Wednesday
January 31st, 14:30
CCC
conference room 874/1-011
There were no comments on the minutes of the 18th LHCCWG meeting during this meeting. However, Stefano Redaelli has sent a few comments per email afterwards. An updated version of the minutes of the 18th meeting has been posted on the web.
Roger summarized the presentations and topics treated by LHCCWG in 2006, and highlighted the items which have not yet been addressed. He next presented the comments from the LHC Machine Advisory Committee, which stressed that the beam studies during the 450-GeV engineering run should be thoroughly completed, and should include a first look at the beam behavior on the ramp.
Roger then reported news on the triplet heat exchanger problem. The repair procedure for all 24 triplet quadrupoles is still under investigation. Roger observed that 18 of these quadrupoles are already installed in the tunnel. The baseline solution is a replacement and an in-situ installation of entirely new heat exchanger tubes, which should arrive at CERN by end of February. A plan B also exists, and it consists of a reinforcement of the tube extremities. The repair starts with the triplet 5L. Sector 8-1 will be commissioned after sector 4-5. The triplet in 8-1 should be fixed before cooldown of this sector. The 450-GeV run in 2007 is maintained according to the present schedule. However, hardware commissioning to top energy in 2008 will take longer.
A short to ground was found on a QD circuit, which later disappeared at lower temperature. Oliver mentioned that he discussed this short with Peter Limon and Karl Hubert Mess, who both recalled that similar magnet behavior had been observed at the Tevatron and at HERA, and that at both these accelerators the implicated magnets all worked fine later on.
Roger next announced that the work of the Technical Coordination Committee (TCC) was successfully completed. A new Installation and Commissioning Committee (ICC) is chaired by Lyn Evans, with Roberto Saban as deputy chair and Roger as scientific secretary. The ICC will meet every two weeks. There will also be a change to scope of the LHC Technical Committee (LTC), which Steve Myers would like to refocus more closely on the LHC beam commissioning.
The LHCCWG plan for 2007 is centered around two major axes: (1) continuing the elaboration of the commissioning plan, (2) getting more closely involved on the accelerator systems side. Outstanding topics for axis (1) include two-beam operation (Jan and Ralph), details of the ramp (Mike), measurements at 7 TeV (Frank), collisions at top energy (Helmut), documentation and procedures (Verena and EICs), and prioritization of open-action list. Axis (2) may cover hardware commissioning with power tests of the main circuits, individual system tests, other items not covered by the hardware commissioning committees, integration of hardware into operation, and elaboration of details for the 4th quarter 2007.
A draft agenda exists for the next nine LHCCWG meetings, until June. The number of items allocated per meeting appears challenging. On February 14, Elias and Werner will discuss alternative filling schemes and their impact in the injectors. Mike suggested that timing be added here as an additional topic. Also, after the meeting, Thijs offered an additional presentation on the use of beam-loss and radiation monitors for brining the 450-GeV beams into collision. The February 28 meeting will address magnetic modeling as well as the behavior of magnets during the squeeze. On March 14 there is no meeting, due to the OP visit to LNGS. The March 28 meeting will address beam measurements at 7 TeV, luminosity and collisions, and the BLM system. On April 11 we will review various types of beam excitation and the vacuum, on April 25 details of the ramp and the RF system, and on May 9 the MAD-X online model and the magnets. The May 23 meeting includes a report from MPSC subgroup. Three open items have not yet been assigned to time slots and need to be scheduled. Roger mentioned that in each of the future meetings we will also look at some of the procedures and documentation, which are being developed by Verena and the EICs.
Jean-Jacques reminded that BI needs to define the interface to LSA for each instrument during Q1, including the required timing information, etc. Verena added that a related question concerns the remote setting of the BLM thresholds. Persons responsible for the instruments should take part in these discussions, which Jean-Jacques offered to organize from the BI side if it cannot fit into LHCCWG agenda before the end of February or beginning of March.
=> ACTION: Schedule LHCCWG
discussion or dedicated meeting on LSA interface for each instrument (Roger,
Jean-Jacques)
Paul Collier commented that, from looking at the 2006 draft agenda, by the end of May we might still have a long way to go. Roger replied that we could increase the meeting frequency if this turned out to be necessary. He asked whether fewer longer meetings or a larger number of shorter meetings would be preferred. Verena suggested introducing more “parallelism”. Jan anticipated that a larger number of meetings with limited scope would naturally lead to more parallelism. Helmut thought that it was possible to extend the number of items covered per meeting, e.g., with stricter chairmanship.
Frank presented a table of tolerances for optics and beam parameters, which he had extracted from various reports and documents and finalized with the help of Stephane and Stefano. These tolerances refer to various characteristics of the linear and nonlinear optics, dynamic aperture, beam energy, beam quality, and beam lifetime. Frank distinguished between nominal tolerances and relaxed tolerances for the engineering run in 2007. Most of the latter are based on educated (and possibly controversial) guesses, as they have not been formally specified anywhere.
Stephane commented that the 0.6 sigma orbit stability tolerance for collimation, which Frank quoted, should be lowered to 0.2 sigma, in order to represent the correct trade off for the combined effect of orbit motion and transient beta beating. Frank replied that he determined the 0.6 sigma in collaboration with Stefano, and that this number had been published in several reports by the collimation team, e.g., Ralph’s contribution to Chamonix XII, or the LHC-Project-Report-598 by Ralph, Bernard Jeanneret and Dobrin Kaltchev from 2002. The 0.6 sigma tolerance here corresponds to a 50% degradation of the cleaning efficiency in simulations. Frank pointed out that, in a similar spirit, also the 8% tolerance quoted for transient beta beating gives rise to 50% increase in cleaning inefficiency. Frank argued that a change of the orbit tolerance should therefore logically be accompanied by a similar change in the beta beating tolerance. Stephane presented his arguments, which can be found in Eq.(12) of Ralph's Chamonix paper. This equation gives the minimum required setting of the primary and secondary collimator in the presence of both closed-orbit error and beta beating. An 8% beta beating implies 4% beam-size change and a change in the normalized setting of the primary/secondary collimators by 6*0.04/7*0.04 (with n1/n2=6/7), i.e. +/- 0.25/0.3. Therefore in terms of the effective retraction of the collimator jaws, this is equivalent to a change of the closed orbit by 0.2/0.3 sigma. In a worst case, the two effects could combine additively, resulting in a change of +0.5 sigma at the primary, and -0.5 at the secondary collimator, which corresponds to the maximum change tolerable. Inspecting Figs.11 and 12 of the above report, it is clear that the sensitivity of the collimation inefficiency to these two perturbations is rather different: when doubling the beta-beat to16%, the inefficency increases by a factor of 2-3, while doubling the closed-orbit change raises the inefficiency more than 20 times with a clear threshold around 0.8-1 sigma. The tolerance in Frank’s slides was modified accordingly.
=> ACTION: Reproduce Fig.12
of Ralph’s report showing inefficiency vs. closed-orbit change for 8% beta-beat
at the worst phase (LCWG)
Next, a discussion ensued on the origin and meaning of the tolerances for bunch-to-bunch intensity and emittance variation. These tolerances were quoted to be about 10% for the nominal beam, and could be relaxed for lower intensity and/or a smaller number of bunches. Frank reported that he had discussed the nominal tolerances with Werner, according to whom they were not originally derived in view of beam-beam effects, but rather reflect the best performance of the injections. Werner had referred him to the LHC design report, Volume 3 (chapter 10), from which Frank had then extracted the numbers. Oliver recalled that the 10% tolerance had originated from discussions in Chamonix 2001 (e.g., see M. Benedikt’s summary of discussion on PS as LHC pre-injector), and at that time had been a specification for the injectors. Jean-Pierre had a similar recollection, saying that the initial intensity and emittance variations from the injectors had used to be of order 20% and their improvement to 10% had then been demanded. Paul remarked that everybody certainly agrees that in 2007 we will inject into the LHC any beam available from the SPS independent of its quality. In further discussion with Werner after the meeting, he pointed to LHC-Project-Report 628, which states that the rms bunch-to-bunch intensity variation should not exceed 10% to avoid unacceptable bunch-to-bunch tune spread due to long-range beam-beam effects. The tolerance on emittance variation is not discussed in this report, but from the SPS experience about 20% would be the upper limit from head-on beam-beam interaction. Werner suggested discussing a collection of pertinent data and information with Massimo.
The tolerance of 1 hr for the beam lifetime in 2007 was questioned. Mike noticed that the expected lifetime was 30 h at the start up. Frank concurred that a better lifetime is expected. However, he explained that the table was meant to quote tolerances in the sense of our minimum requirements. He recalled that the initial beam lifetime in HERA had been of order 20 minutes only. In any case, after the meeting he has added the 30-h expectation value in parentheses to his table.
The discussion then turned to the proposed initial 6-mm tolerance for the peak closed orbit. Jean-Pierre suggested that it may be easy to correct the peak closed orbit to less than 4 mm. Oliver proposed that we could issue a warning that for peak orbits above 4 mm we do not guarantee a safe machine. He recommended that the commissioning goal should be to demonstrate the feasibility of 4 mm peak orbit. Verena mentioned that this condition may be true locally, but that perhaps a tolerance as tight as 4 mm may not be needed all around the machine. There was some uncertainty on this point.
Roger mentioned that certain tolerances for the nominal LHC and for the so-called “early running” used to be available on a web site. He asked whether we require another set of tolerances for 2008, e.g., due to the incomplete installation of the collimation system and lower intensity. Oliver agreed that having specific target parameters for each commissioning stage would be desirable.
=> ACTION: Define or agree
on specific target parameters for each commissioning stage (Roger et al?)
Verena highlighted that all parameters are coupled, which makes it difficult to define tolerances for each one independently, as the above example of orbit stability and beta beating already illustrated. Therefore, she recommended, as an alternative approach, to directly measure the aperture of the machine, which may turn out to be faster and easier than checking all other parameters. Oliver suggested that it needs to be checked whether 6 mm peak closed orbit would be OK for machine protection.
Frank now discussed the beam measurement programme for 2007. He first reviewed the various beam measurements that had been mentioned or proposed in the LHCCWG presentations of 2006, which fill about two pages, and primarily concern various types of calibrations, orbit control, linear and nonlinear optics, and aperture. He suggested that in addition to the previously discussed “baseline measurements” we might consider performing forward looking measurements aimed at detecting problems which may haunt us in the following year, and which could then be addressed and possibly solved during the 2008 down time, thereby giving us a head start for the full commissioning run. Examples of forward-looking measurements include tests related to collimator operation, understanding beam losses, scans of lifetime and backgrounds vs. tune and chromaticity scans, aperture checks, detection of tune shift with current, beam-beam effects and possibly a first look at the electron cloud by injecting a short bunch train with 25-ns spacing.
Helmut commented that the experimental solenoids are a special concern. He recommended that the solenoids are turned off initially and then switched on during a later stage of the commissioning. Jan remarked that the aperture kickers could be used to measure the aperture. Frank agreed and explained that in his list they would fall under the item “aperture measurement by kicking the beam”.
Paul Collier asked whether for each measurement there exists a procedure, application software and documentation. Everybody agreed that having all this should be the goal. A large fraction might be available already. A number of substeps for each nonlinear optics measurement had been detailed in Frank’s paper on flat bottom optimisation and tuning for Chamonix XII, and in his talk on nonlinear field quality checks at LHCCWG#10. The web procedures by Verena and EICs will provide further information. Jean-Jacques asked who would take care of K modulation. The answer was Mike. Verena next asked to which detail the web procedures should be described. She pointed out that the usage of application software will not be explained in the procedures. She suggested that a help menu should or could be added to the application GUI if needed. Oliver suggested establishing a clear recipe for each type of measurement, which would enable everyone to do the key measurements. Mike commented that the software functionality needs to be defined and prioritized. Oliver remarked that a training for the commissioners may be needed. Jan proposed that the procedures be continually updated during the commissioning.
Oliver commented that also the
question of presenting and analyzing information is important, quoting
the beam-loss monitors as example. Roger explained that last year’s goal was to
determine tolerances and procedures, and that we now need to prioritize. Oliver
observed that injection matching should have a high priority, recalling that at
HERA hours or even days were lost attempting to match the energies of
=> ACTION: Establish,
prioritize and check procedures, application software and documentation for
each measurement (All)
Paul asked whether beam-beam studies at 450 GeV would be relevant for the later 7-TeV operation. Frank conceded that this was not obvious. Answering to a question, Massimiliano explained that running with short bunch trains of 25-ns spacing in 2007 could be of interest for the experiments, e.g., for detector alignment. There would be no crossing angle in his mode of operation. He had asked earlier whether the machine could make use of such beams. Frank had mentioned a first assessment of electron cloud in his slides.
After the meeting, Jean-Pierre recommended strongly focusing the 2007 measurement program on items that could have an impact on hardware or software development during the following shut-down. This may include the instrumentation, the control software, and some basic beam dynamics.
Frank listed possible problems we may encounter, ranging from not getting the beam around, or not meeting some of the optics tolerances, to an unexplained poor beam lifetime. He suggested possible responses and attitudes towards these eventualities, arguing that preparation for such eventualities could save us a lot of time during commissioning.
Jean-Pierre remarked that a few parameters may be relaxed in case of problems. Helmut recommended backing up in intensity, and he commented that one cannot reduce but easily blow up the emittance and therefore better start with a beam of smaller emittance. Paul remarked that the emittances for 2007 intensities might be too low rather than too high, and that the SPS may need to blow up the beam for this reason. Helmut reiterated that he would consider lower emittances as an advantage. Massimo pointed out that the lower emittance might be more difficult to preserve in the LHC, and Paul predicted that a natural blow up may occur. Jean-Pierre commented that developing a response to poor beam lifetime would be an excellent intellectual exercise and a real challenge. Mike remarked that getting the beam around may not be easy either.
=> ACTION: Develop response
to poor beam lifetime (Jean-Pierre?)
Rhodri asked for a prioritization of the parameters, wondering to which level the instruments need to meet the tolerances, and the requirements for 2007. It was said that the start of the ramp should be defined for the beam diagnostics. Two possible approaches for instrument commissioning would be either a full optimization for each instrument in a single step, or a staged approach involving repetition of measurements and set up procedures. Stefano brought up the pilot bunches as an example. Verena stressed that some diagnostics and interlocks will have to be commissioned before injecting any beam more intense than the pilot. Oliver recommended setting up milestones for the instrument commissioning. For example, measuring and controlling the tunes with certain accuracy could have an impact on the beam lifetime. Oliver envisioned a specification of what we must achieve before going to the following step. Verena reminded us that entrance conditions and exit goals for each commissioning step are already specified in the web procedures. Stefano suggested that we all agree on the milestones. Roger assumed that the commissioning of instruments would proceed in incremental steps.
=> ACTION: Set up
milestones for instrument commissioning (All)
Rhodri quoted the example of the PLL which is not needed in 2007 at all. Still work on it will be ongoing in 2007, so as to get it working for 2008 and to fix any problems in the downtime. Roger re-expressed his preference for the step-by-step approach. Oliver took up the PLL as an excellent example for the question of how much time to spend on instruments in 2007. Rhodri added that the situation for the orbit feedback is similar. Oliver recalled that the HERA PLL never worked. He suggested us not to worry about emittance growth in 2007.Verena asked which fraction of the instrument commissioning can be done parasitically. Rhodri replied almost none, and that controlled changes are needed for most of the set up. Paul recommended taking advantage of LHC’s two-ring nature, and to perform the commissioning in parallel. Oliver asked how much time other groups, like the RF group, need to commission their system.
Roger proposed that we establish a prioritized list of measurements and beam parameters for 2007, and a similar list for 2008. These lists would be assembled in a dedicated meeting by an LHCCWG subgroup. Laurette asked whether we plan to measure the quench level with beam in 2007, as had been foreseen for the sector test. The answer was probably not, and that naturally occurring quenches may suffice to get a calibration of the BLM system. Gianluigi commented that it would be interesting to establish limits on acceptable losses, e.g., for the abort gap filling at 450 GeV. Oliver suggested that this complex of questions should be part of machine protection. Gianluigi asked whether we will have an abort gap monitor in 2007. Jean-Jacques replied that BI was working on this monitor. Stefano raised the more general point of defining specific requirements which we have for each beam diagnostics device. Oliver suggested that the ramping software be tested in 2007 with all hardware in place, even if without beam. Mike responded that this test is already foreseen as part of the hardware commissioning.
=> ACTION: Establish
prioritized list of measurements and beam parameters for 2007 and for 2008
(All)
Helmut reported on two workshops related to the machine-experiment interface held in the previous week at CERN, one on very forward detectors, the other on luminosity monitoring and measurements. Two points raised are of particular interest to the LHCCWG: (1) There is a request that the machine does not only monitor the luminosity, but also provides an estimate of the absolute luminosity based on optics and beam parameters. (2) The experiments also request a precise measurement of the crossing angle. A possibility of adding button BPMs at Q1 is being proposed to improve the measurement resolution. This second point requires urgent follow up.
Helmut is scheduled for a longer presentation on collisions at top energy and luminosity in the LHCCWG meeting of 28 March 2007. Rhodri commented that installing new BPMs is excluded for the 2007 run, but it could be done in the 2008 shutdown. He stressed that BPM measurement at the Q1 location only work for bunch spacings larger than 25 ns, so as to avoid interference between signals from the two beams. Helmut clarified that the high precision is required mainly for measurements of the total cross section, e.g., for the ATLAS high-beta optics, without crossing angle and a small number of bunches.
Helmut is in discussions with the experiments. It is clear that the present hardware does not provide the resolution required by the TOTEM experiment, which assumes that the distance between the two beams at Q1 can be measured with a precision of 5 micron. With the presently installed diagnostics, one might hope to achieve a resolution of about 50 micron. Stephane remarked that the proposed location of the Q1 BPM had strongly degraded over the years, for nominal bunches. He pointed out that, however, position readings from PACMAN bunches could always be used to measure the crossing angle. The alternative filling scheme, which increases the number of PACMAN bunches, might be even more favorable in this regard. Rhodri commented that the expected unknown electronic offset is more or less equal to 50 microns, and, importantly, different for the two beams in case of directional stripline monitors, whose signals are processed in different electronic channels.
Rhodri proposed that he could cable the existing IR stripline pick ups in a different way so as to improve the resolution for the crossing angle. Paul Collier asked whether any such measurement was foreseen for the 450 GeV run. Helmut replied that this was not the case, and that, in his understanding, the precise knowledge of the crossing angle is required only for the measurement of the total cross section at 7 TeV. Therefore a Q1 BPM installation in 2008 or 2009 would be adequate. Rhodri commented that re-cabling the stripline BPMs would also help us in centering the two beams and bringing them into collision, which could be of interest already for the 2007 run.
=> ACTION: Follow up
re-cabling of IR BPMs for bringing the beams into
collision (Rhodri)
Wednesday February 14th, 14:30
CCC conference room 874/1-011
Provisional agenda
Minutes of previous meeting
Matters arising
Alternative
Filling Schemes (Werner)
Implications
for the Injectors (Elias)
Timing Issues
(speaker designated by Mike)
Beam Losses and
Radiation Monitoring for Steering the Beams in Collision during the Engineering
Run at 450 GeV (and at High Energy?) (Thijs)
Documentation
and Procedures
AOB
Reported by Frank