Estimates of residual gas pressure in the LHC

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


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


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


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


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


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


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


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


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)


ATLAS D1


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

<[email protected]

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


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)


CMS

H2

CO2

COCH4

H2 equivalent


Scenario in 2004:44 bunches x 1.15·101011 p/b

7TeV


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)


D1


—— 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


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


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


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


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


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


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.


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.


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

Documents

Estimates of residual gas pressure in the LHC