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LIU Beam Parameter WG meeting #10
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Europe/Zurich
874/R-018 - MEETING ROOM PRIORITY BE OP (CERN)
Follow up from last meeting
- Transverse feedback system in the PS is not yet operational on LHC beams
- Potential to damp headtail instabilities and injection oscillations (including contribution from kicker rise times and ripples) is still to be assessed on an operational basis.
- High intensity 25 ns beams in the present state can be used to study the transverse beam stability at 26 GeV and the need for the feedback system at this point of the cycle.
Proton beam parameter tables (Hannes Bartosik)
- Following the discussions in the other LIU beam parameter WG meetings, budgets (5% loss and emittance growth in the PS and 10% in SPS) and intensity/brightness limitations in the different machines are reviewed. The risks of the assumptions are also ranked as low (under control, existing margin), moderate (minor uncertainties, tight) or high (bottleneck identified, action needed) at the present state of art.
- Brightness line of PSB with Linac4. Simulations have been carried out by E. Benedetto and the expected double brightness is confirmed (depending on the working point and initial longitudinal emittance, one could in principle even reach brighter beams). Elena also showed that the present multi-turn injection into the PSB at 50 MeV for the current LHC beams is well reproduced using the same simulation tools. Although it would be desirable to reproduce with simulations the full existing brightness line and not only one point, this gives some confidence that the simulations are correct. Low risk
- Large longitudinal emittance at the exit of the PSB (~3 eVs for standard LHC beams). Different options have been explored: 'standard' C16 blow up, multi-stage parametric resonance excitation, noise injection on C02. The first two options lead to reproducible results but are not yet operational, the third option requires LLRF work but it would have the advantage not to depend on C16. Until proven reliable operationally: Medium risk
- Transfer of long bunches (220 ns) from PSB to PS. In principle, measurements on the recombinations kickers (both with coil and with beam) seem to suggest that this should be possible. However, direct measurements of the transfer of long bunches without degradation have not yet been done. These are necessary not only to prove that long bunches with large longitudinal emittances can be transferred, but also to show the gain we can have in the PS in terms of space charge. The effect of the injection error (including z-dependent effects from ripple and rise time of the kickers) with the transverse feedback should be studied both in the machine and in simulation (with all the ingredients contributing to the decoherence, i.e. nonlinearities, space charge, impedance) for the future intensity and brightness. Medium risk
- PS space charge. The assumption that Delta_Q=0.31 is acceptable keeping losses and emittance growth within the budget is based on <2012 measurements with few points and does not have any margin. It also assumes that this tune spread will be reached in the future with the new longitudinal parameters and low transverse emittances at the PS injection (at 2 GeV). Mitigations exist (hollow bunches, split tunes), but they are not straightforward. Further studies are needed. Medium risk
- Headtail instabilities at PS injection. They are presently cured with high chromaticity and coupling, but it has been proved that they can be suppressed with the transverse feedback, while the tunes can be moved off the diagonal and the chromaticity can be lowered favouring lifetime. Low risk
- PS longitudinal instabilities. It was assumed that with the 1-turn delay feedbacks on the RF systems and the new Finemet cavity as broad-band feedback system, the intensity reach for 25 ns beams could be increased well beyond the ultimate values needed by HL-LHC. In reality, present experience with the major part of the upgrades already in place shows that unexpected issues can still appear when increasing the intensity beyond 2e11 p/bunch at the PS extraction (e.g. higher order coupled bunch instabilities). Stabilisation methods need to be applied, e.g. larger longitudinal emittance (but this is not good for the SPS) or second harmonic cavity (would require the installation of new tunable cavity to be active all through the ramp and flat top). High risk
- Losses at the SPS flat bottom. They are longitudinal, as particles are lost from the bucket at injection, during the flat bottom and also at the beginning of the ramp. The cause is still uncertain, but these losses scale unfavourably with intensity. This means that increasing the intensity, it would become impossible to preserve the loss budget of 10% all over the SPS cycle (losses would amount to well above 10% only until the beginning of the ramp and then there would still be the ramp and the scraping close to flat top). Further studies on the PS-SPS transfer are needed both in the machine and in simulation with the high intensity effects. High risk
- Electron cloud in the SPS. The experience with high intensity in 2015 showed that the beam could be kept stable at 26 GeV (with chromaticity, octupoles, feedback system), scrubbing could be a solution for future operation but if that does not work a-C coating will be put in place during LS3. Medium risk
- SPS beam loading and longitudinal instabilities. Recent simulations suggest that the high intensity needed for HL-LHC would be stable at flat top (also with some margin coming from the use of the 800 MHz RF system) for the required bunch length at extraction (1.6 ns) if the longitudinal impedance reduction is put in place. However, simulations should prove that the beam is also stable during the ramp and the magnetic cycle might have to be longer in the future (and with different optics, Q22). Medium risk
- TMCI at injection in the SPS. Q20 offers sufficient margin, while Q22 would be about marginal for the needed intensity. However, high chromaticity can help. Low risk
- Protection devices in the SPS-LHC transfer lines. LIU upgrade of protection devices and new beam dump compatible with HL-LHC parameters (although future HL-LHC-type BCMS beams could be marginal). Low risk
- In conclusion, although there are a certain number of medium risk items that require further studies and full validation (and need to be closely followed up within this working group), the bottleneck seems to come presently from a combination of PS intensity limitation and SPS losses at flat bottom for high intensity. To gain more confidence that these limitations can be overcome in the HL-LHC era, some more hardware might have to be planned, like additional cavities in PS/SPS or a collimation system (also staged post-LS2). One worrying trend in 2016 is the absence of MD requests in the PS and consequently the lack of follow up on space charge, working point optimisation, coupled bunch instabilities, etc.
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