FELSI Meeting 28. October 2008

Tuesday 28 Oct 2008, 14:00 → 15:00 Europe/Zurich

WBGB 019 (PSI)

1 Andreas Adelmann: New Features in Opal

BB: Are secondary effects in the pepperpot planned for the future? AA: If we need it, we could probably easily implement simple elastic scattering effects, for instance. True secondary would be more difficult. YK: Is the mesh recomputed after collimation? AA: Yes. Note that for the moment there is only the elliptical collimator to approximate the true aperture, but with the new solver it will be possible to implement any geometry! BB (drooling): Including wakefields? AA: No, the next version of the field solver is still an electrostatic model. The only way to add wakefields is with a function. TG: Does the pepperpot have a finite thickness? AA: Yes, it is given as the length of the element. YK: In a drift there is no CSR in the current version? AA: No, not at the moment. This is essentially just a bookkeeping problem (need to know what was the element before etc.) and will be solved in the near future. YK: In the CSR calculation, what do you mean by 3D? AA: Essentially that CSR and space charge are calculated together. Ref: Saldin & Schneidmiller Phys. Rev. A 398, p373 1997 and G. Stupakov & I. Kotelnikov Phys. Rev. STAB, 6, 034401, 2003 YK: How can we control the binning in the CSR calculation? AA: There is a default binning, but you have an argument, where you can increase the binning (it is called 'NBIN'). By the way there is the 'help;' command that lists all parameters available (see also the OPAL manual).

Slides pre1.4.pdf

2 Yujong Kim: Alignment Tolerance and Performance Comparison of Two New Gun Solenoids

Selected questions and discussion: BB (on p. 10): Why is the emittance growth different vertically and horizontally? YK: move only horizontally, emittance growth is mainly vertically due to the vertical dispersion. AO (still on p. 10): The scale is millimeter, how do you get a tolerance of only 14 μm? YK: Simply by linear extrapolation. It is not exact, but a good indication. MB (about solenoid tilt): One can find correct solenoid tilt by checking the beam size. RG: currently not foreseen to tilt the solenoids AO: Are you using field profiles from Marco Negrazus? Can he build these solenoids? YK: Yes. RG: We started out with 0.4 T, which was too high, but 0.35 T seems to be feasable, but still at the limit. TG: Does the angular momentum you add to the beam with a rotated single solenoid pose a problem? YK: All other labs use single solenoids for the RF gun based injectors. AO: This may not be an issue at 0.34 μrad emittance, but could be an issue at 0.05 μrad, which is why we have double solenoids in OBLA (design by Rene Bakker). RG: Where do you determine the 0.34 μrad emitttance? YK: After the booster, in front of the bunch compressor. I am afraid if I use a more downstream emittance as my criterion, the tolerances will be even tighter! RG: Why do you set the limit at 1% emittance growth? YK: We could choose a higher value, let's say 10%, but then the beam size becomes significantly different and large, and you end up with a completely different machine! MB: By how much can you move the beam that goes into the solenoid.? YK: At FLASH we first aligned gun and solenoid with beam-based alignement. To control the beam position we should have a steerer in front of the solenoid but there is not a lot of space. With two steerers we could control the angle, but there is no space for two steerers. MB: How big is the actual mechanical alignment tolerance? The 14 μm are not the mechanical alignment tolerance, since you can always correct the incoming beam. YK: The 14 μm is actually a tolerable offset between beam and magnetic center of the solenoid. Here, I assumed that beams go through center of solenoid by the steerer, then we started to move solenoid. We will have only one steerer set in front of the solenoid. We will certainly need beam-based alignment which is why we need your help, Michael! We should perform the beam-based alignment about once per year. To perform the beam based alignment of the gun solenoid, we need Micro-movers for the solenoid, BPM or YAG screens to find out beam positions. While scanning only one solenoid current, we will monitor beam position with a BPM or image center on an YAG screen. By using measured beam positions and measured beam parameters, we will simulate the machine status to find misalignments of the solenoid and laser beam offset. Then, we will move the solenoid with the Micro-movers to correct the misalignment of the solenoid. In case of laser beam, we will correct its misalignment by adjusting angle of mirrors in laser beamline. By repeating those steps above continuously, we can reduce misalignment of the solenoid and laserbeams. Normally, it took one or two weeks to finish all those process. RG: Could the steerer possibly affect the cathode area? YK: We have not thought about that yet. But its field strength is only a few tens Gauss. It will be OK. BB: Will the gun cavity be on movers? YK: No. AO (on tolerances of the solenoid, p. 20): Are the 200 μrad tilt tolerance reasonable to align? YK: In the case of the gun solenoid it is about 200 μrad, so I just used the same number for the bucking solenoid Besides, Fuqiang said it is okay! MB: At the SLS it is a standard value, not difficult to achieve. RG: What about tolerance of the bucking coil? YK: See my talking material on bucking solenoid. It will be somewhat looser than the gun main solenoid. TS: Did other labs (e.g. SLAC) arrive at similar tolerances? YK: In case of the main gun solenoid, its transverse alignment tolerance is 50 μm for LCLS gun. TS: Has the decision single/double solenoid already been taken? YK: Yes, Marco took the decision. We will choose the single solenoid for the CTF3 RF gun. AO: Which field profile were you using for the gun? YK: We are using a field map from Micha, which is the field map for the current gun setting. Cathode has a slightly lower gradient than the 1st and 2nd cells. AA asks YK to update Steve Russell with the new fieldmaps and any other changes. YK: I will supply the new fieldmap when magnet design is finished and all things are optimized.

Slides FEL-KY84-018-1_Performance-and-Tolerance-of-Solenoids-in-250MeV-Injector.pdf