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Possible Vacuum Issue

V. BaglinCERN TE-VSC, Geneva

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011

1. LHC present collimator’s base line2. Cold / Warm Collimator

3. Some though and observations4. Conclusions

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 20112

1. Present LHC Collimator’s

Base Line

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 3

Vacuum Specification• Ref : Vacuum requirements for the LHC collimators, EDMS 428155 and Invitation for tender FC 553

• Bake able to 250 deg during 24h. Minimum cycle of 60 over 20 years• Cleaned, no traces of hydrocarbons, organic and inorganic residues (partial pressure < 10 -11 mbar)• Leak tight : global helium leak rate < 10-10 mbar.l/s• Outgassing rate 10-12 mbar.l/s.cm2 i.e. furnace treatment at 1000 deg under vacuum (carbon surface, stainless steel,

ferrites …)• Total outgassing rate ~ 10-7 mbar.l/s

• Transition to the standard aperture (ID80) in the collimator tank• Permanent bakeout system and remotely controlled (reduce dose to personnel, ALARA)• Avoid trapped volumes

• No in-situ welding (requires in-situ cleaning)• Number and length of weld should be minimised (leaks due corrosion)• No crossing welds• Fully penetrating welds and brazing between vacuum and cooling circuit are not allowed

• Easy access to potentially leaking components (reduce dose to personnel)

• Bellows should be bake able and leak tight after 4000 cycles• Quick release flange with a Conflat knife design

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 4

Material List

• Each components were carefully selected and vacuum cleaned according to UHV CERN standards (EDMS 798034)

• Stainless steel vacuum chamber• Stainless steel welded bellows• Tungsten, copper and graphite jaws• Glidcop• Ferrite to damped HOM• Cu/Be Ag coated and Cu/Ni RF contacts• Ceramic temperature sensors• Temperature sensors feedthrough• Kapton cable• ….

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 5

LSS layout• At each end of a collimator is connected a pumping dome (VPIA)• It allows an exchange of a collimator when required

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 20116

2. Cold/Warm Collimator

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 7

Warm Collimator

• This solution offers :- A bake able collimator- A vacuum sectorisation to allow a “fast” exchange of a collimator- A port to allow RF ball test to check beam aperture before cool down (PIM !)- Access to vacuum system to boost the pumping speed if required (allows to upgrade the vacuum

performances)

Cryostat (“by-pass”)(QTC)

Collimator Module (TCLD)

A. Bertarelli et al.

This solution needs to be shorten and revisited

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 8

What about a Collimator Operating at Cryogenic Temperature ?

• This solution :- Does not offer any of the previous aspect - But is smaller

D. D. Ramos et al.

This solution needs to be fully evaluated theoretically and experimentally by VSC

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 9

Outgassing rate baked vs. unbaked

• Total outgassing rate:- 2 orders of magnitude lower with bake out

10-10

10-9

10-8

10-7

10-6

10-5 Without bake-outAfter bakeout

Out

gass

ing

rate

[mba

r l/s

]

Total H2

CH4 H

2O CO N

2 CO

2

J. Kamiya et al., EVC 11 Salamanca, 2010

0

10

20

30

40

50

60

70

80

90

100

H2 CH4 H2O CO N2 CO2

Before bakeoutAfter bakeout

Outgas composition

• Residual gas analysis:- H2O is the main component before bake out (65 %)- H2 is the main component after bake out (85 %)

Q (N2 eq) [mbar l/s]

Unbaked 7.3x10-6

After 1st bake-out 6.7x10-8

After 2nd bake-out 5.2x10-8

After 3rd bake-out 4.0x10-8

1001.31.3

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 10

Unbaked/baked what does it mean ?

• Unbaked state :- The total outgassing rate of an unbaked collimator is equivalent to ~ 400 m of unbaked cold bore

concentrated in 1 m length

- In terms of gas load, it is equivalent ~ 300 x the synchrotron radiation gas load

- During bake out, the total amount of removed H2O is equivalent to at least 100 monolayers desorbed from a 1 m long cold bore

• Baked sate:- This is the LHC base line- The total outgassing rate of a baked collimator is equivalent to ~ 10 km of stainless steel ID80 vacuum fired beam tube

This quantity of gas will be available for future stimulated desorption

Despite all precautions taken for LHC, there are still operation issues from time to time …

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 11

Jaws : RF contact’s Movement• Moving the collimator’s jaws induce gas desorption (due to friction)• Hydrocarbons show up and gas load x 100

• The outgassing rate is linear with the displacement length ~ 10-7 mbar.l/s.mm• A lot of hydrocarbons are degassed under displacement• No differences with an unbaked collimator (except water)

J. Kamiya et al. IPAC 2010, Kyoto, Japan.

• Almost no conditioning is observed after a few tens movements• Observed in the LHC machine

• CO and CO2 outgassing rate are ~ 10-7 mbar.l/s for a 30 mm displacement • LHC life time: 20 000 cycle (1.2 km) i.e. large gas load

J. Kamiya et al. EVC-11 2010, Salamanca, Spain.

The desorption due to friction must be reduced

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 201112

3. Some Though

and Observations

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 13

Operating temperature of Beam Screens• LHC beam screens operate in the range 5 to 20 K (EDMS 107716)

• Holes are produced through the beam screens to allow pumping of H2 vapour pressure onto the cold bore

• The H2 saturated vapour pressure decrease from 10-6 mbar at 4.2 K to 10-16 mbar at 2 K

C. Benvenuti et al. J. Vac.Sci.Technol., Vol. 13. No6, Nov/Dec 1976

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 14

Why perforated Beam Screens ?

• SSC studies in 1994

V.V. Anashin et al. J. Vac.Sci.Technol. A. 12(5) , Sep/Oct 194

No perforations With perforations

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 15

Vacuum Transients• Transients are due to an excess of physisorbed gas onto the beam screen : beam screen’s surface must be bared

• Their level varies with the gas species, the local pumping speed, the temperature, the driving mechanism (temperature excursion, electron cloud, synchrotron radiation, ion bombardment, particle loss …)

• Appropriate cooling scenario with decoupling between cold bore and beam screen with possibility of BS warming up to 80 K have been implemented in the LHC base line

V. Baglin, Chamonix 2004

In a LHC-type mock –up (SR driven) In LHC (T driven)

Fill 2177, 1st October 2011

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 16

Possible operating temperature of cold collimator• To provide necessary cooling power (~ 150 W during 10 s and ~ 30 W in steady state) the only available cooling

line is 50-70 K / 20 bar

• The zone around 80 K must be avoid to allow flushing of CO2 from outside the beam tube and avoid that of CO2 is physisorbed in view of the beam

• We need to operate above ~ 90 K

Based on measurements by V.V Anashin et al.

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 17

Implication of a non uniform source of gas in the arc

• Without beam, static gas load are dominated by unbaked collimator scheme

• During operation, a baked collimator or an unbaked collimator will bring dynamic gas load

• However, it is expected that the stimulated gas load from baked will be much less than unbaked collimator

• Moreover, desorption yields of physisorbed gas are much larger than chemisorbed gas

• En vrac :- Photon stimulated molecular desorption (synchrotron radiation)- Electron stimulated molecular desorption (photoelectron, electron cloud)- Ion stimulated molecular desorption (gas ionisation by protons, electrons …)- Friction induced molecular desorption (RF bridges)- Particle loss stimulated molecular desorption (by essence of a collimator)- Vacuum stability- …

All this must be studied in detailed

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 18

Observation of proton loss in the LHC• Stimulated outgassing is observed at the (baked) TDI due to proton losses at perpendicular incidence

J. Kamiya et al. IPAC 2010, Kyoto, Japan.

• Extrapolation of 107 -108 p/m/s loss yields to ~ 10-7 to 10-6 mbar.l/s i.e. 3-30 times the SR gas load

• Similarly, proton losses at collimator produce stimulated gas desorption but at grazing incidence ….

G. Bregliozzi, private communication

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 19

Observation at Betatron Collimation in the LHC• A pressure increase of ~ 10-8 mbar is observed at …. 50 m from the TCPs

• It consists only of hydrocarbons not pumped by the NEG

• According to literature, gas load to other gas species is 10-100 times larger (see e.g. E. Mahner et al., Phys. Rev. ST Accel. Beams. 8, 053201 (2005))

Fill 2181, 4st October 2011

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011 20

Observation at RHIC

I guess that RHIC advice to CERN would be “ bake your collimator”

…. after rebucketing, the pressure increases exponential with a time constant of about 10 seconds until the vacuum interlock system aborts the beams.

A vacuum instability in the Blue ring

The Blue beam intensity was limited by pressure rises in the collimator region. The collimators were not baked due to scheduling conflicts during the last shut-down. The Yellow collimators were baked, and no vacuum instabilities were observed there.

Remark : it reminds me my everyday work !

Conclusions

21Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 2011

• LHC base line is to bake all components operating at room temperature

• Unbaked system i.e. system operating at cryogenic temperature have continuous pumping, that means a perforated beam screen

• A solution at room temperature exists and need to be consolidated/upgraded

• The proposal with a collimator operating at cryogenic temperature must be studied in detail (issue with resources & time)

• Our present observation with LHC shows that collimators areas (TDI, TCTVB-D1 ….) are delicate regions subjected to pressure excursions

• RHIC, which suffer of electron cloud like LHC. has a bad experience with unbaked collimator operating at room temperature !

Vincent Baglin Preliminary Meeting on Interface 11 T – Cold Collimation - 5 October 201122

Thank you for your attention !!!