Minutes of the ICE section
18th meeting on Wednesday 23/02/2011 (08:40-10:30, 6/2-004)
ICE members: Benoit Salvant (BS), Christian Hansen (CH), Carlo Zannini (CZ), Hugo Alistair Day (HD), Elena Benedetto (EB), Ewen Hamish Maclean (EHM), Elias Metral (EM), Elena Wildner (EW), Frank Schmidt (FS), Giovanni Rumolo (GR), Jean-Luc Nougaret (JLN), Kevin Shing Bruce Li (KL), Maria Carmen Alabau Pons (MCAP), Nicolo Biancacci (NB), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Tatiana Pieloni (TP), Werner Herr (WH).
Present/Excused: BS, CH, CZ, HD, EB, EHM, EM, EW, FS, GR, JLN, KL, MCAP, NB, NM, OB, TP, WH, Eirini Koukovini Platia, Giuliano Franchetti, Hannes Bartosik, Octavio Dominguez Sanchez de la Blanca, Rama Calaga, Roderik Bruce, Sandra Aumon.
1) Newcomers / visitors
- Giuliano Franchetti (GSI).
2) Comments on the minutes of the 17th meeting + Actions
- 1 comment: In most of the previous minutes it seems that I wrote Maria Carmen Alabau Pons in the list of present people instead of Miriam Fitterer (sorry for that Miriam! I corrected that... Thanks BS!).
3) General infos
- 2 comments:
- TP for LBOC => Discussions ongoing on the IP8 leveling and 25 ns beam, in which the beam-beam team has to give its input.
- OB for the SPS movements of some quads => Discussions about these displacements and the possible follow-up for the layout. As these displacements are in the transverse planes (and not in the longitudinal one) the layout database is not concerned. This re-alignment (in the transverse planes) is done every-year at the start-up.
- SL meeting last week:
- Follow-up of EPFL stages:
- At the level of master degree => Equivalent of TS but for 5 months and paid as a trainee (1500 CHF/month instead of 3200 CHF/month => MG will check this with the person who triggered all this).
- Procedure:
project descriptions should be sent to the
following address
stages.sb@epfl.ch with CC to Lenny Rivkin.
- The fellows request for our
group has been
submitted
-
- SPS => Obstacle found in 218: BBSH.21778 which was IN!
- AP fora (organized by WernerH) => Would be a good idea to have them always on THs (but it is not every TH!) at 11:00 in 6-2-004 (this is also the current plan of WernerH). Everybody is encouraged to attend these meetings!
- News on the LHC:
- Insulation vacuum leak in 4-5. Seems to be stable now but to be monitored as it could be a big issue (several months of downtime of a repair is needed as we have to warm the whole sector up).
- 1st injection of the year with the LHCPROBE bunch last Saturday (19/03/11) at 21:30. Since then, everything is going well (some ramps already done as well as squeeze).
- Plan to move to LHCINDIV bunches (4) during this week.
- 1st beam-beam meeting took place last Monday and the impedance and HD will take place next Friday.
- Alexey Burov might not come to CERN due to budget restrictions at FNAL, or could be delayed. Initial arrival date was April.
- 2 questions from RiccardoDM and StephaneF to be answered by ICE:
1) Effect of Q" on the head-tail instability of the LHC => Action1.
Infos from Stephane:
- At 100A and 3.5 TeV the Q''_x is 8500. I suppose that the energy spread is about 1.5E-4 rms at 3.5 TeV, which gives a tune spread of Q''/2*sigma_d^2= 1E-4 in the H plane and four times less in the V plane.
- As for the anharmonicity the Q'' shows cross-talks between the H and V plane when both the OF and OD are switched on. However the sign rules are not the same, e.g. a negative K for both OF's and OD's will give a negative amplitude detuning in both planes but a negative and a positive Q'' in the H and V planes respectively.
- With a beta* of 1.5 m, we will start to see a very sizeable Q'' coming from the triplets (several 1000's) again mainly in the H plane (because the IP1/5 phase advance is close to 90 degrees in the V plane for the V6.503 optics). So concerning the detuning, the question remains, what is damping what and I would say is rather "d'actualite", possibly to better benchmark the theory and the thresholds and possible H/V dissymmetries not completely understood.
- Assuming we start to have rise time faster than an half-synchrotron period (which should presently be rather long, i.e. several hundreds of turns, at 3.5 TeV and not yet the nominal RF voltage of 16 MV), then I think a Q'' can drive head-tail instabilities because during half of a synchrotron period, the chromaticity seen by half of the bucket is negative. This is possibly for a long term study to establish a theory both with Q' and Q''.
2) Addition of 2 localized impedances => Action2.
Info from Alexey Burov (when he was at CERN last year): According to Alexey Burov (these are notes from EM => To be checked) only the betatron functions are important and the absolute betatron phase advances of the 2 localized impedances are not important (note that it is also what we applied up to now). The reason being: Lets consider 2 particles spaced by a distance z, the source and the test. At the 1st impedance, the test undergoes a kick form the 1st particle. Then at the 2nd impedance, the test particle undergoes another kick which only depends on the "relative betatron phase advance between the 2 particles", which did not change and therefore the 2nd kick adds to the 1st. In the past we compared with HEADTAIL the case of the sum of 20 kickers in 1 kick or the 20 kicks with the real phases, and Benoit and Diego came to the conclusions that the same results were obtained (looking at the transverse tunes + rise-times if I remember well => It is on Benoit's PHD thesis) => This was thought to be a justification of what we did in the past (i.e. put all the impedance in 1 kick). May be, as StephaneF said, the phases are peculiar... To be checked...
- Note that also in the case of 1 localized impedance, from one turn to the next the betatron phase advance (or tune) is not peculiar (to avoid resonances etc.) and this effect should already happen. Over many turns one can imagine that there is a smoothing (to be studied in detail for both dipolar and quadrupolar impedances) but what is the situation over few turns? All this has to be clarified.
Info from Stephane:
- Assume two purely dipolar impedance spaced by pi/2 in phase. When the drive particle arrives at a peak amplitude at the first impedance, it has zero amplitude at the second and therefore drive a wake only at the first impedance. When the drive particle arrive at a zero amplitude at the first impedance, it has max amplitude at the second one and therefore drive a wake at the second location only. Therefore it is easy to see that the witness particle sees an effective kick of 1 [per turn (and not 2 if the impedances would be spaced by pi or 2pi). Then the beta functions are just weight nothing more. So I definitely thing we gain, at least for frequencies higher than the revolution frequency (i.e. for single turn effect). Then the impedance sources that we generally focus on are objects with specific phase advance in between, e.g. kicker (perhaps pi in the SPS???) or collimator (more closer to pi/2 in the LHC!). So if a factor of 2 can be gained or sqrt(2) for impedances randomly distributed in betatron phase.
- In my previous e-mail (see above) I compared two sources of impedance spaced by pi/2 or by a integer number of pi. I then conclude that in the first case the kick given to by the witness particle was around 1 for each turn (in arbitrary units), and 2 in the other case. On the other hand I completely forgot that for the other case, there are also some turns where the kick is 0 when the drive particle arrives with zero offset at the first and therefore also at the second impedance. So I do no longer know. I think a simple 2 particles analytic model should be easy to be done in order to conclude. Perhaps for dipolar impedance, the averaging vs turn makes that we are at the end not sensitive on the phase between impedances (at least if the rise time is not a few turns). On the contrary, perhaps for quadrupolar impedance (where the kick depend both on the drive and test particle offsets), this averaging gives on the contrary different results depending on the distribution in phase of the sources of impedance?? So again, I think this is interesting enough (and the potential gain very attractive for machine design) to be carefully understood.
- Vacuum observations in the PS with the new WS in SS68 => To be followed up. See pictures from RendeS: picture 1 and 2 (correlation between beam in the machine and outgassing).
- u_ICE queue for simulations => For the next 6 weeks we have 5 times more processors (500 instead of 100)! Many thanks FS and Bernd Panzer-Steindel (IT/DI)!
4) Coherent tune shift measurements at PS injection and extraction (Sandra Aumon): ppt
- Sandra presented the motivations for this study (some high-intensity beams remain unstable at injection; what is the PS impedance after the introduction of the MTE kickers etc.?; PS 2 GeV upgrade, etc.) and showed some measurements of instability at PS injection with the LHC and AD beams. It seems that it is the usual head-tail instability of high-order mode which is not stabilized anymore by linear coupling when the intensity is increased too much. Action3 (Sandra and MD team): Optimize the working point and linear coupling on the long injection flat bottom to have minimum (no) transverse emittance growth and beam stability (as linear coupling was optimized in the past for the ~ nominal LHC beam!)
- The tune shift measurements were done with the BBQ (note that in the past, see below, the measurements were not done with this system).
- Applying the classical Sacherer formula (the j should be removed from the formula as it is expressed with the imaginary part of the impedance) to the measured transverse coherent tune shift vs. intensity, the imaginary part of the transverse effective impedance can be deduced.
- At injection (1.4 GeV kinetic energy), this leads to ~ 3.5 MOhm/m in H and ~ 12 MOhm/m in V.
- At top energy (26 GeV/c), this leads to ~ 1 MOhm/m in H and ~ 6.8 MOhm/m in V. These values are different from injection because (i) the space charge contribution is different, (2) the bunch length is also very different (factor ~ 3), meaning that a different frequency range is sampled, (3) other reason(s)... => The measurement at injection and top energy should be done with the same bunch length (to sample the same frequency range). Action4: Sandra (and MD team) should try and redo these measurements with the same bunch length at injection and top energy. Note that a strange mechanism is observed as the horizontal tune (mainly) is decreasing with time by a large amount (in the example shown: ~ -0.01 in ~ 100 ms!). This is followed up by Bernard Vandorpe from OP, and it seems that it is independent of energy.
- Sandra then estimated the coherent tune shift due to the space charge at injection and extraction => It seems that ~ 1/4 of the coherent tune shift measured at injection is coming from the coherent space charge coming from the images (at top energy it is almost 0). Comment: don't forget also the incoherent space charge contribution coming from the images (as the impedance is proportional to the difference between the coherent tune shift and the incoherent one). Action5: Remove the incoherent space charge contribution from the images.
- These values seem quite different from measurements performed in the past by R. Cappi et al. (1993) and EM et al. (2001) => ~ 1 MOhm/m in H and ~ 3 MOhm/m in V => To be followed up.
- Action6 (EM): Answer to the question from SandraA => What is the effect of closing or opening the relay gaps on the PS 40 and 80 MHz RF cavities?
5) LHC BLM (Humberto Maury Cuna and Octavio Dominguez Sanchez de la Blanca): ppt or pdf
=> POSTPONED TO NEXT MEETING.
6) LHC BBQ (TP and BS): ppt or pdf
=> POSTPONED TO NEXT MEETING.
7) Preliminary investigation on SPS space charge effects (Giuliano Franchetti): pdf
- After the SPSU meeting etc., Giuliano discussed mainly with BS and HannesB (who want to understand the transverse emittance growth of the SPS with high intensity bunches) and he started to have a look to possible space charge effects in the SPS.
- Giuliano showed first the possible SPS resonances on the tune diagram with the nominal working point for LHC beams (Qx = 26.13 and Qy =26.18) and then the harmonics excited by the SPS optics (taking the FFT on the betatron motion) => Mainly harmonics 108 (which is the periodicity of the beta functions) but also 52.
- Reminder: In the SPS, there are 18 (FODO) cells per superperiod and 6 superperiods (sextants) in total, i.e. there is 108 cells in total. A typical SPS cell is composed of the following elements: QF-MBA-MBA-MBB-MBB-QD-MBB-MBB-MBA-MBA.
- Qx = 26, Qy = 26 and Qx = Qy can be excited by space charge.
- Giuliano made a first analysis with a coasting beam with peak intensity doing self consistent simulations with a 2D PIC SC code. Fixing the horizontal tune to 26.13 and scanning the horizontal one from 26.0 to 26.2, a nice picture is obtained with a region of vertical emittance growth (between ~ 26 and ~ 26.1) and a region of emittance exchange (between ~ 26.1 and ~ 26.2).
- Looking at the transverse phase space (after 1500 turns), 2 islands clearly appear (was not so clear during the meeting :) but then Giuliano made better plots where this is clear!) in the vertical plane when the vertical tune is increased: when the vertical tune is too low, the islands are too far and cannot be populated. Then there is a region where these islands are populated (see example picture in the slides, with Qy = 26.038). Finally, when the vertical plane is too high the islands are also populated but are already where the particles are, so the distortion in phase space is not clearly seen. However, there is still some emittance growth => The excitation of the 2nd order structure resonance might play a role for a high intensity bunched beam (Periodic crossing of a space charge structure resonance. to be checked).
- To avoid this, the working point should be below 26. Note that this is the case with the new low gammat optics (of Yannis and Hannes) where the tune is around 20.
- Reminder:
- CS = Courant-Snyder in his slides.
- Giuliano made 60 simulations for this study and each simulation lasted ~ 1.5 day on 1 processor.
8) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action1 (for HDWG): Effect of Q" on the head-tail instability of the LHC.
- Action2 (for HDWG): Addition of 2 localized impedances (effect of the transverse betatron phase between the 2).
- Action3 (Sandra and MD team): Optimize the working point and linear coupling on the long injection flat bottom to have minimum (no) transverse emittance growth and beam stability (as linear coupling was optimized in the past for the ~ nominal LHC beam!).
- Action4 (Sandra and MD team): Try and redo these measurements with the same bunch length at injection and top energy.
- Action5 (Sandra): Remove the incoherent space charge contribution from the images.
- Action6 (EM): Answer to the question from SandraA => What is the effect of closing or opening the relay gaps on the PS 40 and 80 MHz RF cavities?
9) Miscellaneous
- The space charge working group led by Frank Schmidt should start somewhere in April.
- The next (19th) meeting will take place on 02/03/2010 => Agenda:
1) BE-IT meetings and outcome (FS)
2) LHC BLM (Humberto Maury Cuna and Octavio Dominguez Sanchez de la Blanca)
3) LHC BBQ (Tatiana Pieloni and Benoit Salvant)
4) Look at the list of Actions.
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 23/02/2011.
