Minutes of the LRFF Task Force
11th meeting on Tuesday 26/06/2012 (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), Sergio Calatroni (SC), 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, RV, RJ, RL, SC, SR, VB, VP, WW, Massimiliano Ferro-Luzzi.
1) Comments on the last minutes + Actions
- Sent just before this meeting...
- No particular comment.
2) More information about the wake field suppressor in the LHCb VELO (Massimiliano Ferro-Luzzi, PH-LBD): pdf
- Reminder of VELO WF (Wake field suppressor) studies. The Wake Field Suppressor is made of two 0.075 mm thin CuBe foils, compressed with gear wheel and rack. CuBe is chosen for the good electrical and elastic properties. The 0.075 mm thin CuBe foil can be hardened at 320 degrees Celsius to get better spring properties => Production: http://www.nikhef.nl/pub/departments/mt/projects/lhcb-vertex/production/wakefieldsuppressor/.
- Reminder: VELO = Si strips at the interaction point (in vacuum). LHCb measures mainly forward particles. The best is to be material-less with a very good conductor.
- On the right of the picture (slide 2) we see the wake field suppressor connecting to the beam pipe and the interaction point is on the right.
- 5 notes summarize all they did on this subject (work from the student N. Van Bakel from NIKEF): 1, 2, 3, 4 and 5.
- 1st study in 1998: dream design with perpendicular plates etc. => They were finally convinced that they had to put the strip shielding and then they lost the advantage on being perpendicular => They moved to a 2nd design without strip shielding but reduced corrugation.
- The detectors are cooled. There are 2 boxes (left and right) and on each box we have 21 modules and each module dissipates ~ 40 W. The screen is not cooled but only the detector (therefore one relies in this case only on black body radiation). On the slide 6 they draw also the curve of 100 W just to have an idea and at this value things are worried...
- VB said that they observed pressure rises in the VELO but is it due to e-cloud (as there is no NEG coating => In fact, Massi reminded us that there is NEG coating on the Al RF box but not on the CuBe WF suppressors (although the NEG coating is likely to be non-pumping, it should still have a beneficial effect on the SEY))? Is there also some contribution from RF losses? Normally during the run the cooling is always ON and we are not aware of any issue (so far).
- Impedance simulations were performed showing that losses due to resonant modes become acceptable for a corrugation depth smaller than 20 mm. Im[Zl/n] at low frequency was estimated at ~ 5.5 mOhm.
- The production of RF boxes and wake field suppressors was then discussed.
- Concerning the movements: It can move vertically only a little (~ +- 5 mm) and it is mainly horizontally that it has to move by +- 30 mm. Longitudinally there is also almost no movement.
- Massi reminded us that for the VELO sliding contacts where never considered, but it is clear that the stroke range requirements for VELO and LHC PIMs were very different.
- It seems there is aluminum there and not beryllium.
- It seems there is another kind of RF fingers used in KEKB according to VB (see Action 1 below).
- The slots, between the RF fingers, are for vacuum pumping and for them it is better to minimize the material so it is good (2 reasons not to use sheets).
- Reminder: for the physics run it is closed. It is open when we are not in stable beam and then when we are in stable beam we close the VELO gap (with a radius of 5.5 mm when it is closed). On some LHC pages we can see the VELO gap. The beam pipe is ~ 50 mm there and 2 beams are circulating in this part.
- Summary and outlook:
- Present:
- Well-studied VELO design in terms WF/Impedance effects paid off: VELO works like a charm.
- No RF pick-up from beam ever observed.
- No RF heating due to beam ever observed (predicted to be order of 1 W).
- No impedance effect on beams ever observed => One could maybe try, as proposed by BS, to look at the synchronous phase shift and tune shifts when they close the gap in stable beams.
- Future:
- LHCb Upgrade (in LS2): we intend to reduce the inner radius of the foil (5.5 => 3 − 4 mm).
- Impedance aspects will be followed-up by the impedance team and reported through the LEB.
- Massi reminded us that the experiment can benefit a lot from reducing the beam pipe radius because then the extrapolation distance to the vertex is smaller and the result more precise.
- BS asked if there is a temperature sensor => Yes and it should be possible to find it on Timber. Massi reminded us that it is the VELO gap which should be in TIMBER but he is not sure the foil temperature is (but it is archived in LHCb).
3) Temperature history of the TDI and VMTSA (VB): pptx
- VB showed 2 slides revealing:
1) No temperature increase in the VMSTA in 2012 => Excellent! The modifications implemented during the crash program in the shutdown 2011-2012 worked very well.
2) Temperature increase in both TDI (LSS2 and LSS8), i.e. confirming the observations of last year (when the TDI was fully retracted in parking position).
- 1st slide is for LSS2, where we see that the 2 curves of the TDI (upstream and downstream) are on top of each other (with a max of ~ 39 deg) and that there is a beam-induced increase. The almost constant curves correspond to a reference chamber and the VMTSA.
- 2nd slide is for LSS8, where we see that the 2 curves of the TDI (upstream and downstream) are not exactly on top of each other (but this is almost the same result )and that there is a beam-induced increase. The almost constant curves correspond to a reference chamber and the VMTSA.
- No temperature increase visible on VMSTA is confirmed by the absence of vacuum pressure spikes since 2012.
- Reminder for the TDIs: There are 2 sources: thermal heating (due to RF) and e-cloud. There is for sure e-cloud as when the beam disappears, the signal decreases very rapidly and then there is a second regime corresponding certainly to the RF heating.
4) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1 (VB): Send a note on another kind of RF fingers in KEKB.
- Action 2 (BS): Follow-up of the LHCb Upgrade (in LS2) and impact on the LHC impedance if the radius is reduced from 5.5 mm to 3-4 mm.
5) Miscellaneous
- The next (12th) meeting will take place on 31/07/2012 between 09:00 and 11:00 (max.) in room 6-R-018 => Agenda:
1) (First) review of ferrites used at CERN to damp HOMs: references, manufacturers, thermal treatments, etc. (all the groups: TE/VSC, EN/STI, TE/ABT, BE/RF, BE/BI, BE/ABP, EN/MME etc.).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 26/06/2012.