4th ICFA Mini-Workshop on Space Charge November 4th – 6th, 2019 CERN (Geneva, Switzerland) Minutes of Q & A: Wednesday, November 6th, 2019 – Guided Discussion, afternoon session reported by Michele Carlá *** LINAC: Alessandra: Space charge neutralization: we have no solution. Beam matching is less critical from linac4 view. Kiersten: Chose optics to minimize losses and optimize diagnostics, losses could depend on optics. Yannis: Should we do something regarding simulation codes? Kiersten: We think codes are ok, but maybe info is lacking? Giuliano: Beam mismatch/oscillation can result in halo, this could be a problem for SNS. Kiersten: Yes there is an extensive halo in SNS, but is not an operational problem (at least for now). Alessandra: What could be the best layout for space charge neutralization? Daniel: We should look at measuring the particle distribution (for compensation, not the H- beam) Kiersten: There is no specific diagnostic for space charge neutralization. We need specific design. Pellico: We have a test stand for neutralization studies (fermilab) *** NOISE: Yannis: Shinji presented his code and talked about two types of noise: heating and finite number of macroparticles Giuliano: Not clear if the taylor map method is advantageous, you don't get enough high order terms and if you have beam oscillation they are not included. Hannes: Maybe it could be still useful for special distribution with no analytic expression. Malte: Tobias is working on MAD-X, they are going to implement sigma-matrix method. Yannis: Stresses the importance of modular approach in codes (Adrian talk) Frank: Modular codes (as in Adrian talk) work only for frozen Eric: At Fermilab the idea was to combine 2 codes: space charge + TPSA but was not really working well so they wrote synergia from scratch that is somehow modular but is not the main point. Frank: Is similar to PTC + pyorbit. Yannis: Can you run PIC on GPU? Stephen: NO! The problem is the memory access. PIC requires sparse memory access. Yannis: (to Stephen) Optimization of the grid, have you tried with your code? Stephen: Our code is not optimized for speed is just plain-python. Giuliano: Noise in PIC/frozen... depends on the problem: for linac we don't care about simplecticity, in ring is different. The question of good or bad depends on the problem. Malte: In PIC bending magnets are easy. *** General Issues II: Frank: (To Eric on the locality of spacecharge) How many kicks did you used in the IOTA simulations? Eric: 12, 1 for each cell. Looks like you can compensate locally close by the lens but not globally. Giuliano: (To Eric) In your simulations the tunespread is 1: how do you match it? Electron lens should be modulated with local density? When you see emittance growth is because of mismatch? How many kicks for betatron wavelength are you using? Eric: 6, I tried also with 12 but got the same results. Shinji: Suggests that Eric should try a simpler model: ideal 12-fold lattice Eric: With perfect symmetry no problem. you don't need to compensate, and with the compensation it stops working The compensator is a driving term! Shinji: Is it because of mismatch? Stephen: (To Eric) How do you know the compensator is compensating spacecharge and not correcting linear optics? Try a simple test: shrink tune spread only, after you look at defects. Eric: Tune spread has nothing to do with emittance growth. Yannis: Build a small model where you are sure to excite space charge resonances and only after try to compensate. Giuliano: Be sure that you are matching the beam. If you need very high current e- lens we must be sure that e- beam is stable. *** 9: Giuliano: Space charge driven incoherent resonance can be distinguished. Yannis: In simulation is clear, in the experiments no. Shinji: Suggest to do as in j-parc ....? follow particles in phase and real space to optimize the injection. *** 10: Angela: (On IPM) for PS we should have magnetic field! Giuliano: What is the error bar in IPM? (Looks like at GSI is a "dreaded question"). Angela: Statistical noise is very small. Cycle-to-cycle LHC IPM is very flat Malte: Envelope oscillations can be measured with IPM to match optics. *** 11: Yannis: Does space-charge just induce tune spread and therefore landau damp or does it shift the modes. Do we need full simulation to find this out? Giuliano: Vlasov equation discussion is made using KV and not Gaussian distributions. Coherent vs incoherent depends also on the type of the beam and KV is a very special case. We should focus on what we can measure/have in the machines, therefore not KV beams but Gaussian beam. Jeff: What about "thick KV"? is interesting for injected beam with painting Giuliano: At SNS they want to paint an uniform beam but not a KV. SNS: Is ~4D KV *** Tune ripple 12: Yannis: Are there other observation of modulation + spacecharge (similar to Hannes work)? Giuliano: Try to do an experiment where you sample the sidebands of the modulation. *** 13: Yannis: Ask Hideaki to comment on the 3d mapping of the injection magnet: did it help on the optimization of losses ? Hideaki: The injection magnet has large fringe fields so it affects a lot the beam. Shinji: Not a big effect in terms of absolute losses? but if you want to get to ~1% losses level this what you have to do. Elisabeth: Ask about simulating carbon/graphite/graphene foils. Hideaki: At j-parc we use carbon foil. *** General: Giuliano: Suggests a review of where we stand to keep track of what was done in the past Yannis: Can someone review the past studies? Hannes: Maybe a session in the next workshop.