Wednesday April 5th, 14:30
CCC conference room 874/1-011
Greek symbols are not recognized by Netscape browsers. They will be avoided in these and future summary notes.
LHCf: Helmut, Daniela, and Massimiliano are following up this detector. The ongoing discussion considers e.g. to limit the detector motion. The LHCC has requested a short statement from the LHC machine side before its next session on May 10th. Helmut will present a draft statement in the next LHCCWG meeting. ACTION: Helmut.
75 ns: The conclusion from the last meeting is that this commissioning step should be kept. Roger will inform the LTC. ACTION: Roger.
Energy matching: Gianluigi and Jean-Pierre are looking into this item, e.g., estimating which difference in circumference between the two rings is expected. Jean-Pierre had commented by email that two separate rf systems were introduced intentionally during the design of the LHC in order both to separate control of the two beams and for reduced beam loading. Andy repeated that once the rf systems of the two rings are unlocked, they cannot be locked again. A re-locking requirement would imply a change of the low-level rf system, which has not been built. Andy argued that one should keep the rf system as simple as possible. Paul pointed out that different rf frequencies for the two rings are needed for proton-ion collisions, which are however not yet in the LHC baseline. Ralph emphasized that e.g. a 1.5 mm difference in radial position would have to be subtracted from the 4-mm orbit tolerance. Stephane mentioned that 1 or 2 units difference in b1 between the two rings gives a momentum error of 2e-4 and a tune difference of about 0.02 due to the natural chromaticity, though this may not directly be linked to the question of the two rf systems. Andy highlighted that the phase is a free parameter and can be changed without a problem within 1 rf period, independently for the two rings. ACTION: Gianluigi and Jean-Pierre.
A web site with open issues and actions will be created.
Rhodri discussed the set up of BPMs, dc and fast BCTs, and BLMs with circulating beam, as well as measurements of tune, chromaticity, coupling and emittance.
He pointed out that most BPM polarity errors are already found during the threading and first turn measurements. Each BPM has to be “timed in”. This is currently a manual, but fairly fast process (a 2-ns dead time out of 25 ns needs to be avoided). For the BPM system ‘slot 1’ is defined as the first after the dump gap. Brennan mentioned that the pilot bunch will not always be in ‘slot 1’, since the timing of the dump kicker system requires pilot bunches in several time slots. The calibration of the BPMs and CODs for the orbit feedback is done by exciting the CODs one by one. The total time required is estimated by Jorg as about 30 s per COD or 9 h in total per ring. The same procedure was applied for TI8, with on-line analysis. After the meeting, Jorg stressed that for LHC a "fast on-line analysis" fitting ALL BPM calibrations + corrector calibrations + optics model is neither feasible nor of interest, and that manual interventions by a real human will always be required. Special attention will be paid to BPMs in IP6 and at injection regions. Information from the survey database will be included from the start. Paul suggested that data be taken for both rings at the same time, speeding up the commissioning. The orbit feedback is likely to use correctors and BPMs in the common regions. Stephane pointed out that the phase advance per cell close to 90 degrees implies a high degeneracy, which could lead to non-local corrections in an SVD-based orbit feedback. He recommended using nearby correctors if possible. Orbit feedback functioning and calibration may be discussed in a future meeting. Possible ACTION: Jorg and/or Ralph Steinhagen. Ralph Assmann commented that necessity of an orbit feedback is determined by the observed stability of the LHC orbit. Rhodri pointed out that noise from the cryogenics could be important.
The about 4000 BLMs require a fine adjustment of their threshold. It is foreseen to set the initial thresholds a factor 3 below the estimated quench level at 450 GeV, which one could qualify as ‘dump & learn’. Ralph, Paul and Brennan suggested setting the thresholds higher initially, and to only protect against damage, since one can learn more with increased thresholds (‘quench & learn’). BLMs can be masked with safe beam at 450 GeV. The threshold change procedure is under discussion at the MPWG.
The BCTFRs are already commissioned during the 1st turn. With circulating beam, the DCCT response is checked, and BCTFRs and DCCTs are cross-calibrated. For a single pilot bunch the DCCTs are at the edge of their resolution limit. The BCTFRs can measure a 10-h pilot-bunch lifetime with 10% accuracy over 1 minute.
The MKQ kicker needs to be available from the beginning. It is used to measure the tunes and coupling by the BBQ monitor and chromaticity by the head-tail monitor. The measurement repetition rate can be increased by using chirp excitation via the transverse damper. Such a system will be operational at the SPS in 2006. PLL tune tracking will be commissioned in parallel. It requires either a special kicker or the damper. Rhodri suggested commissioning the tune and coupling feedbacks early on, since this may greatly reduce commissioning time later. The potential time saving was demonstrated at RHIC, where an identical feedback has greatly speeded up ramp development. The chromaticity feedback requires a stable PLL tune measurement and rf frequency modulation. The PLL excitation level in RHIC corresponds to a beam motion at the micron level.
Concerning emittance measurements, wire scanners will be available from day 1. The synchrotron-light monitor requires the undulator to be turned on and its effect on beam orbit or optics to be corrected. The two systems will be cross-calibrated. The residual gas monitor needs an artificial pressure bump if the natural ring pressure is too low, i.e., below ~1 ntorr.
Finally the abort gap monitor is gated over full 3 microseconds, or over 30-ns or 100-ns time slots. Threshold, timing and interlock need to be calibrated and checked. Stephane asked whether the abort gap monitor is needed without the possibility of cleaning by the damper. The reply was that whether or not active cleaning is possible, monitoring of the abort gap will still be required. Paul asked for the minimum intensity needed for commissioning each instrument. He and Brennan recommended a periodic dump&inject mode, where the beam time is maximized with sufficient intensity, rather than an inject&dump mode where the no-beam time is maximum. A possibility for relieving the instruments is to operate with intensities higher than the pilot-bunch charge.
Brennan recalled his comprehensive overview at Chamonix 2006. He summarized the various commissioning steps. These involve a verification of the ring optics in IP6, the commissioning of critical screens and BPMs, careful aperture measurements in the extraction line, and a confirmation of the waveform for extraction and dilution kickers. Simultaneously using several pilot bunches would speed up the dump commissioning. The fine timing of the kicker requires two pilot bunches spaced by 3 microseconds. Once circulating and extracted beams are qualified, the IR6 BPM interlock can be set. It becomes active for orbit changes larger than 4 mm. The BPM reading will be calibrated with respect to the quadrupole center. There is a 40% budget for beta beating.
Brennan proposes to set the TCDQ/TCS system at roughly +/-10 sigma and to retract the TCDQ by 10 mm. With 156 bunches during an asynchronous beam dump only a single bunch will then hit the TCS without impacting the TCDQ. This is safe at 450 GeV and also at 7 TeV if beta* is not smaller than 2 m. Further commissioning could be delayed until beta* is squeezed to smaller values or more bunches are stored.
Brennan stressed that presently all dumps are requested via the interlock system. Some of the consequences, like PM trigger and switching off the rf, are unwanted when a beam is dumped for reasons other than emergency, e.g., for performing measurements with the second beam.
The inject&dump mode needs a hardware turn counter. The current pre-pulse is issued 8 microseconds before injection, which makes it difficult to dump in the injected turn. Rhodri and Gianluigi commented that 8 microseconds were an extremely short notice, and that by contrast the pre-pulse timing is about 150 microseconds in the SPS. Injection, collimation and beam dump may all be set up with beam circulating over only a few turns. Paul remarked that one instrument requiring a dump after a small number of turns is the matching monitor. Stephane suggested injecting a “virtual pilot” bunch to produce an earlier pre-pulse. ACTION: Brennan will organize a more restricted discussion on the pre-pulse, the need for it, and the associated time requirements.
The initial measurement program comprises polarity checks for BPMs and correctors, aperture scans, and basic optics verification. Orbit, coupling, tunes, and chromaticity will be adjusted in this period. The BPM resolution in orbit mode is 20 micron for a pilot bunch and 5 micron for a nominal one. In trajectory mode the resolution is about 10 times worse. Frank recommends starting on the special working point proposed by Stephane Fartoukh in order to avoid problems with coupling. Going to the nominal working point later will require a coupling correction based either on the response to two skew-quadrupole families or on multi-turn BPM readings. Simulations by the HEADTAIL code suggest that a large chromaticity does not necessarily imply a degraded tune resolution. Maximizing the decoherence time appears to be an efficient and fast method of correcting the chromaticity. Gianliugi suggested that by detecting the motion of the bunch center the signal decoherence may be avoided even when the rf is on.
The measurements of BPM-corrector responses for polarity checks and orbit-feedback calibration also provide plenty of redundant information on the beam optics. In addition, multi-turn BPM data can be taken for an independent optics cross check. Time permitting K modulation may be used at critical locations, e.g., near beam-size monitors or at the extraction point. Radial steering is employed for measuring the dispersion, the chromaticity and off-momentum beta beating. Frank also illustrated how local sources of strong beta beating can be identified.
A scheme for measuring the transverse and momentum apertures was presented. Frank also calculated the expected apertures and bottlenecks, with the help of Stefano. Stephane and Verena commented that this type of analysis should include the contribution to the beam size from momentum spread and dispersion, and that it may be difficult to determine the center of the beam from loss measurements. Frank replied that the dispersive part of the beam size was a small correction in this type of measurement and that the beam center should be found with a resolution of a fraction of the rms beam size, which is much smaller than the difference between tightest restrictions and typical arc apertures. After the meeting Frank found that including the dispersive part of the beam size, when normalizing, would make little difference for the aperture restrictions. The two ring center frequencies should be measured early on.
He concluded that 4 days per ring was a realistic time estimate for the initial measurements, and that rapid progress depended on the availability of procedures and (ideally online) software. He noticed the non-existence of a comprehensive optics version simultaneously including aperture, misalignments and measured errors, and he asked whether such model would be desirable for LHC flight simulations. ACTION: Massimo will organize a more restricted discussion on the commissioning optics model.
Wednesday April 19th, 14:30
CCC conference room 874/1-011
Provisional agenda
Minutes of previous meeting
Matters arising
LHCf statement (Helmut)
450GeV commissioning: increasing the beam intensity
AOB
Reported by Frank