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LIU Beam Parameter WG meeting #4
→
Europe/Zurich
874/R-018 - salle de réunion activité CCC (CERN)
Follow up from last meeting
- Ion parameter table: The EDMS document with the LIU achievable performance and estimated integrated luminosity with the LIU parameters has been released after a few rounds of reviews. The scenario with 4 bunches at 50 ns through batch compression at flat top remains interesting and Heiko is investigating its costs. However, it won't be much detailed in the TDR, because a first technical limitation has emerged in the handover between the 20 and 80 MHz RF systems (bunches are too long), which needs deeper analysis. Heiko will follow up on this subject later on during the year (after MDs in the PS are done and a cost/resource estimation is put in place)
- SPS protection devices: In general, no showstopper has been found to limit the bunch intensity below 2.3e11 p/b within 2.1 um. BCMS has been checked only up to 2e11 p/b (with 1.3 um), extension to 2.3e11 p/b would be ok using simple scaling laws for attenuation and robustness, however detailed simulations should be run to confirm this.
- There was a first meeting between Giovanni, Bettina, Alan, Steve, Simon to discuss the question of the longitudinal emittance blow up in the PSB and path to follow. Steve believes that one of the main issues to have it work now is the fact that the phase between C02 and C04 cannot be controlled to the degree, as it would be needed. Elena said that simulations have been already run and show that in principle the longitudinal emittance blow up in the PSB should work beautifully. We should then review the hardware specifications to implement the scheme and see what is missing and what should be done this year in terms of MDs (involving Finemet?). This will be one of the subjects in the next meetings, tentatively 23 April.
PSB-PS recombination kicker rise times (Wolfgang)
LIU beam parameters assume future bunch lengths at PSB extraction of 205 ns for standard beams (transferred into h=7 in the PS) and 135 ns for BCMS beams (transferred into h=9 in the PS). The specification is therefore 110 ns rise time.
The fields of the KFA10 and KFA20 (recombination kickers) were measured during the shutdown.
- KFA10 was found out of specs, but some improvements have been envisaged to improve this (new vacuum tank with blocks of 'good' ferrite, simulate new PFL configurations and test in lab on KFA20 spare to reach specs, reducing length of transmission lines). If all fails, further improvements would not be cost neutral and are not presently funded
- KFA20 was found to be in specs without any margin. Some improvements are also envisaged here to remove the bumps on the flat top, gain some margin by reaching 100% earlier and lowering somewhat the kick strength to reduce the magnet breakdown risk. If the upgrade work on KFA10 brings it within specifications, KFA20 could then be also reconfigured in the same way to avoid magnet breakdowns. No LIU unit existing for this item.
The injection kicker KFA45 was measured in current. Controlling the ripple at flat top here is important in order to keep the induced emittance growth low (i.e. not eating up all the 5% emittance growth budget for LIU beams in the PS). The requirement on the rise (fall) time is the same as for the recombination kicker. Permanent short-circuit configuration looks promising, parts of the planned upgrades already tested and further tests during the EYETS. More measurements with beam necessary?
Some measurements with beam were conducted in the past to try assessing the emittance growth due to the kicker waveforms.
- KFA45 was fired at 26 GeV with three bunches and different kick strengths. For kick strengths above 100 kV, tails get populated, although the general emittance growth seems to stay lower than what is predicted analytically. Macro-particle simulations with non-linear model of the machine and kicker waveform should be conducted to have a better prediction of the emittance growth and tail population.
- Wire scanners measurements at the PS injection with bunches badly kicked by KFA10 were made to assess the effect on emittance growth. However, even when bunches were half kicked out, hardly anything was visible on the emittance measured in the PS. Looking at the quality of the longitudinal profiles, it would seem that ~85 ns distance between bunches would be sufficient for a clean transfer. Nevertheless, the transverse signal shows that the tails of the bunches can be badly kicked in the vertical plane when the distance is below 100 ns. These measurements were not systematic and should be re-done.
More measurements are foreseen in 2016 (e.g. with short bunches to probe the KFA10 and 20 waveforms or with longer bunches than 180 ns to check when the impact of the recombination kickers becomes visible), list in Wolfgang's slide
SPS intensity limitations (Elena)
Best performance reached so far in SPS is 1.35e11 p/b at extraction with 1.63 ns average bunch length but non-negligible bunch length spread along the four batches
Presently a programmed RF voltage of 7 MV (already reduced due to beam loading) leads to a bunch length of ~1.7 ns at extraction for nominal intensity (~1.25e11 p/b). With the power upgrade, only 2.1e11 p/b will be available (prior to scraping) for the same bunch length, and the voltage available at this intensity will be 12 MV. For 2.5e11 p/b, only 10 MV would be available, which would not be sufficient to reach the required bunch length at extraction. From simple scaling, however, allowing for 10% longer bunches at extraction would make 2.5e11 p/b possible at SPS flat top.
BLonD simulations with the SPS full impedance model reproduce well the measured thresholds for trains of 24 and 74 bunches in single and double RF at 450 GeV (with 7 MV available). Moving to 10 MV, the intensity threshold for a bunch length of 1.65 ns becomes 2.1e11 p/b as was anticipated by simple scaling. Different scenarios of impedance reduction yield in principle different gains in terms of achievable intensity, e.g. the case of enhanced HOM damping + VF shielding seems to give also enough margin. However, one has to take into account that the voltage assumed in simulations was always 10 MV (without taking into account that for higher intensities the available voltage is lower due to beam loading) and that in reality the batches will always have a spread of bunch length over the bunches, extending to regions in which the instability threshold could be much lower (the stability boundaries are very steep curves, because the range of the x-axis, i.e. bunch length, is very small - they would however be much flattened if the voltage reduction from beam loading was taken into account in the intensity scans). This means that even with average bunch lengths of 1.65 ns, the beam could still become unstable due to the outlier shorter bunches.
Many more studies still need to done with simulations to complete the picture, e.g. after further refining the impedance model and how it would evolve after reduction, other points of the ramp, accounting for bunch-by-bunch intensity and bunch length variation in the batch, with increased 800 MHz voltage (20%), with bunch rotation (optimal minimum voltage), ...
For the moment, it seems that the HL-LHC intensity is in reach with the impedance reduction program, but we are at the limit. One of the risks is actually that the RF upgrade might not be able to deliver the promised power and therefore the available voltage will be lower than we believe it will be at this stage (the discrepancy between the design and the available power of RF systems is not new). We could however gain some margin adding the option of bunch rotation at flat top or with just 5% more tolerance on the bunch length into LHC (more studies, 200 MHz in LHC?). This margin obviously does not translate into making potentially available larger intensities, because e.g. the limitations on the protection devices (total intensity) and RF power limitations during the ramp would remain.
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