estimates of residual gas pressure in the lhc
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Estimates of residual gas pressure in the LHC. Adriana Rossi AT-VAC Workshop on Experimental Conditions and Beam Induced Detector Backgrounds 03-04 April 2008. Outline. Vacuum calculations for LHC runs Input parameters Machine layout - PowerPoint PPT PresentationTRANSCRIPT
Estimates of residual gas pressure in the LHC
Adriana Rossi AT-VAC
Workshop on Experimental Conditions and Beam Induced Detector Backgrounds
03-04 April 2008
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Outline
Vacuum calculations for LHC runs Input parameters
Machine layout Results for “static pressure” in the experimental
LSS and comparison with measured values Comparison between filling factors Arcs Effect of collimators Ion operations Discussion Questions
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Vacuum calculations for LHC runs
dx
+p
i
e- e
ph
qth
ADAD
SR
Simulations code
Cylindrical geometry Time invariant parameters Multi-gas model Finite elements
Dynamic effects in LHC
Beam induced phenomena : ion, electron and photon induced molecular desorption.
Ion induced desorption instability Electron cloud build up
Input parameters depend on: Incident energy State of the surface (baked-
unbaked) Dose
The sources of gas depend on the surface properties and on the operating scenarios.
Estimates are only a snapshot of specific conditions
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Photon Induced gas Desorption
[Gröbner et al. Vacuum, Vol 37, 8-9, 1987] [Gómez-Goñi et al., JVST 12(4), 1994]
Evolution with dose Energy dependence
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Electron Induced Gas Desorption
J. Gómez-Goñi et al., JVST A 15(6), 1997Copper baked at 150ºC
G. Vorlaufer et al., Vac. Techn. Note. 00-32Copper Unbaked
Evolution with dose
Evolution with dose Energy dependence
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Secondary electron emission
N. Hilleret et al., LHC Proj. Rep. 472, 2001For “as received” Copper and
electron energy of 99eV and 500eV
1.2 1014 e/cm2/s
1.3 1013 e/cm2/s
1.1 1013 e/cm2/s
Evolution with dose
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0 500 1000 1500 2000 2500 3000PE energy (eV)
SE
Y
A. r.
2 h 120 C 2 h 160C 2 h 200 C 2 h 250 C 2 h 300 C
120°C x 2h
160°C x 2h
200°C x 2h
250°C x 2h
300°C x 2h
C. Scheuerlein et al., JVST A 18(3), May/Jun 2000.
TiZrV sample
0 500 1000 1500 2000 2500 3000Primary electron energy (eV)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
Energy dependence at different activation temperature
At few 10-10 p/b no e-cloud expected
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
NEG properties
[P. Chiggiato, JVC-Gratz-06-2002] [P. Chiggiato, JVC-Gratz-06-2002]
10-2
10-1
100
101
10-7 10-6 10-5 10-4 10-3
10-3
10-2
10-1
1001013 1014 1015 1016
Pum
pin
g S
pe
ed
[ s-1
cm
2 ]
CO Surface Coverage [Torr cm-2] S
ticking
facto
r
[molecules cm-2]
TiZrV on smooth Cucoated at 100 °C
TiZrV on rough Cucoated at 300 °C
CO
101
102
103
0 5 10 15 20
H2 p
um
pin
g s
pe
ed
[
s-1 m
-1]
Number of heating/venting cycles
200°C
Heating duration 24 hours
beam pipe diameter = 80 mm
TiZrV/Alheated at 180°C
TiZrV/Alheated at 200°C
TiZrV/St. Steelheated at 200°C
Pumping speed Aging
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Machine layout
Cold magnets provided with beam screen actively cooled between 5 and 20K :
Avoids ion induced pressure instability and guarantees a low background pressure.
In the CB at 4.5K, H2 will be cryosorbed on dedicated materials placed on the rear of the BS.
LSS room temperature sections are copper chambers coated with ~1 to 2 m of TiZrV sputter NEG
NEG coating is employed to prevent electron multipacting, given the low secondary electron yield after activation at a temperature between 160 and 200ºC for 2 hours, and to ensure low desorption and the gas pumping necessary for ion induced desorption stability and low background pressure
All room temperature sections are being baked-activated
Q5
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
LSS 1/5static pressure (w/o dynamic effects)
in terms of nuclear scattering
TCT
1.E+08
1.E+09
1.E+10
1.E+11
1.E+12
1.E+13
1.E+14
1.E+15
0 50 100 150 200
Den
sity
(m
ole
cule
s/m
3)
Distance from the interaction point (m)
ATLAS D1
D2-Q4Q1-Q2-Q3 Q5 Q6recombinationchamber
H2 equivalent
(nuclear scattering)
H2
CH4
COCO2
Nuclear scattering cross section: CH4/H2=5.4; CO/H2=7.8; CO2/H2=12
2
2
2
2
4
2
4
22 . COH
COCO
H
COCH
H
CHHequivH nnnnn
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Pressure as measured around the machine
TCT
1.E+08
1.E+09
1.E+10
1.E+11
1.E+12
1.E+13
1.E+14
1.E+15
0 50 100 150 200
Den
sity
(m
ole
cule
s/m
3)
Distance from the interaction point (m)
ATLAS D1
D2-Q4Q1-Q2-Q3 Q5 Q6recombinationchamber
H2
CH4
COCO2
5E-12 mbar@ 120m
<1E-11 mbar@ 151m
4E-12 mbar@ 178m
< 1E-11 mbar@ 58.9m
~10-9mbar@TCT
mainly H2
5·10-12mbar@120m
<10-11mbar@151m
4·10-12mbar@178m
1·1011 molec/m3 = 2.7 ·10-14 mbar at 2K
1·1011 molec/m3 = 4 ·10-12 mbar at 293K
TCTs (145 to 148m) not taken into account in any
of the plot presented
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
LSS 1 at machine startup
A. Rossi, LHC Project Report 783
Present scenario (MARIC) 2008: 43 bunches – few 1010 p/b – 5TeV photon flux reduced by ~50 expected pressure profile static
2008/9: 43 bunches – few 1010 p/b - 7TeV
2009: higher number of bunches
TCT
1.E+09
1.E+10
1.E+11
1.E+12
1.E+13
1.E+14
1.E+15
1.E+16
0 50 100 150 200
Den
sity
(m
ole
cule
s/m
3)
Distance from the interaction point (m)
CMS
H2
CO2
COCH4
H2 equivalent
D2-Q4Q1-Q2-Q3 Q5 Q6recombinationchamber
Scenario in 2004:44 bunches x 1.15·101011 p/b
7TeV
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
LSS 1 — 44 x 1.15·1011 p/b — 156 x 1.15·1011 p/b — 2808 x 3·1010 p/b
TCT
1.E+10
1.E+11
1.E+12
1.E+13
1.E+14
1.E+15
1.E+16
0 50 100 150 200
H2 e
qu
ivale
nt
den
sity
(m
ole
cule
s/m
3)
Distance from the interaction point (m)
D1
D2-Q4Q1-Q2-Q3 Q5 Q6recombinationchamber
—— 44 bunches x 1.15E11 p/b
—— 156 bunches x 1.15E11 p/b
—— 2808 bunches x 3E10 p/b
Calculation parameters
NEG with 1/10 of maximum pumping speed (to be conservative) Desorption yields as for surfaces never exposed to photons-electrons
A. Rossi, LHC Project Report 783
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Is this the upper limit?
Pressure estimates have a factor ~ 2 uncertainties and depends on assumptions made for calculations
Photon flux is an upper limit The desorption yields considered on Cu and uncoated parts
corresponds to what is expected at the beginning period The yields decrease with dose (ph and e- bombardment) :
conditioning Cryo-pumping is neglected (i.e. beam screen pumps only via
pumping holes)
The given estimate is the upper limit
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Effect of collimators
Collimator outgassing fully characterised: main gas H2
Experience to be made during operation: outgassing depends on jaw temperature
1.E-10
1.E-09
1.E-08
1.E-07
10/18/04 17:31 10/18/04 18:00 10/18/04 18:28
delt
a p
art
ial pre
ssure
(to
rr)
- non
calib
rate
d
1.E-10
1.E-09
1.E-08
1.E-07
delt
a p
ress
ure
(to
rr)H2
CH4
H2O
CO
C2H6
CO2
C3H8
VGI2
VGI3
Measurements in SPS
Insertion of Graphite collimator jaw ~1mm into beam
p ~5·10-9 mbar
H2 main gas – unbaked system
A. Rossi - unpublished
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Effect of collimators: heating of vacuum chamber due to energy deposition from particle losses
Temperature rise estimated to 150°C (R. Assmann) in momentum and betatron cleaning insertions, at nominal intensity
The pressure will recover when back to room temperature
Standard section - atomic concentration of <1·10-3
H2 pressure at 150°C ≤ 10-10 mbarA. Rossi - EPAC, Edinburgh UK, June 2006
RTHPsc HH exp2
MBW worst case scenario - atomic concentration of 2·10-2 System not pumped from extremities In proximity of graphite collimators, after 9 month operations
H2 pressure at 150°C ~ 5·10-9 mbar < 100h beam lifetime
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03 1.E-02 1.E-01
hydrogen atomic concentration
H2 p
ress
ure
(m
bar)
H = 52.66 0.7 kJ /mole
150°C
200°C
250°C
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
COLD ARCS (with proton beam)
Assumptions Beam screen pumping only via holes (as in LSS) Photon critical energy about 3 x in LSS Photon (and photoelectrons) flux about 10 x in LSS Photon and photo-electron gas desorption about the
sameResults Pressure expected in the arcs ≤ 20 x in the cold sections
of the LSS
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Residual gas pressure for ions operations
Gas sources: only ions from beam losses Residual gas ionisation neglected + ion estimated energy ~ 2eV (no gas desorption expected); Synchrotron radiation desorption neglected at critical energy ~ 2.8eV No photoelectron or electron multipacting expected (low current and long bunch spacing) Desorption yield for ions ~ 105 molecules/ion [E. Mahner, lhc-project-report-798] for each gas
species considered (H2, CH4, CO and CO2)
Beam screen holes pumping only = neglect cryopumping (worst case
scenario)
Ion losses 2·106 ions/turn density for 100h beam lifetime
real lifetime < 2s
Estimated localised losses for quench limit 200x100 beam lifetime if lost over a sec.
2h if lost in 1 turn, but pressure recovery <1s
Vacuum not expected to be limiting factor to beam lifetime
A. Rossi, presented at I-LHC meeting on December 9, 2004
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Discussion
Calculations of residual gas pressure strongly depend on surface properties and on the operating configuration, and give only a snapshot in time.
Estimates made so far are for stable beam and do not include collimators.
Their effect is well understood on the static pressure. Long term and beam effects will be studied.
In the cold arc: The gas density for 100h lifetime (1015 H2 equiv./m3) gives an upper
limit and is the value to be used for experiment b.g. estimates. The gas composition is expected to be similar to what calculated in
the LSS.
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Discussion
Machine layout will evolve: Part of 2nd phase collimators under design (2010) Inner triplets with larger aperture (2012/2013)
The pressure during stable beam may also depend on a transient during the beam cycle that causes particle losses, beam displacement, collimator setting, …
In order to estimate the gas density profile it is necessary to study case by case.
Detector Background WS – 3-4 April ‘08 Adriana Rossi AT/VAC
Questions - Answers
Is the lifetime a useful information to renormalise the pressure estimate:
The vacuum group will be working close to the operation to learn how to use this information
What happens if we have a He leak in the arcs: It is expected to have a magnet quench before any effect of pressure
can be seen. BLM will give us some information. We have to learn if beam lifetime
can give us an early warning What could go wrong:
Fast temperature gradients could open leaks (in LEP, with beam at 80GeV due to synchrotron radiation hitting transitions)
Damage caused by loss of beam ……
Can HOM in the experiments cause temperature rise? No, according to estimates (L. Vos) made at time of design:
Cu coating, conical transition, RF contact, RF screen for pumps Matter under investigation