Minutes of the ABP-LCE team meeting on 19.12.03 present: GA, JJ, EM, FR, DS, FZ ---------------------------------------------------------------------- (1) Minutes & Pending Actions ----------------------------- OLD ACTION -> FR will contact LV concerning factor 4 Pi. Done. LV has been contacted by email, but no response so far (LV is thought to be in Australia). ACTION -> ZBASE progress & examples Pending. Problems were reported in previous meeting. Tutorial by O. Bruning had been suggested. ACTION -> FR will check the Zbase afs access rights FR will grant access rights today. ACTION -> CB head-tail growth rates for updated LHC impedance (FR) Pending. Requires response to JPK. ACTION -> AK will check CB rise time for Roman-Pot impedance. Pending, as AK was on leave. ACTION -> Stability Analysis for TOTEM Beam (EM) Done. See EM's presentation below. ACTION -> Study McGinnis paper and impedance of of Schottky monitor for LHC (EV) Done. EV's email should be attached, if available. ACTION -> Organize ABP-RF-BDI meeting on beam-beam diagnostics with feedback (EV). This idea is to be re-discussed due to some concern about who should be contacted. (2) News: Impedance, CERN Planning and BNL Workshop (FR) ----------------------------------------------- B. Spataro has new results on the LHC B recombination chamber. He will send them to FR. FR reported on new perspectives for R&D at CERN based on material from the SCP and the new DG. The LINAC4 may be constructed by 2007, and experienced staff is considered to be required for LHC upgrade studies. The topic of the BNL workshop was pressure rise in rings, which includes the electron cloud. RHIC observes an electrn cloud in the straight sections. They attempted a scrubbing run similar to the SPS. The vacuum pressure of 1e-6 Torr was too high, and scattered particles burnt the BPMs. The BPM electronics will be taken out of the tunnel. It is not excluded that there may be an alectron cloud inside the dipoles as well, but the present RHIC diagnostics (pressure gauges, and cryogenics) would not be sensitive enough to detect it. JMM urged RHIC to install more diagnostics. An outcome of the workshop is that the role of residual-gas ions needs to be revised. They could be important in two ways, by (1) creating secondary electrons and desorbing molecules at the wall, and (2) ions desorbed from the wall could trap electrons. The ionization cross section is high for the low-energy electrons of the cloud. Beam ions that are lost to the wall could generate a large number of electrons. FZ pointed out that the ion generation, losses and sputtering were studied in 2001 for the SPS and the LHC. The density of ions and their effect on the electrons or the beam were found to be completely negligible. The results were published in: G. Rumolo, F. Zimmermann, Interplay of Ionization and Sputtering with the Electron Cloud, CERN-SL-2001-014 AP, http://doc.cern.ch/archive/electronic/cern/preprints/sl/sl-2001-014.pdf At BNL Ubaldo Irizo is active, simulating electron cloud build up in RHIC for large bunch spacings. L. Wang presented excellent results, but his presentation was difficult to follow. He is now attending an Englich course. FR remarked that the main focus of the next LHC MAC in July 2004 will be the electron cloud. The date is chosen to have the maximum number of results. The Colorado company Tek-X is writing modules for eelectron cloud simulations, which will simplify comparisons of various codes. (3) BPM Impedance (FZ) ----------------------------------------------- FZ had a meeting with RJ. The number of and types of BPMs between BDI and MAD are still not fully consistent (10% discrepancy). ACTION -> The optics team will be informed by FZ. Various formulae for the impedance of a shallow cavity exist (L. Vos, Palumbo/Zobov). The Palumbo/Zobov expression for a tapered chamber gves the opposite sign to that for a shallow cavity with steep edges. ACTION -> FZ will contact M.Zobov. DS mentioned that he could simulate the cavity with Gdfidl (ACTION?). FR pointed out that M. Dyachkov had a web site with an online program for computing this type of impedances. ACTION -> FZ find web site. (4) Electron Cloud Simulations (DS, FZ) ---------------------------------------- In FZ's simulations for the LHC at injection, the ECLOUD results strongly depend on which geometry is chosen for the vacuum chamber (circle flattened in the vertical direction or ellipse). The differences are particularly large for dipoles and drifts. Neither description is correct. DS has developed a new routine which can be called once at the start of program execution, and which provides for the correct boundary conditions. In the 'corners' of the beam screen the field is 4 times larger than at the outward wall. The cells he uses are of 100 micron size. In preliminary simulations, the routine changes the simulated heat load by a factor of 2. The same routine still needs to be implemented for the electron space charge. FR recommended to plot the equipotential lines rather than the field lines. FZ presented a series of simulations for the SPS WAMPAC1 and WAMPAC3 calorimeters, looking at 25-ns and 75 ns bunch spacing. He computed electron line density, energy spectrum, flux at the wall for electrons above 30 eV, and heat loads, considering the paddage of 4 batches. For WAMPAC1 simulations show no multiapcting with 25-ns spacing, but strong multipacting with 75-ns spacing. This is opposite to the observation, and might be related not only to the sapcing but also to the emittances and bunch lengths assumed (simulations were done for 2003 parameters, while this measurement refers to 2002). For WAMPAC3 with smaller radius the agreement is better. The simulated absolute heat loads for 25 ns spacing may agree the meaurements. here, the simulation seems to show a lower heat load at 75-ns spacing than what was observed (measurements by V. Baglin show a factor 2 difference between the two spacings). FR suggested that all information (emittance, bunch length, heat load, intensity) should be stored together on the same file. GA will find out the exact date of the 2002 measurement. DS will present the electron cloud simulations at the LTC. FR recommended not to show too many pictures. DS showed simulations which reveal a clear threshold of electron cloud build up, at about N~1.5e10. The threshold is the same for goood and poor vacuum conditions. A surprising result is that even below this threshold the simulated line density saturates at a level which hardly changes with pressure, indicating that also here multiapcting is the dominant process. DS has improved the simulation for a rectangular chamber (121 image charges instead of 2 formerly). This results in a larger electron cloud avtivity with a magnetic field, in accordance with observations. (5) Automatic Collimator Impedance Calculations (EM) ----------------------------------------------------- EM considers all collimators, using version v26top6n1b1 for IR 7 and June version for IR3, and in addition the effect of resistive wall, and broad-band impedance. The beam is stable at 40% of the nominal intensity. When the bunch spacing is increased the coherent tune shifts move almost parallel to the vertical axis, which means the actual tune shift stays constant, but the instability rise time decreases. This remains true even for a single bunch, and is a consequence of the inductive bypass. For the nominal LHC even a single bunch lies outside of the stability diagram. (6) Stability of TOTEM beam (EM) --------------------------------- The TOTEM calculations follow the lines indicated above. If the collimator gaps are decreased, the real part of the coherent tune shift becomes large, and the beam can end up far away from the stable region, which is not thought to be healthy. EM presented combinations of intensities, emittances, and gaps which would ensure stability. If the gaps should refer to the nominal rms beam size or to the beam size at each emittance value was unclear. ACTION -> Clarify gap requirements for TOTEM and definition of sigma (EM,FR?) Attached: Slides by EM.