----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 21.04.2006 present: FC, Heiko Damerau, WH, JJ, EM, TP, FR, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of Last Meeting, Pending Actions -------------------------------------------- ACTION: EM will put all contributions to the broad band impedance in the stability diagram both at injection and at top energy to see where we are actually standing with all the objects so far examined. Status: completed; see EM's presentation below. ACTION: AG will write a short note on the TCDS impedance estimates that can be referenced in the Functional Specifications. Status: Alexej is writing. ACTION: EM will follow-up the impedance of LHC septa Status: delayed due to PS start up. EM briefly reported on the PS start, which has been very successful so far, although 26 magnets were changed during the last shut down. The energy between booster and PS was matched. The PS commissioning is done from the CCC. In the old PS 5 magnets were voluntarily displaced vertically and 5 other horizontally. The reason for the displacement has been partly forgotten and in the new PS no magnet is displaced intentionally. The performance looks ok. EM illustrated this with a pot of the horizontal and vertical closed. The maximum horizontal orbit excursion is about 8 mm, and it is two times smaller vertically. FR announced that the collimator crisis is over. More springs were added behind the lateral metallic plates to increase the contact force of the rf fingers, as confirmed by visual inspection and by future measurements. The collimator production has thus started. (2) Bunch Shortening in the SPS with LHC Beam (EM, GR) ------------------------------------------------------- EM showed some animations of the measured longitudinal profile evolution for the 1st, 35th and 72nd bunch in the train with LHC beam in the SPS on the injection plateau. The animation includes 38 traces, spanning a total time range of 10.4 minutes. Only the data for the 72nd seem to contain evidence for bunch shortening. For comparison EM presented profiles from controlled 2003 space-charge MDs in the SPS, where bunch shortening and shoulder development are obvious. It was commented that the SPS longitudinal profile data had unusually poor quality, containing very few and noisy points. The suspicion was these data stem from the HEDATAIL monitor and that higher quality data should be available from the RF wideband pick up. After the meeting, GA informed us that the bunch longitudinal profile have not been taken with the head-tail monitor, but with an RF longitudinal pick-up. As the bunches are short there are few points. It should improve with the faster scope that the RF colleagues are going to install. In particular the situation gets worse when the bunches are loosing intensity. (3) Update of LHC Stability Diagrams at Injection/Top Energy with/without Collimators (EM) ------------------------------------------------------------------------------------------ EM first compared the tune shifts induced by a single round collimator made from either Copper, graphite, or copper-coated graphite. FC commented that a graphite copper collimator could also be investigated (referring to copper bars embedded inside the graphite). The most critical modes are 3499 for copper and 3489 for graphite. EM computed real and incoherent tune shifts for both. It appears that the copper collimators reduce the magnitude of the complex tune shift by about a factor of 2, but provide little change in the growth rate (imaginary part of the tune shift). For flat collimators the result needs to be multiplied by the corresponding Yokoya factor. FC proposed to study the case of a corrugated or segmented collimator, which would push the image currents further away from the beam. EM next presented a splendid review of all stability diagrams for the LHC at injection and at top energy with and without collimators, for single and multiple bunches, plus varying chromaticity. At injection space charge is taken into account. For example, EM showed the single-bunch and coupled-bunch stability limits for the three or four lowest-order head-tail modes at both top and injection energies as a function of chromaticity. The collimators represent a dominant part of the impedance, not only at top energy, but also at injection. At slightly higher values of Q' (say Q'>3) higher-order head-tail modes become unstable. At top energy the stability region is defined by the Landau damping octupoles. At injection the stability region is computed from the maximum detuning with amplitude which is compatible with a good dynamic aperture. The longitudinal degree of freedom is not taken into account when computing the dispersion relations (i.e., no z dependence and no synchrotron tune spread). The stability diagrams for injection illustrate the subtle interplay of space charge and nonlinearities. FZ pointed out that the positive tune shift from the electron cloud would be in the direction opposite to space charge, and, therefore, it would always cause a stable region far away from the coherent tune, the latter being shifted to negative values. By summing over the exact parameters for all collimators, EM more or less recovered the collimator broadband impedance which had been estimated in the LHC design report, using average beta functions and factor of 2 margins. EM gave simple rules of thumb for the tune shifts induced by a certain impedance. Namely, 0.1 MOhm/m gives a tune shift of 1e-5 at injection and 1e-6 at top energy. The difference between single-bunch and coupled-bunch tune is about 5e-4. The difference between the tune of a pilot bunch and the nominal coupled-bunch tune is about twice this. FZ asked which tune a feedback should keep constant. FR replied that we have no choice but to stabilize the coherent tune. FC raised the question whether the positive impedance could be compensated by adding components with negative impedance. JJ recalled an attempt at SPEAR of installing a capacitive impedance for suppressing bunch lengthening, based on a proposal by Karl Bane, which was only half successful. FR mentioned the inductive and capacitive inserts at LANL and Japan. He suggested that we attempt to shift the coherent tune in the direction of the electron-induced spread, e.g., by capacitive inserts or possibly using the damper. (4) Tune Shift for FP420 (EM) ----------------------------- The FP420 experiment consists of a 8-m long Cu beam pipe which is only 3 mm from the beam. EM computed the tune shift induced by this device and found that it is only a few 1e-6. He assumed a Cu thickness of 2.5 cm. FR recommended to check this number, as the thickness might be only a few microns, which could change the result. Replying to a question from FC, EM explained that his estimate contained a factor 1/4 due to the fact that the FP420 is located only on one side. This factor can be found in the 1999 PRL article from Burov and Lebedev. Lastly, EM mentioned that FP420 is looking for a CERN-based postdoc and that Federico Roncarolo has been recommended by Oliver and Francesco. (5) Super B-Factory Schemes (Crabbed Waist, etc) and Possible Application to the LHC Luminosity Upgrade (FZ) ------------------------------------------------------------------------------------------------------------- FZ summarized a recent email exchange with Kazuhito Ohmi and Mikhail Zobov, concerning the SuperB scheme in Frascati. The goal is a luminosity of 1e36/cm^2/s^1 using technology from the linear collider. Two workshops were held in November 2005 and March 2006, at which two different scenarios were developed. The first one had large disruption in the collision, high power consumption, strong damping, and energy recuperation in a s.c. linac. The new one has much less disruption, higher collision frequency, no longer any linacs except for e+ top-up injector, an consists mainly of two 6-km long rings with an ILC final focus. The main novel ingredients in this new scheme are (1) operation with a large Piwinski parameter (at the beam-beam limit the luminosity increases with this parameter, and the horizontal tune shift decreases, (2) small beta* which has to be larger than the beam overlap region, but can be smaller than the bunch length, (3) a "crab focus" or "crabbed waist" which avoids resonances due to the large crossing angle, (4) a shift of the vertical waist along the bunch, which reduces the hourglass effect, (5) the possibility of operating with higher current, and (6) the absence of parasitic collisions. Ideas (1), (2) and (6) look extremely similar to the LHC upgrade scenario in the regime of high-Piwinski parameter. WH commented that the calculations are due to Augustin in 1967. None of these reports are quoted in the INFN roadmap. FZ presented the Hamiltonian for the crabbed waist, in which the vertical waist position depends on the longitudinal position along the bunch as well as on the horizontal position and the crossing angle. The z-shift could be realized by an RFQ (TM210 mode) as pointed out by Ohmi, the x-shift with a sextupole at the right phase distance from the IP. Both together can also be achieved by crabbing the beam at a sextupole using a crab cavity. FR asked what the advantage of the crabbed waist is compared to a crab crossing at the IP. Concerning a possible application to the LHC, this scheme could be of interest for a long-bunch upgrade (scenario with large Piwinski angle); it could be realized in combination with local chromatic correction, crab cavities, and flat beams; the luminosity gain is to be determined (the primary gain may come from pushing the beam-beam limit). All of these ingredients are likely to be required. Posted on the web: Slides by EM and FZ Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/