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v3.3.2-pre
L* change request
Presented reasoning for change to single L* configuration optics: to reduce risk due to increased tuning time across push/pull operations, simplify background optimization and ensure the ability to deliver comparable luminosity to both detectors. Change request documentation has been submitted. Provisionally we will look at an L*=4m solution being a reasonable compromise between the 2 detector options and probably the minimum possibility given the ILD design limitations. Okugi presented work demonstrating the optimal choice is strongly effected by the position of QF1. Ability to move QF1 is constrained by the mechanics of the push-pull solution and more work is required to see if this can be reduced. A comment from Parker was that there may be the possibility to reduce the length of QF1 which could help (maybe up to half?).
Work required: generation of 4m L* optics and comparison with 3.5m and 4.5m options with QF1 in TDR position and in a position preferred by the accelerator optics. Also usual tolerance, backgrounds, collimation & tuning studies required. Okugi and SLAC will look into this, aiming for preliminary presentations at LCWS.
Request for prioritization of 250 GeV (CME) optics study made by Walker
Previous assumptions were made that a solution with adequate collimation requirements with a shorter FD might be possible, needs to be studied.
Commissioning
Walker presented some thoughts on possible problem areas for BDS commissioning. Main concern is how to quickly recover beam collisions initially or after significant off-time. Commissioning is assumed to take place with detector at IP with use of beam-beam deflection and beamstrahlung radiation which is ok once beam is within ~5-10um (x & y) capture range. Need to think a little about how to quickly capture the beam into this “commissioning envelope”. Idea of using an additional BPM (downstream of QD0) to form a “virtual IP” using a pair of BPMs (new QD0 one + fast feedback BPM) made. Should add QD0 BPM to decks.
Work required: Think about alignment of IP BPM pair and ensuring readout of BPMs is fiducialised with respect to the IR mechanically and electrically, what are the BPM requirements (resolution, gain errors etc + alignment and fiducialisation requirements) etc? Boogert offered to think about this one.
Backgounds, Collimation
SLAC presented ongoing work looking at collimation requirements for 3.5 and 4.5m L* optics. Based on prevention of SR hits in IR, some fairly tight collimation required in betatron collimation section for 4.5m optics (~5 sigma at tightest location). For 3.5m optics the magnet apertures shadow all tracked particles which can generate harmful SR in IR and collimator jaws just need to be set to protect magnets.
SLAC & RHUL to collaborate on background studies and muon flux suppression requirements etc through use of BDSIM and Lucretia/Placet, possibly looking to develop use of AML as common deck standard to provide additional descriptors required (geometry & materials etc).
Main beam dump line
SLAC presented ongoing work with studies for the 500GeV and 1TeV CME cases. Losses in magnets for high-lumi 1 TeV case probably too high, also large power loss in some collimators (>40kW). Cases for on-axis and max beam-beam deflection shown, but assume beam-beam deflection largely mitigated by fast feedback and needs not form the tolerance requirements.
Optics/decks
Woodley presented status of decks. BDS decks significantly tidied up, parameter overwrite errors corrected. Error discovered in definition of LW gamma detection chicane, ~30m too long. Left as-is to match TDR for now but should be flagged as potential for small length saving.
General question from Walker: why is the CLIC system so much shorter (energy scaled) than ILC. Someone needs to look at a comparison and present the differences.
Timing/beam length issues
Paterson discussed the ~300m timing mismatch which potentially needs to be subtracted from somewhere. Request made to list all possibilities for recovering length by reducing e+ BDS system length and consequences. SLAC agreed to provide this.
Crossing-angle
Parker presented possibility for a re-design of the QD0 magnet which would in principal allow for a reduced crossing angle (down to maybe 10mrad). Discussed possibility of providing ability to (post-construction) change crossing angle in BDS. Could be possible to make a ~5mrad change by switching the polarities of the FFS bend magnets and allowing for ~few m extra transverse space in IR. The downside of this would be that the requirement for the polarimeter to be co-linear with the IP is lost. No strong support from detector community for reduced crossing angle apparent at this time. Only a weak statement made from the accelerator community that things become less risky as the requirements on the crab cavity become weaker. From CFS, it seems that few-100m change to linac positioning not a huge deal given the site geography and geology so this not considered a priority just now, though Parker will continue to pursue QD0 re-design anyway as it has other benefits.
Fast feedback corrector downstream of QD0
It was stated that there are strong benefits to moving the fast feedback corrector downstream of the QD0 cryo system as this eliminates the offset of the beam in SD0 magnet during operation of the fast feedback system which limits its dynamic range due to induced beam aberrations. This looks doable for the SiD detector at least, but would need more consideration for the case of ILD. Moving the corrector would increase the fast feedback dynamic range to that of the maximum deliverable kick and strongly elleviate vibration concerns for the final doublet.
Need to follow up with both detector groups to try and realize this and submit a potential future change request.