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LIU Beam Parameter WG meeting #5
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Europe/Zurich
874/R-018 - salle de réunion activité CCC (CERN)
Follow up from last meeting
Recombination and PS injection kicker rise time:
- Concerning the recombination kicker rise time, from direct measurements one of the two was found to be in specs, the one was not. Some corrective actions have been implemented during the YETS, to be checked now with beam. Past measurements with beam were not conclusive, an intensive campaign is needed in 2016.
- Concerning the emittance growth induced by kicking bunches out of the kicker signal flat top or from the ripples, macro particle simulation studies are needed.
- The transverse damper could play an important role in mitigating possible detrimental effects.
- Action: Malika to follow up on these items with Wolfgang and Vincenzo.
SPS intensity limitations:
Allowing for longer bunches at flat top does not gain much margin for increasing intensity, because
- due to RF power limitation, the voltage available with higher intensities is lower and therefore the stability boundaries in the bunch intensity vs. bunch length plots are much flatter than in the usual plots assuming constant voltage
- the RF power exceeds the available value already while accelerating larger longitudinal emittances.
However, new simulations with 800 MHz voltage larger than 1/10 of the 200 MHz voltage value show that actually the instability threshold at flat top can be even further increased, meaning that there is already more margin at 450 GeV. MDs in 2016 will be carried out to confirm these predictions (now that larger voltage wth 800 MHz is available together with the new cavity control). The intensity bottleneck because of acceleration unfortunately remains. It needs to be studied more carefully through simulations, but simulating the full ramp is very demanding in terms of CPU. The CNAF cluster, for which LIU has paid a part of the commitment, can be used for these simulations. ABP is collecting the information to run simulations on this cluster with the goal of making it available.
PS intensity limitations (Heiko)
The main performance limitations in the PS come from space charge (brightness) and instabilities (intensity).
- Besides the 2 GeV, space charge mitigation already relies on the injection into PS of long bunches (see above on kickers) and large longitudinal emittances from the PSB (this subject will be discussed in detail at the next meeting).
- Transverse instabilities
- Headtail at injection, which appears for high working point (far from coupling but needed to accommodate the space charge tune spread) and low chromaticity (needed because of the large dp/p due to the desired large longitudinal emittance at injection). It can be cured with transverse damper. This is an important role of the transverse damper to determine the performance of LHC beams in the PS. This study is intimately related to the space charge studies.
- TMCI at transition: quick tests with beam this year suggest that the threshold is Nb > 2.0e12 ppb for ez = 1.2 eV in h21 (only one bunch rebucketed). This is above the value needed for LIU. More dedicated studies should be carried out in 2016, possibly with the proper LHC beam configuration, i.e. more bunches. Action: Guido to follow this up within the PS beam dynamics studies
- Electron cloud instability at 26 GeV. The transverse damper is also useful to mitigate this effect. In one of the next meetings, it is important to re-discuss all the uses of the PS transverse damper (e.g. injection oscillations, headtail instabilities, e-cloud instabilities, 80b production, etc.) and assess its impact on the ultimate performance of the PS in terms of LHC beams. Do we understand all the requirements and have the necessary upgrade items in the baseline for the future LHC beams? Action: Guido to make the point on the PS transverse damper within LIU-PS beam dynamics and report back to this working group
- Longitudinal beam quality and instabilities
- Reduction of transient beam loading and better bunch-to-bunch equalisation, achieved thanks to the 1-turn delay feedbacks. Studies carried out presently with lower longitudinal emittances show that this mechanism of compensation will work efficiently also with higher intensities.
- Mitigate coupled bunch instabilities by
- Reduction of the cavity impedances (improvement of the feedbacks);
- Use of the broad-band Finemet kicker as kicker for feedback system. Simulations, which agree quite well with the observed instabilities using an impedance model including only the cavities, would anticipate that this should be enough to stabilise the beam for intensities well beyond the HL-LHC target. The system could provide 67% of the final voltage in 2015 and will be close to full functionality (~85%) in 2016. MDs in 2015 showed that a beam with the required quality could be produced at the PS with up to 1.7e11 p/b. One of the problems encountered was that, even if the dipole instabilities were cured as expected, a quadrupolar oscillation appeared as an unexpected intensity limitation. More studies will be needed in 2016, both experimental and in simulations (e.g. try to improve the impedance model including kickers and other elements, reproduce the new instability). Action: Heiko and Letizia, to continue machine and simulation studies on the coupled bunch instability;
- Increase of longitudinal emittance compatibly with the SPS requirements. In fact, if a larger longitudinal emittance is allowed at the PS extraction, e.g. 0.5 eVs, the bunches can still be rotated with two 40 MHz cavities and the intensity reach becomes significantly higher. This value seems to be too high for the SPS, there would be losses along the ramp. More transmission studies are needed in 2016. Action: Heiko and Elena to coordinate new transmission studies aiming at minimising losses in the SPS as well as enhancing the intensity reach out of the PS (what is the maximum longitudinal emittance that can be received by the SPS?)
In summary, the required 2.6-2.7e11 p/b with 25 ns spacing for LHC is well beyond present achievements, transversely and longitudinally. Although on paper it seems that the PS upgrades should be enough to reach in post-LS2 era, some new instabilities have been already observed that may limit the parameter reach. Machine studies in 2016 are crucial to determine the future performance. Action: Giovanni, follow up regularly within the LIU beam parameter WG the advancement of all these studies.
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