• Coldhead temperature: 15.5K (14K before BNNT)
• 39K: pressure drop – Cryopumping
0
50
100
150
200
250
300
350
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
0 6 12 18 24
Cry
op
um
p T
emp
era
ture
(K
)
Pre
ssu
re (
To
rr)
Hours
P(Extractor)
T1
T2
Marcy Stutzman IUVSTA Cryo Workshop 2016
Hydrogen pump Speed vs. gas dose
• High initial pump
speed
– Modification of
test dome
procedure
– Conductance
limiting orifice
– 1 g BNNT
– 15.5 K base
temperature
• Poor capacity as
expected
0
5000
10000
15000
20000
0.000001 0.0001 0.01 1
Pu
mp
Sp
eed
(L
/s)
Gas dose (TorrL)
Red: First run, varied pressure
Blue: Second run, constant inlet pressure
Marcy Stutzman IUVSTA Cryo Workshop 2016
Mass spectra comparison
1.E-14
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1 4 7
10
13
16
19
22
25
28
31
34
37
40
43
46
49
52
55
58
61
64
67
70
73
76
79
82
85
88
91
94
97
10
0
10
3
10
6
10
9Parti
al
pre
ssu
res
(Torr
)
AMU
BNNT warm
BNNT cold
1.E-141.E-131.E-121.E-111.E-101.E-091.E-081.E-07
1 4 7
10
13
16
19
22
25
28
31
34
37
40
43
46
49
52
55
58
61
64
67
70
73
76
79
82
85
88
91
94
97
10
0
10
3
10
6
10
9
Parti
als
(T
orr
)
AMU
Leybold XHV pump
Cold, unbaked
1e-9
GP VQM, total pressure with
Leybold extractor gauge
Marcy Stutzman IUVSTA Cryo Workshop 2016
BNNT limitations
• First BNNT prototype: 1 g of material = 300 m2
• Typical Cryopump: up to 100,000 m2 installed charcoal surface area
• Hydrogen specific cryopumps
– T ≤ 10K
– Charcoal on underside of fins
• BNNT first prototype
– T ~ 15K
– Heat load due to copper mesh absorption
– BNNT applied only to tops of fins
• BNNT second prototype improvements
– Fine wire attachment, no copper mesh
– Total 4.4g BNNT, 1000 m2
– Attachment on both sides of fins
Marcy Stutzman IUVSTA Cryo Workshop 2016
BNNT simulations
Simulated pressure profile: Upgraded BNNT cryopump
Sticking coefficient: top and
bottom of fins
α =0.003 α =0.012
Simulated Gauge Pressure 6.2x10-9 Pa 1.55x10-9 Pa
First BNNT prototype
Sticking coefficient
calculated from
measured pressure
α =0.003
Measured pressure 6.2x10-9 Pa
mbar
as built α = 0.003 α = 0.0122nd
Prototype
model
Marcy Stutzman IUVSTA Cryo Workshop 2016
Summary
• JLab Cryomodule vacuum system
– investigating particulates, addressing hydrogen pump speed
• Evaluation of Leybold XHV cryopump
– Three gauges characterize pressure near XHV
– Experimental results: only small improvement
– Simulations, calculations and measurements find conductance,
valve outgassing limit results
– Mass spectrum: adhesive contamination?
• BNNT cryopump testing
– Potential hydrocarbon-free cryopump
– Relatively low surface area, low effective α, minimal
cryotrapping?
– Next prototype assembled, to be tested shortly
Marcy Stutzman IUVSTA Cryo Workshop 2016
Backup
Marcy Stutzman IUVSTA Cryo Workshop 2016
BNNT properties
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.na
sa.gov/20140004051.pdf
Purity 40 to 50% by mass
Residual hBN flakes and micro-droplets of
elemental boron, by TEM
Walls 1 to 5 walls, mostly 2 or 3 walled tubes
Tube Length up to 200 microns by SEM
Surface Area up to 300 m2/g by BET
Bundles single through bundles of 5 tubes, TEM
Band Gap 5.7 eV (semiconducting) EELS
spectroscopy
Strength in air 800°C (CNT is 400°C)
Thermal
oxidation
resistance
Stable to at least 920˚C in air
Thermal
Conductivity
3000 W/mK (Cu = 400 W/mK,
CNT 60-40,000 W/mK)
TEM
Impurities: B and hBN
Marcy Stutzman IUVSTA Cryo Workshop 2016
BET surface density comparison
Charcoal properties* m2/g Density (g/cm^3)
Roth 1700 0.3
Chemviron GFF30 1600
Degusorb 2300 0.44
Chemviron SCII 1500 0.45
BNNT 121-300 0.001
*Christian Day, “The use of active carbons as cryosorbent”,
Colloids and Surfaces A 187 (2001) 187-206.
Lower surface area, but
Low density = high porosity: effect on capacity, speed?
Marcy Stutzman IUVSTA Cryo Workshop 2016
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
1.E-10
1.E-09
1.E-08
1.E-07
0.01 0.10 1.00 10.00 100.00
Pu
mp
sp
eed
(L
/s)
Pre
ssu
re (
To
rr)
Time (hours)
Pump Speed and pressure: 1g BNNT, 15.5K
P(Torr) Run 2
Speed (L/s) Run 2
H2 pump speed vs. time, 1g BNNT, 15.5K
Marcy Stutzman IUVSTA Cryo Workshop 2016
0
5000
10000
15000
20000
25000
30000
35000
1.E-10
1.E-09
1.E-08
1.E-07
0.01 0.10 1.00 10.00 100.00
Pu
mp
sp
eed
(L
/s)
Pre
ssu
re (
To
rr)
Time (hours)
Pump Speed and pressure: 1g BNNT, 15.5K
P(Torr) run 1
P(Torr) Run 2
Speed (L/s) Run 1
Speed (L/s) Run 2
H2 pump speed vs. time, 1g BNNT, 15.5K