LPC meeting summary 21-03-2016 - final
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LPC meeting summary 21-03-2016 - final |
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Main purpose of the meeting: Discussion of items related to the first steps in commissioning LHC with beams.
In the introduction Jamie summarised the highlights of the commissioning schedule for the next weeks:
During the commissioning of the machine there might be collisions on several occasions. The scenarious were discussed in some details.
During the alignment of the TCTs the machine will bring the beams to collision. This will happen shortly before the planned Quiet Beam in week 16. Jamie remarked that this is an exercise for the machine and no significant extra time should be requested by the experiments in order not to slow down the commissioning. Witold Kozanecki remarked that parasitically these collisions can be used by the experiments for setting up the timing. The duration of these collisions will still need to be discussed.
It is possible to give some collisions at injection energy to the experiments at an early stage of the commissioning (week 14). The machine would find this interesting if experiments could give some feedback on the IP position during these collisions. In this case the machine experts can extrapolate the information of the experiments to the ramped and squeezed beam and corrections could be applied to the optics at an early stage before the TCTs would be aligned. This could potentially save some time. Jörg remarked that especially in the case of CMS the situation was critical since the correctors at this IP do not have a lot of margin to implements large corrections and therefore it would be good to get as much information as possible early in the commissioning.
ATLAS stated that they could give some feedback on the vertex with these collisions. CMS expressed doubts that it would be possible to give some valuable feedback with only calorimeter data.
Jörg concluded that in this situation the machine would not be interested in these collisions since they would not get feedback on the CMS IP.
All experiments were asked about a statement concerning the 450GeV collisions: ATLAS would be able to give some feedback on the IP position but they would not push for these collisions. CMS could make use of these collisions (probably without being able to give feedback on the IP position) but they would not push for this. ALICE believes that these collisions would not be very useful for them and would need more internal discussion on this to give a definite statement. LHCb could do some timing alignment of muon detectors and calorimeters, but they can do this with any kind of collisions and therefore would not push for this.
The possible overhead of skipping the 450GeV collisions and do any IP position checks in Quiet Beam in week 16 was discussed. If corrections would be necessary at a collision point it would need time to simulate the cost in aperture of the correction and the TCTs would have to be re-aligned. It was stressed by Stefano Radelli that the TCTs now all have BPMs integrated which makes the alignment less time consuming.
The topic was concluded that we do not foresee collisions at 450 GeV since the benefit of this would be very limited and it was felt that it would be better to save this time.
These collisions will be set up once the TCTs are aligned. Stefano mentioned that one could save some time by not aligning the TCTs but since the experiments should give possibly precise IP feedback in short time, the Tracking detectors need to be switched on for which the experiments want the TCTs be aligned. Witold proposed to do the collisions with VdM optics since this might save some time since the TCT requirements would be a bit relaxed, but Jörg was not sure that significant time could be gained with this. During this discussion it was remarked that the setup for the VdM scans is not included in the current commissioning plan but needs to be done significantly before the VdM scan takes place. This needs to be followed up by the LPC.
Experiments are requested to give feedback on the IP position to the machine within 1 day.
It was clarified that Totem in the last years were allowed to take some data after the alignment, which could be done again this year however this will be a dedicated period for TOTEM and will not be called "Quiet Beam". During the Quiet Beam period TOTEM will NOT take data. ALFA will not take data during the commissioning period since there is no high β* optics foreseen during this time. Nobody was able to give a statement on AFP.
Alessandro Cerri asked if the experiments magnets should be switched on during the Quiet Beam period. Jörg stated that the corrections due to the ATLAS magnets are very small but ideally conditions should be as close as possible to the final running configuration.
The conclusion on the topic is that there will be Quiet Beam with aligned TCTs during which the experiments determine the IP position within 24h to give feedback to the machine. The conditions will be such that all collimators are put into the nominal positions, as during Stable Beams, but the TCT settings are not yet validated with loss maps and no async beam dumps will have been done yet. However loss maps at Flat Top will have been done. The beams will have three nominal colliding bunches and will be squeezed. The beam mode during the Quiet Beam will be "Adjust".
Jamie summarised the requests for early filling schemes during the ramp up and some requests for the entire year and explained that some requests are conflicting with each other (see slides). Greg Rakness reminded that CMS had asked for a single isolated colliding bunch during the year as long as their is space for this in the machine. (If this bunch cannot be put into the scheme without loss of luminosity anymore, CMS would request to have such a bunch inserted only at some regular interval (about two weeks, tbc).
It was concluded that the initial filling schemes should be optimised to bring the machine as soon as possible into luminosity production mode.
Jamie requested that experiments should request special configurations early during the commissioning so that they can be taken into account in the planning at the most adequate point in time. (E.g. high pileup runs with few bunches, low pileup runs for ATLAS and CMS, ...)
Stefano summarised the scope, and the strategy of the initial commissioning including important commissioning steps. The presentations contains a link to the shift by shift commissioning plan which is continusouly updated.
Alessandro Cerri remarked that the machine should tell ATLAS when to mask the BCM interlock signals during the commissioning to avoid unnecessary beam dumps.
Sune Jakobsen mentioned the problem of ALFA getting potentially much more radiation due to the collimator settings when AFP is inserted. This problem is currently being studied with simulations but results are expected on a timescale of months. Therefore ALFA asks if it would be possible to have a BLM installed behind the ALFA electronis to monitor the radiation doses. After a short discussion on whether a BLM or a radiation monitor would be the best suitable device it was concluded that experts would look into the issue to decide if something can still be installed and which type of device would be the best suitable.
In the same context ALFA proposed to move the high β* run to week 34 so that ALFA could be removed during TS2, and AFP would not be moved into the beam before this, to reduce the accumulated radiation. TOTEM will discuss this proposal within the collaboration.
Experiments underlined that they would like to be informed whenever there will be collisions during the commissioing period, since they could parasitically profit from this.