Minutes of the ICE section
46th meeting on Wednesday 21/12/2011 (08:40-10:30, 6/2-004)
ICE members: Benoit Salvant (BS), Christian Hansen (CH), Carlo Zannini (CZ), Hugo Alistair Day (HD), Elena Benedetto (EB), Elias Metral (EM), Elena Wildner (EW), Frank Schmidt (FS), Giovanni Iadarola (GI), Giovanni Rumolo (GR), Jean-Luc Nougaret (JLN), Kevin Shing Bruce Li (KL), Nicolo Biancacci (NB), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Tatiana Pieloni (TP), Werner Herr (WH), Xavier Buffat (XB).
Present/Excused: BS, CH, CZ, HD, EB, EM, EW, FS, GI, GR, JLN, KL, NB, NM, OB, TP, WH, XB, Fritz Caspers, Miriam Fiterrer, Sasha (from KEK).
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 45th meeting + Actions
- Comments / corrections from AlexeyB and OB after the last meeting already implemented in the last minutes => OB sent also a new version of his slides which is already linked.
3) General infos
- No particular comment from anyone.
- SL meeting:
- Administrative students: new category of students available.
- Quality of IPAC12 abstracts was not always up to the required level (PaulC had to correct the English!). Earlier preparation of the abstracts would have avoided this problem.
- HL-LHC room and time slot reservations:
- Fridays: Conference room 6-R-018 from 2 pm to 5:30 pm.
-Wednesdays: Conference room 6-R-018 from 10:30 am to 12:30 pm.
- MARS: all forms to be signed by GLs by 27/04/2012.
- New saved leave scheme is being approved. Old schemes will be stopped. There will be a short and a long saved leaves. The first account has a ceiling of 22 days and everything beyond this will be transferred at the end of the year to the second account. The interest rate is negative (-0.8%)!
- Operation in 2012: Rex Isolde is taking 35-36% of PSB beam, only. DIRAC will stop. Ions should be ready for NA61 in September.
- Budgets: -2.5% (adjustment), -0.8% (IT contribution), -5% (DG reserve), in total -8.3% reduction on all budgets.
- Stefano Redaelli will be transferred to ABP as of 01/01/2012.
- IPAC12: Reminder to register on time to get reduce registration fees.
- As I already sent by email, there are 2 additional vacancy notices => 4 positions in total.
- Many thanks to everybody for the help to organise the ABP Christmas Party!
- Evian workshop last week => Talks by NM, BS and GR as foreseen:
- NM: LHC impedance => Factor ~ 2-3 worse wrt simulations (the impedance model has been modified recently).
- BS: RF heating => Many follow-ups and in particular on some RF fingers (VMTSA) where a solution was proposed and is being followed-up by BS and JLN (measurements and simulations). Discussions with AdrianaR and we will continue to work in close collaboration with vacuum at the beginning of next year (see discussion below).
- GR => Evolution of SEY during scrubbing runs + synchronous phase shift => Nice agreement between meas. and simulations for the latter.
- Comments from one of the 2 new MD coordinators (Benedetto Gorini) replacing Massi:
- 5 sigmas discovery down to 115 GeV at sqrt(s) = 8 TeV requires > 15 fb^-1 => Ideally they need ~ 20 fb^-1 (and 1.5 fb^-1 for LHCb).
- Is it worth from physics point of view to go to 4 TeV? => Yes, see slides from ReyesA (# 3).
- Bunch length => Increasing it by 10% is acceptable (if needed due to RF heating etc.) but all the experiments would like to know that asap to adapt themselves and then keep it constant for the whole year!
- Philippe and Themis took the Fourier transform of their measured spectra and it revealed some kind of triangular profiles. To be followed up.
- RoderikB => Could reach a betastar of ~ 60-70 cm.
- VerenaK => On average ~ 20-30% growth between SPS flat-top and LHC collisions.
- After the meeting, VincentB sent us some X-rays observations close to CMS: X-rays near CMS (Vincent Baglin, 20/12/11). All vacuum modules with RF fingers are conform except the module located at 18 m right (Z-side). This is in agreement with the pressure excursion frequently observed on the right side of CMS. The origin of the non-conformity is not understood yet. As shown, the RF fingers are placed inside the RF insert. As a consequence, the aperture is locally reduced from 40 to 36 mm. This reduction is still in the shadow of the TAS aperture which is installed beside this module. The TAS is 2.13 m long with 34 mm inner diameter.
4) Discussion about the Large Piwinski Angle MD (Frank Zimmermann) => Postponed to 2012
- Reminder from/for me:
- Definition of Piwinski angle: Phi = ThetaC * Sigmaz / (2 * Sigmax)
- Nominal LHC: Phi ~ 0.64. And Ohmi's simulations seem to indicate a broad minimum for the lumi at a Piwinski angle Phi of ~ 0.5, where we operate at the moment => Room for improvement.
- ATS upgrade => Aiming for Phi ~ 2.5.
5) Discussion about the LHC RF heating (BS, FritzC, EM) => Current hot topics
- VMTSA RF fingers:
- Reminder: MiguelJ took the decision on 15/12/2011 (as we could not wait without impacting the LHC schedule) to use shortened RF fingers => Being manufactured in urgency, and they will start installing them as from Monday 9th January.
- BS showed some slides about his current simulation work on the VMTSA RF fingers. Jean-Luc Nougaret (helped by Fritz Caspers, Gregory Cattenoz, Bernard Henrist, Benoit Salvant and Alexis Vidal) made some measurements on a spare module and Benoit Salvant performed some numerical simulations => The current situation is the following (we already informed our vacuum colleagues):
1) According to the measurements with the wire, a quite big impedance (of the order of ~ 100 Ohms) could be feared at ~ 650 MHz, and seems to be due to the cavity behind, coupled through the slots between the RF fingers.
2) The fact that the (large) cavity behind plays a role means that reducing the length of the RF fingers will not be the solution. Some simulations seem to indicate that it should be indeed better but might not be sufficient for the future high intensities we want to achieve.
3) Benoit could not reproduce the results of these wire measurements as he sees nothing if he assumes that the RF contacts are perfect. He needs to remove some contacts to start to see something. He simulated some bad contacts by cutting the tip of 4 RF fingers. A very large impedance can then be obtained (~ 500 Ohms). Note that without all the RF fingers it would be even ~ 100 times more critical, which clearly reveals the importance of the RF fingers.
4) A possible explanation for the difference between the measurements and the simulations (with perfect contacts) is that the contacts between the RF fingers and the pipe (as well as the azimuthal arrangement of the RF fingers) are not perfect in the measurements.
5) Therefore, we recommend:
5.1) Try and improve as much as possible the contact between the RF fingers and the pipe.
5.2) Install some ferrite, typically 2 tiles (of ~ 5 cm * 5 cm) per bellow. As there are 2 bellows per module, it means 4 per module, and as there are 10 modules in total (to be checked), it means 40 tiles in total. Fritz is in contact with Skyworks to find some ferrite and we contacted RhodriJ to see if we could recuperate some ferrite tiles bought for some BI equipments.
5.3) Make quantitative impedance measurements asap in 2012 with the old and new fingers with and without ferrite to try and have the best estimate of what will be installed in the LHC => Action 1: BS-JLN-FritzC-EM to follow this closely at the beginning of 2012 with top priority.
6) Remark: As only 5 bellows out of 10 (opened) are destroyed and the others are perfect (if we understood correctly), we are wondering if there is a non conformity problem for those equipments. Another possibility to explain the fact that some of the modules are fine and some are bad is the influence of the relative phase between the 2 beams (can have constructive or destructive interferences). To be followed up by the impedance team with 2-wire measurements and simulations. Finally, it is feared that bad contacts between the fingers and the pipe create more EM fields, which cause more heating, which cause worsening of the spring, which cause worse contact, so that a catastrophic failure can be predicted for a very small difference in initial state of the contacts.
- ALPHA roman pot => Meeting yesterday afternoon with BS, FritzC, Per Grafstrom, Christian Joram, Patrick Fassnacht:
- Reminder:
- Observations (Sune Jakobson, 03/11/2011): slides.
- Past work:
- Impedance design and references: http://accelconf.web.cern.ch/AccelConf/e08/papers/tupp063.pdf.
- ALFA TDR chapter 4.3: https://cdsweb.cern.ch/record/1095847?ln=en.
- BS showed some results in time domain and frequency domain.
- FC reminded us what the role of the ferrite: it can be not totally intuitive but by adding a lossy magnetic material (ferrite), we decrease the Q factor and therefore the (R / Q) * Q = R (as R / Q depends only on the geometry) and therefore we decrease the power loss proportional to R! The ferrite will decrease the total power by a factor of few tens, maybe 50. But, then the ferrite will absorb the remaining power. If the pot is welded then the shielding is more than 100 dB.
- With the most critical mode at ~ 960 MHz, BS found a Ploss ~ 8 W with the ferrite already here. Without the ferrite it would be kW.
- The observation is a temperature increase by ~ 30 deg until a total of ~ 50 deg. They made a test by putting ~ 10 W (with a heating tape at the outside in the air, but with vacuum in the pot) and they saw ~ 50 deg inside the pot => Seems consistent.
- But, we need to understand how the heating goes from the outside to the inside. This is very important for them to know how the heat exchange happens (conduction? convection? radiation?) as it will define the cooling system!
- FC proposed to make a coating of the outside of the pot. Convection is very small in vacuum and for the radiation, FritzC reminded us that we should not assume a black body (as usually assumed).
- The first observations they made seem to go to radiation as the only possibility for the heat exchange. To be followed up.
- According to BS's simulations, the ferrites are not in the good position => Adding ferrites at the optimized position we could reduce the power loss by a factor ~ 5.
- Reminder 1: Adding ferrite decreases also slightly the resonance frequency, which goes in the bad direction.
- Reminder 2: For 2012, we will have not time to prepare and install the ferrites, so we should not rely on that for 2012 => We will have to deal with this power loss.
- It is very important to work altogether and find a solution otherwise they will have to stop their physics programme.
- Reminder 3 from Per: In the Design Report, it was mentioned that the roman pot should be removed for high intensity beams => Could be a solution but then they think they need a stop of ~ a week to re-install it and it cannot be done during a technical stop of 4 days!
- Best solution maybe to be explored in detail: put the roman pot not in parking position (~ 40 mm) but at ~ 20 mm => The cavity would be smaller and maybe the resonance frequency would shift to a higher frequency leading to a smaller power loss => To be checked.
=> Action 2: BS-EM-FritzC to send a summary to the ALPHA team.
- MKI => Might be close to the interlock threshold next year which would imply to wait few hours. Could maybe be reduced by increasing the bunch length (if nothing else can be done, being followed up) => We could compare the power loss increase expected next year by using more particles and estimate the bunch length increase needed to compensate (at least partly) for it => EM will do this for Chamonix12.
6) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1: BS-JLN-FritzC-EM to follow the VMTSA RF fingers closely at the beginning of 2012 with top priority.
- Action2: BS-EM-FritzC to send a summary to the ALPHA team.
7) Miscellaneous
- The next (47th) meeting will take place on 11/01/2012 (to be finished at 10:00) => Agenda:
1) Follow-up of the LHC RF heating (impedance team)
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 21/12/2011.