e - cloud studies for different LHC upgrade scenarios ( HL-LHC and (V)HE-LHC )

e- cloud studies for different LHC upgrade scenarios (HL-LHC and (V)HE-LHC)

C. Octavio Domínguez

Thanks to Stephane Fartoukh, G. Rumolo, G. Iadarola, F. Zimmerman

8 April 2013 - e- cloud meeting

Outline

1. HL-LHC1.1 Motivation1.2 Microbatches1.3 Reduced bunch spacing1.4 Bunch “doublets”1.5 Summary HL-LHC

2. (V)HE-LHC2.1 Machine and beam parameters2.2 Photoemission yield choice2.3 HE-LHC: Results for different bunch spacings 2.4 HE-LHC: Scrubbing?2.5 VHE-LHC: Results for different bunch spacings 2.6 VHE-LHC: Scrubbing?2.7 Summary (V)HE-LHC


Outline





• Luminosity goal: ~250 fb-1/year

• Potential stoppers: LRBB interactions Excessive pile-up in the detectors e-cloud

• Different filling schemes have been suggested (S. Fartoukh) to minimize the first two: Microbatches Reduced bunch spacing Bunch “doublets”

• It is necessary to check whether e-cloud activity would prevent these schemes to be used

HL-LHC: Microbatches


Illustration for 50 ns(courtesy G. Papotti)

• PACMAN bunches: First and last bunches from a train They experience less LRBBI

• This can affect dynamic behaviour and beam life time Direct impact of Luminosity

• The effect will scale up for the HL-LHC due to: a longer inner triplet, i.e. more LR encounters:

typically 20-23 instead of 15 for 25 ns a higher bunch charge (stronger kicks):

typically 2E11 instead of 1.15 for 25 ns

• Removing properly the central bunches and separating the trains sufficiently would cancel this effect

• If so all the bunches will see strictly the same number of LRBBIs, of course not at the same location, but the integrated kick would be very similar for all bunches

• Assuming 23 LRBBI for the longer HL-LHC triplet (most pessimistic case with 100 T/m NbTi), one gets the most symmetric (25 ns) micro-batch filling scheme:

10 x ( 7 x (24b+24e) + 12e ) +84e (corresponding to 10 LHC injections and 1680 bunches)

(Idea by S. Fartoukh, presented at the HL-LIU Brainstorming, 30.03.2012)



25 ns* 50 ns* Microbatch (25 ns)

# Bunches 2808 1404 1680

p/bunch [1011] 2.2 (1.12 A) 3.5 (0.89 A) 2.2 (0.67 A)

Minimum b* [cm] 15 15 15

eN [mm] 2.5 3.0 2.5

sz [cm] 11.8/7.5** 11.8/7.5** 11.8/7.5**

Lpeak (Virtual) [1034cm-2s-1] 24 26 14.5

Llevel [1034cm-2s-1] 5.0 2.5 3.0

Events / crossing*** 135 135 135

tleveling [h] @ Llevel 8.5 17.1 8.5

topt [h] 10.2 18.1 10.0

Availability**** 59% 99% 99%

* Stretched parameters ** injection/top energy *** sinel = 85 mb (COMPETE scaling law)**** Needed to achieve 250 fb-1 in 150 days

• The reduction in the number of bunches (40%) obliges machine availability to increase up to 99%




Injection Top energy

• There is a clear reduction of the e-cloud activity with respect to the 25-ns scheme

• Nevertheless still far away from 50 ns (well outside the plots)

• If the 25-ns filling scheme can’t be implemented due to e-cloud issues, this scheme could be another fallback option since it offers a very similar performance to the 50-ns scheme

dmax, thres. MB dmax, thres. MBdmax, thres. 25ns dmax, thres. 25ns


HL-LHC: Reduced bs

• Reducing bunch spacing Straightforward way to reduce pile-up in the detectors• Bunch spacings explored: 12.5 ns, 5 ns, 2.5 ns and 1.25 ns

Difficult Not possible (a priori)

25 ns* 12.5 ns 5 ns 2.5 ns 1.25 ns

# Bunches 2808 5616 14040 28080 56160

p/bunch [1011] 2.2 1.1 0.5 0.25 0.125

Minimum b* [cm] 15 15 15 15 15

eN [mm] 2.5 2.5 2.5 2.5 2.5

sz [cm] 11.8/7.5** 11.8/7.5** 11.8/7.5** 11.8/7.5** 7.5/7.5**

Lpeak (Virtual) [1034cm-2s-1] 24 12.1 6.3 3.1 1.6

Llevel [1034cm-2s-1] 5.0 10.0 > Lpeak > Lpeak > Lpeak

Events / crossing*** 135 135 34 8 2

tleveling [h] @ Llevel 8.5 0.8 - - -

topt [h] 10.2 5.9 9.2 14.6 25.9

Availability**** 59% 56% 74% 109% 161%

* Stretched parameters ** injection/top energy *** sinel = 85 mb (COMPETE scaling law)**** Needed to achieve 250 fb-1 in 150 days


HL-LHC: Reduced bsInjection Top energy

• Lowest threshold 12.5 ns

• Peak activity 5ns. Similar for 2.5 – 12.5 ns. Behaviour changes slightly with energy

• Sharper behaviour at the threshold for shorter bunch spacings (especially at top energy)

• 1.25 ns marks the backward tendence for e-cloud activity for further reduced bunch spacings

• 5-ns scheme presents an almost identical behaviour at both energies (above threshold, which is the same)

• All these schemes must be discarded for operation


HL-LHC: Reduced bsWhat about scrubbing?

• A 12.5 ns scheme would scrub efficiently almost the entire region for 25 ns• A 5 ns scheme would scrub efficiently only the central part of the chamber (due to

reduced bunch charge), but: + We could use steering to displace the beam horizontally and scrub more

- Steering takes more scrubbing time (a higher machine availability needed)


HL-LHC: Bunch doublets

• 800 MHz RF cavity could split a bunch into two “bunchlets” separated by 1.25 ns

• 400 MHz + 440 MHz RF cavities could split a nominal bunch into two “bunchlets” separated by 2.5 ns

• Only possible at top energy thanks to a lower phase space filling factor

Not interesting:- Additional LRBBI- Incompatibility with crab cavities- Incompatibility with detectors

(Idea and RF simulations by S. Fartoukh)

RF cycle used for the splitting and recombination processes. One second appears to be a time sufficient for carrying out the two processes adiabatically.



(Cortesy S. Fartoukh)



• One advantage of this kind of beams is the possibility of recombination• With 25 ns, the optimum run time is shorter than the recombination time• In terms of luminosity performance, it is identical to pure 12.5-ns bunch

spacing (56% machine availability needed)



• Similar behaviour as observed with regular bunch spacings (backward tendence for 1.25 ns)

• Longer bunch length explored (10 cm, as nowadays in the LHC) Negligible reduction

• These scenarios can be also discarded



And what if instead of 25 ns we use 50-ns bunch spacing?

• Luminosity performance similar to pure 25 ns with Nb=1.75 10∙ 11 ppb (~70% machine availability), although still trecomb. > topt

• e-cloud activity much worse than pure 50 ns but considerably better than the base line 25 ns

• In case of long fills (for whatever reason), recombination could still be used• It could be another backup option for operation


HL-LHC: Summary

• From all the explored filling patterns, only two show a reduced e-cloud activity with respect to the base line 25-ns bunch spacing:

Microbatches (with 25 ns) Minimizes LRBBI potential issues 50 ns doublets (with 2.5 ns bunchlet spacing) Reduces pile-up

• Both present a worse performance in terms of integrated luminosity compared to the base line 25-ns bunch spacing (99% and 70% vs. 60%)

• Both could be considered as fallback solutions in case 25 ns cannot be injected

• Other 25 ns schemes with “holes” generated at the PS can be exploreddmax threshold at 450 GeV dmax threshold at 7 TeV

50 ns 2.4 2.3

Microbatches (25 ns) 1.7 1.6

25 ns 1.5 1.4

12.5 1.17 1.1

5 ns 1.2 1.2

2.5 ns 1.26 1.16

1.25 ns 1.42 1.32

Doublets 25/1.25 ns - 1.2



Outline





(V)HE-LHC: Parameters

LHC HL-LHC HE-LHC VHE-LHCc.o.m. E (TeV) 14 33 100B (T) 8.33 20Dipole coil aperture (mm) 56 40Beam half aperture (mm) 18 (x), 22(y) 13 (x & y)Bunch length (cm) 7.6SR arc heat load (W/m/aperture) 0,17 0,33 4,35 43,4


PY* choiceLHC HE-LHC VHE-LHC

E (TeV) 7 16.5 50.4

Ec (eV) 45 575 5475

r (m) 2784.32 8412.29

Ng (photons/m/p+)

0.028 0.067 0.067

Rg 10 %

PY* 0.03-0.053 ? ?

• PY* (effective photoelectron yield) is a controversial parameter: There are not many measurements in general and none in particular for Ec=575

and 5475 eV Different labs measure different values ([1], [2], [3], [4]) Ec divides the photon energy spectrum in two. Does it assures an average PY*? So:

PY* E∝ c ?

PY* ≈ constant ?PY* (E∝ c)-0.4 ?

Can we really establish a certain dependence between PY* and Ec?

(Considering Cu-sawtooth)

∝ E

∝ E3

We normally use at 7 TeV the value 0.049 from V.V. Anashin et al. / Vacuum 60 (2001) 255-260

http://ac.els-cdn.com/S0042207X00003924/1-s2.0-S0042207X00003924-main.pdf?_tid=bb4193aa-9f91-11e2-bb07-00000aab0f6b&acdnat=1365346083_8635ee8da1d715c6d34a009c087b38f7

http://conference.kek.jp/two-stream/Suetsugu-electron.pdf

http://indico.cern.ch/getFile.py/access?contribId=5&sessionId=4&resId=1&materialId=slides&confId=125315

http://dcain.etsin.upm.es/~goni/conferences/aseva_99.pdf








• We also have to take into account that high energy photons could penetrate into the material reaching deep layers where no e- could be liberated

• Hence a non monotonic behaviour with energy can be expected

• I’ve considered an intermediate dependence (same as for h): PY* (E∝ c)2/3 E∝ 2

• In any case the number of photons is big enough to need to think about suppression measures (saw tooth pipe, coatings, biased photon stops at warm temperature, etc.)

F. Zimmermann, SLAC-PUB-7238

PY* choice

http://arxiv.org/ftp/arxiv/papers/1108/1108.1642.pdf

Outline





HE-LHC: ResultsFirst suggested bunch spacing: 50 ns

• Very high heat load Photon absorption techniques must be applied



• If a high photon trapping efficiency can be achieve, e-cloud would not be a problem with a 50-ns bunch spacing

• In addition, pile-up rates are also very high (295 events/crossing!)

We could think of shorter bunch

spacing




spacing

50 ns 25 ns 12.5 ns 5 ns

Nb ( 10∙ 11 ppb) 1.89 0.94 0.47 0.19

nb 1404 2808 5616 14040

eN (mm) 2.76 1.38 0.69 0.28


HE-LHC: Results

• Shorter bunch spacings show a sharper behaviour at threshold (as for the HL-LHC)

• The thresholds for 25 ns (1.55) and 5 ns (1.45) are higher than 25 ns at the LHC

• Operation with 12.5 could be in principle discarded (highest e-cloud activity)

• For shorter bunch spacing the photon trapping efficiency does not play a major role


HE-LHC: Results

I also have considered different values for R


HE-LHC: Results

• The higher the e-cloud activity, the less important are the values of R and efficiency

• SEY thresholds:

• The bunch spacing adopted for the European HEP Strategy has been 25 ns, although the pile-up is still high (147 events/crossing)

• If compatible with experiments, 5 ns would be a good backup option for operation: Acceptable heat load levels after some scrubbing Its lower bunch charge constraints the e-cloud activity to a central region, what

eases the placement of clearing electrodes

Threshold at R=0.3 Threshold at R=0.5 Threshold at R=0.7

50 ns 3.15 2.9 2.6

25 ns 1.75 1.65 1.55

12.5 ns 1.22 1.22 1.22

5 ns 1.52 1.48 1.45


HE-LHC: Scrubbing?

• The different bunch charges places the stripes in different regions of the chamber

• The central stripes can be very easily scrubbed with any of these filling schemes

• For operation with 25 ns, beam steering would be needed with 12.5 or 5-ns beams

Outline





VHE-LHC: Results

First suggested bunch spacing: 50 ns

• Very high heat load Photon absorption techniques must be applied

All results and conclusions are very similar to the ones at the HE-LHC



VHE-LHC: Results

• If a high photon trapping efficiency can be achieve, e-cloud would not be a problem with a 50-ns bunch spacing

• In addition, pile-up rates are also very high (342 events/crossing!)


spacing



VHE-LHC: Results


spacing

50 ns 25 ns 12.5 ns 5 ns

Nb ( 10∙ 11 ppb) 1.94 0.97 0.49 0.19

nb 1404 2808 5616 14040

eN (mm) 4.3 2.15 1.1 0.43


• Shorter bunch spacings show a sharper behaviour at threshold (as for the HL-LHC)

• The thresholds for 25 ns (1.55) and 5 ns (1.45) are higher than 25 ns at the LHC

• Operation with 12.5 could be in principle discarded (highest e-cloud activity)

• For shorter bunch spacing the photon trapping efficiency does not play a major role

VHE-LHC: Results


VHE-LHC: Results

I also have considered different values for R


• The higher the e-cloud activity, the less important are the values of R and efficiency

• Since machine parameters are very similar to the HE-LHC, SEY threshold are almost identical

• The bunch spacing adopted for the European HEP Strategy has been 25 ns, although the pile-up is still high (171 events/crossing)

• If compatible with experiments, 5 ns would be a good backup option for operation: Acceptable heat load levels after some scrubbing Its lower bunch charge constraints the e-cloud activity to a central region, what

eases the placement of clearing electrodes

Threshold at R=0.3 Threshold at R=0.5 Threshold at R=0.7

50 ns 3.15 2.85 2.65

25 ns 1.75 1.65 1.55

12.5 ns 1.22 1.22 1.22

5 ns 1.52 1.48 1.45

VHE-LHC: Results


VHE-LHC: Scrubbing?

• The different bunch charges places the stripes in different regions of the chamber

• The central stripes can be very easily scrubbed with any of these filling schemes

• For operation with 25 ns, beam steering would be needed with 12.5 or 5-ns beams

Outline





(V)HE-LHC: Summary

• Results for both accelerators are very similar due to similar machine parameters

• Due to very strong SR, anti-photon measures must be considered for machine operation (warm biased photon absorbers, saw-tooth surface, coatings, etc.)

• If high photon trapping efficiency is achieved, e-cloud build-up would not present problems for a bunch spacing of 50 ns (but very high pile-up)

• 25 ns has been the bunch spacing adopted by the European HEP strategy for both accelerators (still high pile-up)

• 12.5 ns can be discarded highest e-cloud activity, very low SEY threshold

• 5 ns could be a nice backup scheme (if compatible with detectors): Reasonable e-cloud activity after some scrubbing More suitable for clearing electrodes Low pile-up (~LHC @ 7 TeV)

• For new machines, suppressing techniques shall be preferred

• If, after suppressing techniques, scrubbing is needed for 25 ns operation, beam steering with 12.5 or 5 ns would be needed

• Other additional measures must be considered (higher operating temperature of the beam screen, solenoids where possible, NEG coating in warm straight sections, etc.)

Thank you for your attention!


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e - cloud studies for different LHC upgrade scenarios ( HL-LHC and (V)HE-LHC )

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