Minutes of the 6th LHC Insertions Upgrade Working Group held on 29th November 2007
Present: V. Baglin, F. Cerutti, S. Fartoukh, M. Giovannozzi, J.-P. Koutchouk, H. Mainaud-Durand, K.H. Meß, R. Ostojic, V. Parma, L. Tavian, E. Todesco, R. Van Weelderen, E. Wildner, F. Zimmermann
Excused: O. Brüning, A. Ferrari
Invited: M. Karppinen, P. Limon
1. News and approval of the minutes of the last meeting
The minutes of the last meeting are approved without comments.
2. Options for the resistive D1 dipoles (M. Karppinen, ppt file)
Taking as basis the previous discussion on the aperture of D1 magnets in IP1 and IP5 (minutes of LIUWG #2 and LIUWG #3), Mikko presented several possibilities for the case where D1 is a resistive magnet (a similar analysis for the case of superconducting D1 will be discussed in one of the next meetings). One of the possibilities he considered is the proposal by JPK and DT to modify the pole gap of the existing MBXW magnets (Internal note AT-MCS 2007-08; EDMS 875274).
The requirements for D1 (resistive magnet option) are the following:
Integrated strength
Assuming no change in the layout of LSS1 and LSS5, the integrated strength of D1 should be increased from 26.1 Tm to 29 Tm, as the separation between D1 and D2 is reduced by about 8 m due to the longer inner triplet foreseen for phase I (estimate for a 130 mm aperture symmetric triplet). On the other hand, if the Q4/D2 magnets are moved towards the arc (as is presently considered for improving the tunability of the insertion), the integrated strength of D1 could be as low as 24 Tm.
Gap height
The gap height of D1 should be increased from 63 mm to 110 mm and 120 mm in IR5 and IR1, respectively (H and V crossing, respectively) to give the same beam clearance as the one expected in the 130 mm aperture triplet quadrupoles. JPK commented that a 120 mm gap should be considered as an extreme requirement, implying a slight change of the target beta* (25 cm in the present case). SF replied that increasing the gap of D1 by 57 mm in IR1 (47 mm in IR5) is quite comparable with the 60 mm aperture increase presently considered for the inner triplet quadrupoles (i.e. from 70 to 130 mm). As an example, he commented that the minimum beta* would be increased by 20% (i.e. to 30 cm) if the vertical gap of D1 was reduced by 10 mm w.r.t. the above targets. The D1 separation dipoles would then become an aperture bottle-neck, which should be avoided. JPK agreed.
Good field region (GfR)
At present, the horizontal extension of good field region (i.e. dB/B lower than a few units) is requested to be +/- 50 mm, compared to about +/- 40 mm in the present design.
Based on the above requirements (24 Tm < BdL < 29 Tm, 110 mm < Gap <120 mm), Mikko then analyzed several scenarios involving MBXW magnets and associated equipments. The operating current of D1 needs to be increased from 850 A (ultimate current of the present D1) to 1200-1400 A. This requires new power converters and cooling stations, both technically feasible, but implies a total cost of about 1 MCHF (for two IPs). Inserting a 57 mm magnetic spacer between the half-cores of the present MBXW magnets would allow to meet the 120 mm target for the D1 gap but with a poor field quality (GfR ~ 10-15 mm). New cores would be needed to improve the field quality, which would require an investment of 3.2 MCHF. In case new coils are made, the total cost of the resistive D1 would be 4.7 MCHF. As far as planning is concerned, only the second option ("new core & coil") is compatible with installation of D1 dipoles during a normal LHC shutdown (3 months). This would, of course, require that the magnets are produced and tested in the time preceding Phase I. In all other cases, removing, remachining and testing of the present D1 could only be done after the LHC start-up. Finally, the power consumption of D1 would be substantially increased (2.8 MW for the whole machine compared to ~1 MW presently). In case that a completely new design is considered, the power rating of D1 magnets could be much better optimized. The existing powering and cooling stations could be compatible with this new D1. In this configuration, the total cost is estimated to be around 7 MCHF for 24 MBXW's and 4 spares, possibly reduced to about 5.5 MCHF assuming a modification of the LSS layout (BdL=29Tm -> 24 Tm, see above).
Concerning radiation aspects, Mikko reminded the high activation dose expected in D1, typically 10 mSv/hour after one year of LHC operation at nominal intensity and 1-day cooling, decreasing slowly to 2-3 mSv/hour after several months of cooling. These numbers are certainly overestimated for the first years of LHC operation, but remain four orders of magnitude higher than the safe limit of 1mSV/year given by the present legislation. Machining even moderately radioactive material has certainly an impact on the cost, implementation time, perhaps even on the feasibility of any recycling D1 options, and has not been taken into account in the above analysis. The meeting agreed that any D1 recycling can only be done before the start-up of the machine, which is impossible a priori. Rebuilding a new D1, normal conducting or cold, seems therefore to be the only possible option for phase I. JPK further commented that this new D1 should be ideally compatible with the upgrade phase II, for which no clear specification exists yet.
3. Summary of IR'07 Workshop (F. Zimmermann, ppt file)
Frank reported on the three days workshop IR'07 held beginning of November at INFN Frascati. The workshop was organized around nine sessions, with several intermediate round tables and a final round table at the last session. The goal of the workshop was to narrow the possible choices for LHC IR optics, converge on magnet parameters, identify the different ingredients for the LHC upgrades, and strengthen the collaboration with DAFNE/SuperB in order to explore applicability of advanced IR concepts to LHC.
In general, all methods for reducing the geometrical losses of luminosity, and in particular crab cavities, received a strong boost at the workshop. It was recommended to launch (or complete) their studies, development and implementation in the LHC independently from the different "phases" of the upgrade. A topical workshop on crab cavities is being organized in February 2008, to streamline the large interest in this option for nominal and upgraded LHC.
Several aspects of the two optics solutions retained for Phase I (symmetric triplet and low-beta_max options with 130 mm aperture quadrupoles but for Q1 in the low-beta_max option, see minutes of LIUWG#3) were discussed. In particular, the first results of energy deposition studies were presented which show that beam absorbers might be needed in all quadrupoles (and not only in Q1, as in the present triplet). If confirmed, this requirement may have a strong impact on the aperture definition for the new triplets. The option of a hybrid triplet proposed recently in the US-LARP meeting by L. Rossi, (Nb3Sn supplied by US, and NbTi quadrupoles by CERN), was brought-up in several presentations and discussions, with varying opinion among the participants.
4. Highlights from THERMOMAG (R. van Weelderen, ppt file)
Rob reported the main highlights from Thermomag, a CARE-HHH Workshop on heat generation and transfer in superconducting magnets, held 19-21 November in Paris. The goal of the workshop was to review the latest developments in the field, in particular related to the new accelerator projects (LHC upgrade - injectors and IR -, fast-cycling magnets for FAIR). The workshop was an occasion to present the latest experimental results, as well as discuss the advance in modeling. Besides CEA, which was for a long time the only place for systematic studies of heat transfer in superfluid helium, experimental results were reported from KEK, CERN and WUT, as well as Rutherford lab. Promising new insulation schemes, including ceramic, polyimide and epoxy wrappings, were recently tested and show a considerable potential of increasing the power extracted from carefully prepared cable stacks. Further experiments are planned, as a guideline and test-bed for numerical models, both in the regime of heat transfer in superfluid and in supercritical helium. A number of subjects have been identified that would need streamlining, and an initiative has been launched for strengthening the collaborations. A follow-up workshop on the subject will review the advances and collaborations in autumn 2008.
5. A.O.B. and follow up of actions
Next meeting scheduled for 13 December 2007: Powering and magnet protection aspects for the new inner triplets (K-H. Mess and F. Bordry).
S. Fartoukh and R. Ostojic