Minutes of the ABP-LCE team meeting on 06.02.04 present: EB, WH, AK, JJ, EM, FR, DS, EV, FZ ---------------------------------------------------------------------- (1) Minutes & Pending Actions ----------------------------- Following a pending action, FR informed L. Tavian about an earlier e-cloud heat load plot by FZ and asked for an update of the official beam screen cooling capacity, to be included in the new LHC design report. The reply of L. Tavian was received only after the LCE meeting and is currently being analysed. The LTC e-cloud presentation by DS was shortly discussed with FR last Monday as foreseen. (2) Geometric Collimator Impedance (DS) ----------------------------------------- DS presented Gdfidl calculations of the LHC collimator impedance. The thickness of the collimators was increased to close a gap. There are still slits around the moving jaw. Resistivity is not taken into account in these simulations. So the purely geometric impedance is computed, due to limitations of the Gdfidl code. DS computed all resonances at frequencies up to 2-4 GHz. The results do not depend much on whether or not the tank is included, or whether the slits are closed, i.e., the tank contribution can be neglected. The effective impedance was first computed for each mode individually. The resonator impedances extend up to 35 MOhm/m. In particular there is trapped mode between the jaws at frequency 2.25 GHz. Making the conservative assumption that all resonances drive the same multibunch mode, DS estimated the total effective impedance per collimator. It corresponds to an imaginary tune shift of about 5e-6. This value could then be multiplied by the total number of collimators to get a worst-case scenario. FZ remarked that the trapped mode between the jaws might affect the electron-cloud build up. (3) Resistive-Wall Impedance with Inductive Bypass (AK,EM) ----------------------------------------------------------- AK presented curves of the resistive-wall impedance for thin and thick pipes with and without the inductive bypass. For 'normal' parameters where the beam pipe radius is larger than the beam-pipe wall, the thick and thin formulae with and without bypass intersect roughly in a single point. For the LHC collimators the pipe (collimator) thickness is much larger than the pipe radius (collimator gap), and the results with and without the bypass are widely apart. Without inductive bypass the crossing occurs around 8.8 kHz. Here the formula for the thick wall with inductive bypass, considered for the LHC baseline, is a factor of 2 lower than that for the thin pipe. EM showed that using the thick pipe formula is reasonable for our parameters. The correct behavior at the transition point is currently under study by AK and Bruno Zotter. The real parts of both impedances with inductive bypass show the correct behavior (approaching zero) as the frequency goes to zero. AK presented an impressive movie illustrating the effect of a decrease in pipe radius b. (4) Stability Diagram for a Beam with Non-Gaussian Tails (EM) ------------------------------------------------------------- EM first commented on the minutes of the last meeting. Not only for the higher order distribution function n=15, but also for the standard n=2, the beam stability exceeds that of a Gaussian for the bad sign of the octupoles. He next studied the stability of a distribution with enhanced tails, where 2% of the beam was described by n=16 and the remaining 98% by n=2. The distributions were chosen to correspond to collimator jaws at 6 sigma. A factor 4 was gained in stability for the real tune shifts, compared with the baseline. The next step will be to use the real transverse profile, fit it to a sum of distributions of various order, and then obtain the stability diagram for the real beam. (5) Follow-Up on Electron-Cloud Presentation at LTC (DS) --------------------------------------------------------- DS presented the slides he had shown at the LTC. FR mentioned that there was a concern expressed by the management that the simulations had no predictive power. FZ objected that the code does have a predictive power, if the input parameters are well defined, and that the LHC predictions had not much changed since 2 years. -> We should measure all parameters at the same time as the e-cloud flux, heat-load, energy spectrum etc. are measured, DS illustrated the dependence of the build up time and threshold on beam size, bunch length, delta_max, and bunch intensity. -> FR raised the question whether the build up time along the batch changes during scrubbing and that we should aim to get the pertinent information. -> The decay time of the cloud is sensitive to our model of elastic reflection, and measurements should be conducted for various batch spacings. FR stressed that untrapped coasting beam could influence the results, and that the intensity calibration might have a large error. FZ pointed out that the different detectors may be at locations with different delta_max. -> Changes in e-cloud signals with bunch length could be a valuable calibration and benchmarking tool. In the new LHC design report the bunch length at injection was erroneously listed as 17.5 cm. The electron cloud simulations will need to be repeated for the correct length. FR deplored that after 7 years of study there is no analytical formula for the dependence of the heat load on the bunch length. The general conclusions of the LHC simulations are that with 25 ns spacing the luminosity is limited to 1/4 of the design. With 75 ns spacing there is no electron cloud heat-load problem expected and the luminosity would be at least 1/3. (6) BPM Impedance with and w/o Copper Coating (FZ) -------------------------------------------------- Following a request by Gerhard Schneider, FZ tried to answer the question whether the warm BPMs in LHC should be coated with a 100-micron copper layer. He noted that various formulae for the resistive wall impedance make widely differing predictions, and the formulae with inductive bypass typically predict 10 times less impedance than formulae based on a solution of Maxwell-s equation in 2D, such as the derivation by Burov and Lebedev at EPAC02. Nevertheless, even in the most pessimistic case the uncoated BPMs do not contribute to more than 1% of the total impedance. Using the Burov/ Lebedev expression for a coated chamber, the impedance actually increased slightly at low frequencies, reversing its sign. There is a concern that neither of these formulae may be applicable (private communication by F. Caspers). However, they should represent the worst case. In view of these results, the answer is that coating the warm BPMs in LHC is not strictly necessary =>ACTION: FZ will inform Gerhard Schneider. -> FR and EM underlined that L. Vos' formula for the resistive wall impedance with inductive by-pass applied to a collimator is in excellent agreement with HFSS results obtained by H. Tsutsui, based on a numeric solution of Maxwell's equations in 3D. According to EM and FR, this was already discussed at the LCE meeting of 1st August 2003. (7) Touschek in MAD-X (FZ) -------------------------- With Catia Milardi, visiting from INFN Frascati, and FS, a Touschek module was developed for MAD-X. It is based on the general theory of Piwinski and computes the rate all around the machine. Momentum spread, bunch length, and rf acceptance are presently considered as constant (as it is for the IBS calculations). This is not correct for the planned upgrade DAFNE-2 operating with strong rf focusing. An extension to this general case is intended in the near future. (8) E-cloud Simulations at Injection and Top Energy in LHC and E-Cloud in DAFNE (FZ) --------------------------------------------- The LHC results were already shown by Daniel. For 75 ns spacing the heat load will be ok up to nominal intensity. With 25 ns spacing, delta_max values of 1.1 are required to operate at nominal intensity. M. Zobov informed FZ that DAFNE is now observing a horizontal electron-cloud instability in the positron ring. The instability growth time increases along a bunch train, and an additional positive tune shift is observed in both planes. The threshold increases by a factor of two, when the rf frequency is changed bu 10 kHz. The reason is unknown. The instability is suppressed in collision by the beam-beam interaction. (9) Suppression of E-Cloud Instability by Chromaticity (EM) ----------------------------------------------------------- Parametrizing the e-cloud wake by a broadband resonator and appropriately scaling with bunch length and beam size, EM computed the instabiltiy thresholds in SPS and LHC as a function of chromaticity and bunch length. For the SPS a large chromaticity xi close to 1 is required to suppress the instability, consistent with observations. For the LHC at injection, a similar chromaticity of 0.8-1 is needed for 0.7 eVs longitudinal emittance and a somewhat lower chromaticity of 0.5 for 1 eVs. A shorter bunch length for the same emittance also acts stabilizing. At top energy, the LHC beam is always stable. Enclosed: Slides by AK, FZ.