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Europe/Zurich
A discussion was held on intial assumptions and guidelines for the beam dynamics design of RFQ3.
Improvements to be explored are:
1) longer/smoother RMS input section to ease matching from the LEBT: allow smaller divergence beam at the entrance of the RFQ, matching over 6-8 cells instead of 4 as is presently done.
2) symmetrize the beam at the exit of the RFQ using a Crandall cell instead of exiting in the middle of a FODO. This will require a re-adjustment of the MEBT dynamics.
Target is a minimum current of 45mA at extraction from the RFQ minimizing transmission losses.
Some guidelines should be given on what is feasible in terms of beam current/emittance/energy at source extraction.
Increasing the RFQ transverse acceptance could potentially lead to moving transmission bottlenecks downstream. It is preferable to have losses at lower energy (LEBT), hence the goal should be to increase the current in the present acceptance.
Concerning the RF design, a preliminary guideline is that we should aim at a maximum electric field on the vanes of 34MV/m (not dissimilar from the present 33MV/m and original Linac4 RFQ assumptions). Final resulst on the electrodes testing and breakdown behaviour (WP10) will be presented at the next meeting (Alexej).
Comparative studies of different vane shapes were shown. Trapezoidal shapoe increases acceleration efficiency at the expense of focusing. Sinusoidal shape yield lower max electric fields. Concerning the vane design, AG pointed out that machining with 6D drilling is now possible, hence one could look at a 3D vane design instead of a 2D one, and look at other types of cross-section other than round to reduce the max surface field.
Minutes by GB (07/05/2021)
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