Summary notes of the forty-second meeting of the LHC Commissioning Working Group
Tuesday March 11th, 14:00
CCC conference room 874/1-011
Minutes of the Previous Meeting and Matters Arising
There were no minutes of the 41st meeting.
Roger introduced the schedule for the 42nd meeting, which focused on three topics: the maximum strength of the LHCb spectrometer at injection (Werner and Malika), the bunch number distribution (Philippe and Mike), and the commissioning of the beam instrumentation (Jean-Jacques).
Responding to a remark by Roger, Massimiliano commented on the intended switching frequency of LHCb and ALICE: In the long term LHCb would want to switch the polarity once per week, ALICE once per month (both solenoid and dipoles together). However, in 2008 the switching could be more flexible. For the early running, both experiments wish to have one polarity flip, simply to get some data with both polarities. Massimiliano also mentioned that both ALICE and LHCb plan to run with full spectrometer field even if the machine collides at lower energy, e.g. 5 TeV.
How Do We Have to Ramp the LHC Spectrometer Magnet? (Werner)
Werner prepared this presentation together with Malika Meddahi and Yannis Papaphilippou. He noticed that the question about ramping the LHCb had come up in regular intervals, with slightly different details since the boundary conditions had been changing. Instead of the originally suggested title of his presentation “can we excite the LHC spectrometer with ½ strength at injection”, Werner addressed the more general question “how do we ramp the LHC spectrometer magnet?”
He recalled his pertinent presentation on “The Effects of Solenoids and Dipole Magnets of LHC Experiments” at Chamonix 2006. In IP8 the crossing angle is in the horizontal plane, and bunches always have to cross from the inside to the outside. Namely, the sign of the effective crossing angle is fixed to avoid additional crossings. The effect of the LHCb spectrometer is compensated with three close-by dipole magnets. A polarity switch of the spectrometer changes the sign of the internal crossing angle, which must be overcompensated by an external crossing angle. The effective crossing angle always is the sum of the two.
Massimiliano asked what exactly was meant by “negative” polarity. Werner replied that for negative polarity the spectrometer bends the beams to the left when looking from above, or, put differently, in the "problematic" polarity (or "bad" polarity) the LHCb spectrometer B-field is pointing up.
For the bad polarity the external angle must be much larger than the angle introduced by the spectrometer. The effective crossing angle is different for the two polarities (smaller for the bad one) due to aperture limitations. The full spectrometer field at injection would result in an internal crossing angle of +/-2.1 mrad. Werner posed the question whether for positive polarity the overcompensation was possible at all.
To answer this question he looked at different scenarios corresponding to the various commissioning phases. For 43 or 156 bunches no external angle would be needed. However, more bunches with bunch spacings of 75, 50 or 25 ns require the external angle. Werner assumed no beta* values smaller than 10 m for all cases with crossing angle.
For operation with 43 and 156 bunches, the spectrometer at full strength generates +/- 10 mm orbit bumps at injection energy. The aperture nevertheless was OK. Werner concluded that in cases without a crossing angle we are safe with respect to aperture, and we could accept the spectrometer at full strength in this case. The reason is that the longitudinal extent of the internal crossing angle bump is rather small.
Next, Werner showed that in cases where we need a crossing angle, the case of negative polarity would also be OK at full spectrometer strength with +/- 170 microrad external crossing angle. Mike asked if Werner really advocated this scenario. Werner repeated that from the viewpoint of aperture there would be no objection.
Then Werner demonstrated that for the bad sign of the spectrometer polarity (“plus”) and with the spectrometer at full strength we would need a very large external angle for providing sufficient separation of the parasitic encounters, so large indeed that the beam would be sent through the triplet magnet.
Werner demonstrated that with the bad polarity we cannot even accept 20% of the spectrometer field at injection, which would still yield too low an aperture of n1=5.5. By scanning the field, computing n1, and trying to optimize the crossing angle bumps for maximum n1, Werner obtained a case with 8.2% of the maximum field and n1=6.1, which might be almost OK. He drew the conclusion that realistically the maximum field we can hope for is 7.5%. This would make no big difference to ramping the spectrometer field linearly with the beam energy, which yields an injection field of 6.4%. Werner stressed once more that the aperture problem arises only for one polarity.
Roger commented that the experiments would need to accept this conclusion. Massimiliano remarked that in the beginning LHCb will request to have full field at 5 TeV (whether ramped with the beam or not). Full field means the 7-TeV nominal LHCb field. LHCb would prefer to keep the magnet fixed at full field, even at injection if possible (but this depends on the LHC commissioning group: one has to balance ease of operation, risks with beam, etc.). Andreas Schopper asked if LHCb can hope for lower beta*values than 10 m. Werner responded that without crossing angle the beta* value is uncritical, since the spectrometer bump is completely independent of the optics, and that there was no beta* limit from the aperture in LHCb. Roger commented that operationally other aspects could be important. Werner clarified that the internal crossing angle bump at injection with full spectrometer field was not “huge”, only 11 mm. Helmut suspected that the aperture alone might not be the full story, but that detector background and other constraints could enter.
Werner turned to collisions at lower energy, asking at which energy the full spectrometer field would be possible. From the previous results it was clear that without a crossing angle the full field was always possible. The answer for cases with crossing angle depends on the beta* and again on the polarity. As an example, Werner looked at 4 TeV and beta*=10 m. He found that under these conditions the full spectrometer field would be possible for both polarities.
Werner commented that for beta*<10 m, one would need to increase the crossing angle to provide an efficient separation. The resulting limits on beta* at 4 TeV would need to be assessed. Massimiliano suggested to check beta*=2 m at 5 TeV beam energy. Werner expressed doubts that this beta* value would be possible for the bad spectrometer polarity, but he was ready to explore it.
After the meeting Massimiliano summarized his understanding of the bump aperture problem for LHCb as follows:
1. Is it possible to keep a fixed full field at any energy, i.e. without bump field ramp ? (we only talk here about the aperture problems)
A. zero external angle filling schemes: no problem for both polarities,
at any energy, at any foreseen beta*.
B. external angle filling schemes:
a. B-field down: no problem, at any energy, for beta* of 10m.
b. B-field up: problem. This may depend on beta* and energy.
Option to cure this with mixed vertical/horiz crossing was mentioned and yet to be investigated (but not most urgent).
2. At what energy can the full field be used ? Is 5 TeV OK ? (with ramping the
bump field if needed)
Given the answers to 1.A, we only need to talk about a possible problem
with external angle filling schemes (75-50-25ns).
The answer depends on beta*. For 10m: OK at 4 TeV, hence 5 TeV as well.
For 2 m: this may be more difficult.
3. A (not so urgent) question: what is the minimum possible beta* at 5TeV with external crossing angle and full field for Alice and LHCb? (with ramp allowed). For Alice this is only relevant in the “new” 50ns scheme with few colliding bunches; for 75ns and 25ns they do not want any squeezing.
Roger concluded that there was no problem for the 2008 physics runs, namely we could always leave the magnet powered at full strength, from the aperture point of view. Werner added that for ALICE the situation was even easier, thanks to a smaller crossing angle and the vertical crossing plane. John cautioned that in ALICE some new constraints related to tertiary collimators had recently emerged for heavy ion runs. During a subsequent discussion on the spectrometer ramp, Mike recalled that the maximum ramp rate of the spectrometer was 4 A/s, which could limit the overall speed of the LHC ramps. Jean-Jacques observed that the undulator ramp would take some time too, and one such ramp would need to be done prior to injection. The spectrometer ramp could conceivably be performed in the shadow of the undulator ramp.
Bunch Numbering (Philippe Baudrenghien)
Philippe described the proposed numerology of LHC bunches, in terms of several conventions. Convention 1: The 400 MHz RF defines 35640 buckets, spaced by one RF period, and numbered from 1 to 35640. Convention 2: Bucket 1 is the first bucket after the 3 microsecond long abort gap (defined from bucket 34442 to 35640). Convention 3: bunches in bucket 1 of the two rings collide in IP1 (or any other IP to be agreed upon).
Massimiliano and Roger commented that Philippe’s proposal was accepted. John cautioned that for ions one might prefer to have the bunches no. 1 collide in IP2.
Philippe presented several examples, e.g. for 25ns operation the bunches will occupy buckets 1, 11, 21, etc., with gaps at every PS or SPS kicker gap. The Revolution Frequency is a train of pulses with one 5-ns long pulse per turn. At a given place in the machine, and at a given beam energy (that is at a fixed RF frequency) the delay between the pulse and the passage of a bunch in bucket 1 will be fixed from run to run. Drifts during the ramp are insignificant. The Bunch Clock is a square wave at 40 MHz obtained by dividing the RF by 10. The divider is synchronized with the Revolution Frequency. At a given place in the machine, and at a given beam energy (RF frequency) the delay between the edge of the Bunch Clock and the passage of a bunch will be fixed from run to run.
Alick asked who was fixing this delay. Philippe replied this would be addressed later during his presentation.
Now turning to the clock generation, Philippe described how all signals are generated in SR4, and then are directly transmitted to CMS (point 5) and BT (beam dumps). Philippe pointed out that the requested delay for injection pulses to the kickers is very different from the delay requested for BI, a difference which prevents using the same modules for BT and BI, and which should be re-discussed.
A reference clock is generated for the physicists. The experiments only care about this signal during physics. RF will provide a stable 400 MHz reference. On the flat top the beam would be rephased to that reference. The presence of the signal and its validity will be guaranteed only from shortly before the filling until the beam dump.
The RF synchronization equipment is being installed and commissioned in SR4. Philippe expected that all this equipment would be running by mid-April. He reviewed the RF synchronization for the SPS-to-LHC bunch-to-bucket transfer and its associated functionalities. Two different rf frequencies for the two rings can be used to rotate the two LHC rings against each other prior to an injection.
Philippe outlined the commissioning procedure of the synchronization and bunch clock systems. The first bunch injected will be bunch no. 1 by definition. Then one will adjust the delay in the reference frequency sent to the beam dump, in order for the dump kicker rise to occur just before bunch no 1. Philippe assumed that BT must have the tools to make this adjustment. Jean-Jacques commented that for the abort gap monitor a similar adjustment is done in BI. He added that it was important to keep the relation between bucket 1 and the RF frequency signal fixed from run to run. Massimiliano remarked that all experiments will adjust this phase, and that this delay would indeed never change. Replying to a comment by Alick, Andy clarified that the bucket selector setting changes at every injection to inject bunches into different buckets.
Philippe stressed that in order for bunches in bucket 1 to collide in IP1, we must rotate ring 2 with respect to ring 1, and for this we need information from the experiments or from BI on the time difference between passages of the two pilots in IP1 or IP5, e.g., we will need one pick up which sees both beams.
Helmut commented that the installation of a special pick up and electronics near the IP had been agreed upon with BI earlier. Helmut added that an extra card had been added for the purpose of detecting the time difference of the two beams. Jean-Jacques cautioned that nevertheless assumptions on the cable lengths were needed, and that one would need some calibration measurement. After the meeting Gianluigi and Rhodri explained that BI plans to measure the time delay between beam 1 & beam 2 at the first BPM on both sides of the IP (in fact, BPMSW 1,2,5,8 L & R, BPMR-Q6L2 BPMR-Q6R8, + 2 BPMs in Pt4). If the BPMs, cables and geometries are symmetric then the time difference between both should be the same when the beams collide in the centre of the IP.
Frank asked whether the final resolution of the collision adjustment at the IP was not better than 1.25 ns. Philippe replied yes, it was, and that this better centering was achieved by fine tuning with phase shifters. Alick asked for the target value of such fine tuning, and Philippe replied better than 1 degree, adding that we would use information from the experiments to make this fine adjustment. Massimiliano asked when this would be happening. The tolerance of experiments was said to be of the order of cm.
During and after the meeting John commented that when the idea of adjusting the longitudinal position of the IP with BPMs had come up in Jan 2007, the accuracy hoped for was of order of a few mm. However, Rhodri had possibly reported in the extended LTC held in the previous week that the extra electronics needed would not be installed at startup (to be confirmed with him). John added that this discussion had started because he had heard that ATLAS (and presumably other experiments too) would be able to measure the longitudinal position of the IP with their zero-degree calorimeters. This should perhaps be kept in mind as possible verification for the accelerator tuning.
Philippe addressed the question if we could change the convention that bunches no. 1 collide in IP1. His answer was no, but he suggested that changes also would not be necessary, since one could always use the bucket selector to shift the collision point, e.g. by one quarter or octant. Massimiliano remarked that in this approach, however, the abort gap would always collide in IP1-IP5, while for ion collisions it would be better to have abort gaps collide in IP2 to maximize the luminosity. On the other hand ALICE might also plausibly prefer the opposite configuration for beam-gas background studies. John stressed again later that one might sometimes want to collide in IP2 instead of IP1.
Finally, Philippe answered a couple of other FAQs: Counting in 400 MHz or 40 MHz is equivalent. We should not number the bunches, but stick with bucket numbers or bunch clocks.
Massimilano raised a question about satellite bunches. According to Jean-Jacques in this case the bucket number would be used. Gianluigi pointed out that most of the instrumentation would not see any signal of the satellites. Massimiliano recommended that we agree on the terminology between the experiments and accelerator. Alick recalled that the orbit marker was fixed to bucket no. 1. Massimiliano asked how the filling pattern was distributed to the experiments. This was answered in the following presentation by Mike.
Distribution of Bunch Configuration to Experiments (Mike)
Mike described how the LHC bunch configuration will be transmitted. The communication of the machine with the experiments proceeds via three channels: (1) the General Machine Timing (GMT) network [see e.g. J. Serrano et al, “Nanosecond level UTC timing generation and stamping in CERN's LHC,” ICALEPCS-03 Korea], (2) the Beam Synchronous Timing (BST) network [see D. Dominguez et al, “An FPGA based multiprocessing CPU for Beam Synchronous Timing in CERN's SPS and LHC,” ICALEPCS-03 Korea], and (3) the CERN Data Interchange Protocol (DIP) [https://edms.cern.ch/file/457113/2/DIPDescription.doc ].
Mike highlighted that 21st century tools will be used for the data transfer: ORACLE, JAVA API for Parameter Control (JAPC), and Controls Middleware (CMW) with get/set methods and publication&subscription facilities [see e.g. K. Kostro et al, “Controls Middleware - The New Generation,” EPAC02 Paris]. All these aspects are well covered. Basic timing concepts include the following: Events arrive asynchronously and can be subscribed to. Each event is a 32-bit quantity. The payload is 16 bit. Telegrams are sent at fixed frequency. Safe Machine Parameters (SMP) are distributed in the form of events and telegrams. Mike presented examples of telegram groups descriptions (slide 4) and distributed events (slides 5-6). He also recalled Philippe’s three conventions.
The injection request involves several events: BTNI (next injection beam type), BKNI (next injection rf bucket), RNGI (next injection ring), and BCNT (number of CPS batches). 2x4 telegrams are used for defining the beam type (spacing, intensity. particle type etc). Telegrams exist both for injected and circulating beams.
Roger asked if the beam type would change from cycle to cycle. Mike replied that, yes, the beam type could change, for example, if we go from the pilot bunch to an intermediate beam. Gianluigi inquired whether this information would be sent out at every injection. He suggested that one must also specify the number of batches as part of the beam type. John asked if the particle type could have other values than 0 (protons) or 1 (ions) to encode ions other than lead. Mike answered that the data structure allowed 16 bits to encode the particle type, so that “2” might eventually represent argon, etc.
Mike presented a proposal for defining the bunch configuration (slide 10), while also referring to an alternative proposal by Andy. Mike’s proposed scheme covers all cases. It is set on the database, which stores the LHC run configuration and which is published to DIP (response time ~second) and via JAVA Messaging Service (JMS) to any high level application. He recommended to set the RF FESA property rather than to distribute it over the timing system.
Philippe asked if the distribution is not done via the timing, how one could be sure to get this information on time. Andy pointed out that the time was not critical, since a time interval equal to a full SPS cycle is available to do it. Massimiliano posed the question how much could be distributed over the timing. Andy answered 4 integers – sufficient to describe the incoming beam. Mike announced that a dedicated discussion on this topic would happen immediately after the 42nd LHCCWG meeting.
The next item addressed was the bunch intensity from the FBCT. Mike stated that he would push the average bunch intensities over the DIP.
Massimiliano resumed his earlier question on satellite bunches and he asked whether those would show up. Jean-Jacques replied that the satellites will not be seen here (due to the 40 MHz data acquisition rate); the only monitor that would notice them is the longitudinal density monitor - which would not be available for phase 1. However, Jean-Jacques offered as a temporary replacement the dedicated pick-up (APWT in point 4) that BI will connect to an oscilloscope. This should hopefully be able to detect satellite bunches of a few % of nominal intensity. He added that if the satellite bunches were big enough they could lead to a strange behavior of some applications, but there was no easy way to find them.
Roger asked if all questions from the experiments had been answered. Massimiliano replied that all these points and conventions should be defined in an up-to-date document e.g. in EDMS, since it was difficult to refer to presentations. Roger asked Philippe whether he would like to document the bunch numerology in this way.
Jean-Jacques pointed out that BI counts in 40 MHz slots, not in buckets. The BI instruments would therefore not resolve the individual RF buckets. Gianluigi suggested that one might possibly infer the presence of satellites from the DCCT reading. However, Jean-Jacques qualified this option by saying that the DCCT and fast BCT will both integrate satellite bunches, so the difference will not say anything. The difference could give theoretically an idea of the ‘de-bunched’ intensity (that we may lose during the ramp) but the final resolution of the DCCT (a few 10^9 p) and the uncertainty on cross-calibration will tell what we can do. Jean-Jacques also reminded that the BCTDC data transmitted via the timing system for the validation of a safe beam will only have 1 bit data corresponding to 1e10 protons, and that e.g. the intensity shown for the pilot beam on the general machine timing network will be zero.
Recognizing that this theme concerns different groups, Massimiliano stressed that at such interfaces weaknesses usually occur. He reiterated that, therefore, a write up would indeed be marvelous. He recommended a continually updated official documentation. Philippe agreed to write a document for definitions and conventions.
Philippe inquired if all the information shown by Mike will be available from the start up. Mike replied in the affirmative, saying that it was already available in large parts.
Commissioning of the Beam Instrumentation (Jean-Jacques)
Jean-Jacques explained how the machine check out for the beam instrumentation will be organized. It will be scheduled after the BI HWC. The goals of the instrument machine check out are several: to verify the integration in the control system, to check the functionality, to debug the devices as much as possible before beam, and , last but not least, to ‘have people involved’ (‘commissioners’ from OP/ABP – already identified) which will then know and ‘own’ the systems. This activity will be based on the AB-BI LHC Technical Board Wiki : Dash Board . Jean-Jacques provided important background information for the various columns of the dash board (slide 4).
The BI SW responsible of the instrument knows best what can be tested and when. He/she will organize the continuous assessment of his/her instruments, which implies to find/organize time slots outside official dry runs where conditions allowing testing of the dashboard are met, to invite the instrument team members to the tests, and to maintain & update the dash board information. BI would participate in the LSA dry runs, and the dash board should show to the LSA dry-run organizers the instruments available. In addition, to work efficiently, BI was asking for certain timing events which LSA should play regularly outside the dedicated dry runs. Some example events were mentioned – all of these were normal events.
Roger asked whether there could be any interference with the hardware commissioning Jean-Jacques gave the reassuring answer that in principle no, none had been identified yet. The only tricky event is the post-mortem event and it’s not part of the list for the moment.
The requested timing table contains events related to injection beam#1, injection beam#2, energy ramp, beam dumped, etc., which are interleaved with minutes-long sleeping periods. The underlying idea is to simulate a realistic sequence and play it for testing all functionalities of the instruments. A “start simul table” event (TBD) is sent prior to the simulated beam injection to pre-check the instrumentation. The ramp events would be used e.g. to test BLM threshold usage or to protect wire scanners. Jean-Jacques pointed to the need of sending the next injected beam telegram group RNGI in LSA and of agreeing on a temporary central timing (CTIM) for the “start simul table”.
Assuming that his proposal was accepted, Jean-Jacques presented a corresponding planning for the BI access. He pointed out that due to RF tests there could be no access in point 4 in the 2nd half of May and June. Most BI instruments should be ready for checkout much earlier, during April. He recommended that the LHCCWG and BI work together on this. As immediate next steps, LSA has to create, test and run the agreed-upon timing table, and BI will organize dedicated meetings for each instrument before the end of March.
Jean-Jacques also suggested that a few events for BT and/or RF could be interlaced in the proposed sequence, allowing e.g. BI tests between the dry runs. Reyes confirmed that a similar tool had been prepared for BT, which was now in place for the EMC test, and the next reliability run. The same could be done for BI. Jean-Jacques fully agreed and he advocated to integrate Etienne’s needs with those of BI.
Gianluigi asked whether an energy ramp was possible. Jean-Jacques responded that no real ramp was needed, but that the ramp could be entirely simulated. Mike commented that Julian could provide this simulation. Jean-Jacques said that he did not know of any side effects of the “beam dumped” events and the energy ramping. Roger asked if the potentially interfering events could not be replaced by fake events. Jean-Jacques replied that this was not easily possible, e.g. for the injection of beam#1 Lars would not like to change his 64 crates to a dummy trigger. The easiest solutions would be if all the teams concerned would disable their response to certain events, so that one e.g. would not perform a wire scan at every event. Lars mentioned that one would also need to test post-mortem crates with possible side effects on the HWC of power converters and QPS, and this latter test must be executed in any case.
In conclusion, and RF, BI, BT will agree on a common list of events to be played regularly by LSA. Gianluigi highlighted the organization of the mini-teams.
Roger announced that the next LHCCWG meeting would take place on Tuesday April 8 to avoid a possible conflict with Easter and to be able to take into account the feedback and conclusions from the extended LTC.
Tuesday April 8th, 14:00
CCC conference room 874/1-011
Minutes of previous meeting
Other agenda items yet to be determined
Reported by Frank