----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 26.08.2005 present: Romain Basset, UD, AG, EM, TP, FR, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions, announcements ----------------------------------------------------------- The list of actions is posted at http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Actions/actions.htm . There were no comments to the minutes of the previous meeting. Modifications of the IPM design have been agreed upon with BDI, guided by FC's ideas and AG's calculations. (2) Follow-up of single bunch instability in the SPS (EM) --------------------------------------------------------- EM reported new developments on the vertical instability in the SPS. Previously thresholds were compared with theory. Now EM is looking at the detailed time evolution, in particular the time of the beam loss (100-150 turns after injection; the synchrotron period was 300 turns) and the phase relation between different slices of the bunch. The latter was measured by the HEADTAIL monitor during the SPS experiment. These monitor data were analyzed by GA. With MOSES, EM has found a rise time of 43 turns for a broadband resonator of 10 MOhm/m, at 700 MHz and Q=1. The loss time corresponds to about 3 rise times, i.e., 129 turns, assuming a 1 mm injection offset and an aperture limit at ~20 mm. EM computed the rise time versus chromaticity, and also here found a dependence in good agreement with the measurement. Next, EM derived the expected phase shift from slice to slice using the two theories of TMCI and BBU, respectively. Both theories predict a constant betatron phase difference between different longitudinal positions of the bunch, exactly what is observed, i.e., all phase difference become constant at the onset of instability. The agreement is further improved by lowering the resonator frequency to 400 MHz. The beam-loss times predicted by the TMCI and BBU calculations are 130 turns and 105 turns, respectively, which are similar, but not identical, and both consistent with the observation. EM pointed out a 1988 paper by Brandt and Gareyte where a similar loss for positrons after injection into the SPS was explained by the same type of BBU instability. Preliminary calculations by the HEADTAIL code gave results which are also consistent with the theoretical models. In conclusion, there is a good understanding of the SPS observation. The new results indicate that the resonator frequency in the SPS is around 500 MHz rather than the canonical value of 1.3 GHz. (3) Data base for CARE-HHH simulation codes web repository (RB) --------------------------------------------------------------- RB described the motivation for the data base development, the meaning of 'code repository' and the tools used to develop it. Requirements were that the database should contain a search engine, provide a uniform format for all code, be easy to use and be easy to maintain. The database was developed in ORACLE, mainly because there exists a local ORACLE support team at CERN, which could help with the maintenance. The ORACLE Designer CASE tool, the ORACLE Database Management System, and the PL/SQL Web Toolkit were employed to create the database, the applications, and the code which produces the web page dynamically from the database. RB wrote two different tools, namely the repository proper and the database management tool. He gave a live demonstration how to add a code (here MAD-X) to the database, and how to link it to the correct contact persons and categories. The work goals for his last week at CERN are to move the database from the development to the production server, to write a user guide and a developer guide, and to complete the entries in the database. (4) Nonlinear wake fields for flat resistive wall (FZ) ------------------------------------------------------ FZ derived a set of final expressions for the coherent and incoherent tune shifts from the nonlinear resistive wall wake field of Piwinski. The resulting coherent tune shift predictions differ by only 20% from the SPS measurements for the smallest gaps. At larger gaps the agreement is perfect. The Burov-Lebedev theory differed by about a factor of two for small gaps. The remaining 20% discrepancy could be attributed to the assumption of an infinitely thick wall in the Piwinski model, since there is also a 20% difference between the classical thick-wall theory and the Burov- Lebedev one. The latter includes the effect of the finite wall thickness. In summary, most of the measurement can be explained by the nonlinearity of the wake field and there may be a 20% contribution from the wall thickness. It is foreseen that EM's student, starting later this year, will try to develop the general theory combining the two effects. FZ hss also presented the analogous analytical expressions for the incoherent tune shift and tune spread. They were benchmarked against particle tracking. For the smallest gaps there is a full tune spread of a few 1e-4, which leads to decoherence and additional Landau damping. The tracking simulation indicated a re-coherence after more than 10,000 turns, the origin of which is not exactly understood. (5) Progress with ZBASE model of the LHC (FZ) --------------------------------------------- The ZBASE program was made to run. Several paths had to be modified. Two errors were corrected in the routine calculating a transverse impedance from the resonator parameters. Impedance data did already exist for the four LHC experiments, but in many cases multiple sets, computed sometimes by different people, and showing a large spread in values and patterns. FR recommended that all experiment impedances be recalculated for the final geometries. R. Veness is the contact person. Presently the LHC ZBASE model contains impedance data for LHC-B, ALICE, CMS, ATLAS (all already present since several years, and most of them computed by O. Bruning), to which FZ has added the impedances of the transverse damper electrostatic plates (from Wolfgang Hofle) and of the 400 MHz rf system (provided by Deepa Angal-Kalinin). The impedances of these components can be added. A preliminary sum impedance was shown for the longitudinal plane. For the transverse plane, a missing weighting with the local beta function has been introduced in the routine which sums the group impedances. FR suggested that other team members contribute to the ZBASE effort. Ultimately the impedance value of every LHC component shall be taken into account, including all the elements listed in the LHC MAD optics model. (6) AOB ------- The next RLC-LHC team meeting will be on Friday 9 September. It will start with a ~40 min presentation by Rama Callaga (BNL) on "SUPERCONDUCTING RF CAVITIES FOR HIGH CURRENT ENERGY RECOVERY LINACS". Posted on the web: Slides by RB, EM, FZ Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/