Contacts around LHCb Velo RF box Benoit Salvant
Contacts around LHCb Velo RF box Benoit Salvant, Branko Popovic, Christine Vollinger Thanks to colleagues from IWG, LHCb, SMOG 2 and NIKHEF impedance meeting 04/10/2019
Context • Major upgrade of LHCb detector: new VELO • Discussed with impedance team/IWG since years • Focus since 2017 on the addition of the SMOG 2 cell on one side of the VELO
Complete VELO model with SMOG 2 (Open Position) Closed Open SMOG 2 Model Branko Popovic
Context • Major upgrade of LHCb detector: new VELO • Discussed with impedance team/IWG since years • Focus since 2017 on the addition of the SMOG 2 cell on one side of the VELO • Ad-hoc meeting organised yesterday with SMOG 2 and VELO colleagues to discuss contact issues on the VELO/SMOG 2 side
situation on the SMOG 2 side Observation by SMOG 2 colleagues of RF contact foil deformation (sept 17 th) Request for IWG recommendation
situation on the SMOG 2 side RF box RF foil SMOG 2 Worrying deformation. No point of fixation on RF box side Not clear that contacts are well defined
situation on the SMOG 2 side RF box RF foil SMOG 2 Worrying deformation. No point of fixation on RF box side Not clear that contacts are well defined
situation on the SMOG 2 side SMOG 2 RF foil RF box Worrying deformation. No point of fixation on RF box side Not clear that contacts are well defined
Additional files presented yesterday Worrying deformation. No point of fixation on RF box side Not clear that contacts are well defined
Additional files presented yesterday RF box mockup RF foil SMOG 2 Worrying deformation. No point of fixation on RF box side Not clear that contacts are well defined
Changes to SMOG side WFS Mounting LHC-UNPOL-TARGET_ASB-V 5. stp LHC-UNPOL-TARGET_ASB-V 7. stp Branko Popovic, files from Vittore Carassiti
Picture of the mock-up (Branko Popovic)
Ideas? Make sure that contacts are well defined Ensure that the foil will not fall off on the RF box side (true for the free SMOG 2 arm, the other one has a star and is fixed) To be discussed at IWG
situation on the detector side We have assumed perfect contact so far between the wakefield suppressor and the RF box
situation on the detector side Vittore said that the situation with SMOG 2 was better than the configuration without SMOG 2
situation on the detector side And could even have degraded to this if the foil slides on the mushroom pins Mushroom pins
situation on the detector side And could even have degraded to this if the foil slides on the mushroom pins Worrying situation for the other side of the VELO, if really the situation: - Impedance will increase due to this 2 mm hole, that would be 3. 5 mm from the beam when closed - The contact could be very poor : only the thickness of the foil contacts with the small pin. Could lead to local heat and damage after several cycles.
Where we beginning of October? • We warned the LHCb colleagues • We are assessing the exact geometries with them and NIKHEF. • We are looking for solutions to ensure contact, in case it is needed. • RF measurements on a mock-up were done, and the foil was contacting well then, but no cycling was performed to avoid stressing the wakefield suppressor.
Meeting with SMOG 2 colleagues mid-October • Saw the mock-up • The foils is more rigid than we thought • Freek (NIKHEF) said that the current contact is not acceptable from his point of view either (potential scratching) • New design being worked on: http: //www. fe. infn. it/~vito/LHCb-target-cell/IMG_3164. MOV • Did not know if would coat the wakefield suppressor with carbon discussions ongoing
Three questions • Is the new design from SMOG 2 acceptable? • Is this ~2 mm gap ok from impedance point of view? Simulations afterwards and by Branko • Is the contact ok? can we measure and compare to existing specs?
Toy model to see broadband impedance from VELO contact gap
Toy model 3. 5 mm half gap 1 mm x 1 mm square rods with perfect contact
Longitudinal impedance Resistive wall shape Im(Z/n)=2. 1 Ohm/(1 GHz/11245 Hz) Im(Z/n)= 0. 02 m. Ohm Small contribution compared to the VELO
Vertical impedance 1 mm displacement Simulation initially unstable Required mesh refinement to converge Broadband contribution Zt~12 Ohm/1 mm Zt~12 k. Ohm/m Not a worrying contribution
Some scans: Re(Z//) No gap 1 mm gap 4 contacts near the beam 2 mm gap 5 contacts Resistive wall shape not affected by contact, as expected Resonant modes strongly affected by contact numbers and gap
Some scans: Re(Z//) No gap 1 mm gap 4 contacts near the beam 2 mm gap 5 contacts Resistive wall shape not affected by contact, as expected Resonant modes strongly affected by contact numbers and gap
Some scans: Im(Z//) No gap 1 mm gap 4 contacts near the beam 2 mm gap 5 contacts Clear impact of gap: Im(Z/n) increases if gap increases (as expected). No impact of “star” contact on Im(Z/n) encouraging signs that contributions could be decoupled.
Some scans: Re(Zy) No gap 1 mm gap 4 contacts near the beam 2 mm gap 5 contacts Broadband contribution not affected by contact Resonant modes affected by contact numbers and gap
Some scans: Im(Zy) No gap 1 mm gap 4 contacts near the beam 2 mm gap 5 contacts Clear impact of gap: Im(Zy) increases if gap increases (as expected). No impact of “star” contact on Im(Zy) encouraging signs that contributions could be decoupled.
Can we conclude something? Very coarse toy model Indications that the resonant modes and the environment do not affect too much the broadband contribution from the gap and the contacts. Broadband contributions in the toy model are not large and are perturbations on top of the full VELO contribution To be recalled that the full VELO contribution is enormous when compared to its total length (it represents 5% of the total longitudinal Im(Z/n) for ~1 m out of 27 km).
- Slides: 31