Post on 10-Jan-2016
description
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Electron Cloud in LHC
V. BaglinCERN AT-VAC, Geneva
1. Mecanism and Recipies2. LHC Vacuum Chambers
3. Diagnostics4. Conclusions
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 2
1. Mecanism and Recipies
F. Ruggiero et al., LHC Project Report 188 1998, EPAC 98
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 3
Electron Cloud : the Mecanism
• In modern machine with dense bunches and large positive current : KEK-B, PEP-II, SPS, RICH, LHC …• Emittance growth, gas desorption and heat load in cryogenic machine
S
QP
ElectronsElectrons
• Key parameters :bunch structurevacuum chamber dimmensionsecondary electron yieldphoton electron yieldelectron and photon reflectivities…
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 4
Electron Cloud : the Recipies
• Play with the key parameters• Reduce photoelectron yield (perpendicular vs grazing incidence)• Reduce secondary electron yields (scrubbing, TiZrV coatings, carbon coatings, geometry ..)• Reduce the amount of electrons in the system (solenoid magnetic field, clearing electrodes, material reflectivities …)• Adapt the bunch structure or the chamber geometry to reduce multiplication• …
Secondary Electron Yield
N. Hilleret et al., LHC Project Report 433 2000, EPAC 00
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 5
2. LHC Vacuum Chambers
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 6
LHC Dipole Vacuum System• Cold bore (CB) at 1.9 K which ensures leak tightness• Beam screen (BS) at 5-20 K which intercepts thermal loads and acts as a screen
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 7
LHC Dipole Vacuum System
• Rely on scrubbing at cryogenic temperature• With dense bunch current : kick for high energy electrons ! • But large bunch distances to avoid multiplication : 50 to 75 ns or more
O. Gröbner. PAC 1997
Courtesy R. Cimino
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 8
Room Temperature Vacuum System
• ~ 1 μm thick, Non Evaporable Getter TiZrV coated vacuum chambers ensure the required vacuum performances for LHC• Some vacuum chambers were constructed and getter coated …
Courtesy R.Veness AT/VAC and P. Chiggiato TS-MME
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 9
Room Temperature Vacuum System
• ….. and installed inside the LHC tunnel
Warm magnets
Collimators
Vacuum chambers
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 10
TiZrV Vacuum Performances
Courtesy P. Chiggiato TS/MME
10-3
10-2
10-1
100
101
10-7 10-6 10-5 10-4 10-3
1013 1014 1015 1016
Pu
mp
ing
Sp
ee
d
[
s-1 c
m-2]
CO surface coverage [Torr cm2]
[molecules cm-2]
N2
H2
CO
Pumping Speed
PSD Yields
V. Anashin et al. EPAC 2002
Secondary Electron Yield
C. Scheuerlein et al. Appl.Surf.Sci 172(2001)
ESD Yields
C. Benvenuti et al. J.Vac.Sci.Technol A 16(1) 1998
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 11
Room Temperature Vacuum System : Demonstration
Ultimate Vacuum Pressure Distribution after NEG Activation of theLHC Room Temparture Vacuum Sectors
G. Bregliozzi et al. EPAC’08, Genoa 2008
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008 12
3. Diagnostics
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Vacuum Gauges
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
Courtesy J.M. Jimenez
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Vacuum Gauges
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
Courtesy J.M. Jimenez
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Current measurements
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008Courtesy J.M. Jimenez
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Current measurements
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
Courtesy J.M. Jimenez
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Current measurements
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
Courtesy J.M. Laurent
18Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
T_uph - T_downh m Q HeHe
T_up T_down
Flowmeter
mHeater
T_BS
Q.
.
COLDEX #101- Scrubbing run 2004 - part 2 - 21 June 2004 13:00-16:0025 ns - BS = 12 K, CB = 3 K - Intensity scan
4 batches
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.00E+00 2.00E+10 4.00E+10 6.00E+10 8.00E+10 1.00E+11 1.20E+11 1.40E+11
Bunch current (protons/bunch)
BS
hea
t lo
ad (
W/m
)
above electron cloud threshold
V. Baglin. APC, 26/5/2005
Heat Load Measurements
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Heat Load Measurements
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
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Pilot Sectors
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
• Under definition ..
G. Bregliozzi et al. EPAC’08, Genoa 2008
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4. Conclusions
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
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• After repair of sector 3-4 …..
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
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• And successfull operation with beam …..
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
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• Followed by …
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
… P. Higgs in the CCC
HIGGS BOSON DISCOVERY
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• Electron Cloud could show up !
Vincent Baglin Electron Cloud Mitigation Workshop ECM’08- 20/21 November 2008
We are waiting for it !
Meanwhile we (continue) to prepare ourselves