Effects of ultra-peripheral nuclear collisions in the LHC

and their alleviationR. Bruce, S. Gilardoni, J.M. Jowett,

CERN, Geneva, Switzerland

Abstract: Electromagnetic interactions between colliding heavy ions at the LHC are the sources of specific beam loss mechanisms that may quench superconducting magnets. We propose a simple yet efficient strategy to alleviate the effect of localized losses from bound-free pair production by spreading them out in several magnets by means of orbit bumps. We also consider the consequences of neutron emission by electromagnetic dissociation and show through simulations that ions modified by this process will be intercepted by the collimation system, without further modifications.

Abstract: Electromagnetic interactions between colliding heavy ions at the LHC are the sources of specific beam loss mechanisms that may quench superconducting magnets. We propose a simple yet efficient strategy to alleviate the effect of localized losses from bound-free pair production by spreading them out in several magnets by means of orbit bumps. We also consider the consequences of neutron emission by electromagnetic dissociation and show through simulations that ions modified by this process will be intercepted by the collimation system, without further modifications.

Conclusions of paper:

•Beam losses caused by BFPP during 208Pb82+operation might quench magnets in the LHC through localized heat deposition

•Distributing the losses over several impact locations by means of N+1 orbit bumps decreases the maximum heating by 1/N

•Losses caused by EMD intercepted by collimation system, no risk for quenches

Conclusions of paper:

•Beam losses caused by BFPP during 208Pb82+operation might quench magnets in the LHC through localized heat deposition

•Distributing the losses over several impact locations by means of N+1 orbit bumps decreases the maximum heating by 1/N

•Losses caused by EMD intercepted by collimation system, no risk for quenches

Electromagnetic processes between colliding PbElectromagnetic processes between colliding Pb82+82+ beams in the LHC, beams in the LHC,

producing ions with a different charge to mass ratio:producing ions with a different charge to mass ratio:

6σ envelope at IP2, no kicks6σ envelope at IP2, no kicks

6σ envelope at IP2, 4 kicks6σ envelope at IP2, 4 kicks

Bound Free Pair Production (BFPP), 281 barnBound Free Pair Production (BFPP), 281 barn

1n Electromagnetic Dissociation (EMD), 215 barn1n Electromagnetic Dissociation (EMD), 215 barn

•Secondary beam of wrongly charged ions from BFPP leave the wanted trajectory

•Lost downstream where dispersion has grown sufficiently large, causing heating and possible quenches of the superconductors.

•Secondary beam of ions from 1n-EMD inside momentum acceptance and intercepted by collimation system. See 2nd page for details.

•2n-EMD has a factor 5 lower cross section than BFPP, no risk for quenches

Alleviation of BFPP through orbit bumpsAlleviation of BFPP through orbit bumps

•Idea: Introduce orbit bumps to displace the orbit at each dispersion max

•Distribution of losses and heating over several impact locations

Condition for equal losses at m=1..N locationsCondition for equal losses at m=1..N locations

•Example: 4 kicks at IP2, 3 loss locations

•Each aperture limitation defines cuts in initial phase space.

•Example: 4 kicks at IP2, 3 loss locations

•Each aperture limitation defines cuts in initial phase space.

nominal beam

BFPP beam

You can also see this poster and the paper at

http://cern.ch/rbruce/epac2008/

You can also see this poster and the paper at

http://cern.ch/rbruce/epac2008/

Operational aspects, BFPPOperational aspects, BFPP

•Orbit bumps introduced before reaching full luminosity through van der Meer scan.

•Correctors need fine-tuning around predicted value using BPMs and BLMs

•BPMs method unreliable, due to uncertainty in aperture. Might see BFPP losses directly

•BLM signals estimated by convolution of energy deposition from 1 particle with loss pattern from signal particle tracking

Impacts locations at IP2 with 4 kicks.

1/3 of total lossesat each impact

Impacts locations at IP2 with 4 kicks.

1/3 of total lossesat each impact

Estimated BLM signals

Estimated BLM signals

•Maximum deviation of nominal orbit 3.8 mm

•Orbit bumps move loss locations by a few metres, possibly into other elements. Need to investigate quench limits

•Beta beating of 1.7 % possible to correct

•Using up to 27% of total corrector strength

•Adjustments of 0.1% of total strength needed for fine-tuning

Tracking of 1n-EMD particlesTracking of 1n-EMD particles

•Affected ions within momentum acceptance

•Ion collimation predicted to have a high inefficiency

•EMD particles tracked with ICOSIM, which combines optical tracking with particle-matter interaction in collimator.

•99% of 1-n EMD particles intercepted by collimator

•Remaining 1% cause negligible heat load (~0.055 mW/cm3), far below quench limit

Simulated loss map caused by 1n-EMD at IP2

Simulated loss map caused by 1n-EMD at IP2