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LIU Beam Parameter WG meeting #16
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Europe/Zurich
874/R-018 - MEETING ROOM PRIORITY BE OP (CERN)
General
Vincenzo's talk on the beam-based measurements of the kicker rise times for the PSB-PS transfer will take place next Friday (3 March). This meeting has been maintained anyway to discuss the question of the bunch length and longitudinal emittance entries in the new LIU beam parameter table, and shortly address already the question of the emittance budgets assumed throughout the injection chain in the light of the 2016 measurement presented in Evian and Chamonix.
New LIU beam parameter table
A draft version of what should be the new LIU beam parameter table has been shown. It contains both the parameters for the standard and BCMS(+) beam, with lines for the achieved values and LIU targets. Since the LIU target numbers have been finally matched to the HL-LHC request, there is no line for the HL-LHC request, as in previous versions of this table.
The bunch length and longitudinal emittance at the SPS injection should be both those right at injection (i.e. those extracted from the PS after bunch rotation) and those after filamentation. Elena remarked that the table should also contain explicitly the capture voltage assumed, because the filamentation might lead to different final values of bunch length and longitudinal emittance according to the considered voltage. For the rest, baseline assumptions should be considered, i.e. bunch intensities as reported in the table and Q20 optics. It is proposed to keep for bunch length, momentum spread and longitudinal emittance both the values right at injection (because they define what the PS provides) and those after filamentation (because they determine space charge on the injection plateau of the SPS as well as beam stability). For the moment, the values after filamentation are reported in parentheses. Several questions could give rise to confusion. First, the longitudinal emittance right at injection, which should be that from the PS after bunch rotation, is actually measured before the bunch rotation. From the PS experience it is well-known that a bunch of 4 ns at extraction comes from a measured longitudinal emittance of 0.35 eVs before rotation. These values anyway do not take into account that the bunch rotation already produces an S-shape in the longitudinal phase plane with a potential emittance blow up already in this process. Besides the value of 0.35 eVs is also the one quoted in the SPS after filamentation. Here the reason could be the different conventions used for the longitudinal emittances in the PS and SPS. The value measured in the PS is a matched area from the tomoscope. The value from the SPS is 2sigma, where the sigma is evaluated from the HWHM and disregards the existence of long tails actually filling the SPS bucket. It is agreed anyway to leave one single value for the longitudinal emittance, i.e. 0.35 eVs putting in the asterisk that this is consistent with both the PS and SPS conventions at the different stages. The values of momentum spread at SPS injection will be provided by Heiko. Action: Heiko.
For the LHC injection, the values relevant for LIU are those right at injection. Presently, we have the values measured in the LHC after filamentation (capture in 6 MV instead of the matched 3 MV), and they appear to be different for the standard beam and BCMS. This might be due to the fact that, although the target bunch length at SPS extraction is 1.65 ns, the BCMS beam might be actually extracted with shorter bunches, e.g. 1.5 ns, and lower longitudinal emittances. LIU target values are indeed 1.65 ns bunch length and 0.60 (or 0.63?) eVs longitudinal emittance at SPS extraction. However, Elena will also have a closer look at these values and give feedback of which numbers should be actually quoted in the official tables. Action: Elena
Emittance growth at the PSB-PS transfer
The emittance evolution along the injector chain, as purely extracted from the logged data for the LHC beams during the LHC filling periods, was already shown and discussed in Evian/Chamonix and in a few other places. It shows that the horizontal emittance blow up between the PSB extraction and PS injection amounts to 40% for the standard and 50% for the BCMS beams (10 and 20% expected, respectively, from betatron/dispersion mismatch). The measured emittance blow up in the vertical plane (10-15% for both beam variants) is much more in line with the expectations from the mismatch. For the future beams, we have to consider that large longitudinal emittance bunches will need to be transferred to alleviate space charge in the PS, and therefore any dispersion mismatch would become even more severe due to the larger momentum spreads. The compatibility of the kicker rise-times with the requested bunch lengths at transfer will be addressed at the next meeting by Vincenzo. The expected emittance blow up from mismatch is predicted to be negligible in the horizontal plane after the upgrade and rematching of BT-BTP, but it will still be 10-15% in the vertical plane for both standard and BCMS beams. While this should be not a problem for the BCMS beam (because the PS space charge limitation line lies more than 10-15% above that of the future PSB brightness), for standard beams it would be a real problem as it could limit the achievable brightness of the LIU beams. The emittance blow up budget always assumed in our tables was 5% for the PS, however we know now that only from dispersion mismatch at injection we could have 10-15% in the vertical plane. The obvious mitigations with respect to this assumption are:
- The 'excess' emittance blow up at the PS injection is absorbed by other 'pessimistic' budgets;
- The brightness line of the PSB is actually better than half slope of today (simulations suggest that this might be the case);
- Any action to be taken to improve the vertical dispersion mismatch at PS injection?
Several factors can contribute to the emittance blow up at the PS injection, for example if the collapse of the injection bump is not perfectly synchronised between the different kicker modules, which can result in non-adiabatic closed orbit distortions. In the future this will be more critical because the collapse of the bump will be faster and one of the bumpers will be of a different type based on a new technology (eddy-current bumper). To minimise the non-adiabaticity, the possibility of slower ramp down of the bumper magnets for LHC type beams at 2 GeV should be investigated by ABT. Steering errors should be taken care of by the transverse damper. However, it would be important to inject with low chromaticity to avoid decoherence before the damper can even play its role. It could be interesting to benchmark all the analytical formulae for the estimation of the emittance growth from mis-steering/mismatch with simulations in which all nonlinearities and intensity effects are taken into account.
Another question that was raised is that the BCMS beam seemed to be within the expected blow up and loss budgets before LS1. This might suggest that the additional blow up observed in 2016 could be indeed corrected.
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