----------------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 01.10.04 present: EB, EM, WH, TP, FR, EV, FZ ----------------------------------------------------------------------- (1) Follow Up on Pending Actions and News (FR) ---------------------------------------------- FR informed the LCE team that the ABP group will be restructured in 2005. The number of news posts in ABP may be much less than previously expected. Existing senior staff is assumed to contribute to CARE and EUROTeV. PAST ACTION: EM will check the effect of the lattice on the Montague resonance. EM contacted I. Hofmann, who shared EM's opinion that the PS lattice is sufficiently smooth that a constant focusing approximation is valid for space-charge effects. Nevertheless, the GSI group is performing new simulations which will test this assumption. Concerning the dynamical effect of the resonance I. Hofmann believes that longitudinal mixing alone does not suffice to reproduce the PS measurements, and that an additional ingredient, perhaps intrabeam scattering, may be needed to achieve agreement with observations. FR mentiond that during Massimo Giovannozzi's AB seminar presentation, Steve Myers asked whether we can simulate space charge effects on the island shape for a multi-turn injection scheme. Presently no good simulation tool exists at CERN. FZ recommended exploring this question with the ORBIT code, which has been benchmarked on the LANL PSR and on the PS and which can read a MAD input file. EM suggested collaborating with GSI. EM announced that a new intensity record was reached in the SPS with 5.6e13. FR asked for the reason behind this intensity record, e.g., whether it is related to the increase of linac intensity, and what the present limitations are. One reason is that the injection energy of the PS has been increased from 1 GeV to 1.4 GeV. The SPS intensity may be limited longitudinally. EM informed the team that studies with EB et al. on the TMCI threshold with space charge make good progress. (2) MKE kicker impedance and heating (EM) ----------------------------------------- EM explained that earlier calculations of the kicker impedance performed with A. Burov were invalid, since both the ferrite material and the geometry were incorrect. A revised calculation is being worked on. FR emphasized that earlier computations of the longitudinal kicker impedance likely also were wrong. He asked whether there exists a simple relation between longitudinal and transverse impedance. EM answered that the present scheme of computing exhibits no obvious relation. FR stressed that coating of the kicker is not possible due to high voltages which could lead to sparking. FZ mentioned that all SNS kickers are coated and suspected that the SNS extraction kickers operate with similar voltage. EM discussed the amplitude and power spectra for various distributions following a request by J. Uythoven. The power spectrum shows little qualitative difference between a Gaussian and other distributions. If the full length of other distributions is chosen as 4 sigma (where sigma is the standard deviation of the Gaussian), the heating of the kicker varies by up to 50%. FZ pointed out that if the width of the distribution function would be adjusted to match the shape of the beam core in each case, the variation of the power would be much lower. An alternative approach would be to interpolate the measured beam power frequency distribution by splines and integrate numerically over the product of beam power and impedance. EV asked whether the measured bunch length is representative for all bunches and over a longer time; this may not be the case. (3) LCE future work on LHC and SPS impedance database (FR et al) ---------------------------------------------------------------- FR's presentation to the LTC was postponed due to a computer problem and the news on the MKE kicker impedance. Another complication is the lack of a complete updated impedance model for the LHC and the SPS. Recent systematic work on the SPS resistive-wall impedance was done only by A. Koschik who compiled apertures all around the machine. The work on LHC and SPS could go in parallel. The LCE team should make a new effort to proceed with this task. This work requires a proper interface with several other sections and groups, in particular the rf group, which contributes both impedance measurements and some simulations. (4) Torino School on Plasma Physics and e-cloud simulations (EB) ---------------------------------------------------------------- EB reported highlights from a school on NON EQUILIBRIUM SYSTEMS: TURBULENCE IN FLUIDS AND PLASMAS (see web site http://www.isi.it/42/events_detail.html ) Among other items, the plasma dynamics in a Tokamak was discussed, with currents of the order of MA and plasma densities of 1e20 m^-3. FR asked whether part of the electron cloud dynamics can be described by a fluid approach. EB reported new HEADTAIL e-cloud instability simulations for LHC where she compared with nominal PIC simulations with a hybrid method where the field of the electrons is computed by the PIC module but the field of a beam slice is deduced in a soft-Gaussian approximation. The emittance growth is not the same. The PIC code shows initially larger growth horizontally. At later times, the trend reverses and the soft-Gaussian scheme leads to a fast instability. EM asked whether the emittance growth seen below TMCI threshold could be explained by a regular head-tail effect. FZ answered that regular head-tail instabilities had been seen in earlier microbunch simulations with K. Ohmi, but were never seen with the codes HEADTAIL, PEHTS and ECI. For confirmation, EB will re-run one of the cases considered with zero chromaticity instead of the chromaticity Q'=2. EB also performs simulations with a frozen cloud, where electrons do not move. The emittance growth for this case can be compared with one using an analytical expression for the electron field. Any additional emittance growth in the former case must be due to the PIC calculation. Appropriate techniques for suppressing PIC nose will be explored. (5) Highlights from SNS Review (instability diagrams, kicker impedance, resistive wall wake, and e-cloud) (FZ) ---------------------------------------------------------- FZ presented accelerator-physics highlights from the SNS ASAC review 27-29 September, in particular slides and studies by S. Henderson, J. Wei, D. Raparia and V. Danilov. The topics addressed were stability diagrams with space charge and momentum spread for bunched and coasting beams, impedance of the coated ferrite kickers, low-velocity resistive wall wakes, and electron cloud. Following an earlier recommendation, V. Danilov, together with A. Burov, studied the EPAC 96 paper by Karliner et al., which described a novel resistive-wall wake effect for the LHC. VD and AB found the results in this paper to be invalid. Being asked, the authors of the 1996 paper have distanced themselves from their earlier result. This effect may thus not occur in the LHC. The SNS injection kicker consists of a ceramic coated with copper and TiN on the inner side of a ferrite-filled tank. The longitudinal impedance is given solely by the thickness and resistivity of the coating and the transverse impedance follows from the usual scaling, suggested by the Panofsky-Wenzel theorem. This impedance is valid for a kicker of finite length and much higher than what would be expected from the Piwinski solution for an infinitely long object. The extraction kickers are placed inside the vacuum chamber, and the ferrite is directly coated with TiN. The impedance of these devices was computed (Davino et al.) and measured at BNL (Hahn et al), including the full assembly with PFN and the coating. The coating made no difference to the impedance (this is very different from the injection kicker where only the coating determines the impedance). The coating of the extraction kicker is applied in strips. The SNS ring instability threshold is at 2-3e14 with a nominal intensity of 1.4e14 protons. The threshold depends on energy spread, chromaticity, beta functions and tunes, transverse distribution, etc. A first attempt was made to compute a stability diagram, for a coasting beam using the simulated energy distribution due to painting and space charge. Without space charge the result agrees with ORBIT simulations. With space charge the stability diagram suggests that the beam would always be unstable by a large factor, while ORBIT simulations for a bunched beam predict stability. It is now planned to compute stability diagrams for the bunched-beam case, and to start from the Landau solution to the initial conditions instead of the van-Kampen approach. L. Wang simulates the 3D e- evolution at the stripper foil with the CLOUDLAND code. One input is the backscattering probability of electrons with energies between 10 keV and several MeV, which was estimated by EGS. It is of the order several percent and depends on the material. The stripped electrons are guided by a tapered magnetic field towards a carbon catcher with grooves which retain secondary electrons. The crossing of the Montague resonance was simulated by S. Coursineau and J. Holmes with ORBIT. The result is close to the experimental observation and may give a better agreement than other simulations using the IMPACT code. The cost of the last SNS magnets has dramatically increased due to high steel prizes driven by China boom and use of steel for humvee armor plates. Attached: Slides by EM, FZ