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LNO-NDC joint meeting on ion losses and optics in the ramp
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LNO-NDC joint meeting on ion losses and optics in the ramp
LNO-NDC joint meeting on ion losses and optics in the ramp
News.
Rogelio reminded that we had the last LNO meeting on ion optics on the 18 October. See the https://indico.cern.ch/event/1336105/ presentations and minutes.
Stefano recalled that news were presented in yesterdays ABP information meeting, and that the last NDC meeting was held the day before.
He reminded that IPAC topics are needed now as input for the IPAC planning on the group level on Monday. LHC operation has just finished for this year with the ion run. Discussions on a possible future oxygen run can be expected soon.
Losses in the ion energy ramp, Roderik Bruce, slides
Roderik said there were mainly two types of losses : losing the un-bunched particles at the beginning of the ramp and transverse losses towards the end of the ramp. He said that the combined ramp and squeeze (R&S) used in this years ion run has in fact been the most aggressive R&S used ever in LHC. The losses towards the end of the ramp exceeded the dump thresholds in several cases such that fills got lost before producing any collisions, and at times limited the maximum useful intensity in the ion run. Crystal collimators were used for improved cleaning but probably often outside the tolerances required for channeling during the R&S in the less efficient amorphous mode. Beam loss monitor (BLM) thresholds were increased in several steps. After a more quiet week with few fills lost in the R&S, losses increased again at some point. The reasons are not well understood and may also originate in the beam quality provided by the injectors. Primary beam losses are located in IR7 and are believed to be mainly horizontal. Secondary losses reaching the tertiary collimators (TCT) increase with energy with the beta* squeeze. The 208 isotope lead ions provided by the injectors can lose one neutron in the primary interaction resulting in 207Pb82+ ions that appear as 0.5% off energy particles. They can travel through LHC arcs and get lost in one of the following straight sections. Beam dumps were mainly caused by beam 1 and only in two cases by beam 2.
Time location of losses in the ion energy ramp, Sara Morales Vigo, slides
Sara works with Belen on an analysis of the beam losses during the ramps of the ion run, based on BLM and beam current transformer (BCT) data. She looked at all fills of the ion run with beam intensities of at least 1.e12 charges. Her slides show the intensity lost or the BLM or losses (blue for beam 1 and red for beam 2) together with the beam energy as a function of time during the ramp for several selected fills. The times at which the match points used in the R&S are reached are shown as dashed vertical lines. For beam 1, loss peaks appear to coincide with the match points and to be highest at energies between 6.3 - 6.4 TeV. For beam 2, the losses are more distributed in time and more visible already in the earlier part of the ramp.
Stefano comments that Björn is working on an analysis of the orbits during the R&S. The LHC is believed to be quite reproducible. The beam quality provided by the injectors maybe less reproducible. Except for few wire scans that are only allowed up to intensities reached by the first trains injected, there is unfortunately no reliable emittance data available to judge the beam quality. In some later fills, the orbit correction and control was such that crystal channeling could be maintained during R&S. Two fills towards the end were very clean. A list of fills with extra information to be further analyzed will be given to Sara.
Optics measurements in the ion energy ramp, Tobias Persson, slides
Dedicated optics measurements in the ramp were performed on the 26 October. Measurements were mostly done at matching points were optics models are defined and where the AC dipole can be used. Some measurements were also done using excitation by the ADT for coupling corrections, and to obtain some (less precise) indications of the optics behavior between match points. The beta beat was found to remain within 20% for all points in the ramp. A clear increase is seen between 5.4 and 6.3 TeV and can largely be accounted for by an energy mismatch of the triplet strength compared to the beam energy of order 2.e-4. The origin of this and its energy dependence are currently not well understood. The mismatch could always be present and just become more visible when the beta* is reduced in the squeeze.
Roderik commented that the positive beta-beating shift between 5.4 and 6.3TeV of about 6-8% could drive some losses as it would make the collimators cut at a lower number of sigma. This is to be further investigated.
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