Minutes of the LRFF Task Force

2nd meeting on Tuesday 27/03/2012 (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), 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

- No comment on the last minutes.

- See list of actions.

- RJ could not join today and will present the start of the BI equipment review in 2 weeks.

- Discussion as foreseen between EM and Pierre Strubin yesterday afternoon => See 2). Pierre will come on 17/04/12 to give more detail.

2) Summary of past work on the PIMs, PIMs' crisis and PIMs Working Group (EM): pdf

- Question from AlessandroB: what is the Cu-Be grade used? AlessandroB reminded us that the grade is a key player each time we have heating. In the present case there is no bake-out but in some other cases in can play an important role. After the meeting EM found in the specs that the grade is: CuBe 17410 1/2 HT (page 11).

- Reminder: SSS = Short Straight Sections => These sets of magnets to focus the beams contain, among others, the main superconducting quadrupoles (but also insertion region quadrupoles and a large variety of corrector magnets, diagnostics and cryo and vacuum systems...

- Reminder from the specs:

- This specification concerns the supply of the mechanical components for the LHC beam vacuum arc interconnects.

- 1232 dipoles + 410 SSS => PIMs are everywhere between cold magnets and the most critical ones are between SSS and MB.

- 3 different types of interconnects (PIMs): dipole - dipole, SSS - dipole and dipole - SSS.

- Requested quantities: 1830 dipole-dipole + 914 SSS-dipole + 914 dipole SSS => 3658 in total.

- Some of the components require rhodium and gold coating.

- A bellow is needed to allow for thermal expansion as well as for mechanical and alignment tolerances between 2 adjacent beam screens at the location of the RF-contacts.

- An RF-contact has to ensure smooth cross-section with low impedance when the LHC is at operating conditions. Gold and rhodium coatings are foreseen to optimise the contact resistance of these RF-contacts allowing the global DC resistance to be under 0.1 mOhm at room temperature.

- All the welds shall be leak tight to UHV working conditions => No leak exceeding 10^-11 Pa.m³/s shall be observed on a recorder connected to the leak detector during a global leak test with helium with a deltaP of 0.1 MPa during at least 10 minutes.

- To avoid stresses in the cooling tubes due to differential thermal expansion between the cold bore and the beam screen on the downstream interconnect side, a nested bellow is installed.

-The lifetime of the LHC from first circulating beams is expected to be 20 years. The interconnects shall function reliably all this time.

- What is the PIM stroke? => Action 1 (VSC).

- Increasing the contact pressure you increase the contact friction...

- It was therefore not a design pb but a manufacturing pb.

- The SSS have been displaced by 2 mm => RF fingers will be more compressed (tbc) => Action 2 (VSC).

- VB reminded us that the RF ball is used before warm-up and before cool-down as well .

- JMJ reminded us that in particular the PIMs at the extremity of continuous cryostats and vacuum barrier are more critical due to the change of temperature. The strategy was to stay like this and change the PIM when something has to be opened or changed etc. by a conform PIM (new ones have been ordered and built at Novossibirsk). Old ones are modified to be conform and are then used as spares (with proper tools, after measurements etc.).

- In the presentation of JMJ at Cham12 (vacuum upgrade), the current planning is explained.

- Important remark:

- For the PIMs, no transverse movement is foreseen (which also increases the probability of buckling), the magnets are fiducialized etc., and only the longitudinal elongation and contraction are foreseen => Best solution found to have the RF fingers contained between the (inner) copper transition tube and the (outer) PIM transition tube and have good contact pressure. Seems in this case impossible to have a huge gap (leading to a huge resonance) between the RF fingers and the inner tube as observed with the VMTSA => Much better in this respect!

- This is completely different from the case of the VMTSA where a solution had to be found to allow transverse displacements => Solution to use long RF fingers with a spring to keep them in contact with the (inner) copper insert. But there is no outer tube in this case => In case of pb with the spring etc. a huge gap can be created and therefore a huge resonance may appear leading to a significant RF heating. 

3) Start of the equipment review: some equipments from VSC => Vacuum Modules in LHC Experiments (VB): pptx

- VB started to review the vacuum modules in the 4 LHC experiments => See also Vacuum modules in LHC experiments where the info has been summarized.

- VB provided for the 4 experiments, the name of the equipments (modules, from database), the name of the vacuum sectors, DCUM = longitudinal position around the ring, the distance from IP and the associated CDD drawings.

- VMTSA is one from the family VMABB.

- Typo: VC1JP, X => VC1JP.

ATLAS VBX => 2
- Edge-weld bellow and not a hydroformed (more common) bellow. Long elongation longitudinally but nothing transversally => It is a weak point of the design. Marc Galille is in charge and the impedance team should be involved. It is under specification.

- There is a spring to touch but it is not represented on the VBX drawing => It is currently redesigned and will be changed for the next shutdown.

ATLAS VC1JP => 2

- On top there is a tube which explains the sliglty different name from CMS 18 m.

- Flange on the TAS (on the right) and therefore IP on the left.

- Tube in SS =< Cu coating of 10 microm.

- Edge weld bellow also.

CMS

- 3 vacuum sectors also, symmetric around IP.

- 4 special ones for CMS.

- Be careful => mentioned 481 mm (free) but installed at 500 mm => Overlap is reduced!

CMS VBX5A

- All this is surveyed by the survey team => Very accurate tracking of the components => We don't expect huge transverse displacements.

- The stroke is for bake-out.

- Reminder: all the experiments are baked-out => The grade of Cu-Be can be important as mentioned by AlessandroB!

ALICE

- 4 vacuum sectors, 1 beside the triplets L and R, 1 for IP (displaced by -5 m) and 1 for RB24 where we have the compensator magnets.

- All these modules have inserts.

- Reminder: X describes the flange, Y describes the RF insert topology, Z describes the body (1 port, 2 ports etc.).

- 80/80 standard for vacuum pipes and the other 3 are standard for vacuum valves.

ALICE VMABA

- B = 63/63.

- Hydroformed bellow => Allow a bit more due to the length stroke = +-20 mm => Be careful: Not to be installed at the same position depending if we increase or decrease temperatures! => Some errors at D1 and Q3: will be solved at next shutdown.

LHCb

- At the VELO chamber, wakefield compensator.

- Reminder: again asymmetric as it is for forward physics.

- Inside the secondary vacuum there is a tapering. Thickness of Berylium chamber = ?.The window in aluminum is 2 mm => To be clarified.

- After discussion, it was decided that we should start to perform bench impedance measurements on the ALICE VMABA, which is quite simple, to validate our method (comparing these measurements with electromagnetic simulations) => Action 3 below.

-The current plan for the impedance team is therefore to work on 2 parallel things:

- VMTSA (where issues were observed in operation in 2011) => Try and fully understand the bench measurements (comparing in particular with simulations).

- ALICE VMABA (where no issue was observed in operation in 2011 and which is quite simple) => Perform first bench impedance measurements to see if we find some problems. Then benchmark with simulations to validate our procedure.

- Question from JMJ: what will happen in case of bad manipulation in the tunnel? Regularly there are some re-alignment under vacuum in the tunnel and this is always a risky operation. One can end up with a configuration where we have bad contacts etc. => Reminder: There is the design, the installation and what the people are doing with it...

- AlessandroB proposed to think about all the possible failing scenarios (coupling between RF heating and buckling etc.) and rank them => Use the Failure Mode, Effects and Criticality Analysis (FMECA).

4) Some equipments from BI (RJ)

- Postponed in 2 weeks.

5) Actions to be taken for the next meeting

- Old actions.

- New actions:

- Action 1 (VSC): What is the PIM stroke?

- Action 2 (VSC): Confirm that the SSS have been displaced by 2 mm such that the RF fingers are now more compressed.

- Action 3 (VSC): BS and OB to prepare with VB etc. the bench impedance measurements on the ALICE vacuum module called VMABA.

6)  Miscellaneous

- The next (3rd) meeting will take place on 03/04/2012 between 09:00 and 11:00 (max.) in room 6-R-018 => Agenda:

1) Past development work on RF contacts (Sergio Calatroni).

2) News on the VMTSA impedance (BS).

- See preliminary agendas for the next meetings.

Minutes by E. Metral, 28/03/2012.