Minutes of the LRFF Task Force
4th meeting on Tuesday 10/04/2012 (09:00-11:00 max, 6-R-018)
LRFF members: Alessandro Bertarelli (AlessandroB), Alexej Grudiev (AG), Benoit Salvant (BS), Elias Metral (EM), Fritz Caspers (FC), Giuseppe Bregliozzi (GB), Hugo Alistair Day (HD), Jose Miguel Jimenez (JMJ), Marco Garlasche (MG), Mike Barnes (MB), Olav Ejner Berrig (OB), Oleksiy Kononenko (OK), Oliver Aberle (OA), Ralph Assmann (RA), Raymond Veness (RV), Rhodri Jones (RJ), Roberto Losito (RL), Stefano Redaelli (SR), Vincent Baglin (VB), Vittorio Parma (VP), Wim Weterings (WW).
Present/Excused: AlessandroB, AG, BS, EM, FC, GB, HD, JMJ, MG, MB, OB, OK, OA, RA, RJ, RL, SR, VB, VP, WW.
1) Comments on the last minutes + Actions
- Not yet available. Should be released today...
2) Equipment review: some equipments from TE/VSC => Vacuum modules in the LHC septa (VB): pptx
- VB discussed today the vacuum modules in the LHC septa, i.e. MSI at injection points (LSS2 and LSS8) and MSD at extraction/dump point (LSS6) => See summarized info with drawings at http://emetral.web.cern.ch/emetral/LRFF/InfoOnLHCEquipmentsWithRFfingers/VacuumModulesInLHCSepta/VacuumModulesInLHCSepta.htm.
- All the equipments discussed today have the "usual" RF fingers characteristics (to be checked by VB): CuBe RF fingers (gold plated?) on Rhodium coated (3 microns) copper insert etc. Furthermore they also all have a spring.
- Reminders from the LHC Design Report:
- A series of 5 Lambertson-type septum magnets deflects the beam horizontally by 12 mrad under the outer ring (in the direction of the beam to be injected 3 MSIB magnets are followed by 2 MSIA magnets and including the inter-magnet gaps, the whole system stretches over 21.8 m). A series of 4 MKI kicker magnets deflects the beam vertically by 0.85 mrad onto the orbit.
- The MKD kickers deflect the entire beam horizontally into the high-field gap of the MSD septum. The MSD provides a vertical deflection to raise the beam above the LHC machine cryostat before the start of the arc sections. The dilution kickers are used to sweep the beam in an e shaped form and after the appropriate drift distance the beam is absorbed by the TDE assembly. The TCDS and TCDQ serve to protect machine elements from a beam abort that is not synchronized with the particle-free beam gap.
- See some slides about injection and dump septa.
- LSS2:
- Beam going vertically but with different positions at each MSI => Shape outside is the same but inside it is different.
- The difficult point is on the OUT pipe as we have the injected and circulating beam => Will focus on this only.
- VMSIR0002 after the 1st MSIB, VMSIR0001 after the 2nd MSIB, VMSIS after the 3rd MSIB and before the 1st MSIA => There is a step between the last MSIB and the 1st MSIA. VMSIP0003 after the 1st MSIA, VMSIO after the 2nd MSIA and before the BTVSS => Recombination of circulating B1 and injected B1. VMSIA accommodates the BTVSS offsets.
- The injected beam has no RF fingers (as it is passing only once) but it can burn the RF fingers from the circulating beam if it is too close => Certainly what happened in the past (~ September 2009) when it was found that the step was not mounted correctly (opposite direction). It was concluded that the halo of the injected beam heated up the RF fingers which got loose, went down, generating an aperture restriction for the injected beam and OP had to make a bump to go around and be able to inject. See for instance pictures 1 and 2, and some discussions during the 72th LMC held on 20/10/2010.
- VMSIO=> A spare is available to make bench impedance measurements => See Action 1 below.
- LSS8: Same logics with only slight differences.
- VMSIR0004 after the 1st MSIB, VMSIR0003 after the 2nd MSIB, VMSIQ after the 3rd MSIB and before the 1st MSIA => There is a step between the last MSIB and the 1st MSIA. VMSIP0002 after the 1st MSIA, VMSIN after the 2nd MSIA and before the BTVSS => Recombination of circulating B2 and injected B2. VMSIZ accommodates the BTVSS offsets.
- Reminder: all the offsets discussed are fixed ones => The RF fingers are only here for the bake-out (for the thermal dilatation / contraction). The survey people know that they should not realign under vacuum as it can be very dangerous for the bellows (only ~ 2 mm maximum allowed, tbc). If survey people do corrections it can only be very small displacements.
- Reminder from VB: all these equipments have been Xrayed and they are all fine. Note that the Xrays were done by radiography and not by tomography (not easy to handle).
- LSS6:
- Circulating beams and ejected beams => Only special modules and no normal ones.
- It is symmetric (for IN and OUT).
- VB started from the end (VMSDU, as the beam 2 is going towards that): VMSDU, VMSDT, VMSDS, VMSDR, VMSDQ, VMSDP => Question raised by VB: do we need a recombination insert (as we have at LSS2 => VMSIO and LSS8 => VMSIN)? => See Action 2 below.
- FC commented that he never saw all these special vacuum modules in the past => It is important to review all of the them for the future high-luminosity operation of the LHC.
- Reminder: when the geometry is round it is much better for the contact force than when the geometry is elliptical or has a race-track shape => In the latter case the contact force is very small on the straight parts. AlessandroB mentioned the possibility to put a flap (metal sheet) on the flat part instead of fingers. FC mentioned that with a metal sheet we will have only 2 points of contact (or 3 points at max) whereas we can have many individual independent points of contact with the RF fingers (and this was the idea).
- VB is quite confident with the normal equipments as they were studied in detail in the past but the special ones have to be checked.
- FC suggests to replace all the springs using the funnel concept, i.e. keeping the RF fingers between 2 fixed parts, one inner and one outer part to assure the contact to one or the other, i.e. it is the double-contact concept (as it is done with the PIMs) => Was already discussed in our past meetings as it seems indeed a good idea.
- AlessandroB mentioned that in the LHC collimators we use also the funnel concept with 2 coaxial cylinders. Seems that it works well for the moment.
- FC reminded us that the specifications for the DC contact resistance (< 0.1 mOhm for the PIMs) were made in the past without a good knowledge of the impedance a low frequency which resulted into unnecessary tight constraint, which resulted in buckling etc. => We will review what could have been the specifications with our present/new understanding => See Action 3 below.
3) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1 (HD): Contact VB to have a spare VMSIO module (recombination of circulating B1 and injected B1) to make bench impedance measurements.
- Action 2 (Impedance team): Do we need a recombination insert for the VMSDO at LSS6 (as we have at LSS2 => VMSIO and LSS8 => VMSIN)? => Contact VB to have a spare VMSDO module to make bench impedance measurements.
- Action 3 (EM and FC): Review of past specifications for the PIMs DC contact resistance and thickness of copper / good conductor layer on some equipments.
4) Miscellaneous
- The next (5th) meeting will take place on 17/04/2012 between 09:00 and 11:00 (max.) in room 6-R-018 => Agenda:
1) Past work on the PIMs, PIMs' crisis and PIMs Working Group (Pierre Strubin).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 10/04/2012.
