----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 17.03.2006 present: AG, EM, WH, TP, FR, GR, RT, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions -------------------------------------------- FZ commented that the SPS debunching measurements of 2002 had been successful. The minutes of the last meeting were already slightly modified by FR. FZ will contact Joachim Tuckmantel and Bruno Zotter shortly, regarding their definition of transverse resonator impedance. A short discussion followed on the meaning of the real part of an eigenvector. OLD ACTION => GR will contact GA and/or Bruno Spataro to clarify whether the high Q's of these modes include the effect of a load. Fritz had questioned the Q value for two reasons: The BPMs are made from stainless steel and not from copper, and the load was not taken into account. GR contacted Bruno Spataro via Gianluigi Arduini. Bruno agrees that the real Q value will be lower than what he had obtained, but he will not recalculate it. There was another remark by Fritz that peaks at 500 MHz and 1 GHz seen in the headtail-monitor signal on the oscilloscope may be due to sweeping in steps of 500 MHz and GA is following-up this possibility. OLD ACTION => EM will collect existing literature on the effect of resistive coatings by FC, check with AG the scaling and the damping required for half of the nominal LHC intensity, and clarify the logics for a correct reply to the IPM team. DONE. See following presentation. (2) IPM Coating (EM) -------------------- EM reported the result of work by FC, AG and himself. FC had proposed applying a resistive coating of 100 Ohm per square on the IPM plates. Jan Koopman informed EM that this value implies too small a thickness. Fortunately, the 100 Ohm per square are not a rigid number, but according to the report CERN/PS/86-20 by Carter and Caspers the attenuation curves are flat and 30 Ohm is also acceptable. Natural aging will increase the resistivity during operation. EM has sent an email to Jan Koopman suggesting an initial resistivity window between 30 and 50 Ohm per square. (3) Follow-up of TCTV collimator impedance (AG, FR) --------------------------------------------------- Dimensions of the vacuum chamber were obtained from Manfred Mayer. FR discussed the design with Miguel Jimenez, according to whom the foreseen chamber dimensions do not exist in the LHC, which hints at a communication problem. On the other hand, the TDI chamber does exist and is called X2ZDC. AG computed the impedance both with closed and open rf contacts for several values of the gap size. The low-frequency "broadband" impedance is about 300 kOhm/m for a single device with the nominal full gap of 3 mm. It is ten times smaller for 12 mm gap, 100 times smaller for a gap of 30 mm, and 1000 times lower for a 60-mm gap. Without rf contacts the contribution of the lowest higher-order mode to the "broadband" impedance Im Z(f=0) is enhanced, by about a factor of two. Above 100 MHz the impedance is reduced when the rf contacts are opened. Below it increases. It therefore is not entirely obvious that opening the rf contacts is advantageous. However, since an opening of the contacts is required to allow for damping by ferrites mounted on the outside, and since the increase of the low-frequency impedance is not one by orders of magnitude, AG still recommends the removal of the rf contacts. The added ferrites will damp all modes. FR pointed out that the number of TCTV devices needs to be clarified with the collimation team, since it is not logical that only one or two collimators are installed although there are 4 IPs. FR reported from the last LTC that a loss of 1e-5 of the beam can destroy the TCTV protection collimators, which thus appear weaker than the triplets they are meant to protect. EM recalled a few numbers: The total LHC impedance of 2.7 MOhm/m without collimations corresponds to a tune shift of 4e-5 at top energy, which is ten times smaller than the tune shift for which Landau damping would be lost. => ACTIONS => Contact Stefano to clarify number of TCTV (EM,AG) Contact M. Mayer to question chamber dimensions (EM,AG) Contact D. Macina to organize meeting with experiments (FR) Inform collimation team that a gap of 12 mm is needed for two devices and 30 mm in case of more than 2 devices (FR) (4) Trapped modes in the LHC collimators (FR) --------------------------------------------- OB requested an update on collimator trapped modes for the LTC. FR responded that collimator trapped modes have been simulated and compared to measurements, with pertinent presentations at the ABP-RLC meetings of 28-Jan-2005, 4-Feb-2005, and 11-Feb-2005. He added that heat loads induced by the LHC beam and RF finger heating have also been computed and the collimation team has been informed. The effect on the LHC beam at 7 TeV has been estimated too: slow multi- bunch instability rise times between 20 and 100 sec are expected, which can be damped by Landau octupoles or by the feedback system. AG added a reference to AB-Note-2005-042, which summarizes the collimator trapped-mode impedance computations. FR emphasized the usefulness of the RLC minutes and meeting web site in responding to this and similar requests. He suggested that also 2003 meetings be added to the web site. => ACTION => Add 2003 minutes and presentations to the RLC web site (FZ). (5) Transverse Resistive Wall impedance for SIS100 at GSI and LHC collimators (EM) ---------------------------------------------------------------------------------- EM had been contacted by Slava Zhabitsky, a former student of Balbekov, who is working on the GSI damper system. Slava noticed a similarity between the Balbekov formula of resistive-wall impedance and the one by Bruno Zotter, which both differed from the 'official' GSI expression, since the latter did not include the inductive bypass effect. EM showed that the Balbekov formula from 1985 is indeed similar to the Zotter formula. A derivation for the latter is now known, but Balbekov stated that his is the 'exact formula'. The thickness of the vacuum chamber wall in the SIS100 ring is only 0.2-0.3 mm. In this thin-wall regime, the Balbekov equation, multiplied by beta, and Bruno Zotter's formalism give identical results for the frequency-dependent impedance over the full frequency range. FR remarked that the magnets outside of the vacuum chamber should influence the impedance at the SIS100. EM also made a comparison for the LHC collimators. In this thick-wall regime, the two formulae (Balbekov's and Zotter's) do no longer agree, but differ by more than a factor 2 at 8 kHz. EM also discussed the official impedance description used at the GSI. One impedance is given in the appendix of a draft paper by Kornilov et al. This does not include an inductive bypass term. In a 4-author paper at ICFA-HB2004, by R. Hasse and coworkers, the transverse impedance does not contain any resistive-wall term, but it includes only the space charge (despite of referring to 'resistive wall' in the title of this paper and in its abstract). Finally, R. Hasse has sent an impedance plot to EM, which seems to raise more question than it answers. The issue can be clarified and details be discussed at the CERN-GSI meeting end of March. (6) Bunch Shortening along SPS Bunch Trains due to Electron Cloud (GR) ---------------------------------------------------------------------- GR analyzed the 2004 coasting bam data. 4 batches were stored at 26 GeV/c for about 15 min. Bunch currents, bunch profiles, emittances (at start, middle and end of each batch), and e-cloud monitor data were recorded. GR demonstrated that the lifetime at the end of a train was much shorter than that for the 1st bunch. In particular, the loss in intensity coincided with a drastic reduction in bunch length, indicating that the off-energy particles are lost. This seems to corroborate the e-cloud induced loss mechanism via tune shift and synchrotron motion which is proposed by G. Franchetti, EB and FZ. Typically the last bunch lost 75% of its intensity within 10 minutes, the first bunch about 25%. GR showed example analyses for coasts 6 and 12. (7) Highlights from RPIA2006 (FZ) --------------------------------- FZ presented highlights from the RPIA2006 workshop, which included KEK induction synchrotron, RHIC superbunch upgrade, FNAL barrier rf systems, RIKEN RI facilities and Nishina Accelerator Center, all-ion accelerator, DAHRT-2, high-gradient insulators, fast kickers, and bio-macromolecule and GeV accelerators. Of particular interest was working group 3, which addressed the LHC wire lens from the current pulse-power technology point of view. With the help of E. Cook of LLNL and K. Torikai from KEK two technical solutions were developed on the blackboard. Issues to be checked are transmission line effect, radiation hardness of MOSFETs and SiC MOSFETs, and reference timing stability. It is considered to build a prototype pulser at CERN, which could be shipped and installed at RHIC before the 2008 run. Posted on the web: Slides by EM, AG (soon), GR, and FZ Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/