Minutes of the LRFF Task Force
12th meeting on Tuesday 21/08/2012 (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, MarkG.
1) Comments on the last minutes + Actions
- Discussion about new RF fingers for light source according to JMJ.
- Ferrite issues: until now we were saved by FC and his stock of ferrite. We should start to purchase some ferrite and anticipate the needs.
- FC said that he has a stock and that we should keep the TT2-111R and not 8C11 or any other due to the much high Curie temperature.
2) Beam impedance of 63mm VM with unshielded Bellows (OB): pptx
- CST model of the VM module (with diameter = 63 mm) and of the RF fingers. This model was in fact not approved. To be followed up.
- From a theoretical formula (whose exact reference is being checked), which overestimates the impedance (as it assumes a square bellow and not a sinusoidal one) and which is only valid up to the first cut-off frequency (~ 3.6 GHz), one finds:
- Im [Zl / n] = 0.072 mOhm, if the RF fingers are pressed as in operation.
- Note that when b' = b (i.e. inner radius equal to the outer radius), then no impedance is found, as expected
- This formula shows that a bellow with a deeper corrugation has a smaller impedance as the length is then smaller => We should express L as a function of b' => To be followed up (and possible trapped modes as well which are not included in this simple formula) => Was discussed during the next talk.
- Comment/reminder from JMJ about the new RF fingers design: this was studied in the context of CLIC etc (see http://emetral.web.cern.ch/emetral/LRFF/9thMeeting_05-06-12/LRFF_CG_5_VI_12.pptx). We want to use it where we are very tight in space and for lateral movements (near Q1 in fact) => The goal is not to replace all the RF fingers by this.
3) Longitudinal impedance of new RF fingers (BS): pptx
- CST simulations for the longitudinal impedance from an ideal shielding: Im [ Zl / n ] = 0.008 mOhm.
- CST simulations for the longitudinal impedance from a bellow with shielding:
- Difficult to mesh correctly. Blue cells => Code is not resolving very well.
- Thin shielding very difficult to simulate but similar value obtained => Conclusion: the shielding is working very well in longitudinal.
- From the eigenmode mode solver, no harmful longitudinal modes were detected.
- Reminder: simple theory tells that beam does not talk to TE mode.
- Questions from VB (see Action 1 below):
- What happens if we have a transverse offset (~ 10 mm transverse and ~ 10 mm compression in longitudinal) as this is like this that we want to use it?
- What about the tolerances?
- What about transverse plane?
- Reminder: We should assume ~ 10 of these equipments.
4) Electromagnetic Simulations of VMTSA Equipped with the Longer RF Fingers (OK): pptx
- New simulations for the longer RF fingers (i.e. old design in the machine in 2011) to compare to the measurements we made. As already mentioned in the past there are many ways to deform the RF fingers, the simplest way has been introduced in CST and different gap sizes have been studied.
- OK reminded us with the different models he simulated in the past and the ongoing simulations.
- With no RF fingers on both sides, a resonance at ~ 650 MHz is found in CST and HFSS in good agreement with measurements (done if I remember well with RF fingers on one side and none in the other).
- With RF fingers but with a gap, resonances are found and the best agreement with the measurements is obtained for a gap of ~ 3 cm, which is certainly close to what we had. In the measurements the resonance was at ~ 200 MHz, whereas in the simulations we have something between 250 and 300 MHZ, but the gap shape is not exactly the same in both cases and we cannot expect a perfect agreement but it seems that we understand what happened.
- OK made then some CST TD (Time Domain) simulations with conforming RF fingers and 3 cm gap:
- No surprise: introducing a gap we are getting unwanted modes at high Q.
- Some of the resonances could also be seen in the wire simulations.
- Looking at the tangential component of magnetic field, it is seen that unwanted modes with high Q have been excited near the deformed RF fingers.
- Reminder: losses proportional to imaginary part if the frequency.
- Conclusions (same as before):
- Long RF fingers with good contact => No impedance pb, no surprise.
- Bad contact => Impedance pbs depending on the gap size/shape.
- Next step (see Action 2 below): better approximate the shape of the gap and introduce the ferrite.
5) Typical NC (Non Conformity) in Warm Modules Following X-ray campaign (VB): pptx
- X-ray campaign started in Jan 2011 and should be finished at the next TS (many people working on this):
- 1800 X-rays taken.
- 95 NC (~ 5%).
- Reminder: There are 190 vacuum sectors at room temperature (58 are concerned, i.e. ~ 30%) + 88 sectors at cryogenic temperature.
- VB reminded us with the "Criteres d'acceptation" about the "ressort de traction", "doigt RF" and " distance de recouvrement" => 0 < L <= 15 mm (see picture).
- VB showed an example of documentation with a conform situation.
- Reparation campaign:
- Started during winter technical stop 2011-2012 => 52 vacuum sectors remaining.
- To be continued during LS1 => Strategy:
- All vacuum sectors to be open for activities other than VSC will be repaired.
- Currently ~ 110 vacuum sectors are foreseen to be opened. Reminder: opening a sector relies on time / manpower and availability of materials.
- 18 vacuum sectors will not be opened, i.e. 29 NC which need to be classified use as is / repair according to:
- X-ray,
- Pressure activity in the vicinity,
- Impedance simulations/measurements.
- VB showed us then several cases of NC:
- Case1 (VMAAB, sector A7R4.B): Spring out, deformation, zero overlapping length => Is this acceptable impedance wise?
- Case 2 (VMAAF, sector C5L6.R): Spring out, deformation, aperture restriction (beam is passing trough since years) => Is this acceptable impedance wise?
- Case 3 (VMAAB, sector C5L6.R): Spring out, RF fingers blocked, overlap acceptable => Is this acceptable impedance wise?
- Case 4 (VMAND, sector C5L6.R): Spring in place, 1 RF finger out, overlap acceptable => Is this acceptable impedance wise?
- Case 5 (VMAAE, sector A7R1.R): Spring in place, some RF fingers out, overlap acceptable => Is this acceptable impedance wise?
- Case 6 (VMACC, sector A7R7.B): Spring out, RF fingers blocked, overlap acceptable => Is this acceptable impedance wise? Triggered a BD of fill 2978. Increase vacuum interlock => Use as is for beam vacuum performances.
- Reminder from FC: Anything not touching is a serious issue because we can have runaway effects => We have to define what serious is etc.
- FC reminds us that with X-rays it is difficult to see if the contact force is OK or not.
- In some case the RF fingers are touching the bellows => Question from JMJ: can there be an arc between the RF finger and bellow? With the arcing / sparking can this make a hole in a bellow? => Could be then a big pb.
- This committee has to make recommendations and gives priorities as it seems it will not be possible to repair all the RF fingers.
- Conclusions:
- X-ray campaign of warm module is (almost) finished!
- 95 NC disseminated in 58 vacuum sectors (30 % of total vacuum sector).
- 66 NC will be repaired (70 % of total NC).
- 29 NC are actually foreseen not to be repaired in 18 vacuum sectors.
- The repair of all NC would imply 15 % more activities in the tunnel => Not acceptable.
- A sorting must be done by LRFF to
- Propose the repair of critical VM,
- Minimise the number of interventions.
- Reminder: All the sectors are not equivalent in terms of repair.
- JMJ: all the bellows which are moving (transversally) are not safe because the spring can always move outside. So we need ferrite in this case to be able to continue the operation.
- JMJ reminded us that after the repair campaign we might have similar issues even if the quality is improved as there will still be some NC.
6) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1 (OB and BS): Follow-up for the new RF fingers design: What happens if we have a transverse offset (~ 10 mm transverse and ~ 10 mm compression in longitudinal) as this is like this that we want to use it? What about the tolerances? What about transverse plane?
- Action 2 (OK): Finish the simulations for VMTSA by introducing a more realistic gap shape (if not too difficult/long as we already see what happens) and introduce the ferrite to see how/if it damps the mode in case of non conforming RF fingers.
7) Miscellaneous
- The next (13rd) meeting will take place on 11/09/2012 between 09:15 and 11:00 (max.) in room 6-R-018 => Agenda:
1) Status report of this task force (EM),
2) 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.),
3) Discussion about the ferrite for the BSRT.
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 10/09/2012.