Minutes of the ABP-LCE team meeting on 29.08.03 present: TdA, EM, FR, EV, LV, FZ excused: DS ------------------------------------------------------------ (1) Stability with Space Charge and Octupoles (EM) -------------------------------------------------- EM pointed out that in his paper on the PS instability threshold with octupoles, the space charge contribution was neglected both in the tune spread and in the impedance part. If he were to include these two components, the beam would always be predicted to be stable, in contrast to observations. He next reviewed the classical paper by D. Moehl, who for the 1-dimensional case showed that, depending on the sign of the octupoles, space charge may change the instability threshold for a coasting beam by factors of 2 or 0.67, under certain general assumptions. For the 2-dimensional case, only simulations were presented in the same paper. FR regretted that we have no validated theory for space charge and octupoles in the LHC. FZ mentioned that in LHC also the beam-beam interaction could greatly influence the instability thresholds, as it is known to be the case for either B factory. FR asked for a complete assessment of tune shifts and tune spreads in the LHC at injection. (2) LCE Contribution to the LHC Design Report (FR) -------------------------------------------------- The bulk of the design report is finished and FR presented it to the team. The sections on beam-beam and electron cloud are complete, and the impedance section is handled by cut-and-paste from earlier reports by LV and others. FR pointed out that the logics of the impedance part is not yet clear, however. In response, LV summarized the strategy that was adopted as follows: the high low-frequency transverse impedance (mainly resistive wall requires a transverse damper. This damper can deal with instabilities due to other sources (pumping holes, shielded bellows, sawtooth, collimators). Only the emittance preservation was a concern transversely. In the longitudinal plane there is no feedback. The loss of Landau damping occurs at Z/n=0.2 Ohm. Accordingly the goal of the impedance budget was to stay below 0.1 Ohm. Responding to a question by FZ on the role of higher order modes. FR and LV said that most of these had been treated in a paper by D. Angal-Kalinin and LV. One of FR's key questions is which nonlinearity is needed to damp higher-order head-tail or any other single bunch modes. LV describes that the collimator impedance is not additive to the other impedance, since the frequency dependence is quite different. FR remarks that the tune shifts are additive. The further plan is the following. FR will work full time through the weekend and then send a final draft to LV on Sunday afternoon. Negotations with John Poole will likely be needed. Two impedance tables should be included, one referring to injection the other to collision. Also needed is a table of tune shifts for the different modes, or, best, a plot of their location on the stability diagram. The ZBASE cannot be used for this purpose, as the underlying impedance numbers are out of date. LV summarizes the key question: we know that LHC is stable with feedback; the only remaining question is whether it can be stabilized at top energy without feedback, but using octupoles. For most elements the transverse impedance can be obtained by scaling from the longitudinal plane. The design report also comprises a section on parasitic losses. LV states that all contributions are small compared with the collimators, for example, the beam screen contributes some 1 MOhm/m. FR reminds the team that heating could be important for indivudal elements such as bellows, collimators, and kickers. To estimate these, the resistive part of the impedance is needed for the various elements. This will be provided by LV. (3) Other Items --------------- There was a pending action item, whether the ALS could measure the photon backscattering as part of the US LHC contributions. FZ has not yet contacted M. Furman. Recently, Noel Hilleret recalled that similar results by R. Cimino may already have been published at this year's Berlin conference. He is investigating this possibility. LV shows the first page of his report SPS-87-4 (MAS) where he found the same factor of 2 as D. Moehl. Since predictions using this factor gave excellent agreement with observations both in the PS and in the SPS collider, he urges a re-assessment. EM then explains how the factor of 2 came about in D. Moehl's paper, and why it appears to be wrong (a confusion between amplitude and rms amplitude).