BNL-CERN-UK collaboration meeting on DQW Crab Cavity - April 20, 2015
Minutes taken by Silvia and reviewed by Carlo (last update: 29 April 2015)
Attendees:
- UK: Graeme, Nick.
- FNAL: Tom Nicol.
- CERN: Ofelia, Rama, Carlo, Marco, Paula, Kurt.
- LBNL: Alex.
- BNL: Binping, Qiong, Sergey, Silvia.
Agenda:
1 - Updates on RF design of HOM filter (Binping)
- High impedance mode at 1.75 GHz ~109 (which kind of mode is it?, which is its R/Q?), but not very sensitive to tolerances. Some possible solutions are presented to reduce this mode impedance. These options will be explored for the LHC cavity, not for SPS.
- Good agreement of Qext calculation using waveguide port and absorber mode in CST.
- Actions for Binping:
- Check which is the frequency shift for some critical modes of PoP cavity from simulations and compare to measured values.
- For HiLumi-LARP meeting, check frequency shift due to cooldown for some critical modes. Discuss how position of couplers will influence frequency shift.
- Main freq shift for SPS DQWCC comes from cooldown: 573 kHz according to ANSYS-APDL simulations. Is this value comparable to the measured frequency shift of fundamental mode for PoP cavity?
- Separate transverse and longitudinal modes in impedance plot.
- Add plot of 1.75 GHz HOM to these minutes.
- Update the R/Q table in EDMS.
2 - HOM thermal update (Graeme)
- Thermal study takes into account that surface resistance is thermal dependent.
- Stress for Lorentz detuning when plates are free is shown.
- Lorentz detuning is 44 Hz/MV2, but FPC was not fixed and tuner is a simplified version where only central rod is considered, but not beam-shaped interfaces. Besides, the He tank was in stainless steel instead of Titanium. Deformation is in the order of micrometers. The APDL simulation was performed for a simplified cavity model with no vessel, where ports were assumed to be completely fixed by the vessel. The simulation was performed for two scenarios: i) central plates are free, Lorentz detuning is about 350 Hz/MV2; ii) central plates are fixed, Lorentz detuning is below 1 Hz/MV2. The CST results are in between.
- Lorentz detuning results from CST show that vessel deformation is negligible, so one could assume that ports are fixed as Silvia did in a simplified simulation conducted with APDL.
- Actions:
- Graeme found that some cavity models are corrupted when trying to open them or meshing in CST or ANSYS. Follow up with Graeme if SPS cavity model can be modified and imported correctly (Carlo, Silvia, Graeme, Raphael).
3 - Mechanical update for HOM filter (Nick)
- Cold magnetic shield into vessel. Panels with rounded corners. Shown different configurations.
- Sealed magnetic plates instead of overlapping and attached by screws.
- Ofelia comments that they are considering changing slightly supporting system for cavities in cryomodule.
- Actions:
- Carlo is working on the modal structural analysis. Very small response but excitation from vacuum pumps and cryogenics is not realistic. Carlo will send last calculations to Nick about this.
- Bolted He vessel modification suggested by Nick; to be considered by CERN.
4 - Update on CERN activities (Carlo)
- Stress calculations on Ti bolts and screws are ongoing and OK, although the margin factor is limited. Simplified model used for bolts: cylinder constrained at the extremities.
- Mechanical simulations include gravity, fixed supports (FPC and support system) and bolt tension, beside the internal pressure applied during cool down. The stresses shown in the slides are calculated for a pressure load of 0.18 MPa plus a pretuning of 0.2mm (about 0.12 mm on the cavity). 75 MPa is the yield limit found in stress studies for Nb. 50 MPa is the stated yield field for safety reason (according to PED). Cavity reached about 120 MPa for bottom part of cavity in the opposite side of the two asymmetric ports. Ofelia remarks that these simulations show improved stress results for the cavity than previous versions. Besides, the high stress induces plastification, which is a desired effect for pretuning.
- Efforts to reduce He vessel plates mass by carving some parts away.
- Latest HOM filter case design includes bellow to reduce stresses. HOM filter case is made of stainless steel. Currently qualifying EBW, machining and tools. Plans to evaluate shape response to BCP.
- Ofelia points out that there is not heavy BCP planned for HOM hook because it is machined by bulk Nb. They believe that machining hook from bulk Nb will not show as bad surface as formed pieces from Nb sheets, so the machined hook does not need so heavy BCP. The different light BCP steps will sum up a total of 100 micrometers.
- Non-standard welds are foreseen for HOM filter case, so further mechanical checks are on-going. SS welds of 2mm. Simulations show an intensification of stresses. The amount of a possible localized plastification is upper-bounded by the dimension of the mesh, which has been gradually reduced down to 0.1 mm.
- On-going tests: hardness measurements on Niowave samples.
- Two alignment systems for positioning are shown. BCAM uses targets mounted on the He tank. Alumina bar to avoid reflections. Calibrated CCD measures the position of targets on which a laser is reflected. Another alignment system is FSI. This FSI is the responsible of additional chamfers in flanges.
- Niowave plans to send first parts to Sciaky for welding during 2nd week of May. What about weld quality? Non-conformity weld means a hold point for Niowave from joint CERN and LARP management to proceed welding.
- Alex says that we should also request weld samples from JLab, because CERN will still need to qualify them.
- A HOM filter prototype is being fabricated at CERN to validate fabrication steps and chemistry. Getting prepared to manufacture all filters at CERN. CERN has Nb for all the hooks and is launching order to acquire Nb for outer part of the filter.
- Both EBW and TIG could be used as weldings of the stainless steel HOM filter case. However, EBW is chosen by CERN due to better accessibility in their facility to this service.
- Cooling for HOM filter will have two outings. A large diameter outing to connect to 2K line for cooling down phase. A small 8 mm tube outing that could be connected to 4K line in case it is needed.
- 100mm aperture: crabs are main broadband contributors to impedance in LHC. Reduced impedance by increasing aperture? Is deflecting voltage compromised by this large aperture? It is fine to start working on an “easy” case when only cavity waist is constraint by adjacent beam pipe (cavity only for vertical kick configuration) and cavity height is free (useful to adjust resonant frequency of the cavity). HOMs should be studied up to 2GHz.
- Kurt comments that tuner includes pressure compensators.
- Actions:
- Review latest functional drawing by Friday. Main differences from previous version: 1 - modification on port interfaces, 2 - some modelling for rounded parts that should lead to less corruption failures when opening the model with other software, 3 – RF surface is the same (BNL).
- Update frequency studies and tolerance studies in EDMS (including weld tolerances) (Silvia).
