----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 03.03.2006 present: EB, RM, FR, RT, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) LHC HEADTAIL simulations with stripes and dipole field (EB) --------------------------------------------------------------- EB presented results of HEADTAIL emittance growth simulations for the LHC as a function of electron density. She considered both field-free regions and dipole fields, in the latter cases taking a stripe-like electron distribution with parameters obtained by fitting to SPS strip detector data. Chromaticity Q' was set to 2, and the beam was round. The simulation results for both field-free regions and dipoles are similar, with a similar threshold for the vertical fast instability. Only with a dipole the horizontal instability is somewhat suppressed above the threshold, and the incoherent growth below the threshold is a little lower in both planes. The latter can be explained by the fact that in a dipole the pinch occurs only in the vertical direction. The instability threshold is at about 4e11 e- per cubic meter. Growth rates above are of order 10000% per second, the growth rates just below the threshold about 10% per second. At ten times smaller density, the growth has a magnitude of less than 1% per second. The chamber size used in these simulations was 20 times the rms beam size. FZ pointed out that the numbers for the incoherent emittance growth likely represent an upper bound, as only 10 IPs are taken per turn. More precise numbers are expected from Giuliano Franchetti's code. However, Giuliano first needs to implement the stripes and the dipole field. Also, EB observes little effect of chromaticity, whereas, for a field- free case without stripes, GF saw that larger chromaticity increases the incoherent emittance growth. FR asked about the dependence on the synchrotron frequency. The incoherent growth is expected to become smaller for lower synchrotron tune. Without synchrotron motion the growth stops. FR suggested studying the incoherent growth for a barrier rf bucket. (2) Rating and magnet aperture for the LHC IR upgrade (RT) ---------------------------------------------------------- RT summarized the consensus on the aperture calculation for LHC IR upgrades, which has emerged from a meeting with FR, RT, FZ, Jean-Pierre Koutchouk, and Walter Scandale. There are a few open questions, such as the scaling of the closed orbit error with beta*, the value of the momentum range to be considered (at the moment 8 times the longitudinal sigma, which is used for chromaticity measurements), and the impact of spurious dispersion. FZ emphasized that the spurious dispersion from the other IP or from the arcs does not compromise the local chromaticity correction, as long as the geometric sextupole aberration of the chromaticity sextupoles are properly corrected by other compensating sextupoles. FZ also suggested to check whether in the present LHC optics the outgoing beam is closer to the wall. If this were the case, one could envision to invert the polarity of the final triplets for the IP(s) with horizontal crossing. FR mentioned that the tolerance on the vertex position is +/- 3 mm in total, and that we can move the IP by about a mm without running into problems. RT summarized the ongoing work, which includes the implementation of JPK's notebook, the need of full ring lattice descriptions, the role of the spurious dispersion for the local chromatic correction, and the aperture model based on a python script external to MAD. => ACTION => Develop a lattice for crab cavity option [RT, FZ] (3) Two-beam resistive-wall wake field (FZ) ------------------------------------------- FZ has studied the resistive-wall wake field for two beams propagating in opposite direction. He found that the Panofsky-Wenzel theorem no longer holds. Unlike for the 1-beam wake field the magnetic field and the electric field contribute to the transverse wake field. Considering only the long-range electro- magnetic fields computed by Chao yields a result which diverges at short distances. FZ then considered two different approximations for the complex Fourier transform of the Lorentz force, obtained by looking at the limiting cases above and below cutoff. Above pipe cut off, he found two poles and zero contributions from integration over the branch cut of the k plane. this gives an exponentially decaying real wake, which does not have the correct long-range behavior. Below the cutoff he found that there was a single pole only on the second Riemann sheet (hence not contributing) and the wake field followed from the nonzero integral over the branch cut. The long-range wake is consistent with the one originally expected from Chao's long-range solution, but without any singularity at short distances. The long-range wake turns out to be of exactly the same form as the conventional, only with opposite sign and the need for integrating over z. (4) Impedance localization for the SPS - new fitting results (FZ) ----------------------------------------------------------------- FZ followed up two outstanding actions from the APC related to the localization of the SPS transverse impedance. He fitted to local impedance sources located either at all quadrupoles, a selected subset thereof, or at all kickers and rf elements. The fit quality was similar in all cases, with a total impedance of about 30 MOhm/m. The solution is not clearly determined, but 5 or 6 conspicuous locations were identified, including the MKE and MKP kicker areas. FZ stressed that the exact and unambiguous identification of the impedance sources is only possible with an accurate SPS optics model. FZ obtained from ES various literature and data on the lowest transverse mode of the 200- and 800-MHz cavities. The broadband impedance he computed from these single mode data alone is negligible compared with the total SPS broadband impedance. More information on the cavity broadband impedance would be needed before it can have an effect in the fit. In addition, there is an uncertainty in the definition of the transverse impedance. (5) Pros and cons of 75 and 25 ns spacing (FZ) ---------------------------------------------- FZ and GA have prepared a number of slides comparing advantages and drawbacks of 75-ns compared with 25-ns spacing, for the next LHC commissioner meeting. The advantages include less long-range beam-beam effects, less electron cloud, and higher luminosity for the same current, the disadvantages a larger event pile up, more commissioning steps, and a delay in discovering possible electron cloud limitations. (6) AOB: definition of transverse resonator impedance (FZ) ---------------------------------------------------------- FZ discussed an apparent inconsistency in the definition of a transverse resonator impedance, with the rf group, the old ZBase code, and possibly Bruno Zotter using one definition, while Chao and Ruggiero use another one, with a different dependence on frequency. => ACTION => FZ will check the definition with J. Tuckmantel and perhaps B. Zotter. Posted on the web: Slides by EB, RT (soon), and FZ Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/