----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 10.02.2006 present: AG, WH, EM, TP, FR, GR, RT, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions, report from meetings (FR et al) ------------------------------------------------------------ A draft EDMS functional specification for the TCDQ exists, in which the impedance calculations and contributions from the RLC team are not mentioned. FR asked whether action should be taken, to include these considerations. There was no progress yet on the question of the 2-beam resistive wall wake field. FZ will try to develop a theory a.s.a.p. Concerning the comparison of two SPS working points, a simulation result exists from Giuliano Franchetti. He ran simulations with xi_y=0 and xi_y=-1.25. In neither case did he observe a significant difference between the two working points. The real vertical chromaticity was xi_y~0.5, FZ will ask him for simulations of this case as well. In parallel, EB has re-launched HEADTAIL simulations for both SPS working points. Results are not yet available. EB is also performing HEADTAIL simulations of emittance growth as a function of e-cloud density for the LHC and SPS with a dipole field. A result is expected towards the end of next week, as these simulations will become part of her thesis. FR has arranged for a meeting with L. Tavian on Friday 17.02. at 14:15. The exact questions for L. Tavian still need to be defined. T. Linnecar and M. Tigner have discussed possible Cornell contributions to the LHC upgrade, in particular concerning RF couplers and reduction of HOM heat load in 1.2 GHz cavities for bunch shortening. The transverse collimator impedance was extracted from FC&TK's bench measurements including detuning and driving wake fields. FZ reported from the second meeting of LHC commissioners. Different phases of the commissioning will be detailed. GA and FZ were asked to collect arguments for maintaining a stage of 75-ns spacing 'in the absence of electron cloud'. FR announced that the AB committees will soon be restructured and that the new APC will be merged with the LTC led by Steve Myers. (2) Transverse impedance of the TCTV and possible improvements (AG) ------------------------------------------------------------------- There are TCLIA and TCTV collimators. The second are made from tungsten and have a gap of 3 mm, the former consist of graphite with a gap of 12 mm. AG performed calculations for the TCTV type, since its gap is smaller. However, the TCLIA is used at injection while TCTV at collision. Therefore, TLCIA could also prove important and it needs to be checked afterwards. AG conducted simulations with HFSS for the two beams passing inside and outside the jaws. For the outside beam the impedance is 3 orders of magnitude smaller. The cross coupling impedance (2nd beam on 1st beam) was not computed, but it can roughly be estimated as the square root of the product of the two single-beam impedances. With a 15-degree taper and 3 mm gap the broadband impedance is about 300 kOhm per meter, which amounts to 15% of the total LHC impedance for one of the TCTV collimators. In addition there are various trapped modes, which could drive the beam unstable. AG estimated the magnitude of the CB growth rate for one of these trapped modes at 2e-4 divided by the revolution period. FZ asked how many of these devices exist in the LHC. The answer was found in the following presentation by EM, where two low-gap TCTV's per beam could be identified. AG then studied possible means to reduce the impedance. A 10 degree nonlinear taper (circular arc segment) may reduce the BB impedance, but has little effect on the trapped modes. therefore, damping with ferrite 4S60 is recommended. This requires removal of the rf contact, in order to be able to place to the ferrite outside of the vacuum chamber proper (ferrites cannot be put inside the beam pipe). FR suggested a mixed solution where graphite is added to the metal. Next week the possible remedies will be discussed with the collimation team. FR commented that we are taking a high risk with the LHC, and that different safety margins are applied for different objects. FZ met with Elena Shaposhnikova, who provided old impedance data for the SPS 200 and 800-MHz rf systems. The corresponding effective impedance can be included in fitting current-dependent phase advance measurements for SPS impedance localization. Unfortunately, the only data available are the shunt impedance and Q value for the lowest frequency mode for either rf system. FZ suspected that from this one cannot estimate the broadband impedance of the rf cavities, which would be needed for fitting the single-bunch measurement. His suspicion was confirmed by the RLC team. (3) Coupled-bunch instabilities with the TCTV (EM) -------------------------------------------------- EM independently evaluated the effect of the TCTV trapped modes on beam stability, on the request of AG. Considering only two modes and pessimistically assuming that the resonant frequency of the second mode falls onto a betatron line, EM found that the beam stability is lost, when taking into account that there are two objects of this type in the ring. (A table shown by EM listed the number and gap of all collimators foreseen.) The broadband impedance was not included in this estimate. FR stressed that the impedance of TCTV is unacceptable at the moment, and that this is a high priority issue. (4) Follow-up of the SPS MKE kicker impedance (EM) -------------------------------------------------- In 2006 9 kickers will be installed (4 more than before). The impedance was measured by FC with 1 wire and with 2 wires. The 1-wire method shows an imaginary transverse impedance of about 2 Mohm / m for 1 kicker at low frequencies, which increases towards high frequencies. The transverse impedance is obtained from a 2nd order fit of the measured longitudinal impedance as a function of transverse position. The fit quality is not good. A 6th order fit improves it. The 2-wire method more directly gives the transverse impedance. For this method, it is found to be close to the prediction by Bruno Zotter's theory up to the maximum measured frequencies of ~1 GHz, whereas the Burov-Lebedev theory deviates. The result 2-wire method is more believable since the result is close to expectation and not compromised by poor fits. EM pointed out, however, that the 1-wire method indicates a resonance at 600 MHz, similar to a frequency observed in the SPS instability measurements. EM recommended to extend the 2-wire measurement to 2 GHz. The measurement should also be redone with ceramic insert modifications. (5) Follow-up of the impedance design for the Totem Roman Pots (FR) ------------------------------------------------------------------- The green light has been sent. A final test of the Roman-Pot prototype is planned for June. (6) Follow Up Transverse Collimator Impedance with Driving and Detuning Wake (EM, FZ) --------------------------------------------------------------- EM compared the transverse impedance of the LHC collimator prototype deduced from 1-wire measurements of the prototype with the theoretical prediction, taking into account that the measurement includes contributions from both detuning and driving wake fields. The minima of the measured curves nearly coincide with the theory. FZ made a similar comparison and obtained roughly the same result, though it is not identical. FR expressed bewilderment why the 1-wire measurement seems to work for the LHC collimator, but is highly unreliable for the SPS MKE kickers. (7) Vertical trapped modes of SPS BPMs (EM) ------------------------------------------- There are 108 horizontal and vertical BPMs in the SPS, called BPH and BPV. Bruno Spataro has calculated their impedance with MAFIA. The total shunt impedance amounts to 0.84 MOhm/m. For this impedance alone, EM calculated the threshold of mode coupling at a bunch population of 3.5e14 (more than three orders of magnitude above operating value). Therefore, trapped modes cannot explain this type of instability. Rather, a broadband impedance would be needed. FR suggested that a spread of resonance frequencies between BPMs could give rise to a lower value of the effective Q (the value computed and used was 16000). (8) Rating of different LHC IR upgrade options: first ideas (RT) ---------------------------------------------------------------- FR recommended that RdM joins the RLC meetings in which IR upgrades are discussed. RT presented first ideas and the preliminary web site, which presently contains 4-5 different optics, mostly with dipole first. Aperture requirements and limits on beta* will be set by collision debris, chromaticity, magnet, technology, etc. FR asked to take the same free length to the IP of 19 m for all the optics designs, as had been decided in Arcidosso. FZ pointed out that the optics by Stephane Fartoukh refers to the nominal LHC and not to the upgrade. He also proposed to include two conventional quadrupole first options, one with a common bore, the other with separated magnetic channels. FR pointed out that the aperture constraints need to be clarified, as JPK makes different assumptions, which lead to different conclusions. In principle the transverse position of each triplet quadrupole can be optimized so as to maximize the physical aperture. Parasitic dispersion gives another contribution to the needed aperture. FR and RT plan to launch a discussion on rating with other interested parties. (9) EPAC06 participation and AOB -------------------------------- FR reported that three young members of the RLC team will attend EPAC'06, namely Andrea Latina, Rogelio Tomas, and Giovanni Rumolo. Peter Urschuetz has a question mark, as he may attend LINAC'06. (10) Items for the next meeting ------------------------------- As agenda items for the next meeting are foreseen: - FC reporting on reflectometer measurements of the beam-screen ring periodicity - also FC on measuring the impedance of LHC wire scanners - AG with an update on TCTV - UD on beam-beam and compensation simulations Posted on the web: Slides by AG, EM and RT Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/