------------------------------------------------------------ Minutes of the ABP-RLC team meeting of 26.09.2006 present: RA, OB, Chiara Bracco, UD, WH, EM, Flemming Pedersen, Milorad Popovic, GR, FZ excused: FR web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ 1. TMCI Observations in AD (Flemming Pedersen) ---------------------------------------------- Flemming reviewed observations from the 1999 AD commissioning. Specifically, he described the various modes of AD operation, the original and revised commissioning plans, AD ring and beam parameters, instability threshold and instability observations. Lastly he addressed whether the TMCI studies in AD could be repeated. There were three operation modes: protons via the loop with normal magnet polarity, protons directly from the target area with reverse magnet polarity, and normal antiproton operation. The intensity of the antiprotons was low, which caused problems with beam diagnostics. This was one of the motivations for using protons in the commissioning. However, with improved diagnostics from 2000 onwards most setting up was done in the antiproton mode. Direct protons had a large intensity, but suffered a dramatic emittance growth due to 40-m passage through air. Another complication was that the magnetic field did not reproduce when it was inverted. The best mode for early commissioning was protons via the loop. The intensity was higher than in the two other modes, but limited by TMC instabilities. The beam could be stabilized by increasing the momentum spread. However, this prevented tune measurements at low energy. Even after 1.5 years, it was not possible to decelerate protons via the loop down to 100 MeV/c. Shielding of RF and BPMs in 1999/2000 finally allowed measuring the tunes and orbits with the pbars. The rf shielding improved the signal/noise of the diagnostics signals by about 50 dB. Protons via the loop were last used in 2003. The loop and one injection kicker module were decommissioned since then. The Z/n of the ring was estimated to be 1600 Ohm. The transverse emittance was 2.5-3 pi mm mrad below the TMCI threshold and 7-9 pi mm mrad above the threshold. For an emittance of 0.4-0.5 pi mm mrad in the PS, the emittance in the AD was expected to be 3.5 pi mm mrad, due to a 0.1-mm Al window. A controlled blow up in the PS increased the longitudinal emittance from 0.12 to 0.3 eVs, for which the expected transverse emittances were achieved even at high intensity. A fixed high-frequency betatron line was monitored as a function of time. The instability and fast blow up occurred about 100-200 ms after injection. Noisy coherent signals were observed in conjunction with a depletion of the transverse beam density at the center of the bunch. The filamentation signal and the transverse blow up disappeared for larger longitudinal emittances. Normally the instability did not lead to beam loss, but when a scraper was inserted, reducing the aperture, significant beam intensity was lost in a fraction of a synchrotron period, which cannot be explained by classical head-tail modes, but which would be consistent with a TMC instability. Flemming explained that the large transverse impedance and the low synchrotron frequency may have conspired to yield a low TMCI threshold. EM asked whether changing the chromaticity had an effect on the instability. Flemming responded that the chromaticity is controlled by sextupole components integrated in the quadrupole fields, and that the chromaticity, therefore, cannot be varied. By design the ring chromaticity is close to 0. EM then asked for the space charge tune spread. Flemming replied that he does not recall the number, but that this could easily be computed from the parameters given. Frank remarked that the observations appear well compatible with a TMCI like phenomena, in particular the stabilizing effect of the longitudinal emittance. After the meeting FZ added that the fast growth resembles a special case of TMCI known as "fast transverse blow up" [PhD thesis of Ron Ruth, for Isabelle - a proton machine] in which many modes are involved simultaneously, or the even faster transverse beam break up [as in a linac] Milorad wanted to know whether the AD tunes are the same or split. Flemming's reply was that the integer part of the tune is 4 in both transverse planes. Flemming suggested that if one were to repeat this experiment today, he would attempt to use the stochastic cooling system for exciting and measuring the HT modes. EM proposed to simulate the AD instability with the HEADTAIL code. EM asked whether there exist pictures of the evolution along the bunch. A 1-1.6 GHz pick is in fact available, but no such pictures were taken. Flemming continued his presentation by showing the loss as a function of scraper position, from which the beam profile after blow up could be reconstructed. A depletion of the core density and higher density at larger amplitude is apparent. Since 1999 a lot of hardware modifications took place making it difficult to repeat these past experiments now. ACTION: Repeat AD Experiment in Simulations (GR) Frank commented that it may be as difficult to "prove" the TMCI nature of the instability in simulations as it is in reality. Milorad remarked that one could follow the mode frequencies as a function of intensity. GR confirmed that it is not easy to resolve the mode frequencies for the number of turns typically simulated. OB suggested to apply techniques like NAFF or SUSSIX. FZ expressed certain doubts that the basic assumptions underlying these techniques (single harmonic particle motion in a Hamiltonian system ) would make them suitable for the analysis of growing or damping collective oscillation modes. OB referred to controversial discussions on TMCI in hadron machines at a Montreux workshop. FZ pointed to the VLHC TMCI paper of Burov and Danilov, who clearly considered the existence of such instability for protons, and studied its sensitivity to the chamber geometry. EM commented that some results in the Burov-Danilov paper are not correct. 2. Reports from meetings ------------------------- OB reported from ABMB, which had a short discussion on the US-LARP initiative. FP7 will be addressed next Monday. OB also participated in PAF, where Roland Garoby expressed concern about the organization of FP7. OB explained that the proposals submitted for FP7 will be developed at CERN by Jean-Pierre Delahaye and approved by the AB management, and not by CARE or ESGARD. FZ thought Roy Aleksan to be rather knowledgeable about, and experienced in, drafting successful bids for Bruxelles. RA wanted to know whether the collimator proposal was included in the drafts which had been presented by Roy Aleksan at the CERN CARE meeting. It is. Concerning the LARP dicussion, OB collected information from RA, WH and FZ. Milorad Popovic expressed the intellectual interest of LARP in contributing to the LHC injection chain and its future upgrades. He stressed that LARP should be used in a bilateral way for mutual profit. OB remarked that LARP should be LHC related. In the slides which he prepared based on the above input, he covers e- cloud (LBNL), long-range beam-beam (BNL), beam-beam, IR layout (FNAL), and optics (R. Calaga). GR reported from the APC, which addressed the LHC beam in the SPS and PS. It was again remarked that the beam is seen to be unstable on the first turn in the SPS. Whether this instability is a single or coupled bunch instability in the PS has still not been decided. Raw data of the damper pick up should provide bunch-by-bunch information. The PS working point and chromaticity are being studied. The chromaticity becomes negative during the cycle (but only for a short period of time). All PS matrices need to be remeasured. Yannis is involved in this activity. Replying to a question by FZ, EM explained that the bunches are shorter than before since 1st September 2004, at which time the PS activated a longitudinal CB feedback system. A new PS set up (this Thursday) aims at reproducing the pre-Sept 2004 conditions. EM pointed out again that the instability observed could be an e-cloud instability, as the bunch length prior to rotation is now close to the value where an instability was found in 2001. 3. Collimator MD programe (RA) ------------------------------ RA requested support from RLC for the upcoming SPS collimator MDs. Federico Roncarolo [representing FP420] already participated in preliminary discussions. The MD is scheduled in 1 month. The experimental verification of the inductive bypass effect is the key objective and should be worked out in detail. The MD covers 2 times 24 hr, at high and low intensity. RA stated that the baseline energy of the MD is 270 GeV. FZ had earlier advocated a lower energy where the real part of the impedance would show up more prominently. He also recalled his 2004 study which strongly suggested that the 270-GeV experiment is unlikely to measure the imaginary tune shift induced by the collimator (see http://ab-div.web.cern.ch/ab-div/Meetings/APC/2004/apc040402/zimmermann.pdf) as well as the actual 2004 experiment, which indeed failed to do so. It was suggested that GR runs simulations with HEADTAIL for different energies. ACTION: Simulate effect of SPS collimator on growth rate vs. chromaticity for different energies [GR] RA explained that the experimental studies are an essential part of Chiara's thesis, and he encouraged everybody to support her. Next week Monday the MD proposal will be discussed. The programme will be finalized in three weeks' time. One goal of the MD is to quantify aging effects, and to confirm that the measurements from 2004 can be reproduced. 4. Follow-up on TDI Aperture (EM) --------------------------------- EM confirmed with RA that the aperture of the TDI at injection (6.8 sigma) is nearly the same as that of the secondary collimators (6.7 sigma). There is no problem with such configuration. 5. Follow-up RW impedance of LHC collimator (EM) ------------------------------------------------ EM presented a new result from Rainer Hasse on the LHC collimator RW impedance. This time the correct value of gamma was used. The data are noisy, presumably due to numerical problems. The thickness is only 0.1 mm. The result can be compared with the corresponding prediction of the Zotter theory. There are still the same 4 orders of magnitude difference as before. OB pointed out that not even the thick-wall limits are the same. 6. RF Fingers for TCDQ, TCDI and TDI (EM) ----------------------------------------- EM contacted AG and Brennan Goddard. W/o rf fingers the TCDQ impedance amounts to about 15% of total impedance budget, if beta functions are taken into account. We recommend the installation of rf fingers in this device. The TCDS has a smaller impedance thanks to its larger gap. It is ok w/o fingers. The TDI BB impedance can be reduced by roughly 20-30% of the total impedance budget, if rf fingers are used. Therefore, rf fingers are recommended. EM wanted to meet with Oliver Aberle on the following day in order to discuss the technical implementation. 7. A.O.B. --------- FZ passed around a prototype enamel-coated beam pipe, fabricated for Fritz Caspers by a German company. This appears to be the first step towards a technologically clean e-cloud clearing electrode. FZ mentioned that he had sent an email with questionnaire to Tom Roberts at FNAL, concerning the g4beamlines code, but received no response so far. EM reported that in 1978 Riege developed an automatic correction every 3 hours for reducing the numbers of protons scattering off the septum, which has since been forgotten, though it could be of interest today. OB suggested that EM talk about this with Simon. Posted on the web: Slides by Flemming Pedersen & EM Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/