LPC meeting summary 27-06-2016 - final
|
|
LPC meeting summary 27-06-2016 - final |
|
Main purpose of the meeting: LHC Energy determination with the magnetic model of the LHC dipoles, general status.
Jamie summarised the operation of the last week and came to the conclusion that there is a good chance that we will get more than 10/fb before the ICHEP conference. The design luminosity of 1034Hz/cm2 been reached (within the errors of the online luminosity measurements of the experiments). Jamie congratulated to the LHC team. Even though there had been some downtime in the LHC (mainly due to water leaks in Point 3) the LHC resumed very efficient operation over the weekend.
Various small changes lead to the increase in peak luminosity:
The plan for the upcoming weeks is to slowly go to lower emittances by changing to the BCMS scheme in the injectors. Initially emittances would be blown up to the current values so that the change is transparent for the LHC. Once smooth operation will be established with this scheme emittances will be gradually decreased to reach higher peak luminosities.
A further possibility to increase the peak luminosity would be to fill more protons into the machine by moving the Abort Gap Keeper (the last possible position in the orbit, where an injection is allowed and which is positioned such that the longest possible injection will never reach the abort gap itself). Since in the ongoing year there is no possibility to inject trains longer than 144b in the LHC it would be possible to move this AGK towards the end of the orbit, since it is currently being positioned for trains of up to 288 bunches. This possibility is currently being discussed among the experts and the management of the LHC.
Experiments are requested to comment in case the plan to go to higher luminosity with higher pile-up is problematic for them.
In one of the last fills the MKI vacuum was degrading beyond the allowed threshold towards the end of the injection process so that the last injections of one of the beams could not be performed and the physics run was started without these. Experiments confirmed that this was not problematic for them. Atlas commented to have observed different backgrounds in this fill.
In the discussion it was mentioned that we are currently operating within 5% of the MKI vacuum limit. Jörg mentioned that this is a very small margin and that this value is not a guarantee for not running into problems in any future fill.
It was shortly discussed that in one of the recent fills an UFO reached ≈85% of the dump threshold at the moment the Totem pots were inserted. The question was raised if something in the strategy to insert the pots should be changed. Mario Deile commented that anyway in the next fills the vertical and cylindrical pots will not be inserted any more, in order not to irradiate them. (The cylindrical pots will be re-inserted once the new firmware for the relevant electronics becomes available.) After the meeting Mario analysed the BLM data of the dumps and stated the UFO was NOT generated by one of the pots.
Jörg mentioned that the RF group will implement a controlled way for blowing up the bunch length during operation in Stable Beams (requested by LHCb when running with reversed polarity). He stated that this procedure might become production ready only a bit after the deadline of end of July when LHCb wants to change the magnet polarity. LHCb stated that they are in some limits flexible regarding the time of polarity change.
The general strategy on how to deal with severe detector problems which drastically affect the data quality in an experiment was shortly discussed: General agreement is that a fill will not be dumped if not danger for personnel or equipment is apparent. Access will be granted between fills. This general rule however is seen as a baseline and some degree of flexibility case by case is reserved by the LPC. Greg Rakness remarked that also the time of the day should be taken into account when making judgement.
Ezio Todesco gave a talk about the accuracy with which the magnetic field in the LHC magnets (and hence the energy of the circulating beams) is currently known. He discussed in some detail the limits of accuracy due to the tolerances of the field and the magnet constructions and the accuracy limits of related measurements.
The LHC dipoles are electromagnets, where the field is proportional to the current. Non-linearities are of second order and are due to two main effects:
The field precision is given by the Biot Savart law, i.e. the source is the positioning of the superconducting wires in the coils (≈50µm). Since the coil is ≈40mm from the beam, one can have a first estimate of 0.1% precision in the magnetic field. A more refined model based on a thick coil confirms this estimate.
Ezio summarised the measurements that have been done on the LHC dipole magnets. In particular the field measurements have been performed with several rotating coil probes which have been cross calibrated to a precision of 0.1%. The absolute precision of the field measurement with such a probe has been quoted to be 0.1% whereas the relative precision for comparing the fields of different dipoles is estimated to be less than 0.01%.
The measured spread between the magnets (1σ) has been determined to 0.06% and is caused by the differences in the magnet (and not by the precision of the measurements). These measurements have been performed in warm magnets.
200 LHC dipoles have been measured also at 1.9K. With these measurements an error for the extrapolation of measurements from room temperature to 1.9K is extracted. It has been determined to be 0.04% (1σ). It is currently not known if this error is due to the measurements or due to differences in the magnets.
Ezio concluded that the accuracy with which we know the magnetic main field of the LHC can be estimated to be 0.12% at 3σ.
An upper limit to the improvements of this error is a factor two. This is confirmed by stretched wire measurements and by cross-correlations between collared coils and cold masses, where there is an indication of a spread between measurements done by different techniques or in different conditions that can be as low as 0.02%. It is not known with the available information, if this ultimate precision can be reached. This possibility to improve the measurements will only be considered if requested by the experiments.
Finally Ezio added the case of the accuracy of the knowledge of the quadrupole field; measurements have an accuracy of 0.2%, but in this case the beam measurements of the tune can be used to determine in an independent way the field accuracy. The accuracy of the field knowledge has been determined to 0.2% at injection and 0.1% at 4 TeV or 6.5 TeV - so it is in agreement with the expected values.
In the final discussion it was repeated that the final goal is to summarise the work of these measurements in a publication together with the complementary energy measurements performed by Jörg Wenninger in the p-Pb runs in 2013. This will give experiments an ultimate reference for the LHC energy accuracy, which can be used as input to the physics or error, resp. analysis.