Minutes of the ICE section
38th meeting on Wednesday 14/09/2011 (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), Elias Metral (EM), Elena Wildner (EW), Frank Schmidt (FS), Giovanni Iadarola (GI), Giovanni Rumolo (GR), Jean-Luc Nougaret (JLN), Kevin Shing Bruce Li (KL), Nicolo Biancacci (NB), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Tatiana Pieloni (TP), Werner Herr (WH), Xavier Buffat (XB).
Present/Excused: BS, CH, CZ, HD, EB, EM, EW, FS, GI, GR, JLN, KL, NB, NM, OB, TP, WH, XB, Alexey Burov, Alexej Grudiev, Elena Shaposhnikova, Fritz Caspers, Hannes Bartosik, Raymond Wasef, Rama Calaga.
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 36-37th meeting + Actions
- Check the 2 actions of the last meeting!
3) General infos
- SL meeting:
- Main discussion these days concerned the fellows. There was a special BEMB meeting yesterday on fellows.
- New rules in the process of hiring new people at CERN => A maximum of 4 people per board will be convened.
- News on the LHC:
- Aperture confirmed at beta* = 1 m: there is enough aperture for 1 m and a 1/2 crossing angle of 120 microrad with ~ 2 sigmas additional margin.
- Reached the peak lumi of ~ 3E33 consistent with beta* = 1 m and transverse emittances slightly above 2 microm. No beam dynamics issues anymore.
- New peak lumi record this morning: 3.3E33 with ~ 1.35E11 p/b!
- 2 issues:
- ALICE vacuum => ALICE cannot switch on in stable beams. No significant conditioning observed. The highest pressures are observed near the ZDC (special vacuum chamber, no NEG and no solenoid).
- IR8 vacuum which happened after the TDI's parking positions (in both IR2 and IR8) were increased from +- 20 mm to +- 40 mm on Saturday. Note that the vacuum is already present during injection and does not seem to depend on intensity. Note also that in IR2 it does not seem to have created some more losses etc., maybe a redistribution. Do we expect more problems with larger or smaller gaps? Being followed up by the impedance team and in particular AlexejG.
- Talk by HannesB at yesterday's LSWG (Instabilities observed in the LHC ecloud MD 26th August 2011), analysing some ADT PU data of the last 25 ns MD => ADT on (seems to be dominated by high-frequency vertical oscillations, which could be consistent with the fast vertical instability from ecloud) and ADT off (seems to be dominated by low-frequency resistive-wall).
- Talk by WH at yesterday's LSWG (Results of recent beam-beam studies).
- There will be a report by RobertoL at this afternoon's LMC proposing to have new collimators with higher resistivity (due to time constraint etc.).
- Discussion just after the meeting with ElenaS and AlexeyB: We should study with HEADTAIL the head-tail and TMCI instabilities of the LHC with a double harmonic RF (still without space charge first) => We will add a 2nd RF voltage from 800 MHz RF cavities equal to half of the RF voltage from the 400 MHz RF cavities and study both regimes of bunch lengthening and bunch shortening. Comment: Look also at an old paper from S. Myers which showed that the threshold goes down + recent IPAC11 paper from Y.H. Chin. Action 1: HDWG.
- TDI8R observations since the last Technical Stop with added thermocouples (from Vincent Baglin, not discussed in the meeting) => Will/should be discussed with VincentB in the ICE meeting to be held on 28/09/11.
- Beam induced heating on LHC beam screens - Arcs, DSs and ITs (from Laurent Tavian, not discussed in the meeting) => Will/should be discussed with VincentB in the ICE meeting to be held on 28/09/11.
4) Collimator Design With BPM - TCTP (BS): pptx
- Main changes from current design (from impedance point of view):
- Taper => Larger (18 deg tapering angle instead of 11 deg).
- Wish to suppress the contact RF fingers (but adding ferrite) as the design is easier from the mechanical point of view.
- Preliminary simulations for the vertical imaginary impedance in the case of a half gap of 2 mm:
- ~ 20-30% higher low-frequency broad-band impedance compared to phase 1 (installed in the LHC) => Installing ferrite will/should not change this as it is purely geomtric.
- Significant low-frequency resonances due to the absence of RF contacts => Installing proper ferrite at the proper place should help removing these resonances.
- Next steps: Check these results, compare with the resistive-wall contribution and give a recommendation to the collimator design's team => Followed-up by the Impedance team.
5) Past predictions for Longitudinal Loss of Landau damping and Longitudinal Mode Coupling and comparison with the recent results from Alexey Burov (EM): pdf
- EM reviewed the past predictions (reminding some old analyses/results from Sacherer, 1972) and compared them to the recent results from AlexeyB, confirming that qualitatively the same results seem to be obtained but sometimes (depending on the distribution) with a huge numerical factor, traced back to the fact that the rigid-bunch approximation was used in the past (Sacherer's) approach.
- The general quantitative results for the old (rigid-bunch) analysis and for an inductive impedance above transition (or capacitive impedance below transition) are the following: The HP (Hofmann-Pedersen) distribution is better than the Gaussian one by ~ 0.7 / 0.5 ~ 40%, which is better than the smooth Sacherer's distribution by ~ 0.5 / 0.3 ~ 70%, which is better than the circle approximation by Sacherer by ~ 0.3 / 0.25 ~ 20%.
- Qualitatively, the HP distribution is the best (as found by AlexeyB) because we discuss the particles in the centre of the distribution (as we consider the case of an inductive impedance above transition or space charge below) => It is indeed very efficient to flatten the distribution in the centre to increase Landau damping. However, for the case of an inductive impedance below transition or space charge above, it would be the opposite as in this case, due to the sharp edge of the HP distribution, no Landau damping is provided and smooth (long) tails are preferred in this case.
- On page 13, a simple formula is given for the ratio of the shift of the incoherent small-amplitude synchrotron frequency at the intensity threshold over the low-intensity small-amplitude synchrotron frequency (deltawsith / ws0), which depends only the RF harmonic number and the bunching factor.
- Numerical applications for the case of a HP distribution:
- LHC at top energy:
- deltawsith / ws0 ~ - 6.3% (AlexeyB found ~ -2.2%, i.e. ~ 3 times less).
- Nbth ~ 1.9E12 p/b (AlexeyB found ~ 5.7E11 p/b, i.e. ~ 3 times less).
- With the circle Sacherer's approximation (which I usual use), Nbth ~ 6.7E11 p/b.
- Tevatron at injection energy (150 GeV):
- deltawsith / ws0 ~ - 25%.
- Nbth ~ 5.8E13 p/b.
- Tevatron at top energy (980 GeV):
- deltawsith / ws0 ~ - 9%.
- Nbth ~ 4.5E12 p/b.
- Reminder: The measured intensity threshold is ~ 2E11 p/b, which could be explained only by the new theory of AlexeyB, taking into account the coalescing structure of the bunch.
- Comparison between below and above transition (for an inductive impedance):
- HP:
- Me (and Besnier etc.): Always unstable.
- AlexeyB: Same result (at least very very small as it is a numerical result and depends on grid point etc.)
=> Similar result between the 2 approaches.
- Smooth:
- Me (and Sacherer etc.): Same AT and BT.
- AlexeyB: Almost the same (~ 30% higher BT)
=> Similar result between the 2 approaches => I would say then that the important point is the rigid-approximation and maybe not so much the potential well (to explain the difference with usual theories)
- Comparison with LHC measurements by ElenaS: See ICE meeting (01/09/10) + IPAC11 paper => Seems close to usual theories, leading to a Zl / n close to the theoretical value (~ 0.1 j Ω, including the resistive part of the collimators) => To be followed up in more detail (exact shape of the distribution etc. as it is very sensitive).
- In the past a simple model for the LMCI was compared to HEADTAIL both below and above transition in the case of a single bunch with SPS parameters at injection, interacting with a broad-band impedance:
- The HEADTAIL intensity threshold was a factor ~ 2 higher than predicted with the simple formula.
- The Zl / n deduced from the slope in the bunch lengthening regime (with another simple formula) led to a value of ~ j 8.4 Ohms whereas j 10 Ohms were introduced in HEADTAIL, which seems again not that bad.
- It was checked that BL the intensity threshold is higher (by a factor ~ 1.5 found with HEADTAIL compared to a factor ~ 1.7 predicted with the simple formula).
- Next step => Add longitudinal space charge and compare HEADTAIL with the simple formula => HD (ongoing).
- AlexeyB obtained the following results with his new approach:
- ~ j 5.4 Ohms.
- LLD intensity thresholds:
- AT: ~ 1.1E10 p/b, i.e. ~ 10 times lower than the intensity threshold from LMCI.
-BT: ~ a factor 8 higher, meaning that in this case, the LLD intensity threshold should be close to the LMCI one => In this case the mode-coupling analysis should be included in AlexeyBs calculation as it is not negligible anymore!
- To be followed up.
6) May resistive-wall drive longitudinal coupled-bunch instability? (Alexey Burov): pdf
- AlexeyB checked (in a simplistic rigid-bunch approximation), that the couple-bunch resistive wall wake is too week to drive longitudinal couple-bunch instability (LCBI) for any practical case. EM remarked that it is a "well-known result" that the resistive-wall impedance leads to a very small LCBI due to the shape of the resistive-wall impedance (important at low frequency) and the fact that the first mode (1) is already at quite high-frequency, and that it can be computed with Sacherer's longitudinal formula => Action 2: EM to compute the LCBI rise-time for the case considered by AlexeyB (or similar) using Sacherer's longitudinal formula.
- Since the high-order cavity modes stay normally far from the resonance with the revolution frequency, and have low-Q values, they can hardly drive LCBI either.
- Thus, if the fundamental harmonic is properly stabilized, there would be no couple bunch growth rate.
7) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1: HDWG to study with HEADTAIL the head-tail and TMCI instabilities of the LHC with a double harmonic RF (still without space charge first) => We will add a 2nd RF voltage from 800 MHz RF cavities equal to half of the RF voltage from the 400 MHz RF cavities and study both regimes of bunch lengthening and bunch shortening. Comment: Look also at an old paper from S. Myers which showed that the threshold goes down + recent IPAC11 paper from Y.H. Chin.
- Action 2: EM to compute the LCBI rise-time for the case considered by AlexeyB (or similar) using Sacherer's longitudinal formula => See May resistive-wall drive longitudinal coupled-bunch instability? (Alexey Burov).
8) Miscellaneous
- The next (39th) meeting will take place on 21/09/2011 => Agenda:
1) Transverse instability in the AD? (Lajos Bojtar),
2) Summary of Fermilab's MDs (FS).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 19/09/2011.