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11th HiLumi WP2 Task Leader Meeting
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Europe/Zurich
6/R-012 - conference room (CERN)
Minutes of the 11th WP2 Task Leader Meeting held on 01/07/2013
Present: G. Arduini, R. De Maria, M. Giovannozzi, R. Kersevan, E. Métral, N. Mounet, E. Todesco, A. Valishev, A. Wolski
Minutes of the last meeting and follow-up of actions
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Tatiana and Sasha will present the status of the beam-beam simulations (in particular weak-strong) at the next WP2 meeting on 8/7/2013.
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Riccardo will review the aperture bottlenecks for the matching section elements, the associated motivations for the aperture requirements and the implication of the reduction of the aperture to 95 mm and potential mitigation measures at the next WP2 meeting on 8/7/2013.
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Implication of a reduction of D2 aperture on protection and mitigation measures: Gianluigi and Riccardo have discussed with S. Redaelli and F. Cerutti on Thursday 27/6. Stefano is going to review the protection provided by collimators in the present layout. Possible variants including the movement of the TCTs upstream of Q4 for the incoming beam will be also considered. Action: Stefano. Francesco and the energy deposition team are going to extend the estimate of the present radiation levels to D2 and Q4 area including the TAN. Action: Francesco.
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Ezio will present the most recent information on the D2 field quality as a function of magnetic field and aperture in this meeting
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The expected power loss on the components that have already been studied taking into account the HL-LHC parameters will be presented by the end of August. Action: Elias.
General information (Gianluigi)
The RLIUP review will take place from 29 to 31 October 2013 outside of CERN. Intermediate scenarios resulting from consolidation actions and/or partial upgrades will have to be studied. One of these include the study of the performance with a simple replacement of the triplet and D1 with larger aperture ones. A meeting has been called by Lucio to see how we attack these aspects. Regular meetings have been started to address the expected injector performance for intermediate upgrade scenarios.
D2 field quality – Episode II (Ezio)
Ezio presented new field error simulations (saturation component) considering D2 apertures of 95 mm e 105 mm for two different magnetic fields of 3.5 and 5 Tesla. Roughly a reduction of a factor 2 is obtained when reducing the aperture and a similar value when reducing the magnetic field from 5 to 3.5 Tesla in the components going from b2 to b4. Iron shaping can reduce significantly some of the components (e.g. b3). Further optimization of the field errors resulting from saturation seems to be possible. An updated Table for 3.5 T and 95 mm aperture will be provided by Ezio. Action: Ezio.
Massimo will evaluate the maximum acceptable errors for D2 in the assumption that their impact on dynamic aperture should remain in the shadow of the effect of the triplet/D1 field errors on dynamic aperture. Action: Massimo.
In the meantime Ezio will ask the BNL colleagues to further optimize the field quality by iron shaping.
Riccardo noted that for a final design one should take into account the constraints coming for WP5 and WP10.
Gianluigi thanked Ezio for the promising results.
HL-LHC: impedance considerations for the new triplet layout in IR1 & 5 (Nicolas)
Nicolas reviewed the various components to the longitudinal transverse impedance in the triplet/D1 area. The worst case of a 50 ns HL-LHC beam has been considered.
For the beam screen:
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The co-laminated copper sheet is required to maintain the power deposited on the beam screen to values smaller compared to the heat load due to luminosity debris (600-700 W for the triplet/D1).
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Amorphous Carbon coating has no effect on the longitudinal impedance
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The effect of the welding is not significant although visible. The position proposed for the welding to minimize the effect on impedance (one of the corners of the octagonal cross section is not viable for mechanical stability considerations (this is the area with maximum stress in case of a quench). Roberto mentioned that the thickness of the stainless steel could be reduced to 1 mm. This would have no significant impact on the impedance.
Gianluigi asked whether the estimates have been done taking into account that the spectrum will be modified due to the variable spacing between bunch passages as a function of the longitudinal position. Nicolas replied that the impedance due to resistive wall is of broad-band type and it is expected that the heat load should depend quadratically on the bunch intensity and linearly on the number of bunches that should be counted twice because of the two counter-rotating beams. The detailed structure of the spectrum should not be relevant. Gianluigi suggested to look in detail to this aspect, this is also important for the present triplet where heat loads higher than expected are observed. Action: Elias to review the two-beam effect.
Tapering angles of 15 degrees as presently implemented for the LHC are acceptable. The contribution of tapers and pumping slots to the power loss is negligible.
The contribution of the BPMs (geometrical part of the impedance) and of the of the pumping slots to the transverse impedance in the region close to 1 GHz is not negligible even in relative terms with respect to the total impedance of the machine. Possible options to be investigate:
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Revise the geometry of the BPMs but this will very likely reduce the sensitivity. Action: Elias in collaboration with R. Jones and R. Kersevan.
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Revise the geometry and coverage of the pumping slots. Roberto mentioned that the surface fraction covered by the pumping slots (presently 2.6 % could be reduced. Action Elias to optimize pumping slot geometry in collaboration with R. Kersevan. Roberto noted that the presence of the baffles covering the pumping slot might reduce the impact of the pumping slots on impedance. These were introduced to minimize the heat load on the cold bore due to electrons passing through the slots and impinging on it.
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Revise the number and position of the BPMs (already planned)
Andy asked what is the level of accuracy of the present impedance model. Nicolas replied that this is accurate to within a factor two.
News from the Task Leaders (all)
Sasha noted that given the extremely large beta function at the triplets, mechanical stability of the triplets and of the beam screens becomes even more critical. This should be studied. Action: Riccardo to review these aspects based on the measurements conducted with V. Shiltsev.
Reported by: Gianluigi and Riccardo
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