Minutes of the ICE section
7th meeting on Wednesday 08/09/2010 (08:40-10:30, 6/2-004)
ICE members: Benoit Salvant (BS), Christian Hansen (CH), Carlo Zannini (CZ), Diego Quatraro (DQ), 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, DQ, HD, EB, EHM, EM, EW, FS, GR, JLN, KL, MCAP, NB, NM, OB, TP, WH + Roderik Bruce + Theodoros Argyropoulos + Alexej Grudiev + Fritz Caspers + Valeri Lebedev (from Fermilab) + Chandrashekhara Bhat (from Fermilab) + Helmut Burkhardt + Cedric Hernalsteens + 1 person I forgot...
1) Newcomers / visitors
- Valeri Lebedev (from Fermilab) who will give a talk today.
2) Comments on the minutes of the 6th meeting + Actions
- Comments: NM mentioned that his slides added to the previous minutes but not discussed contain an error as the magneto-resistance effect was not taken into account at high energy => They will be updated soon.
3) General infos
- No particular comment from anyone.
- SL meeting last week:
- Discussions on the Finance committee meeting held on 25/08/10, which have been summarized in the 2 emails sent by the DG on 25-26/08/10:
- The Finance Committee held an extra meeting on 25/08/10 to discuss CERNs medium-term plan (MTP) for the period 2011-2015.
- At its June meeting, Council asked to make cost cutting proposals to be discussed by Finance Committee on 25/08/10.
- The strategy has been to protect the LHC programme, and just as importantly to protect the long-term well being of CERN and its personnel.
- Under the new plan, Linac4, for example, will be connected in 2016, one year later than originally planned.
- There will be no running of the CERN accelerators in 2012, allowing us to make savings on running costs while at the same time diverting manpower to consolidation of the accelerator chain and preparation of the LHC for full energy running starting in 2013.
- The MTP has been designed to position CERN as a strong bidder for any future energy frontier project in particle physics. The MTP therefore allows for continuing R&D on both CLIC and a higher-energy proton machine, but at a slower pace.
- Council recognized in 2007 that measures were necessary to return the CERN pension fund to financial equilibrium. This MTP earmarks funds for CERNs social security systems, allowing us to begin the long-term sustainable recapitalization of the pension fund, while at the same time balancing the budget of the health insurance scheme.
- Our proposals were well received by delegations, who overwhelmingly recommend the revised MTP for approval to Council at its meeting on 16 September.
- An accident occurred in France about one week ago, during which a person died. Reminder on the importance to make accident reports as, in the end, only statistics and casualties are looked at.
- The BIOCELL training will become compulsory to access the LHC as of December 6th, 2010. An official email will be sent.
- Reminder of the infos I sent:
- Electrical safety training to be followed/updated => Online for our section (non electricians).
- Course on formal software testing methods with exam and certification
- Forthcoming TSC/Deadline: Sept 8th.
- Secure coding courses in October.
- News on the LHC:
- With the goal to move to bunch trains with 150 ns spacing, some decisions have to be taken for the SPS batch spacing and the LHC crossing angles at injection and top energy for the different IRs. After discussion with Werner it was decided to keep the 1st bunch of each SPS batch at the same place as for the nominal 25 ns beam, i.e. 400 5ns RF bucket in the RF MMI. For the second topics several discussions took place (as the aperture can be an issue, the collimation also as well as the orbit and beam-beam...) and this is followed up by Werner.
- Giovanni made a short report about the latest MD results (https://espace.cern.ch/be-dep/MSWG/2010%20MD%20reports/MDreport_06-09-10.pdf) and in particular the one on ecloud studies in the SPS with 50 ns and 25 ns bunch spacings.
4) Transverse impedance localization (NB): ppt
- Reminder: This is a local analysis instead of the usual global ones to try and identify the main "generalized impedance" (i.e. dipolar + quadrupolar) contributors.
- Assumptions made in the detecting algorithm: (1) Small (~ 0.01) tune shift, (2) Linear tune shift with intensity and (3) Local impedances not coupled.
- The term deltakj(s) / deltaNb (i.e. variation of focusing strength due to impedance j at position s with intensity) needs to be computed looking to the phase advance beating. Note that a pseudo-inversion of the matrix linking the betatron phase advances and the variations of focusing strengths is needed.
- Nicolo reviewed the past results using only the defocusing quads or all the quads. The latter case was already found in the past to improve the results (more elements for the fitting) and therefore one should not put this constraint (to use only the QDs) in the reconstruction.
- New objectives:
- Let the code work again (mainly informatic issues).
- Implement thin lenses scheme in place of quadrupoles (R.Tomas' idea) => Only used in the reconstruction, to treat independently the horizontal and vertical planes.
- Simple simulations of single impedance or quadrupole kick.
- Multi-kick simulations.
- Present results with the case of a single injection kicker MKPA.11936 (with Tsutsui model) and issues:
- Peaks at the beginning and at the end => HDTL code was carefully analyzed and no kick at beginning was found until now. Large error bars suggest a non linear behaviour in phase advance with intensity.
- Nonlinearity in phase advance with intensity => A Sin(Nb)-like behaviour makes the linear fit intensity-dependent... Working with low intensity only does not always solve the problem.... Finally the reason was found yesterday: Oscillation is made shorter taking a higher number of turns in the HEADTAIL simulations: 1024 turns much better than 512 in the present case but the correct number of turns to be used is impedance-dependent. Also peaks at Beg/End seem to disappear.
- Peaks splitting => Was observe and this is due to the betatron function oscillation. Normalizing the response matrix by the betatron function can solve that. In the case showed, no peak splitting appeared near a QF (where there is a large betax, which enhances the signal) but peak splitting was observed near a QD (where the betax is small).
- Response matrix => There is a 90deg phase advance between the BPMs almost everywhere but not everywhere: some cases with ~ 180 deg and some with ~ 260 deg and the jumps introduce some discontinuities. A derived analytical formula enables us to better study the response matrix:
- In the case of 90deg phase advance, in the three possible cases of the kick location (before the 2 BPMs, between the 2 BPMs and between the 2 BPMs) a signal is anticipated.
- On the contrary, in the case of 180deg phase advance, only when the kick is between the 2 BPMs, a signal is foreseen, which makes it easier to identify the kick location => To be followed up.
5) Tevatron luminosity evolution model and its application to the LHC (Valeri Lebedev from Fermilab): pdf
- Valeri described the model which was developed in the past for the Tevatron and which is used in operation since 2006 (since end of August 2006 the model is applied for analysis of each store and results are available on the web).
- The luminosity evolution in Tevatron is driven by:
- Single and multiple intrabeam scattering (IBS)
- Elastic and non-elastic scattering on the residual gas
- Elastic and non-elastic scattering on counter-rotating beam
- RF noise
- Transverse noise (E or B field noise, quad motion, etc.)
- Beam-beam effects
- The model is based on a set of ODE (Ordinary Differential Equations). The details of the evolution for the longitudinal distribution come from a parameterization of the solution of the integro-differential equation describing single and multiple IBS.
=> It includes many diffusion mechanisms:
1) Gas scattering, leading to emittance growth due to multiplt Coulomb scattering.
2) Nuclear scattering => Single scattering: The cross sections are well known but not the vacuum, which has to be measured with beam (it is not sufficient to read the gauges!). The latter can create tails due to EM (ElectroMagnetic) scattering. This can be measured in the machine but one needs a very good resolution for the BCT (1E-3) and the beam should be debunched (IBS is then negligible, there is no issue with the machine protection etc.). This was done in the Tevatron and published at EPAC2006. After quite a large scraping of a usually Gaussian distribution, the distribution becomes rectangular and the tails start to repopulate due to all the diffusion mechanisms => This is due to the normal diffusion (EM). But, the far tails come from the single scattering and this is what they were looking for. For the Tevatron, the EM noise is less than 20% and the rest is dominated by gas scattering. In the LHC, to be seen, but according to Valeri gas scattering should not exceed ~ 10% and most probably EM noise will exceed gas scattering.
3) IBS => Transverse and longitudinal, where the particle loss due to the large synchrotron amplitudes is also taken into account and is a very important effect.
4) EM heating due to the transverse damper (but no hump). However, if the damper is running at full gain it should completely suppress the hump. Valeri is working with W. Hofle on a digital filter for the feedback system. Valeri stressed that it is extremely important to reduce the noise. A factor ~ 2 was already done during his stay. Another factor 2 is needed and will be gained by improving the digital filter.
5) RF noise => An IPAC2010 paper describe the RF noise for the LHC, but according to Valeri the RF noise needs to be re-measured as it is overestimated: according to the paper the RF noise would be ~ 30 times larger than in the Tevatron.
=> THE MODEL DOES NOT INCLUDE BEAM-BEAM!
- But it helps understanding the beam-beam effects as usually when the beam-beam effects are small then the measured and predicted luminosity curves are identical. When they are not, the difference is said to come from beam-beam effects. Note that beam-beam effects are studied off-line (by S. Valishev) with beam-beam simulations using the code LIFETRACK.
- IMPORTANT REMARK FROM VALERI SINCE MANY YEARS: It is very important to keep the "noise" (in the generalized sense, i.e. defined as the sum of all the diffusion mechanisms described above) in the beam-beam simulations as many observations can only be explained by the interplay between the noise and beam-beam effects. Without IBS, beam-beam effects are almost irrelevant => The beam-beam effects leading to the loss of the large synchrotron amplitudes are IBS driven. Do we do this at CERN? To be discussed with Werner and Tatiana, and it could also be discussed at the next meeting devoted to beam-beam predictions and first measurements in the LHC.
- Reminder: There are HO (Head-On) and LR in the Tevatron.
- According to Valeri, the situation with the LR (Long Range) interactions is more critical at Tevatron as only one ring is used there and one cannot for instance even hope for a wire-compensation scheme as these interactions take place everywhere.
- Valeri showed the cases of 2 stores: 4859, where the model agrees very well and 4581, where measurements are worse than the predictions from the model. The difference comes from beam-beam effects and this was solved during the shutdown by fixing the 2nd order chromaticity (which make the betatron functions change with the synchrotron motion), whose interplay with beam-beam led to beam losses. FS said that at the moment in the LHC the 2nd order chromaticity is larger than predicted by a factor ~ 2. Note however that in the future it will be dominated by the triplets. It can also be seen that on store 4859 the bunch increases as predicted by IBS, but on store 4581 the bunch length cannot => Particles are lost by the interplay between IBS (pushing the particles to large synchrotron amplitudes) and beam-beam leading to beam losses => Tail clipping.
- For the model parameters, described in page 6:
- The RF noise is on the synchrotron frequency and Fritz commented that we could also have amplitude noise (from the amplifiers chain). Valeri mentioned that for the Tevatron it was not important.
- Burning 40% of the pbars is a very good result.
- The amplification factor for IBS is the factor to go from the smooth lattice computations to the real one.
- Using this model and some of the LHC parameters, Valeri obtained and very good agreement for the fill 1303, only adjusting the bunch lengths to best fit the luminosity and intensity curves => The bunch lengths were found to be close to 10 cm (but the model is very sensitive to this parameter), which is close to what is measured in operation and said to be almost constant:
- According to Valeri, the decrease in bunch intensity is mainly due to a loss (from the RF buckets) of the large synchrotron amplitudes, due to the fact that the bunch length is too long => His advice would be to reduce the bunch length.
- The fact that the transverse emittances increase is due to the noise on the betatron sidebands, and this is why the noise has to be reduced. Note that according to OP (and Mike) the values expected by Valeri are close to measurements and the values mentioned by Valeri in his talk on page 8 are coming from the BSRT which cannot be trusted.
- A total beam-beam tune shift of ~ 0.015 was used by Valeri, corresponding to 4 HO collisions (4 * 0.0035 = 0.014) but in reality one should use only 3 HO collisions, leading to a total beam-beam tune shift of 0.0105.
- Reminder:
- In the Tevatron, the beam-beam tune shift is 0.025, i.e. much larger but they are running in a regime where the betafunction (=28 cm) < rms bunch length (=40 cm initial and ~ 60 later).
- In the Tevatron, there is a very small difference between the transverse tunes (~ 3-5E-3), as it was found that there is more space near the diagonal (as in many machines, e.g. also in the case with space charge dominated machines). However, in the LHC there is ~ 0.01, but the working point was mainly chosen due to nonlinearities considerations + the fact that there was a very large coupling at some point etc. (to be checked).
- In the Tevatron, they do not run with a transverse damper. They use chromaticity to stabilize the beam.
- Valeri proposed a scenario for the LHC with small beta*, which according to him it the way to go instead of keeping the current large beta*, adjusting the machine for many bunches and then decreasing the number of bunches to adjust the small beta* and then increasing again the number of bunches...:
- Reminder: beta*(Tevatron) = 35 cm whereas beta*(LHC) = 350 cm, i.e. 10 times more! Furthermore, from the optics point of view we are in a much better situation in the LHC as we have 2 rings. In the Tevatron, it is much more complicated as there is only 1 ring and all the things are coupled.
- A beta* of 0.8 m looks a good goal for him => A factor more than 4 would then be gained like that and it would allow to have the same luminosity with less than half intensity per bunch and, consequently, lower total intensity.
- Improvements of the damper noise by about a factor of 3 (in amplitude) should leave the IBS as the major source of emittance growth => It should be the high priority item for next few months.
- Finally, note that a paper was recently published in PRSTAB by R. Bruce et al. on the "Time evolution of the luminosity of colliding heavy-ions beams in BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider" (http://link.aps.org/doi/10.1103/PhysRevSTAB.13.091001), which we could also discuss at some point => 2 different calculation methods are compared:
- The 1st is a simulation based on multiparticle tracking taking into account collisions, intrabeam scattering, radiation damping, and synchrotron and betatron motion.
- In the 2nd, faster, method, a system of ordinary differential equations with terms describing the corresponding effects on emittances and bunch populations is solved numerically.
- Results of the tracking method agree very well with the RHIC data. With the faster method, significant discrepancies are found since the losses of particles diffusing out of the rf bucket due to intrabeam scattering are not modeled accurately enough.
6) Actions to be taken for the next meeting
- Old actions.
7) Miscellaneous
- The next (8th) meeting will take place on 15/09/2010 => Agenda:
1) Review of some beam-beam predictions for LHC (TP)
2) Beam-beam tune shifts measurements in the LHC (Emanuele Laface)
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 10/09/2010.