Follow up from last meeting

The discussion at the last meeting was about our present knowledge of requests/parameters for non-LHC beams -which LIU might want to take into account when providing specifications for BIDs in order not to introduce (or flag) intensity/brightness limitations. In this context, G. Rumolo will also give an overview on this subject at the IEFC meeting on 7 July (15'). The presentation will be essentially like the one given at the last LIU beam parameter meeting, but the final set of slides will be circulated to the LIU-PT well before Friday to be sure that there is a general consensus on the contents. The subject of today's meeting is a review on the observations of tune change due to the injection bump during the injection process in the PS. The question is whether the measured swing could be partly responsible for the observed horizontal emittance blow up seen when analysing the emittance evolution of the LHC beam along the injector chain (2016 data). The next meeting at the end of July will be a mid-year checkpoint for the LIU MDs to discuss what has been done, what is underway and what needs to be done by the end of the 2017 run.

Tune variation due to PS injection bumpers (Alex)

• The PS injection takes place in SS42 and currently relies on the injection septum SMH42, four BSW bumper magnets and the kicker KFA45. Due to the activation of all these elements, when the second injection of an LHC-type beam takes place from the PSB, there is a strong effect on the circulating beam. The duration of the bump (half sine wave) is about 2 ms. In the future for the 2 GeV injection there will be 5 BSW  bumpers (one of which in the same vacuum as the new SMH42 septum) and the duration of the bump will be limited to 1 ms.
• Using a novel technique for the tune measurement, based on combining the measurements of all BPMs for different bunches, it is possible to track the tune evolution of the bunch-by-bunch tune during the injection process with a high accuracy.
• In 2016 it was observed that during the decay of the injection bump the horizontal tune of the injected beam swings by more than 0.01 and follows a shape more similar to the derivative of the beam position at the bump than to the beam position itself.
• In 2017 the measurement has been made on the circulating beam with a 'missed' second injection into the PS on a typical LHC cycle. The horizontal tune measurement clearly showed the shape proportional to the derivative of the beam position at the bump position. The vertical tune seemed to exhibit the same behaviour but with a different sign. The possible explanation is that the eddy currents in the bumper vacuum chambers create sextupole, which induces a quadrupolar feed-down due to the large position offset. The measured tune swing seems to be consistent with the estimation from the eddy currents.
• Measurements done when pulsing only one of the bumpers at a time showed different patterns, but the interpretation is much more complex, as the bump would be open and there would be a feed-down effect allo along the machine.
• Now the important questions for the LIU beam parameters are:
  • Can this tune swing at least partly explain the horizontal emittance blow up observed at PS injection? To be reminded that 2016 measurements showed values well beyond what was expected from injection mis-steering/mismatch alone.
  • In the future, will we have an even more severe effect due to the 2 GeV injection and the faster bump decay? Will other effects also play a role (non-synchronised bumpers, leakage of septum field)?
• The questions can only be addressed with an integrated simulation study of the injection process that includes realistically mis-steering/mismatch, effect of the eddy currents in the bumpers, synchronisation errors, direct and indirect space charge, transverse damper.
• A collaboration with TE-MSC is underway to investigate eddy current effects in vacuum chamber and the measurements of the multipoles during the bump decay will be started within the next few weeks.
• Possible remedies agains this effect are ceramic vacuum chambers (but they would need to be shielded anyway, so the specifications should be carefully given) or passive compensation circuits as in the main magnets.
• It would be useful to develop an application for the tune tracking during the bump decay (and possible feedback), which could be of interest also for the PSB (Q-strips circuits have been foreseen there to compensate the eddy currents from the BSWs evaluated by MSC) and the SPS