4th ICFA Mini-Workshop on Space Charge 2019

M. Zampetakis - Q&A Contribution

Monday, 4/11/2019 – Afternoon session

Presentation 1: Extraction of transfer map including self-consistent space-charge effects in a symplectic tracking code – Ji Qiang
Question 1:
Yannis: Your Taylor map, clearly you can use it for analysis, etc. But you cannot use it for tracking because it is not symplectic, right?
Ji: Yes, right.
Yannis: So, I would like you to comment on the symplectification of this map and on its potential to be used like frozen space charge or even quasi-frozen adaptive space charge and things like that because I think there is a lot of potential, right?
Ji: Yes, when you do tracking this map needs to be symplectified…

Question 2:
Giuliano: The tune footprint in slide 23 has a very strange shape. Can you comment?
Ji: I have to say that this is still in working progress. I asked Etienne about this case. It is pretty much what I get, and I do not fully understand. 
Elias: At least this is a tune footprint and the maximum tune shift does not look bad. 
Hannes: Did you compare with the tracking?
Ji: No, not yet.
Yannis: Just a clarification, this is 5D? 6D?
Ji: No, it is 4D. Only transverse.

Question 3:
Stephen: When you do your tracking through whatever self-consistent code you are using through the single ten cell channel, was the distribution the same at the end and at the beginning?
Ji: No, the distribution evolves self-consistently.
Stephen: Then if the distribution keeps changing, how useful is the single turn map for an evolving distribution.
Ji: I would say you are right. If the distribution changes a lot, I would suspect that it is not very useful, just like your frozen space charge model. But if the distribution changes within limits I think it is quite useful.

Question 4:
Giuliano: how much faster will be the spectrum?
Ji: I have not done that yet so I do not know but I think it will be quite fast. Let me try it first.

Question 5:
Malte: Where did you get the al and βm coefficients?
Ji: Equations in slide 7.
Malte: What about the forms? I suspect that you are using a non-linear map so a few several thousand macroparticles.
Ji: For the TPSA I just go for 1 particle.

Question 6:
Eric: For our own applications we are worried about particles in halo out of 4σ. This does not look like if you go to high order, the halo particles byeond 4σ can be handled easily.
Ji: When we are using a macroparticle simulation usually whatever we get for this space charge map potential we can use it to extrapolate this transfer map.
Hannes: Up to which order do you go in your footprint?
Ji: I have calculated up to 8th order.

Presentation 2: Initial beam studies using symplectic 2D space charge with open BCs – Stephen Webb
Question 1:
Jean-Baptiste: I was wondering regarding the approximations that you took at the beginning; would it be much more complicated to take into account for example fringe fields?
Stephen: Probably not. If you have a vector potential representation for the fringe fields then you just include it. You have to do some tricks to how you include a symplectic integrator on that but there are some papers that came out that show how to do that. But you can do that.
Question 2:
Adrian: I wanted to add a comment to the comparison to SYNERGIA that we discussed in person (slide 22). If you just go back to the book by Hockney&Eastwood and follow their first standard procedure of implementing the PIC that should be conserving the total momentum for a kick. So regardless through what structure you go through, each of these kicks will conserve the total momentum to roughly 10-14. So, I’m not sure what happens here but at least I would expect to happen the same also for a PIC. But then there is all the other stuff about the emittance conservation, all these things that are clearly a beneficial aspect of yours.
Eric: Depends on which space charge solver you are using.
Stephen: Exactly. So, I do not have an answer to that.
Eric: the 2D solver is known to not conserve the transverse momentum because there is this weird interpolation where the kick is modulated by the longitudinal line density. The 3D solver does conserve it.
Stephen: I should note that we did not run the simulations in 3D.

Question 3:
Giuliano: I have a question on the tests that you made because it is an interesting test when you cross a structural resonance that is induced by space charge. You basically went from below up and then you stopped the changing of the tune?
Stephen: The way that we triggered that is intuitively that your Gaussian beam has an octupole term. So, if I cross an octupole resonance I should see an octupole resonance. So literally by trial and error we tuned the tune of our FODO cell up above the resonance and up to the charge until we saw that resonance.
Giuliano: Yes, because when you are crossing it is not completely wrong that the islands disappear. It depends when you stop what you are doing. So, the physics of what is happening at the comparison, must be looked at in detail.
Stephen: Yes, so we did set up the same situation in both codes. That's why we used a simple FODO channel.

Presentation 3: Experience with the symplectic 3D space charge kick – Frank Schmidt
Question 1:
Stephen: This is related to the question I asked Ji earlier. You are supposing periodic boundaries to your Sigma matrix but how do you know that your distribution is periodic? Because imposing the periodic boundary conditions is kind of non-local constrain that changed the way that the Sigma matrix behaves in the entire ring. How do you know that this is a valid thing to do and what is the approximation limit that you want?
Frank: That's a very valid question. This should be further investigated.

Question 2:
Giuliano: You showed a picture that there are 2 islands. If during the measurements this thing is there, it should affect the mean profile.
Frank: The mean profile including the islands is there (Slide 38). You do not really see anything from the islands. The thing we expect is not happening here. If I go to another location of the ring, I might see something.

Question 3:
Yannis: One of the important parts of applying this analysis from MAD-X is to understand what happens to even linear optics like beta functions etc., when you are applying space charge. I am wondering with the method of Ji, TPSA, normally you get this for free. Also, in your case it becomes very difficult when you approach the integer. So, the interesting part is what happens with Ji's method and maybe benchmark these two methods.
Frank: Yes, I totally agree. We must put the space charge kick and get it in PTC. It is one of the most important items in the list.

Question 4:
Malte: You said you had issues with noise. Is this fixed in the new symplectic code?
Frank: We update the machine each turn. During that period, it is symplectic and therefore no noise is introduced. On the other hand, each turn the emittance is changed and this introduces noise that piles up each turn. The noise is small but still, cannot be neglected.

Question 5:
Jeffrey: You impose periodicity over 1 turn. Have you tried to impose periodicity over N turns? Maybe for large N you can get some reasonable results.
Frank: We should put that in the list. We must invest the question. That is a very good idea.

Presentation 4: GPU accelerated space charge simulations using SixTrackLib and PyHEADTAIL – Adrian Oeftiger
Question 1:
Jean-Baptiste: You were saying that in the SIS100 the paraxial approximation is not valid. I was just wondering where that comes from.

Adrian: First, when you are going through some lattice, you have a 3D trajectory. So, the overall path length of this particle does not exactly correspond to what you just track in the longitudinal direction. That's the amount of breaking down of your paraxial approximation.
Jean-Baptiste: So, if you do some 3D tracking you will not have any problem with that.
Adrian: There is no problem with that in that sense. It's just if you model it in the wrong way then you will get the wrong results.
Jean-Baptiste: How much do you know your dipole? Because if you don’t know the 3D map in the magnet, probable you would not know exactly what the path of the particle is.
Adrian: This is a very small effect for that high energy.

Question 2:
Ji: You talked about a speed up of x400. How many cores for that single GPU?
Adrian: This is the V100 with 4500 cores and 7 teraflops. It costs 7000CHF but you can run the same things in, instead of 2 minutes, 5 minutes with a gaming GPU that costs 600CHF.

Question 3:
Shinji: I was wondering if 2.5D and 3D would have any difference.
Adrian: Define what you want to have a look at, and we will discuss if there will be any difference. For example, the bunch rotation case. We have still long bunch but very compressed, so we have transverse space charge of 0.5 and in SIS100 we have a longitudinal very strong space charge which leads to a potential distortion which completely messes up the way that the bunch gets rotated. So, these two facts together, they already make an interesting case where we can test if the 1D longitudinal + slice by slice 2.5D transverse give the same results as the 3D.

Presentation 5: Characterization of resonances - coherent versus incoherent – Adrian Oeftiger et al.
Question 1:
Ji: 1st slide. You said that first you have an incoherent instability and then an envelope instability, but you can see that there is not much envelope growth. I would expect that if you have an envelope instability you would have much larger growth.
Adrian: I will try to clarify this question and come back tomorrow.

Question 2:
Shinji: I wonder why the 3 times the betatron oscillation period equal to synchrotron period because that is not realistic (slide 7).

Presentation 6: Space charge and lattice driven resonances in the CERN PS – Foteini Asvesta
Question 1:
Adrian: for the 2Qx+2Qy resonance since for the rest of these models seems to work very well, have you tried just to crosscheck with the PIC there whether if it is due to non-self-consistency or do we have no idea?
Foteini: we have done some checks with the PIC; we did not really see an effect, but I don't know if we went to that high tunes. We can check.

Question 2:
Shinji: I remember last year that you mentioned that the frozen space charge result and the PIC result are the same right? and that means that the beam distribution should be similar. Have you looked in the experiment the profile of the beam?
Foteini: Yes, they are well reproduced. I do not have to profiles in the presentation now.
Shinji: If it is coherent in the core, we may see something around the center, right?
Foteini: Yes, but we have never seen anything on the profiles so far.
Ji: If in your experiment measurement if you continue to reduce your intensity that should be still there. And then in your simulation you can just turn off space charge, because this is from the lattice. Then you should be able to see if you can reproduce it.
Foteini: I think that in the tune diagram measurements that Alex did last year at some point they were able to compensate some losses from the 2Qx+2Qy resonance.
Alex: Effect from remnant field. This is a rather small effect that comes from the octupoles that we use for the multi-turn extraction. Because we excite them up to saturation levels then we have some remnant fields that are just a small percentage of the maximum field but with some degaussing we could remove this. Did you apply also the degaussing cycle to your measurements?
Foteini: Not in these measurements.

Question 3:
Giuliano: The 3rd order resonance that you say it is compensated, it is a natural one. But is it strong?
Foteini: This excitation is very similar to the one that we have. It is quite strong but not the strongest one. (1st backup slide)

Presentation 7: PSB simulation studies for post LS2 operation – Elisabeth Renner
Question 1:
Alessandra: You based all the simulations on the emittance of 0.4 um, but in reality it is smaller (0.3). Would you profit from 25 % lower emittance or it would not be the case?
Elisabeth: For Hi-Lumi beam according to the current simulations, not really. We should investigate it but I do not think there will be any difference.

Question 2:
Bill: So, you gave an ISOLDE beam. Which is the maximum goal you are planning to go in terms of the beam intensity?
Elisabeth: 1.6 1013 for the ISOLDE facility.

Question 3:
Shinji: In the last point you wanted to optimize the emittance. Do you have to make a Gaussian beam in the transverse direction or isn't that such a constraint?
Elisabeth: For LHC yes. For ISOLDE not so much. If you look at the final profile of ISOLDE beam in the transverse direction, we do not have a Gaussian beam due to the large phase space painting. I would personally say that optimization of the painting is less critical than in other facilities because at the end if we paint with slightly larger beam we will always more or less end up with the same emittances.

Question 4:
Malte: I assume that you are simulating with pyOrbit frozen?
Elisabeth: No, we are using 2.5D.
Malte: At those intensities you are considering that indirect space charge does not play any role?
Elisabeth: For the Hi-Lumi beams it does not make any difference. Currently it is not included but for ISOLDE the effect might be very small.

Question 5:
Adrian: Since you are running 2.5D PIC and the simulations sometimes run for 25000 turns. How confident are you for the numbers you are giving?
Elisabeth: It's comparative. All we do is comparing different operational scenarios. Here we are confident comparing the results from different scenarios and seeing improvements or disadvantages of certain operational choices.

Presentation 8: Tune scan experiment at injection in the PS – Haroon Rafique
Question 1:
Malte: I have a question about your experimental setup. Are you injecting directly on the integer?
Haroon: No. It is a static tune scan. So, we inject the same beam we expect that it will be from the booster and we inject it directly on the tune that we want.
Malte: So, you inject directly close to the resonance.
Haroon: Yes.
Malte: But there are remnant fields, injection bump etc. How do you deal with all these close to the integer?
Haroon: I do not think that we do in terms of the machine.

Question 2:
Malte: You generate the particles for every case?
Haroon: I generate one distribution and then I change the tune. The distribution is matched to the nominal working point.
Malte: So that means that you might see mismatch effects since your distribution is not necessarily matched to the optics.
Haroon: I saw the exact same thing even when matching the distribution each time.

Question 3:
Bill: You do see a Montague resonance when you use the Pole Phase Windings to go to 6.21?
Haroon: For the Pole Phase Windings, you do. (backup slides)

Question 3:
Giuliano: You say that the resonance is quadrupolar. How do you know that?
Haroon: We claim that the resonance is quadrupolar because we see the effect of the low-energy quads.
Giuliano: You believe it is a gradient resonance?
Haroon: the integer resonance is an accumulation of many resonances, so I think that we don’t know where the stop band is etc.
Giuliano: It would be nice to change dynamically the tune and cross the resonance. Because with a larger tune spread the emittance growth will be gigantic.

Presentation 9: Summary of 2019 Fermilab Booster experiments– Jeffrey Eldred
Question 1:
Shinji: About Number 3 in slide 33, the problem is that you do not know much information about the optics of the machine itself. To what extend can you try to see the optics, for example can you measure the beta function?
Jeffrey: We can measure the beta function.
Shinji: Does it agree to whatever you see from the half integer resonance excitation out of the beta function modulation?
Jeffrey: we can measure at the nominal lattice and we get 10-20% agreement with the betatron functions but one of the things that is more interesting, we put a flat lattice that should get rid of the half integer but it does not have this effect. So, perhaps if we measure near the half integer resonance, we will not get good agreement. This is a hypothesis, but I will do the study soon.

Question 2:
Yannis: It is clear that you are dominated from losses from the half integer. In slide 22, it seems that there is a kind of a line. What is this?
Jeffrey: I cannot be sure. It looks like a resonance at 0.8. It might be 5th or 6th order. It is unexpected. We have the dipoles which are supposed to be dipoles and quadrupoles and we have the correctors which are supposed to be up to 6th order, there are no octupoles activated.

Question 3:
Yannis: When you were doing the modulations, you were modulating to 180Hz. This modulation should run through your tune footprint. But you do not see a difference.
Jeffrey: This would be a tune modulation and should move the resonance.

Question 4:
Giuliano: You say nominal intensity. How much is the tune spread here?
Jeffrey: We do not have a good measure of the emittance in the cycle.  We know what the Linac provides and what we extract in term of the emittance but now to get an exact estimate of how much the tune spread will be, I think it is around 0.2.