------------------------------------------------------------------ Minutes of the ABP-RLC team meeting of 07.04.2006 present: AG, EM, WH, TP, FR, GR, RT, Tom Kroyer, Heiko Damerau excused: DS and FZ are in Turin for the defense of EB's PhD thesis web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------------ (1) Minutes of last meeting, pending actions: see the following --------------------------------------------------------------- (2) Progress report on US-LARP Beam-Beam studies (WH) ----------------------------------------------------- Working with M. Furmann, Ji Qiang (LBL) is using his strong-strong beam-beam simulation code based on the concept of displaced Green's function to study the emittance growth in case of beam-beam interactions with a static offset at the central interaction point. Collisions in IR 1 and 5 are considered. He studied 3 cases: head-on collisions with either horizontal or vertical offset, and a third case with head-on as well as lumped long range interactions. He evaluated beam-beam offsets between 0.1 and 0.4 sigma. In his preliminary results for the head-on cases he finds emittance growth in the plane of the offset which increase with increased separation. Both beams behave similarly. At present, it is studied whether this growth is real. In case it is confirmed, it is important to minimize the offset of the two beams at collision. The case with long range interactions is less clear and further studies are presently done to understand the results. FR: Is this emittance growth an effect of numerical noise? Similar simulations run by S. Krishnagopal showed no such effect. WH: The fact that the same behavior is observed for both beams (which have different random initializations) suggests that the effect is not purely numerical. FR: What are the practical implications for the LHC? WH: The offsets must be compensated as much as possible (tolerable values could be < 0.2 sigma), because as the separation gets to values close to 0.4 sigma the emittance increase is significant. The offset of PACMAN bunches with compensated orbit effects by alternating H/V crossing planes is about 0.1 sigma, but without compensation it easily gets to ~0.5 sigma. FR: Do they correct beam-beam offsets at the TEVATRON? WH: They don't have PACMAN bunches and have many other problems before coming to the emittance growth caused by offsets. (3) Increasing the TMCI threshold in the SPS by linear coupling --------------------------------------------------------------- EM reported interesting HEADTAIL simulations about a possible increase of the TMCI intensity threshold at injection in the SPS by linear coupling. Working near the coupling resonance should allow an increase of the intensity threshold by 36%. The simulations assume a broad-band impedance of 20 MOhm/m at 1 GHz with Q=1, and a single bunch with longitudinal emittance of 0.2 eVs (instead of 0.3 eVs for the nominal LHC beam). Using that: 1) thresholds in the horizontal and vertical planes are equal in a round geometry 2) threshold in the vertical plane is half of the one in the horizontal plane in a flat geometry (and it is 12/pi^2 the one of a round geometry), one can think of introducing linear coupling between the transverse planes in order to increase the threshold in the vertical plane for a flat geometry. Simulations have been done with HEADTAIL using the parameters of a single bunch with low longitudinal emittance and Qx=26.180, Qy=26.185, and an integrated skew quad strength K2=0.005 m^-1 (ideally the tunes should be related by Qx+DeltaQx,imp = Qy+DeltaQy,imp). The simulated threshold in the vertical plane goes from 3.3x10^10 without coupling to 4.5x10^10 with coupling. The increase is by 36%, that is comparable to the 48% current increase required to go from the LHC nominal intensity to the ultimate intensity. Linear coupling would also help for the e-cloud instability because, since the e-cloud is mainly located in the dipoles, it causes a vertical single-bunch instability. This could allow running the SPS with lower chromaticity, which is beneficial for the beam life time. Introducing the ratio between vertical and horizontal effective impedances lambda=Zy/Zx, the gain given by linear coupling can be written as: gain=2 lambda/(lambda+1). This formula gives 39% in the case simulated with HEADTAIL, which is very close to the 36% given by simulations. FR: Do results depend on the tune split? EM: Yes, because he also tried to improve the vertical threshold using linear coupling and Qx=26.18 Qy=26.13 and this did not work. AG: The same effect as linear coupling could be achieved by introducing in the beam pipe an object with a negative vertical impedance. This will of course enhance the horizontal impedance but the overall situation for the beam improves because the total impedance gets reduced in the vertical plane, which is the critical plane (it has the lowest TMCI threshold). FR: Tests in the SPS should be done soon! This will require a change of working point. (4) Report about FP420 meeting (EM) ----------------------------------- FP420 is a new experiment not yet approved, located at 420 m from IP1 and IP5, which requires Roman Pots or a 8 m long movable pipe, 1-sided, copper, very close to the beam (3 mm). Message given from the impedance point of view: experience from collimators shows that one should be very careful introducing such objects, because they basically contribute to all degrading effects: resistive wall, geometrical effects, trapped modes, high contributions to the broad-band impedance. Roughly adding the moving pipe would be like adding 5-10 collimators, which would put the coherent tunes far off the stability region. (5) Follow-up of TCTV and TCLI collimator impedance (EM + AG & FR) ------------------------------------------------------------------ There are 4 TCTH + 4 TCTV (2 of them in a 2-beam region) and 2 TCLI (1 in a 1-beam region and the other in a 2-beam region). The three devices in 2-beam regions give a quite high contribution to the broad-band impedance of the machine. Trapped modes are to be checked. The criterion for these objects is that they should contribute to the broad band impedance by no more than 1/100 of the full impedance budget (that means they should not exceed ~0.02 MOhm/m). This translates into a gap of 12 mm for the TCTVs for nominal beta (~70 m). But squeezing in IP2 or IP8, beta goes to ~660 m, therefore the effective impedance would be multiplied by a factor 10, which would require a gap of 30 mm to still meet the requirement on the broad band impedance. Actually both these gap values are too high because, given their position, with gaps > 8 mm the TCTVs become useless to protect the triplet magnets. FR points out that one should consider also the beam intensity when doing these estimations. The TCLIB should also have a gap of 12 mm following the same criterion as above. This is compatible with the protection of the arc. ==> ACTION: EM will put all contributions to the broad band impedance in the stability diagram both at injection and at top energy to see where we are actually standing with all the objects so far examined. Concerning the TCDS functional specifications, AG gave FR a paper copy of the material shown at the RLC team meeting of 10.06.2005. Many of the options discussed and investigated are not mentioned in the functional specification and it would be desirable to add a single reference to the results of AG. ==> ACTION: AG will write a short note on the TCDS impedance estimates that can be referenced in the Functional Specifications. FR informs the team that the impedance of the copper coated injection septa in IR2 and IR8 (MSIA and MSIB, made out of mu-metal, 4 m long magnets, in groups of 5) and of the dump septa in IR6 (15 MSD, 4 m long magnets) should be re-evaluated, even though it should be negligible according to the LHC Design Report. After the meeting Stephane Fartoukh informed FR and EM that there are several transverse steps of the vacuum chamber in correspondence with these septa. ==> ACTION: EM will follow-up the impedance of LHC septa FR, AG, and Ralph Assmann discussed the collimators RF fingers. Apparently many of them come out deformed or broken from production and should therefore be cut, but a number is needed on how many could be cut without affecting too much the impedance of the collimators. AG says it mainly depends on the length of the fingers, but in principle no more than 1-2 in a row should be missing to avoid a significant increase of the coupling to the collimator tank. After the meeting FR sent the following email to the LHC collimation team in connection with RF fingers with bad/missing contacts: Dear Ralph, I summarize below the conclusions of the LHC impedance team for the collimator RF fingers (~5 mm long, ~2 mm wide with ~1 mm spacing): - all RF fingers are functionally important and should not be considered as an optional esthetic ornament of the collimators... - we may exceptionally accept ONE missing finger or maybe TWO provided they are not consecutive. With two or more missing fingers in a row, the beam would "see" the outer tank and excite low-frequency modes very dangerous for the transverse beam stability. Missing "longitudinal" RF fingers would lead to potentially large heat deposition in the collimator tank. - the contact force of all RF fingers (both longitudinal and transverse) should be around 50 g/finger. Apparently this is not included in the functional specifications of the collimators, but is assumed to be the result of a correct mechanical assembly. - since the collimators will be activated in the LHC, we better fix this problem now, before installation in the machine. Of course I hope that the problem can be solved quickly. (6) Follow-up of CARE-HHH CERN-GSI meeting ------------------------------------------ FR shortly reports about the recent bilateral GSI-CERN meeting of last week at GSI-Darmstadt (attended by FR, EM, GR, FZ, Gianluigi Arduini, and Elena Shaposhnikova). Many subjects were discussed ranging from incoherent effects due to electron cloud (emittance growth induced by periodic resonance crossing) to cross-check of different analytical formulae for the resistive wall impedance to TMCI and benchmarking of different codes for collective effects including space charge and impedances. All the talks are available on the web pages http://care-hhh.web.cern.ch/CARE-HHH/Collective%20Effects-GSI-March-2006/ http://www-linux.gsi.de/~boine/CARE/HHH-meeting.htm Summaries and conclusions of the two-days discussion will be available soon. (7) AOB ------- FR has created at web page linked to the RLC web site with a list of references on Accelerator Physics. This list is far from complete and is intended mainly for future students who may need basic (online) references to start working in the RLC team. See http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ap-literature.htm Posted on the web: Slides by EM Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/