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LIU Beam Parameter WG meeting #9
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Europe/Zurich
874/R-018 - MEETING ROOM PRIORITY BE OP (CERN)
Follow up from last meeting
- All rings in the PSB are being set up with large longitudinal emittance (>2 eVs), beam to be given to PS.
- At the LIU Beam Parameter WG on 26 August we will review the parameter table for the proton beams and release a new version (if agreed), enough in advance with respect to the Cost & Schedule Review.
- The impact of LIU upgrades on non-LHC beams, which we tried to identify in the past, is becoming increasingly of interest. Although the declared LIU policy is to apply upgrades that should improve LHC beams without degrading the performance of the other existing physics (non-LHC) beams, some hardware design (e.g. distributor time in the PSB, injection septum aperture in the PS) in the past was made with the goal of opening the way also to a possible upgrade of the Fixed Target beams.
Emittance growth from injection errors (Wolfgang Bartmann)
- The general aim of this meeting is to review the possible injection errors, for which the damper could help, as well as outline the other important applications of the damper (damping of instabilities, production of 80b trains, MTE), specifying the requirements in terms of power and bandwidth and comparing to the present system.
- Some items are not folded in at this stage, e.g. PS closed orbit stability, errors from bumpers, effect of space charge and impedance on decoherence, etc.
- In general, the injection errors are considered to be composed by a 'correctable' component (e.g. magnet misalignment, long term drifts, etc.) and an 'uncorrectable' component (random errors affecting the shot-to-shot reproducibility, systematic errors from power supply ripples and kicker waveforms). The study on emittance growth from injection errors only assumes the latter component.
- The dynamic errors are assigned in simulation in order to calculate delivery precision of position and angle at PS injection, and consequently assess the aperture in the lines (losses, radiation), the margins for CO and beta beat, the emittance growth from steering and mismatch error for LHC and HI beams.
- Apertures and beta beating are o.k., however the emittance growth caused by the different optics in the four lines and dispersion mismatch can be significant (>5% in the vertical plane).
- The required field homogeneity is 1e-4 rms. However, one could use this requirement only for LHC beam size and accept higher field inhomogeneities for HI beams as long as they fit into the acceptance. This would be a compromise both in terms of emittance preservation of the LHC beams and losses for HI beams. The steering error for HI beams would be larger and the main bottleneck would be in the horizontal plane at injection when the injection bump is on.
- The beam/bunch distribution should be folded in with the kicker waveform to determine the weighted effect.
Effect of recombination kickers for transfer of long bunches (Vincenzo Forte)
- A novel beam-based measurement technique of the waveform of the recombination kickers BT1.KFA10, BT4.KF10 and BT2.KFA20 has been used. This technique is based on the time resolved position measurement from a 180 ns long bunch with a wideband pick up downstream from the kicker. Moving the bunch along the waveform of the kicker allows for the reconstruction of the kicker waveform, as long as the bunch is not lost before the pick up due to the mis-kicking. The combined measurements from the bunches from two lines being recombined can be used to fully reconstruct the kicker waveform.
- The measurements have been made with different bunch intensities. Higher intensity bunches provide the best measurements because the BPM response is more accurate. The reconstructed waveforms reproduce very closely those measured in the lab by L. Sermeus with the search coil. In particular, both the rise times and the steady state ripple are reproduced with very high accuracy. A clear difference with the OASIS signal can be observed, which suggests that OASIS should not be used for the fine synchronisation between signals.
- Both the KFA10s have rise times below 105 ns and steady state ripple below 2%. KFA20 has a rise time of 109 ns and a steady state ripple that can reach up to 2.6%. A strange effect was observed when measuring KFA20
- The frequency of the signals excited on the beam is up to 20 MHz.
- The SEM grids in BTM have also been used to check how much longer a bunch extracted from the PSB can be before measuring a clear emittance growth from the kicker rise time. It seems that for the recombination kickers there is still a margin of ~75 ns to keep a target vertical emittance of 1.7 um and ~150e10 ppb (~present LHC type beams). New tests will be performed with the new 220 ns bunches prepared at the PSB, in order to check directly the future LHC bunch lengths.
- PI.KFA45 is under test due to issues with the capacitive BPM in the PS ring (sensitivity to losses) and a proposition of a new non-capacitive BPM after injection has been put forward.
Specs for the transverse damper in the PS (Guido Sterbini)
- System specs:
- The 3 kW per kicker plate were specified to damp 3 mm (peak to peak) oscillations with a damping time of about 50 turns at 2.1 GeV/c.
- The bandwidth between 20 kHz and 20 MHz was specified to cover the lowest betatron side band as well as the effect of the kicker ripple.
- The presently installed transverse feedback system in the PS, meant to damp injection oscillations as well as suppress coherent instabilities, is 90% complete (i.e. what is expected to be in function after LS2):
- In May 2016 the old prototype cards were replaced with the new ones with a much more flexible firmware. After the first beam-based test, new versions of the firmware and the FESA class are currently ready for installation. This will fix some bugs and provide the complete LLRF functionality (GFAs controls).
- The power amplifiers will still need to be upgraded from 3 kW to ≈ 5 kW in order to cope with the higher injection energy. Their power supplies have been already delivered. The matching network between the amplifier and the kicker is expected at the end of 2016.
- The PS TFB is presently being used for the MTE beam production (flat beams, coupling corrected, unstable at natural chromaticity for high intensity > 1500 php) and to make controlled blow-up for the LHC production beams (in the smooth transition from standard 25 ns beams to BCMS). However, LHC beams (and other physics beams) are still relying on strong transverse coupling and natural chromaticities at injection to cure the slow head-tail instabilities.
- It is observed that, apart from the pulse-to-pulse variation that usually does not exceed 1 mm, static orbit errors of several mm exist at injection in both the H and V planes.
- The slow head-tail instabilities at injection energy have rise times of several ms (at least the higher order modes for negative chromaticity) and become slower as the chromaticity becomes more negative. However, also their frequency becomes higher with high negative chromaticity, so even if they are slow, they could end up outside of the damper bandwidth. In general, it would be desirable anyway to correct chromaticity at injection (also to better accommodate the large longitudinal emittances from the PSB required to alleviate space charge at injection), correct coupling and the transverse feedback acting against the slow head-tail motion.
- Miscellaneous:
- The vertical TMCI-like instability at transition (observed on TOF-like and LHCINDIV above certain intensities, 700 MHz traveling wave). The PS TFB cannot cure it, however ongoing measurements show that there is sufficient margin for the LIU beams (1 eVs, <200 pub) --> To be reviewed in the general PS intensity limitations for LIU beams;
- The PS TFB cannot cure bunch motion (and intra-bunch motion) at 40 MHz (beam structure at extraction) but it was shown in the past to be able to alleviate transverse coupled-bunch motion probably induced by electron cloud;
- The injection oscillation dephasing along the bunch due to indirect space-charge (which happens in few tens of turns, as studied in detail in A. Huschauer's PhD thesis) is expected to reduce the efficiency of the injection damping. Further investigations are needed on this point.
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