Summary notes of the forty-fourth meeting of the LHC Commissioning Working Group

 

Tuesday April 22nd, 14:00

CCC conference room 874/1-011

Persons present

 

Minutes of the Previous Meeting and Matters Arising

There were no comments on the minutes of the 43rd meeting. Roger announced that prior to discussing the three main points on the present meeting’s agenda (5 TeV parameters, cold check out, and dry run), a few open issues from previous meetings would be followed up, namely the integration of FiDeL into LSA by Mike, and several collimation questions by Ralph.

 

FiDeL Data for LSA (Mike)

Mike reported that the FiDeL implementation for LSA is being followed up with high priority. To illustrate the progress, he showed some pages from the FiDeL web site, which features a work plan, documentation of meetings, and an updated overall work status. Weekly meetings are organized by Luca Bottura, who will be the point of contact until the end of May.

 

Collimation Issues (Ralph)

Ralph presented an update on the collimators, addressing several questions raised by Roger. He announced that the installation of the collimation is now complete: 88 are presently installed, after 4 had to be taken out again.

A delay was encountered for the four TCTVBs in IR2 and IR8. These are special vertical collimators for regions with two beams, with very large elliptical beam pipes. They were contaminated during production at CERN, and need to be cut open and cleaned. CERN was supposed to produce a total of 15 collimators in house, 9 of which were for protecting warm magnets and are OK. The production of the remaining two collimators of type TCLIA were stopped, after gas composition measurements had revealed the contamination of the TCTVBs. Ralph recalled the function of the TCTVB collimators: vertical triplet protection and vertical background control. He emphasized that these collimators will be needed only for beta*<6 m in IR2 and 8. Therefore, despite of this problem, the collimation system is ready for 450 GeV and 5 TeV beams, without any limit on the beam intensity, and with no impact on the optics in IP1 and 5. Thomas Weiler is presently reviewing the beta*=6 m limit for IR8. Ralph presented the previously calculated aperture in IR1 as a function of beta*, suggesting that the dependence in IR2 and 8 would be similar. A proposed crash effort could have replaced the missing collimators by the fall, but it was incompatible with the general LHC planning. New collimators will be installed after the 2008 run.

A non-critical non-conformity was encountered in the TCTVA.4L5. Collimator production and installation was and is affected by non-conformities, that are solved 1 by 1 and documented in MTF. The primary requirement is that the collimator must function as specified, e.g. the measured torque needed to move the jaw must comply with the specifications. If the torque is found to be OK and if all involved engineers approve, the collimator is accepted. A final visual inspection of TCTVA.4L5 after transport into the tunnel revealed a displaced roller. In total each collimator is equipped with 18 rollers per axis by design. A reduced number of rollers, down to a minimum acceptable number of 14, was approved by TS mechanical engineering based on the weight supported by each axis. A spare collimator would also be available if needed. If torque measurements in the tunnel reveal no problem the collimator will be used as it is. Alick asked whether the missing roller was located at the end or in the middle of the collimator. Ralph replied that it was a roller at the very end, but that dedicated shock monitors had recorded no shock outside the tolerance during transport, so the origin of the problem is not yet understood (no problem during visual inspection on the surface).

Ralph mentioned the recently started urgent study on implementing a temporary betatron cleaning scheme in IR3 for the 2008 run. This is motivated by the installation of unsuitable electronics in the radiation zones of IR7, which was presented by Thijs Wijnands at the last ICC meeting. Ralph highlighted that the collimators are designed to intercept the beam and thereby to create radiation. No proper reshuffling was possible to protect the electronics. The fallback solution therefore is to arrange for betatron cleaning also in IR3. During the run we can then choose to perform betatron cleaning either in IR7 or IR3. In IR3 there is a factor 200 less radiation for the same beam loss. Responding to a question, Ralph clarified that the proposed betatron cleaning in IR3 would imply a coupled betatron-momentum cleaning.

Ralph showed recent results from collimator hardware commissioning, where 14 collimator jaws in IR3 were remotely controlled from the CCC and monitored by 28 independent position sensors, while exectuting the collimator ramp functions. The results demonstrate that the specified accuracy of collimator position control is achieved. Observed differences can mostly be explained by the 20 micron mechanical play that had been specified, and a change of direction in the collimator-jaw motion. The error decreases for the tighter settings, which reflects that the calibration was optimized for high accuracy at small gaps. Jan asked about the interlock level. Ralph answered that he expects the interlock level at 7 TeV to be set to about +/-50 micron (roughly 1/4 sigma). It will be less strict at injection energy and for the larger collimator gaps.

 

5-TeV Stage-A Parameter List (Massimo)

Massimo reviewed the parameter list for 5 TeV, following up his previous presentation on “Beam Parameters for 5-TeV Operation in 2008” from the 43th LHCCWG. There a proposal had been to redefine the target beta* for 5 TeV to 3 m, in order to leave the same aperture margin as had previously been foreseen for 7 TeV, which has now been implemented. For IR3 and 8 the minimum beta* limit is taken to be 6 m imposed by the missing TCTVBs, which is also taken as the target value for IP8. IP2 should stay at 10 m. Longitudinal beam parameters still had to be reviewed, and were presented this time. Massimo recalled that the blow-up hardware would likely not be available from day 1. Three longitudinal emittance values were therefore considered: 0.5 eVs (SPS natural value at low intensity), 1 eVs (with blow up in the SPS and injection mismatch), and 2.5 eVs (nominal, with blow up in the LHC). Massimo presented the momentum spread and bunch length as a function of energy for a constant rf voltage of 16 MV. The nominal rf voltage at injection is 8 MV.

Now turning to a question that had been posed by Oliver, Massimo showed that the synchrotron frequency crosses 50 Hz at about 800 GeV and 1.6 TeV for 8 and 16 MV rf voltage, respectively. No harmful effects from such crossing are expected for the initial run at 5 TeV.

Massimo reminded the team that the longitudinal damping will be lost if the longitudinal emittance is too small. Resulting instability thresholds were provided by Elias Metral, who considered the latest estimate of the expected value of the LHC impedance (previously, the longitudinal beam parameters were studied and reported by E. Shaposhnikova, see LHC PN 242). For the 0.5 eVs emittance the intensity threshold is 2.2e10 per bunch, which is too low to be acceptable even for phase A. For an emittance of 1 eVs the threshold is already 1.3e11, higher than the nominal LHC intensity, which would be perfectly acceptable. Accordingly, Massimo proposed the 1 eVs value as candidate target number for the 5 TeV run. This value also implies a luminous region smaller than nominal, and a lower momentum spread which is good in view of the mechanical aperture. No issue for machine protection is expected, since the geometric emittances are larger at 5 TeV than at 7 TeV. Massimo also commented that the reason why we do not want to use 1 eVs as target emittance for 7 TeV is that the intensity threshold depends on the bunch length, and at 7 TeV the threshold for 1 eVs would correspond exactly to an intensity of 9e10 protons per bunch, as foreseen for commissioning phase A, and less than the nominal one.

Synchrotron radiation effects were also reevaluated for the lower energy. A table compiled by John Jowett summarized the situation. The transverse and longitudinal emittance damping times at 7 TeV are 26 h and 13 h, respectively. At 5 TeV these values increase significantly. Radiation damping rates were compared with IBS growth rates for an intensity of 9e10 particles per bunch, as a function of longitudinal emittance, and in particular for 1 eVs and 2.5 eVs. In the transverse plane the radiation damping always balances or surpasses the growth rates. However, at 5 TeV with 1 eVs longitudinal emittance, in the longitudinal plane the IBS growth time (20 h) is about half the damping time (35 h).

Massimo next compared the integrated luminosity distribution for 5 TeV with 1 eVs with the nominal case. The luminous region is roughly 30% shorter than nominal.

Finally, Massimo presented the overall target parameters for 43 bunches, 156 bunches, 75 ns spacing, 50 ns spacing, and 25 ns spacing. He mentioned that a note summarizing the various filling options is in preparation by W. Herr, M. Ferro-Luzzi and T. Pieloni. Different filling patterns allow varying the numbers of bunches colliding at the various IPs. For 43 bunches, the idea was to maximize IP8 luminosity and to minimize the IP2 luminosity. The 50 ns scheme also included in the table appears attractive. An extended phase-A parameter table now features a 5-TeV central column. Crossing scheme details were added, listing both internal and external angles (the latter being 0 in phase A). Several alternative collision schedules are indicated as well. The luminosity at each IP and the number of events per crossing are shown as well. The tables will be stored on the LHCCWG web site.

Massimo’s main conclusions were: for 5 TeV operation a longitudinal emittance of 1 eVs is OK and a longitudinal blow up in the LHC is not needed.

John commented that for an emittance of 0.5 eVs the IBS effects would be much stronger. Gianluigi suggested that the IBS blow up could be self-stabilizing. Massimiliano asked whether it was indeed possible to have 1 eVs in the LHC for 2008. Massimo replied that, yes, 0.8 eVs had already been demonstrated from the SPS. Massimiliano remarked that perhaps some experiments might prefer to have a larger luminous region. He recommended to preferably choose the nominal bunch lengths for a later 7 TeV run, if there is no reason from the machine side calling for shorter bunches. John pointed out that the 1 eVs emittance could be better for limiting debunching. Massimiliano proposed to discuss these parameters at the LPC. A remaining open issue is the number of bunches for ALICE and how they will be distributed. Massimo remarked that the detailed collision schedule is included in the tables.

 

Preparation for the Machine Check-Out - Status (Gianluigi) 

Gianluigi represented a team of four people, which was looking at the preparation of the machine check-out. The other three members were Reyes, Rossano, and Mike. The presentations at the Extended LTC were used as a starting point. The team already identified the machine check-out contacts, i.e. the relevant people “in the field”.  A first round of discussions collected pertinent information on the planned HW/SW availability, status, requirements for the commissioning time, status of CCC tools, possible dates for preparation tests, interface between systems, and the scheme for standard cold check out. The information flow is still slow, since many people are busy finalizing their hardware. Gianluigi presented the systems table with contact names (slides 3 and 4). He next reported on the various systems one by one.

Access & DSO tests: The access non-conformities found in week 2 are being fixed. A final test is required before sending the beam. The TI8 beam test is foreseen for 24-25.05.2008, and TI2 beam test for 14-15.06.2008. And LHC DSO tests were proposed for 16.05.2008 (TI8), 06.-09.06.2008 (global test 1 and TI2), and 20-23.06.2008 (global test 2).

Roger remarked that Massimiliano had organized a meeting discussing how to get organized in the frame of the LHC Programme Committee, but that it was not clear which committee had the final say on the test schedule. Massimiliano asked whether the DSO tests could be delayed to later dates in order to allocate more preparation time for the experiments. Roger explained that the assumption underlying the proposed schedule was to have beam in July. Rossano pointed out that the access system must be validated, and that this could not easily be done in July or August. Jorg agreed, saying that this must be done early to find and fix problems. Massimiliano asked what would be the latest date when this test could be done. Jorg and Gianluigi answered the absolute latest date would be one day before sending the beam, but this was not to be recommended by any means. Massimiliano inquired whether two weeks before the beam would be a reasonable possibility. Referring to past experience, including the TI2 test, Jan cautioned that there might easily be some problems that could not be fixed within two weeks, and that the two weeks choice would amount to “playing with fire”. He would definitely prefer to conduct these tests two months before the start of beam operation. Massimiliano asked whether any administrative time limit existed. Paul replied that indeed Paris must be notified, adding that committees like ICC and LTC would be involved, and that the Beam Facilities Safety Panel (BFSP) was the only real final authority. He stressed that we should be working towards putting beam into the machine in July, and that there were only two weekends missing. Massimiliano asked whether the weekends could be shifted or whether the other weekends were already blocked. On 14-15 June, for example, a beam test is foreseen. Paul asked if the DSO test really was incompatible with the beam test. Rossano answered that the two could not overlap, since the DSO test would require the whole machine to be closed. Paul suggested that one could check the rest of the machine, i.e. the parts which were not concerned by the TI2 test or TI8 test, respectively. Gianluigi suggested that the DSO tests could also be synchronized with cosmics tests of the magnetic fields in the experiments. Alick asked whether the experiments were required to close at a certain date. This was confirmed by Paul, who quoted the beginning of May. There was some discussion on the detailed requirements for this closure.

Beam instrumentation: Activities around the instruments had been supposed to start by 10.04, but have not yet begun. Gianluigi emphasized that they would need to start a.s.a.p., with the help of Jean-Jacques. He pointed out that by middle of May sector 4-5 will be cooled down, following which access in LSS4 would be difficult. Viewed from the user side, the BTV system is in a good shape. Even for the matching monitor there is excellent news: after the e-LTC is was agreed to provide a turn-by-turn acquisition on BSRT from day 1, as well as the possibility of installing a fast camera at one of the OTR screens in LSS4, if needed. The BLMs are a complex system. BLM data are being logged in some points. EMC tests with kickers were done in 2, 8 and 6. An EMC test still needs to be done in LSS4.

BIC: The installation of optical fibres should be completed by the end of April. There was not yet a complete match with the hardware commissioning. A first version of BIC supervision programme is available.

Collimators: These were already addressed in the report by Ralph. Individual system tests were ongoing. The tests required to demonstrate the machine protection functionality were only sketched so far. Ralph promised to provide an answer during the present week. Logging is being put in place. Alarms were not yet defined. A first version of fixed collimator displays was tested. Gianluigi mentioned that the separation between collimators and injection/dump protection devices for the controls was not totally clear. Verena responded that a difference existed only for TCDQ, and Ralph attributed this to the fact that TCDQ has a different hardware.

Cryogenics: The main issue here is the cryo support. Tools to get the status of cryogenics were, or were being made, available. Insulation vacuum warnings might be useful. This is an issue currently being discussed. Paul remarked that at the moment there was no agreement with TI to monitor the cryo system due to lack of TI resources.

Power converters: There are no major issues; EMC tests are not yet completed. Exercising is done with nominal currents and trimming.  Alarms are available. Reduction criteria are to be defined for them. At the moment more than 1000 warnings exist.

QPS/EE: The main issue is expert support.  No additional tests are planned during the cold check out. Tests will be required after every shut down. Roger asked whether this meant every year or every stop of a few days. Gianluigi clarified that for the most part the tests need to be done every year or possibly after the warm up of a sector. The logging had not yet been looked into.

RF, transverse feedback, beam observation: All hardware is installed in the tunnel. Low-level rf and control SW are lagging behind. Delphine is discussing these items with Andy. Fast signals are not yet distributed, etc.

Timing: The Safe Machine Parameter Controller (SMPC) is not yet in its final version. Verena remarked that the SMPC is used in the way it should be, with simulated input for the energy information. Timing monitoring is very important. Alarms are needed when timing events are not sent or distributed. The entire mechanics of the injection must be tested all through the chain both for the pilot filling schemes and for the nominal scheme.  A dry run on 08.05. is being organized for this purpose.

Vacuum: The installation is almost complete. Gianluigi was waiting for the availability plan from AT/VAC, which was being prepared by Vincent Baglin. Valves and their controls were tested, but for the moment they cannot be moved remotely. The beam info signal from the BIC is needed before one can close the valves. Alarms are presently available only for insulation vacuum, and not yet for the beam vacuum. Verena commented that the whole beam permit loop was needed for the remote operation of the valves. Massimiliano asked who was organizing these checks. Gianluigi replied that this was exactly what he was trying to organize. His goal was to slowly take over the different sectors. Logging was already defined for vacuum pressures, but not yet available for the beam vacuum. For the moment the acquisition rate is once every 3 minutes for the insulation vacuum pressure. The position of the valves is also recorded, on change. The application is ready, except for some minor modifications have been requested, e.g. a display per beam. A presentation of the vacuum displays and PVSS vacuum tools by Isabelle Laugier is planned for the 45th LHCCWG meeting on 6 May. Ralph remarked that an acquisition interval of a few minutes was much too slow, and that vacuum recording at about 1-Hz rate would be required instead. He underlined that vacuum and heating were known limitations in high power machine, and should be appropriately monitored.

General issues: Verification of the UPS connections is part of HWC, but it was not done for sectors 4-5 and 5-6 so far. The tests would have an impact on the cryogenics and the organization. Ralph wanted to know how these tests are done. Gianluigi replied that emergency stops are to be pushed. Roger questioned whether these tests had been done anywhere. Gianluigi answered that they had partially been performed in Sector 7-8. Responding to a question by Ralph, Rossano clarified that several problems were found during these tests in 7-8, e.g. devices that were not connected. Roger assumed that this type of tests had been skipped in the following sectors because of their impact on the commissioning programme. Jean-Jacques remarked that it was known that several components were not connected to the UPS. Ralph cautioned that we should not compromise the safety of the machine without any official statement or clear decision. Paul commented that this issue was on the list of follow ups from the e-LTC to be discussed at the LTC the following day. Other general issues are the status of the compensators and behavior at full load, especially in points 2 and 4, the post mortem, which was not really addressed yet, the logging and alarms, and a few more.

Alick recommended that we avoid storing terabytes, as for HWC. Gianluigi replied that saving all available data would be the safest approach in the beginning.

Next step: This will be to “take ownership” of Sector 5-6. Gianluigi and his team started discussions with HWC. Procedures to be tested include e.g. the generation and loading of the nominal functions. Training could be done in the late afternoon, perhaps only for the dipoles. Other activities concern the trims, the BIC connection, remote vacuum controls, the addition of distributed information, the addition of (a few) collimators, SIS verification, and the verification of the fixed displays, alarms, logging, etc.

Hand-over of a sector: We must receive from HWC lists summarizing the tests done and their results, comprising e.g. maximum currents, non-conformities (a lot of them with decisions pending – shall we go through them before taking over a sector?). The access control to the sectors taken over should be handled by the EIC.  Massimo commented on the issue of the non-conformities. Gianluigi stressed that at least one console should be available for the machine check out, as well as some wall displays. His suggestion was to use a TI corner close to the cryo people.

Roger asked whether the idea that “when a sector is finished we take it over” was generally accepted. Gianluigi responded that he had discussed this so far only with Roberto, who considered this a good idea. The intention of the cryogenics  group  is to keep the commissioned sectors cold. Paul commented that some committee must approve this plan in order to secure appropriate resources. Roger pointed out that these activities would combine cycles and training quenches. Verena asked how these activities would go beyond the scope of the dry runs. Gianluigi replied that they represent a transition from HWC to beam operation. As examples he quoted the SIS (switching off a device) or vacuum (getting an idea about the vacuum and the control of the vacuum).

To be put in place: This includes the names of the people attached to each of the systems tests in 5-6. The information collected so far and follow–up plus a first embryo of the check-out procedures will be put in a web repository/page connected to the LHCCWG and Dry Run pages. Reyes will take care of this. Starting from the middle or end of May, more regular meetings should review the progress in the preparation of the HW with the equipment contacts. A possibility would be merging with ABCIS. Regular reports will be given to the LHCCWG.

Verena asked how the philosophy differed from that of the dry runs. Gianluigi imagined verifying and including all the other still missing systems like vacuum etc. Magnet training was done anyhow only in the evening. Special items would be looked at, like the cryo system. One goal was to arrive at a fixed display presenting the full status of the machine. Verena commented that many items should be looked at system wise instead of sector wise. Gianluigi remarked that the proposed approach would teach us how to prepare for more sectors. Roger added that all the discussion had focused on what we could do in parallel with ongoing HWC, but another question was what needed to be done at the very end. Verena commented that there should be a distinction between the dry runs and the machine checkout. Gianluigi agreed, also mentioning the need to test the machine protection system. Ralph announced that he was drafting a document for the collimator cold check out. Jan pointed out that the cold check out tests should be done at the end, when nobody was touching the machine anymore. Ralph said while the cold check out of the collimation was being prepared, he was concerned by the complexity of the whole machine protection system. Gianluigi summarized that we need to define which tests are required after the individual systems tests, and in particular look at correlations between systems.

Dry Runs – presentation postponed (Mike)

Mike’s talk was postponed due to lack of time. The slides were nevertheless available in his public directory. Mike announced the dates of the next dry run, which is planned for week 19, starting on Monday and continuing for 4 days. It will comprise injection tests, beam dump tests, and include whatever is available in 5-6 (squeeze, pre-cycle, etc., expanding on what had been done in 4-5).

Next Meeting

Tuesday May 6th, 14:00

CCC conference room 874/1-011

Provisional agenda

 

Minutes of previous meeting

Matters arising

Dry runs (Mike)

Demo of vacuum displays & PVSS tools (Isabelle Laugier)

Non-conformities and their consequences (Massimo)

AOB

 

 Reported by Frank