LPC meeting summary 12-02-2018 - final
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LPC meeting summary 12-02-2018 - final |
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Main purpose of the meeting: The meeting discusses the recent schedule updates, the status of the special run requests, the configuration for 2018 pp data taking and the initial plans for the commissioning phase.
Christoph discussed the recent changes to the schedule, the status of the discussions on the special runs, the proposed configuration for the 2018 pp run and started the discussion on the commissioning activities.
Christoph presented the changes to the schedule which were discussed in the LMC meeting on the 7th of February. The opening of the Valves in IP5 will be delayed by 3 days wrt the schedule v1.0 in order to give CMS enough time to exchange all DC-DC converters in their Pixel system. In addition CMS was granted one day of access (currently scheduled at the 20th of April) in order to install a test setup on the CASTOR table which will serve further investigations on the not understood failure mode of the DC-DC converters. In the current schedule the first fill with Stable Beams (3b) is foreseen for the 23rd of April and the physics data taking period (defined as the first run with 1200b) will start at the 11th of May. It was noted that the Machine Checkout falls on the Easter weekend where expert availability is limited and in case of severe problems, delays might be encountered due to this.
The current schedule does not yet take into account the two special runs requested by the experiments this year (only one period of 7 days and one VdM scan (2 days) are currently inserted in the schedule.)
Two special runs are requested for 2018:
The request is limited to 1 week including setup. The originally aimed beam parameters (see LPC presentation in Evian) were considered too ambitious and TOTEM is working together with the machine experts on a more relaxed set of parameters. This will be discussed in a future LPC meeting.
A first tentative to perform this run has been performed in 2017, but background conditions were too bad to perform a precision physics measurement. After evaluation of the data acquired during three test sessions in 2017 machine and experiments decided to try this run again in 2018.
ATLAS and TOTEM do not agree on the energy of the run. For physics reasons ATLAS has a preference for 1.8 TeV and TOTEM has a preference for 900 GeV. Both experiments agree that the physics arguments for one or the other energy are not very strong and the most important question is to find a configuration which allows to take physics data with the required precision at either of the energies proposed.
Christoph showed recent simulations by Stefania Papadopoulou showing that de-bunching of the beam could be delayed significantly when running with an increased RF voltage of 12 MV instead of 6 MV.
Further improvements of the beam quality and the background conditions could be achieved by reducing the brightness of the beams to the minimal required value (the experiments requested bunch intensities of 0.65e11 and emittances of ε=2μm. During the tests in 2017 the bunch intensities were above 1e11 ppb and the injected emittance was ε ~ 1.5μm)
The advantages and disadvantages of going to a higher centre of beam energy of 1.8 TeV were presented. Brian Petersen generated a plot showing the required length of the data taking periods under varying assumptions. From this plot it follows that going to 1.8TeV is only possible if the beam can be re-scraped and multiple data taking periods can be performed in the same fill, or if the data taking period can be extended to much more than one hour before re-filling. This conclusion follows from the large overhead needed to prepare a fill at 1.8 TeV.
The LPC proposes the following:
This proposal will be discussed in the upcoming LHCC and with the CERN management (Research Board).
Chrisioph summarised the pp configuration proposed in Chamonix for the 2018 run. Differences to the 2017 configurations are the PPLP ramp which will gain 110 s per cycle, tighter TCT collimator settings by 1σ and a shortened squeeze segment from 40 cm to 30 cm which will gain 100 s per cycle. The acceptance for CTPPS and AFP will stay unchanged as long as β* is kept at 30 cm since the optics will be the same as in 2017.
It is proposed to commission β* levelling in end-of-fill studies until TS1. If no problems are encountered, calibration data for CTPPS and AFP will be taken after TS1 and the β* anti-levelling will be introduced as a standard procedure during Stable Beams. It has been agreed to use a maximum of 3 β* values in order to limit the amount of calibration data to be taken.
The peak pileup estimations and lumi predictions presented in Chamonix were shown. Finally the integrated luminosity predictions presented by Jörg Wenninger in the LBOC of the 6th of February have been presented. The following assumptions went into the predictions:
With these assumption Jörg predicts an integrated luminosity of 68 fb-1 for ATLAS and CMS and 2.3 fb-1 for LHCb. He pointed out that the assumptions imply some margin.
It was noted that these predictions are not the "official" luminosity targets for 2018.
Finally Christoph presented an idea of Stephane Fartoukh discussed in and after Chamonix, to use crossing angle levelling in IP8 to keep the pile-up density in the limits requested by LHCb (especially when running with Dipole polarity "down" which results in a large crossing angle at the IP). A proposed scenario where the crossing angle is kept constant over the fill could be a possible implementation of the idea. The potential advantage wrt to the bunch length blow-up performed in 2017 would be a slightly higher integrated luminosity in IP1 and 5. It was agreed that the expected gain is small and the necessary commissioning time would have to be carefully evaluated against the potential gain. Discussions on this topic will continue.
Christoph introduced the discussion on the commissioning period by showing presenting the url with the google spreadsheet which will be maintained by OP to develop the details of the commissioning phase:
https://docs.google.com/spreadsheets/d/1nnEb3f15L3NwxJ0RAnh-WkG_eiUUfJ9WGh2yxaT5Kjo/edit#gid=1568510161
In the discussion Witold Kozanecki asked CMS about the Pixel commissioning time for this year. Silvia answered that the detailed understanding of the detector (like timing) is independent of the powering problem in the DC DC converters and therefore the commissioning time is expected to be shorter than in 2017, where the detector was completely new. Silvia expects the Pixel detector to be fully commissioned after TS1.
Sune remarked that it would be important for ATLAS to know the possible scenarios of scheduling for the 90m run so that ATLAS can plan for the scheduling of the low E high β* run.
Witold asked CMS about the motivation for having the VdM scan after a high pileup data taking period. Anne Dabrowski answered that CMS observes changes of the efficiency of the silicon sensors depending on the time the sensors are exposed to a high pileup environment. These effects are not yet understood in detail.
Witold stated that in 2016 ATLAS was performing a VdM scan 12-16 hours after a high pileup data taking period. This was a limiting factor for the precision of the ATLAS scan. Also after 4 days of low activity the systematic error of ATLAS scans is still dominated by effects related to activation of the detectors involved. Especially activation effects in the Tile Calorimeter are difficult to control. CMS stated that they have good handles to deal with Albedo and activation effects but the main issue for them are the above-mentioned effects on the efficiency of the sensors.
It was decided to follow up on this topic in a dedicated meeting between ATLAS and CMS which the LPC will schedule. In this meeting both experiments will motivate their preferences and both experiments will try to find a compromise solution to avoid the necessity of two VdM sessions. It was remarked that such a scenario would also imply some new disadvantages since the welcome pauses between different scans in one experiment which are introduced by interleaving the scans of the two experiments would not be present anymore and scan data of the two experiments would be more difficult to compare since they would not happen in the same fill.
Witold asked for the foreseen ramp-up steps after TS1.
After the meeting Christoph created a web page on the LPC site with the MPP policy for intensity ramp ups in various conditions. The page can be consulted under https://lpc.web.cern.ch/RampupPolicies.htm and is linked from the LPC Homepage.
After the meeting Kristjan Gulbrandsen pointed out to Christoph that ALICE is performing calibration activities in the inter-fill periods. If until TS1 the machine is commissioning β* levelling in end-of-fill studies a solution needs to be found which is compatible with the calibration needs of ALICE and the β* commissioning activities. Kristjan thinks that for the end-of fill studies the High Voltage of the detectors to be calibrated would have to be switched off. The switching on of these voltages after the tests would take rather long O(~0.5h) and the remaining time to perform the calibration in the interfill period might become too short. Kristjan will discuss the details of the calibration fill requirements within ALICE and comes back to the LPC with the result which then can be discussed with the machine experts.