Minutes of the ABP-LCE team meeting on 20.02.04 present: EB, WH, AK, EM, FR, EV, FZ justified absent: DS ---------------------------------------------------------------------- (1) Minutes +ACY- Pending Actions ----------------------------- ACTIONS (mainly DS): +AD0APg- clarify geometry of trapped modes +AD0APg- compute longitudinal impedance +AD0APg- practical advice for collimator design +AD0APg- redo estimate for larger gap +AD0APg- understand stability diagram for m+AD0-4 +AD0APg- what can be measured in the SPS? DS was not at the meeting. He has written a draft memo on the collimator impedance and sent it to his contact in the collimation team. +AD0APg- perform full simulation for the SPS (AK) Not yet done. (2) Review of resistive wall formulae and measurements (AK,EM,FZ) --------------------------------------- EM presented a new general formula for the transverse resistive impedance. It is derived from the multi-layer longitudinal impedance of L. Vos, which is converted to transverse impedance following L. Vos' prescription for the inductive bypass. This new formula agrees well with the measurement of Fritz Caspers and with the Burov-Lebedev formula. Applying this formula to the LHC collimators EM found that the difference to the thick-wall inductive bypass result was negligible (1+ACU- or less). AK evaluated various expressions for the resistive transverse impedance, including the new formula from LV-EM. He confirmed an excellent agreement between the new L.Vos-EM expression and the Burov-Lebedev result for the laboratory measurement of Fritz. For the collimator, the two expressions deviate at low frequency. Why there is a difference in this case, and which one is more correct remained unclear for the moment. It is not relevant for the LHC. The new expression approaches the thin-wall inductive bypass formula at low frequencies, the Fermilab formula is offset. The latter was also evaluated for more than 2 layers, considering copper outside the graphite. The copper enhances the impedance at lowest frequencies. FZ showed similar independent evaluations, not including the new LV-EM formula. He found that Burov-Lebedev's result was closest to both measurements and Tsutsui's simulations, and that thick-wall inductive bypass expression deviated considerably. The discrepancy with the previous two speakers might hint at an error in his mathematica notebook or at a unit problem. Possible actions: +AD0APg- compare results and mathematica notebooks (FZ, EM, AK) +AD0APg- suggest that Fritz may attempt a measurement in a parameter region that is closer to the LHC collimators (FR) +AD0APg- determine what can be measured in the SPS (AK, EM, FR, DS, FZ,...) +AD0APg- check epsilon and mu of collimator material for impedance calculation (mu of the graphite was checked by EM and it is close to 1) It was suggested that CERN adopts a general policy of using low-mu stainless steel for accelerator components near the beam. (3) Electron Cloud: analytic results and code comparisons (EB) -------------------------------------------------------------- HEADTAIL simulations for LHC were compared with QUICKPIC, considering 1 interaction point in both codes and a linear rf. Conducting boundaries were used in both codes. The bunch length in QUICKPIC extends over 5 sigma, while for HEADTAIL both 2 sigma and 5 sigma were simulated. The emittance growth was different from that in QUICKPIC for either case. QUICKPIC shows a fast vertical instability, which is absent in HEADTAIL. Perhaps some other parameter is not the same, or it could be an effect of electron space charge. The latter was not included in HEADTAIL. FZ remarked that GR had performed simulations with and without electron space charge in HEADTAIL and that he saw a negligible effect, in the past. EB will visit USC for 9 days after ECLOUD'04, to continue comparison of codes and to learn the use of QUICKPIC. FR proposed to perform a series of HEADTAIL simulations to study the chromaticity needed, in LHC, for a minimum emittance growth (say less than 0.1+ACU- over the time of the simulation) as a function of electron density. This should use several interaction points, and perhaps be done for both dipole fields and field-free regions. Effect of feedback could be explored in addition. ACTION +AD0APg- perform comprehensive HEADTAIL simulation study for LHC (EB) EB then reported on progress with the analytical treatment of the electron pinch. For a Gaussian longitudinal profile and a linear force she solved the electron motion using the WKB approximation. Inverting the result, the density of electrons is obtained as a function of radius and time during the bunch passage. This can be compared with simulations. Except for the WKB approximation the agreement should be exact, but some differences were visible, perhaps due to small differences in the parameters. If nonlinear forces are added to the simulation, the electron density starts to resemble a step function which assumes a nearly constant value after the first quarter oscillation in phase space. The solution using the WKB approximation preserved the symplecticity. It is not clear whether this is generally the case. (4) Progress on Feedback Simulations etc. (EV) ----------------------------------------------- Work in progress: 1. Create common program for beam-beam simulations and feedback. This could be done in SIMULINK. The option +ACI-accelerator+ACI- compiles the code and gains a factor 10 in speed. 2. Paper on simulations of CNGS extraction with feedback. 3. In contact with J. Tuckmantel regarding technical possibilities of rf amplitude modulation in the LHC. FR recommended to look at the kickers for extraction and injection into the LHC. EV remarked that plans exist to cancel the ripple of LHC injection and SPS extraction kickers against each other. (5) Reports from meetings: LTC and follow-up of Chamonix workshop (FR) --------------------------------------------- In the LTC, S. Myers reported open questions from Chamonix. FR listed topics relevant to us and people who will report back: Pulse-to-pulse emittance variation up to 20+ACU- due to kicker rise time (V. Mertens) Satellite bunch intensities - satellites are created in the PS MKE kicker heating problem. Impedance of all kickers in the LHC and in SPS+ADs- estimates, measurements, how to improve shielding of MKE kickers in the SPS (EM, FR, FZ) Scraper in the SPS+ADs- collimation in transfer line (HBu, P. Collier) 75-ns beam and short bunches in PS booster and PS (needed by end 2004) 156-bunch TOTEM beam in the SPS, 1 test in SPS with equidistant bunches (GA) Beam-loss monitors Beam quench limits of LHC magnets, sector test or slow extraction from SPS? JPK remarked that beam-loss monitors are not sufficient to adjust the collimators. Alternative is to equip collimator jaws with bent crystals. FZ suggested to use the wake field kick and closed-orbit change for determining the distance between collimator jaw and beam and to center the two jaws. EV suggested to look at the background in the experiments. EM mentiond that the collimator distance from the beam is 1.2 mm to 3.8 mm in physics. FR remarked that this distance is significantly larger at injection (4.7 to 11.1 mm) and the corresponding wake field kicks become very small. Beam efficiency for each system on a single paper. Injector chain emittance blow up. Ion operation, lifetime of some magnets operated close to the quench limit. (6) Other News: Computing Power (FR) ------------------------------------- L. Evans agreed to the purchase of 50 new CPUs+ADs- for the next months LCE studies gain priority in the queues. ACTION +AD0APg- FR will investigate whether there exists a special priority queue or whether priorities are automatic. (7) Geometric collimator impedance (EM) --------------------------------------- EM applied L. Vos formulae to estimate the geometric impedance of the LHC collimators, both longitudinal and transverse. The real part is zero until the pipe cut off and then assumes a finite constant value. The imaginary part peaks at the cutoff. The longitudinal impedance is about 0.1 mOhm per collimator for a distance of 2 mm and 15 degrees taper. The transverse impedance is not negligible. It is estimated at 0.2 MOhm/m. This could be compared with DS' Gdfidl results. (8) Stability diagram for m+AD4-0. ------------------------------- EM remarked that the stability diagram should not change with head-tail mode number m. FZ suggested that there could be an effect if the synchrotron tune spread contributes to the Landau damping. Since often the bucket is filled, this could be the case, at least at injection. FR mentioned that the contribution from synchrotron tune spread had been studied by Y.H. Chin (see also LHC-Project-Report-121, enclosed). Enclosed: Slides by EM, AK, FZ.