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LIU Beam Parameter WG meeting #18

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874/R-018 - MEETING ROOM PRIORITY BE OP (CERN)

874/R-018 - MEETING ROOM PRIORITY BE OP

CERN

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Follow up from last meeting

The final version of the proton parameter table is attached to this meeting. It has been uploaded on EDMS and it has replaced the 2012 parameter table. The ion parameter table, which, as presented in Chamonix, was amended with respect to the one published in the TDR, will be again discussed today and then it will be also uploaded on EDMS as the official parameter table.

 

Ion beam parameters

Hannes summarised the rationale for updating the LIU ion beam parameter table, consistently with the  assumptions made to establish it for the TDR back in April 2016 and then following the additional information gathered with the 2016 experience. It was recalled that the beam parameters given in the TDR, which were based on the 2015 experience, would provide ~80% of the required luminosity (but close to this value if the performance efficiency was raised from 50 to the demonstrated 62%).  

  • In 2016 several improvements were made. In brief, Linac3 provided consistently 40% higher beam current, losses in LEIR were further controlled by tailoring the longitudinal bunch shape at RF capture and resonance compensation, losses between LEIR and PS were suppressed thanks to improved steering into PS. All in all, even including bunch splitting at the PS top energy, which was not done in 2015, the bunch intensity at the exit of the SPS was basically the same as in 2015. It also helped that the SPS injection plateau was limited to 7 injections in 2016 against the 12 injections in 2015 (because of a limitation on the flat top length of the LHC injection kicker), which implied reduced losses in the SPS. Keeping the performance of source, Linac3 and LEIR at a constant level requires continuous tuning and this is something that should be improved in the future with some automation, improved instrumentation and OP involvement and training.
  • The SPS transmission around 60% still means that losses in the SPS remain an important bottleneck for the achievement of the future parameters. This is specially true, if we aim at improving further the performance of the injectors upstream - which can be exploited only if the transmission of the SPS, and in particular its unfavourable scaling with the injected bunch intensities, is improved. The SPS performance is known to be affected by space charge (tune spread of 0.3) and IBS on the long injection plateau (due to the bunched structure) plus important losses out of the bucket at the beginning of the ramp and losses at transition crossing. More extended machine studies will be conducted in 2018 to pin down how some mitigation of these effects can be achieved. For future operation, to achieve the required number of bunches in LHC, SPS will have to perform slip stacking of the ion trains and reduce the bunch spacing from 100 to 50 ns. This process has been simulated and works on paper, but will be feasible in the SPS only after the upgrade of the LLRF. Having slip stacking already commissioned and operational by the end of 2021, when the first post-LS2 LHC ion run is presently scheduled, is one of the main risks of the LIU-ions target.
  • Malika confirmed that the request to swap the pp reference run in 2023 with the ion run in 2021 is indeed being considered, but the exchange hasn't yet been confirmed.
  • The new LIU ion parameter table is now available and will be considered as a reference in the future.
  • Without slip stacking we fall to 60% of the desired performance in terms of luminosity production.
    • One back-up scenario already long studied is the possibility to batch compress the 4 bunches out of the PS to 50 ns. Although a proof-of-principle experiment in 2016 in the PS was successful, this option would require two additional broad-band cavities in the PS, a tuner for the 40 MHz cavities + all the related manpower. This would bring to 83% of the desired performance in case of failure of slip stacking, or even to 134% if it could be run in addition to slip stacking.
    • One could also consider having 3 bunches out of LEIR instead of 2 and then avoid splitting in the PS. This would result in higher intensities in the SPS, therefore it could result in a gain only if the transmission of the SPS at high bunch intensity can be improved (would require an improvement from 60 to 75% against the improvement to above 90% required if we would stick to the scheme with 4 bunches). Studies with h=3 in LEIR will start already in 2017 with Xe ions, it relies on some LLRF development that is foreseen.
    • Partial stripping at LEIR extraction could also help because we could accelerate to higher energy in the PS and mitigate IBS and space charge at SPS injection. This would be at the expense of higher space charge in the PS and additional losses due to the anticipated low efficiency of the process.
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