----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 18.11.2005 present: RA, EB, FC, UD, AG, WH, JJ, EM, TP, FR, GR, DS, RT, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, actions, announcements etc. (FR) ------------------------------------------------------------- The team went through the list of actions. Electron-cloud progress was addressed by EB and FZ (see below). The revised list of pending actions is posted at http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Actions/actions.htm. (2) Impedance of tertiary collimators and TLCI's with two beams (RA, FC, AG, EM, FR, ...) --------------------------------------------------------------- The tertiary vertical collimators are located upstream of the D1 dipoles. They are asymmetric. Wake fields are excited by both beams. AG pointed out that the configuration looks similar to that for the TDI, but the distance between collimator and beam is smaller. Issues are trapped modes and resistive wall impedance. The collimator has a sharp edge and a flat top of 1 m. The transition pieces are not yet designed. RLC team input is important and urgent. The distance of bunch arrival time between bunches of the two beams is necessarily smaller than 12.5 ns. => ACTION => Check exact time distance of bunches (RA) => ACTION => literature search for 2-beam wake field effects (EM, FZ) FR recalled a study by Weiren Chou, FZ one by A. Novokhatsky. => ACTION => HFSS simulation (EM, AG), may require 3D drawings, not yet existing => AG will discuss with RA. => ACTION => Attend dedicated collimation meeting on Thursday morning (AG) (3) Follow-up on SLAC design for phase-II rotating collimators (RA, FC, AG, FR, EM,...) -------------------------------------------------------------- A draft design description was sent from SLAC (web link announced in the last RLC meeting of 11.11.2005, see http://www-project.slac.stanford.edu/ilc/larp/). Our response is needed within a few days. In particular, SLAC awaits answers concerning the rf contacts and concerning a list of unsolved issues on grooves, tapering angle, etc. AG presented rough estimates for various impedance effects, namely the broadband impedance of the grooves, trapped monopole modes (rf heating) and dipole modes (transverse stability). The beam sees a sequence of three geometries - circular, square and hourglass shape. The consumable part of the collimator is transversely movable and it can rotate. The rf contacts are assumed to be perfect. AG suggested that the tapering can be chosen as the canonical value of 15 degree. The collimator rolls proper are segmented with 8 grooves for stress release, with dimensions of either gxd=0.5x6 mm or 16x4.1 mm. A rough estimate of the longitudinal impedance is Z/n~100 microOhm per groove, which is small, and Zt~3 MOhm/m per groove, which is significant, for a half gap of 0.5 mm. This estimate suggests that the transverse impedance needs to be checked more carefully. It will be reduced if one considers larger gaps. RA specified that a half gap of 1 mm should be taken for phase-2. FC commented that if one isolates the jaws it would take time to build up image currents, the critical frequency region being below 5 MHz. Therefore, avoiding any rf contacts could be advantageous. Two or three different designs for phase-2 collimators can be pursued according to RA. AG next discussed the trapped modes. Longitudinally an estimate for the TM11 mode gives a mode frequency of 1.8 GHz, and indicates that the rf heating is less than for the phase-1 collimators. The underlying reason is the absence of slots in the transition parts. It was asked whether a similar slot-less design could not be used for the phase-1 collimators as well. FR emphasized that for phase-2 no slots are allowed without re-discussion. The transverse trapped modes have a lower frequency than for phase-1, around 0.4 GHz, and a higher Q value. HFSS simulations would be necessary for precise numbers. An interesting questions is which intensity the LHC could reach if all graphite secondary collimators are replaced by ones made from copper. => ACTION => compute stability limit for copper secondary collimators (EM) => ACTION => A one page recommendation is needed by Wednesday afternoon (FR) (4) Comment on measurement of PS kicker impedance (FC) ------------------------------------------------------ The longitudinal impedance of a PS kicker was measured at various transverse positions using the single-wire method. Interpretation of the results is not straightforward. The note 2002-034 AP by Tsutsui considers a TEM like case, for which a 2-wire measurement yields the dipole impedance and a single-wire measurement the sum of dipole and quadrupole impedance. However, the PS kickers do not correspond to a TEM situation. The mu and epsilon of the ferrite are far from 1, and Tsutsui's theory is not applicable to kicker problems. Tsutsui's simulations should be repeated with proper geometry. Fundamental questions are how to extract the transverse impedance and the quadrupole impedance, and the definition of transverse impedance. HFSS does not simulate a beam. GdfidL could in principle do it, but there are numerical convergence problems. One approach could be to simulate the 1-wire measurement. If the result agrees with the actual measurement, the 2-wire measurement could also be modeled. If there is no agreement, a real 2-wire measurement could be done. AG pointed out that the simulation takes too much time to arrive at a conclusion within two weeks. => ACTION => Draw tentative conclusion for two limiting cases (EM) FC distributed a clarifying email after the meeting. (5) Follow-up on e- Cloud in the SPS (EB) ----------------------------------------- EB discussed simulations of electron-cloud instability with stripes in a dipole field. A movie illustrated how a region of enhanced electron density first forms close to the beam and then moves horizontally outward. A second movie compared the evolution of the central horizontal density for the case of stripes with the one for a uniform distribution. Two puzzles were encountered. The simulations reveal a large effect of the conducting boundary, and the instability does not strengthen monotonically with increasing electron density. The horizontal emittance grows, though the electrons only move vertically. The non-monotony could possibly be explained in analogy to a similar phenomenon for the classical TMCI where modes merge and separate for increasing bunch charge or impedance. Unfortunately, changing the bunch charge in the e-cloud case affects the frequency of the electron motion. EB proposed instead to use the broadband resonator model of the e-cloud instability in order to search for stable regions and non-monotony. EB also presented snapshots of the transverse dipole moment along the bunch for three different average e- densities. At the highest density the dipole motion was absent, consistent with the negligible emittance growth for this case. EB's further plans are to scan the size of the stripe, to increase the chromaticity Q' (it was 1 in the examples shown), and to make predictions for the LHC. DS commented that the e- flux on the wall does not directly correspond to the density distribution inside the chamber, and also that the vertical distribution inside a stripe may be non-uniform. These questions could be explored with ECLOUD simulations. FR asked whether conducting boundaries are a reasonable approximation, given that the chamber is made from stainless steel and the electron motion occurs on a ns time scale. => ACTION => Check adequacy of conducting boundaries (GR) (6) Scaling the e-cloud instability threshold to higher SPS energy (FZ) ----------------------------------------------------------------------- Using simple estimates for the instability threshold, and with a helpful correction by EM, it was found that the e- density threshold scales as (alpha-1/gamma^2)/sqrt(gamma)/sigmaz^(5/2). If the bunch length does not change, at 60 GeV/c, the electron-density instability threshold increases by a factor of almost 2. It is not clear whether this simple scaling law can be confirmed in simulations, which exhibit a complex dependence on cloud density (see the presentation by EB above). FZ also presented a summary of e-cloud observation at the Tevatron. The poor beam lifetime and emittance growth seen there could be caused by the incoherent electron-cloud effect. (7) Intra-beam scattering with nuclear collisions (FZ) ------------------------------------------------------ The transverse rms momentum in the LHC is of order 0.02 GeV/c. The nuclear scattering cross section can be estimated either assuming a Yukawa potential or by extrapolating measured p-p cross sections (from the particle data booklet) to lower energies). In both cases, the beam lifetime due to nuclear p-p scattering in the LHC is estimated to be of the order of 100 hr. This effect should be measurable. A more precise estimate could be made repeating the calculation of Bjorken-Mtingwa for the nuclear scattering amplitude. (8) Follow-up on LHCb crossing schemes (WH) ------------------------------------------- The problematic was explained in an RLC meeting 2 weeks ago. Working scenarios have been specified in the frame of the LHC operations project. The solution will be presented to the LHCb collaboration in two weeks time. Alternative modes of running for the LHCb spectrometer are also under consideration, e.g., -> a vertical crossing plane (i.e. effectively crossing at an angle) would make polarity changes of the spectrometer transparent to operation. Orientation of the beam screens, already installed, would need to be modified for such a solution. (9) AOB ------- FR suggested to check the Touschek scattering rate for ions. Posted on the web: Slides by EB, AG and FZ (the slides will be posted as soon as available and/or when web-server memory problem has been resolved) Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/