----------------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 08.10.04 present: EB, WH, JJ, EM, TP, FR, DS, EV, FZ ----------------------------------------------------------------------- (1) Beam-Beam Simulation Plans (WH) ---------------------------------------------- WH's presentation addressed tune scans for dynamic aperture with beam-beam, beam-beam with linear imperfections, and an assessment of different crossing schemes. Other studies are underway for coherent beam-beam effects and 3-dimensional strong-strong simulations, but were not discussed in this talk. The tune scans for the dynamic aperture with beam-beam are done in collaboration with D. Kaltchev, considering head-on and long-range interactions, without linear or nonlinear errors, but including collimator apertures. D. Kaltchev will present results of these studies during his next visit to CERN. For beam-beam with linear imperfections, field and alignment errors are included. Only information available in the control room is used in the simulation, e.g., MAD matching is not employed and the ideal model is considered as a basis for tuning and corrections. Only chromaticity and tunes are adjusted with global knobs, since the result is identical to the knob tuning of the control room. Several changes to MAD-X were necessary for these studies, as, e.g., the installation of BB elements wiped out all previously defined errors, and a special treatment of correctors caused additional problems. In consequence, the treatment of MAD tables was generalized, now allowing for a fast read-in of errors and for a read-in of orbit corrector settings. The status of the linear-imperfection study is as follows: a suitable "private version" of MAD-X exists, only a few knobs are missing, and the coupling correction will be implemented by adapting the module of S. Fartoukh. The study of crossing schemes compares HH, HV, and VV crossing, comparing both nominal and PACMAN bunches. Different integer tunes will be investigated. The study uses V. 6.4 and the 2 main IPs. Tracking is done with SIXTRACK. In the longer term this simulation shall be combined with that of linear imperfections and corrections. A draft note with D. Kaltchev exists. Input files for the imperfection studies are available. The new MAD-X version will become fully operational with the help of Eric McIntosh and FS. (2) Follow-Up on MKE Kicker Impedance with Different Ferrite (EM) ----------------------------------------------------- Following up on a comment by F. Caspers that the ferrite used in the MKE kickers is of type 8C11 and not 4A4 as previously assumed, EM has reviewed and recalculated the transverse kicker impedance. In LHC Project Note 234 H. Tsutsui computed the transverse impedance for 4A4 ferrite. Between 0 and 1.7 GHz the imaginary part decreased from 0.4 MOhm/m to 0.2 MOhm/m, the real part increased from 0 to 0.2 MOhm/m. The longitudinal impedance has a different behavior and a much lower resonance frequency. In CERN-AB-2003-088 (ABP) L. Vos computed the effective imaginary impedance of an MKE kicker as 1.25 MOhm/m, also for MKE. the number seems about 3 times higher than Tsutsui's. The impedance of L. Vos is inductive until about 0.6 GHz (much lower value than in Tsutsui's model). EM and A. Burov computed the kicker impedance recently, considering a horizontally infinite width and also assumed 4A4 ferrite material They obtained an imaginary effective impedance of 0.6 MOhm/m. The impedance is well represented by a broadband resonator with a resonance frequency of 1.7 GHz. For 8C11 ferrite the imaginary part of the effective impedance is also about 0.6 MOhm/m. The real component of the magnetic permeability, mu', is 5 times higher than for the 4A4 kicker. Data for mu' and mu'' data are available only up to 1.8 GHz. FR commented that higher frequencies are of somewhat academic interest, since SPS bunch spectra for mode m=0 do not extend beyond ~0.5 GHz. However higher order mode spectra extend to higher frequencies and m=2-3 may be relevant for the transverse mode coupling instability. ACTION -> EM will check with BT experts and/or with FC whether 4A4 ferrite data is available at higher frequencies. Otherwise some analytic model will be used to extrapolate the available data to higher frequencies. (3) Emittance Growth by Scattering off the Residual Gas (FZ,JJ) -------------------------------------------------------------- FZ reviewed the effects of the residual gas on the beam: beam lifetime and local losses are determined by inelastic nuclear interactions. The emittance growth is caused by multiple Coulomb scattering. The LHC design vacuum pressure is specified to correspond to a beam lifetime of 100 h. One can thus calculate, for different gas species, the corresponding molecule density and pressure, and, from this, the related emittance doubling time, which depending on the species amounts to a value of a few hours to a few tens of hours at injection energy. These numbers were published in the LHC design report, and recently verified independently by B. Jeanneret. The emittance growth time scales with the square of the energy and the inverse beta function. The effect should be much larger at injection into the SPS. The expected emittance growth time can be estimated from the measured gas pressure and gas composition. The plans for the future include an improved understanding of the limitations imposed by the cryogenic system, an estimate of the effect of elastic nuclear interactions on the beam emittance, and a calculation of beam-halo formation in the LHC based on the formalism of Lebedev and Nagaitsev. Intermediate results may be presented at the HHH-2004 workshop. ACTION: Estimate emittance growth time and beam lifetime from gas scattering in the SPS (FZ?) JJ presented some chapters of the Mathematica notebook computing luminosity lifetimes, which was developed by him in collaboration with Amy Nicholson. This notebook also computes emittance doubling times and beam lifetimes due to gas scattering. JJ uses the formulae by Barashenkov for the nuclear cross section. Assuming the gas densities calculated by A. Rossi and N. Hilleret for the LHC interaction regions, the emittance growth time for protons is much longer than 100 hours. FR commented that the values for the interaction region are necessarily optimistic, and much lower than the densities promised for the arcs. Assuming the same densities as FZ, the beam lifetime and emittance growth times calculated by JJ appeared comparable. (4) Effect of CNGS Extraction Kicker (EV) ----------------------------------------- EV presented results from an SPS MD of 23.09. where he studied the effect of feedback on the 2nd CNGS batch after extracting the first batch. The second batch is excited by the rising and falling edge as well as the flat-top ripple of the extraction kicker pulse. Without feedback orbit oscillations above 0.5 mm result. With feedback the oscillations are damped within about 150 turns. The second batch is extracted about 2000 turns after the first. The remaining problem are proton losses for the first and last bunches. Regardless, the MD result was successful and showed that the kicker ripple has been improved. (5) Electron-Cloud Simulations with HEADTAIL (EB) ------------------------------------------------- Following up on an earlier suggestion by EM to study the possibility of a regular head-tail instability, EB performed HEADTAIL simulations with Q'=2 and Q'=0. Both above the TMCI threshold and below it, the emittance growth is almost unchanged for the two chromaticity settings. This suggests that the regular head-tail instability is not responsible for this emittance growth. EM suggested to perform the same simulations for a BB resonator model. EB showed simulations below TMCI threshold for different numbers of interaction points per turn, up to 100. There is no clear convergence of the number. The result in the horizontal plane seems to oscillate between two levels. FR, DS, and FZ noted that changing the number of interaction points is equivalent to changing the effective tune plus the electron density. ACTION (EB): Plot results as a function of effective phase advance Perform tune scan with a constant electron density Simulate the situation of the recent SPS experiments (6) Future LCE Work on LHC and SPS Impedance Database (FR) ---------------------------------------------------------- EV has offered his help and exchanged emails with FR. However, the man power remains critical. DS is 90% working for CLIC. A. Grudiev will help us to a certain extent. Until the arrival of a new staff, a reorganization of the work is required for the next several months. FR asked whether the impedance of the instrumentation is correctly represented by a canonical number Z/n~1mOhm from L. Vos and whether all instruments are taken into account. FZ suggested that with DS a new user-friendly database program suiting our needs might quickly be developed. JJ suggested that Mathematica would be the ideal tool for such a database. DS and FR brainstormed that a statistical calculation may be needed in many cases, e.g., for varying collimator positions or other uncertainties, and that the future database should facilitate such descriptions. FR also stressed that the database is not our primary goal, but a main objective of the team is to collect all hardware data relevant to the LHC (and SPS) impedance, and then to construct impedance models, and make beam stability estimates. Attached: Slides by EB, WH, JJ, EM, FZ