------------------------------------------------------------ Minutes of the ABP-RLC section meeting of 24.10.2006 present: OB, WH, EM, Federico Roncarolo, GR, Benoit Salvant excused: Fritz Caspers, UD, TP, FR, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ 1. Reports from meetings ---------------------------------- OB reports that a white paper from the DG on "SCIENTIFIC ACTIVITIES AND BUDGET ESTIMATES FOR 2007 AND PROVISIONAL PROJECTIONS FOR THE YEARS 2008-2010 AND LONG-TERM PERSPECTIVES" is now available on the CERN home page, and therefore open for public consultation. the CERN research activities are prioritized according to: 1. LHC, 2. Upgrade of the injector complex, 3. All other R&D. 1. contains also consolidation and improvements to enhance LHC luminosity on the short term (phase 2 collimator system, improvement of LHC beam controls, new power supply for the PS, multi-turn extraction in the PS) 2. emphasizes on the upgrade of PS to a 50 GeV/c machine and replacement of Linac2+Booster with Linac4+SPL. It is striking that now Linac4 and SPL seem to have the same priority. 3. includes all research themes for LHC upgrade and novel R&D. In detail: high-field SC magnets (but also SC quadrupoles for neutrino studies), cryogenic upgrade to cool the LHC IRs, new RF-system in LHC for bunch shortening, development of new tracking detectors and calorimeters, CLIC studies with CTF3. Following the above re-definition of priorities and needs, a comprehensive review on the manpower plan is scheduled for November OB said that a new contract has been signed with the electrical companies. Now our agreed daily limit is 70 MW. In the period 2-27 January 2007 CERN will entirely depend on the French electrical network, operators will be provided with a priority list of components that gives the order in which they should be shut off if necessary. EM reported then from the two latest APC meetings. There was a first meeting on the 6th October with two talks. The first talk was given by R. Steerenberg about the fast extraction without QKE58 in the PS, and the second one was a report on studies on loss mechanisms in the PS at extraction from S. Gilardoni. Another APC meeting took place on the 13th October and was entirely devoted to the collimator tests. Machine protection and safety issues for the TT40 tests were addressed by V. Kain, C. Bracco explained the scope of both the TT40 and the LSS5 tests, instrumentation needs for the LSS5 tests were covered by S. Redaelli and finally EM himself explained what kind of impedance measurements could be done with the LHC prototype collimator currently installed in the SPS. EM concentrated in this talk more on the what could be measured used coupled-bunch effects. He had already covered the possibilities with single bunch in the Collimator Working Group Meeting on the previous Monday, where he had shown all the results from a HEADTAIL simulation study done by himself and GR. Measuring instability growth rates of the single bunch at high energy we would need very small gaps (<1 mm) to disentangle the effect of the collimator impedance with respect to the total machine impedance, and possibly single out the inductive by-pass effect. To study the coupled bunch effects, EM set up a macro-particle model based on wake-field description. Half-gaps of 1.5 mm seem to be enough to have a difference by a factor 2 in the instability rise times predicted with the classical thick-wall resistive wall theory and the inductive by-pass. EM also added the latest news on the CERN-GSI benchmark for the resistive-wall impedance calculation. It seems now that results converge (imaginary part almost perfectly, all over the frequency range, real part within a factor 1.4 at the low frequencies), the steps that eventually led to convergence were: take d=2.5 cm for real collimators, scale the result to 1 m (length of the collimator) and not to the full ring length (27 km!), use the correct value of graphite resistivity (10 uOhm m). The minutes of the last 2 APC Meetings will soon be made available on the web. 2. Follow up of pending actions ------------------------------------------ WH said that this concluded the actions from EM (see above: follow up the difference between resistive wall results from CERN and GSI, propose methods for the measurement in the SPS of the LHC collimator impedance). 3. A.O.B. --------- WH reported from the LOC Meeting that it turns out from aperture computations done by J.B. Jeanneret for the injection regions near IP2 and IP8 that it could be necessary to change the vacuum chamber between the D1 and D2 magnets of the LHC. A statement is desired from our section, how this would affect the ring impedance before taking the proposal to the LTC. J.B. Jeanneret will be invited to one of our next section meetings. WH raised another issue following an email exchange between him and EM where the latter proposed a possible change to the LHC bunch train from 4 batches of 72 bunches to 6 batches of 48 bunches. EM explained the reason that it has been experimentally proven that stable batches of 48 bunches can be delivered from the PS to the SPS, whereas now 72 bunch batches become unstable in the PS just before extraction to the SPS. The observed instability is in the horizontal plane, it could be ascribed to e-cloud in the PS, but the mechanism that causes it is not yet clear. In 2004 it was possible to inject 4 stable batches with 72 bunches from the PS to the SPS, even if already at that time an instability was observed at the end of the run after an improvement of the RF system was implemented, which led to avoid the longitudinal emittance blow up from 3.8 to 4.2 eVs. This year the LHC beam has a smaller longitudinal emittance and the bunch length is shorter, which could explain why it is now unstable at the time of the PS->SPS transfer. The option with batches made of 48 bunches is worth studying because it could give an alternative filling pattern for the LHC. Apart from avoiding the instability at the extraction from PS, it also has the advantage of requiring a shorter flat bottom in the PS, which could allow for higher intensity per bunch into the PS since the space charge limit at injection becomes less stringent (even if perhaps experiments would prefer an upgrade in number of bunches rather than in intensity per bunch because they want maximum integrated luminosity with minimum peak luminosity). A new magnetic cycle to allow for 6 x 48 bunch batches into the SPS needs to be set up, because only 5 such batches can be accommodated within the present length of the SPS flat bottom. The resulting bunch train which should then be injected into LHC is slightly longer caused by the increased number of gaps. There could also be luminosity loss in LHC due to the decreased number of bunches and an increase of the relative number of PACMAN bunches. For the collimator tests this new LHC beam structure will anyway be used because they need 288 bunches. Action -> EM and WH will study advantages and drawbacks of the 6 x 48 option for the LHC beam A new PhD student, Benoit Salvant, was introduced to the group. He is going to work on impedance modeling for the SPS. --