LPC meeting summary 12-09-2016 - final
Main purpose of the meeting: Feedback from the development session held on Wednesday the 7th of September and planning of the physics run.
Hector gave a summary on the collimation scheme for the 2.5km run including the lessons learned in previous development sessions. He explained why the collimation during the run is done with a single stage and that initial tentatives to collimate with the carbon based primary collimators (TCPs) caused high background rates at the pot since the edges of these carbon collimators scatter of protons into the acceptance of the pots at high rates. (It was mentioned that the drawing on page 3 could be improved by replacing the label "Secondary shower" with "scattered protons" and by letting these protons (green lines) originate from the edge of the collimator surface.) The TCTs could not be used for the collimation since the beamsize there is so small that the anti-collision system of the collimators prevents the collimators to be driven in sufficiently close to the beam. Therefore the collimation was done with the TCLAs made of tungsten.
Hector summarised the chosen procedure to prepare the beams for data taking. It has been observed that the re-population of the tails proceeds much slower if the horizontal collimators (TCP.H) are retracted far out. It has been decided that during the physics run they should be retracted to 5.5σ.
During the first part of the session fastly growing tails have been observed. The experts tried to remove these tails by doing a momentum cleaning with the collimators in sector 3. During this procedure an instability developed which lead to a significant loss of a fraction of the beam. The instability was damped and removed by retracting the primary TCP collimators a bit. It is not likely according to the experts that this cleaning procedure was the cause of the instability since the relevant collimators are far away from the beam core. Experts do not know if the instability itself or the momentum cleaing did remove the tails of the beam during this procedure. It was remarked that these instabilities are not completely unexpected with these close collimator settings.
In the end it was concluded that it is not possible to predict how often the beam re-shaping needs to be applied since the repopulation rate if the beam is not exactly known.
First Sune summarised the findings of the 2.5km development session held on the 31st of July. Besides a significant difference of observed background in the two beams, which is not understood, it was found that collimation with only the TCLAs is the preferred scheme since it minimises the background best and allows for the best separation of signal and background.
During the last development session on the 07/09 the much higer backgrounds in beam 2 were confirmed.
Sune showed that the new optics (version 4) with a phase advance of 181 degree improves the dispersion at the pots which is essential for the physics analysis. In spite of the fact that the offline analysis is still pending to extract quantitative results it can be decided already now that the physics run should be taken with the v4 optics.
To find the ideal filling scheme for the physics run the ring was filled with a "high intensity" colliding bunch (10.5e10 p/bunch) and lower intensity colliding bunches (7.2p/bunch). ALFA found higher backgrounds in the high intensity bunch pair but the number of signal events would not increase significantly when using the high intensity bunches. Therefore ALFA prefers to use bunches with 7.5e10 protons in the 2.5km physics fill.
ALFA needs a non colliding nominal bunch for precise luminosity measurements. Since this means a cost of 25% of the luminosity ALFA requests as a compromise to have the first fill and one of the last fills of the programme with one non colliding nominal bunch and three colliding bunches, whereas all fills in between should be filled with four colliding bunches. All filling schemes also contain one non colliding probe bunch with an intensity of 1e10 protons per bunch which is used for background determination.
In the discussion Reyes remarked that a background monitoring needs to be available. The experiments answered that they will have 24hours coverage by an expert in the CCC and that the background will be monitored by these experts.
Finally Sune stated that the minimal requested luminosity for ALFA is 700µb-1.
Concerning the operational procedure it is hoped that the RPs do not need to be re-aligned from scratch in every fill. It should be possible to approach to 100µm before the final settings and then approach these final settings in small steps.
Finally Jamie remarked that if the experiments can digest a bit of background and they do not need to rescrape and refill very often, the integrated lumi would become sgnificantly higher. This should be carefully considered when asking for re-scraping/re-filling.
Mario summarised that TOTEM saw a clear lumi signal and the lumi optimisation succeeded. The algorithm used to determine the online lumi will be improved to be robust against background signals in not filled BX buckets in the ring. TOTEM will anyhow try to get T2 operational until the physics run.
TOTEM agrees with ALFA on the collimator settings the procedures and the filling schemes.
TOTEM requests 400µb-1 integrated luminosity. TOTEM requests a discussion before launching the second fill with a non colliding nominal bunch towards the end of the programme.
In the discussion Jörg Wenninger remarked that a lumi decay by a factor of 2 is possible in during the physics fill due to the high brightness beam which might blow up.
In the final discussion it was remarked that the starting point of the run cannot be
sharply defined since it depends on when the cryo becomes available after the TS2.
The start of the run will be further discussed.
Further it was remarked that it is hard to predict when and how often re-scraping will be necessary. It will have to be done based on monitoring of the data quality in ALFA and TOTEM.
The v4 Optics with the improved phase advance for AFP will be used.
The intensity of the colliding bunches should 7.5e10 protons per bunch.
Two filling schemes will be used: a scheme with 3 colliding bunches, one non colliding bunch and a non colliding probe bunch, and a scheme with 4 colliding bunches and a non colliding probe bunch. The scheme with the non colliding nominal bunch will be used in the first fill. It will be discussed towards the end of the special run if it will be once more used as (one of) the last fill(s).
The filling schemes are available on the LPC web site:
In case TOTEM needs to move some pots (e.g. retract a bit the tilted pots), this would be done in one of he rescraping periods where Quiet Beams is NOT declared.