Minutes of the ICE section
34th meeting on Wednesday 03/08/2011 (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, Chandrashekhara Bhat, Elena Shaposhnikova, Gabriel Mueller, Georges Trad, Gerad Tranquille, Raymond Veness, Raymond Wasef, Theodoros Argyropoulos, William Andreazza.
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 33rd meeting + Actions
- No comment.
3) General infos
- No particular comment from anyone.
- SL meeting:
- BEMB:
- There is a new for TSO for the AD machine (B. Lefort).
- Discussion on the new walkie talkie system (Tetra) for access in the LHC. This includes a distress button, so could be a way around the 2 person rule. They cost 600 Euros each. The scheme is not approved yet.
- Reminder on some cost estimates:
- RCS ~ 140 MCHF.
- PSB energy upgrade ~ 55MHCF (including the consolidation participation).
- LINAC4 ~ 100 MCHF (including the building which is ~ 1/3 of the price).
- News on the LHC:
- LHC 08:30 meeting on MO => New peak lumi achieved > 2E33 due to the small transverse emittances (< 2 microm) from the injectors (without any transverse emittance BU from the injectors) => The LHC can keep the very small transverse emittances delivered by the injectors now at their minimum (between 1.5 and 1.7 microm).
- Some issues with MKI2 erratics, vacuum spikes in L8, UFOs, cryo etc.
- Beam-beam studies (from GA): In the last few days they have
started to take some spectra while in collision and during the physics fill. One
of the observations is that during the fill the highest frequency peak is moving
towards the lower frequency peak in the vertical plane and the opposite for the
horizontal plane (see picture): red = measurement at
the beginning of the fill / blue=measurement at the end. Do we understand why?
They have also started to change the tune at the beginning of the fill (before
going in collision).
- => Working point optimization (before going into collision): Dip in lifetime with 0.31/0.32 With 0.308/0.318 => Worse. With 0.312/0.322 => Better (new values used since few days) => See picture1 and picture2.
- NuFact'11: This week CERN and UniGe jointly host the annual workshop NuFact'11 (XIIIth International Workshop on Neutrino Factories, Super beams and Beta beams, http://nufact11.unige.ch/).
- The detailed agenda is on Indico: http://indico.cern.ch/conferenceTimeTable.py?confId=114816#20110801.detailed.
- Our ICE Section is represented by the BetaBeam Team (Elena
W., is the Conveener for Accelerators WG, Elena B. and Christian) and CERN by
many colleagues for: Neutrino Factories (Simone and Gersende, ABP-LIS),
P-Drivers, BetaBeams, CNGS, neutrinos from PS (Rende, OP), LBNO,...
- New results on the PS Montague resonance by Ji Qiang (IMPACT simulations: fully 3D + nonlinear lattice): 4 slides.
- Measurements from RalphS during the 2nd LHC MD block: emittance evolution vs. Qprime with a single-bunch at 3.5 TeV => Nothing seen as the octupole current was ON (as in physics).
- Raymond Wasef presented a status of his studies at the HDWG meeting on Monday => Possible issue between Sacherer's formula and HEADTAIL / MOSES simulations with a broad-band resonator. Raymond studied 5 cases and the imaginary part of the tune shift of mode 1 could be OFF by a factor 10! EM tried and check this and found 40% error at maximum => Sacherer's formula / implementation to be checked.
- BS mentioned that the new mode of acquisition of the Headtail monitor (which can be triggered when the BBQ signal increases above a given threshold) is available (from RalphS) => Can/could be used for future studies.
4) Bench measurements of the PS fast wire scanner impedance (JLN): pptx
- Reminder on EM simulations done in the past by BS and discussed during the ICE meeting held on 11/08/10 => Slides.
- A reference from Chandrashekhara Bhat (many thanks!): A Generalized TRL Algorithm for S-Parameter De-Embedding by P. Colestock and M. Foley.
- JLN explained in detail the (streched) single-wire and two-wire impedance measurements, discussing the setup used, the calibration, the possible reflections (the wire method assumes that the transmission line is matched, but it is not the case and therefore there are reflections) etc. and the different formulae used (with derivations in the Appendix of the lumped and log formulae) to go from the measured S-parameters to the impedances.
- JLN also reminded the limits of these measurements:
- The stretched wire method should not be applied for cavities.
- The wire forms a TEM line allowing for electromagnetic communication between the end plates of the cavity below waveguide cutoff or below the fundamental mode of the unperturbed cavity.
- The presence of the wire reduces the unperturbed Q values of the longitudinal modes massively.
- Instead Q measurement should be done using on axis probes for the longitudinal modes and probe pairs with 180 deg Hybrid for the transverse modes.
=> We should not have used the stretched wire method for the impedance measurements of the PS WS but at least we could check the frequencies of the trapped modes and compare with the simulations.
- On pages 9 (for longitudinal) and 12 (for transverse), the first equation for the impedance comes from VittorioV, while the second is usually used by FritzC (1st order Taylor development).
- We could do first the measurements on a reference tube to have S21REF. In the absence, we assume the usual exponential term for S21REF.
- The determination of the characteristic impedance Z0 was not that precise: ~ 300-400 Ohms from measurements whereas ~ 200 Ohms were predicted from some theory => To be followed up and discussed in particular with FritzC. Seems normal as it is not well defined in a cavity...
- If one compares to the simulations made in the past by BS, it seems that the resonance frequencies are quite close (without the wire scanner in the tank, see page 11): between ~ 0.8 and 1.5 GHz.
- From the simulations there is a mode due to the wire scanner at low frequency (~ 300 MHz) and this is this mode that we need to damp by ferrite => Only 2 blocks should be sufficient, whereas more have been installed for the moment. This has been clarified with our BI colleagues. The same amount of ferrite should be installed in the future new PS WS tanks.
- Next: Some probe measurements could be made after IPAC11 to check in detail the different resonances and the effect of the ferrite blocks.
5) Theory, observations and mitigation of dancing bunches in the Tevatron (Alexey Burov): ppt
- AlexeyB gave the 1st part of this talk. The 2nd part will be given next week.
- AlexeyB reviewed first the observations of the dancing bunches at the Tevatron, at both injection and top energy => Look like non-rigid (i.e. with change in the longitudinal distribution, core and tails oscillating differently) dipolar longitudinal oscillations (i.e. with 1 node when several consecutive traces are superimposed). It is a single-bunch instability, which is damped by there longitudinal damper (which is a bunch-by-bunch phase loop), but which appears when it is OFF.
- According to their impedance model, this instability appears for a very small value of the synchrotron tune shift compare to the synchrotron frequency, i.e. only ~ 1-2%. ElenaS mentioned that if the phase loop is removed in the SPS an instability could appear for values even smaller than the 1-2% quoted by AlexeyB.
- Reminder from ChandraB: there is a coalescing process of 7 bunches into 1 with a bunch rotation etc. => This means that the final distribution is not that smooth.
- AlexeyB reviewed the general ideas of instabilities on a simplified model, in particular the roles of short-rang and long-range wakes and Landau damping.
- In the classical theories (assuming the rigid-bunch approximation), the intensity threshold for the loss of Landau damping depends heavily on the bunch length, with the 5th power (meaning a factor 2^5 = 32 can be obtained from an error of a factor 2 in the bunch length). This means that it is very sensitive to the bunch length, which means also that the longitudinal bunch distribution should be treated with great care as the bunch length is not always clearly defined. What is the effect of the bunch shape?
- AlexeyB then explained his new general analysis, starting from the Boltzmann-Jeans-Vlasov (BJV) equation and looking at the eigen- system of this equation. This was already done by Van Kampen, for an infinite classical plasma, leading to the Van Kampen modes: continuous spectrum + discrete modes. As the sum of the growth rates of all the discrete modes is 0, it means that some of the discrete modes do not decay => This corresponds to the loss of Landau damping.
- AlexeyB stopped at page 12, where he gave all the equations to be solved which was started by Oide-Yokoya, reducing the problem to linear algebra.
6) Actions to be taken for the next meeting
- Old actions.
7) Miscellaneous
- The next (35th) meeting will take place on 10/08/2011 => Agenda:
1) Theory, observations and mitigation of dancing bunches in the Tevatron (AlexeyB): Part II,
2) Impedance of the SLAC phase 2 rotatable collimator (EM, slides and questions from Tom Markiewicz),
3) Follow-up of the HEADTAIL simulation studies of Landau damping through octupoles in the LHC (RaymondW).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 03/08/2011.
