Minutes of the LRFF Task Force
18th meeting on Tuesday 06/11/2012 (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, StefanoR, ChristineV.
1) Comments on the last minutes + Actions
- Follow-up of the previous actions from 14th, 15th and 16th meetings (everybody) => Starts to become URGENT! Nothing particular in addition to what will be discussed today.
2) Thermal evaluation on VMTSA heating (MG): pptx
- Reminders:
- From LHC measurements => Opening of the concerned vacuum sectors during Xmas break 2011-2012, have shown that the stainless steel spring is deformed and brazed to the Cu/Be RF fingers and that the RF fingers are permanently deformed. The estimated temperature reached during operation is ~ 800-1000°C.
- Simulated power deposited by OK:
- ~ 650 W for a gap of 40 mm.
- ~ 460 W for a gap of 50 mm.
=> Questions:
- Is RF power deposition compatible with expected failure temperatures?
- Limit for power value?
- FC suggest to put all this in an oven to see what happens...
- Reminder: Rh deposition (on copper) and Ag deposition (on RF fingers) for the contact to work.
- The eventual cover does not change the convection coefficient => Can be disregarded.
- 1st approximation: cut in 4 parts.
- Reminder: Anything below 0.1 (for the emissivity) does not contribute anything (from the radiation point of view) => We can take 0 in fact and this will lead to the same result.
- The real geometry of the spring is difficult => Use toroidal shape going along the RF fingers.
- FC said that we should maybe use not ideal thermal contact but AlessandroB mentioned that it is in fact a conservative approach.
- Question from WW that for all interconnects there are copper joints. MG made a simplification but it should not change a lot. He made also an analysis of sensitivity of his results and it is quite robust.
- Results:
- The convergence for 500 W (or higher power losses) was critical with respect to time effective simulation and therefore he used smaller power loss as it was the maximum he could do. Otherwise he should use finer mesh etc. and for the moment he did not reach convergence.
- Conduction only from left to right on the picture.
- For 480 W, it leads to a temperature of the RF fingers of ~ 880 deg C and a temperature of the spring of ~ 750 deg C.
- Fingers heat evacuation can be approximated by a simple formula with 2 coefficients (Kcond for the conduction and Krad for the radiation). Extrapolating to high temperature we see that we can reach very high values even if these coefficients will depend of temperature and here we consider them constant. Krad should increase whereas Kcond should decrease.
- For 650 W it gives ~ 1000 deg C.
- For ~ 1000 deg C it should mean ~ 670 W, i.e. very close to the case of a gap of 40 mm.
- 2nd answer to give: What could be a reasonable power limit due to fingers failure? ~ 100°C => It corresponds to ~ 18 W in the RF fingers.
- Conclusions:
- Simplified model! Watch out especially regarding spring temperature.
- Failure temperatures estimate are compatible with RF power expected.
- 1000°C reached for RF power of 670W.
- RF power of 18 W is the limit to keep the RF fingers below 100°C.
- Comment and conclusion from FC: 5 microns or 50 mm gap will / should give the same results (within a factor ~ 2-3 or so) => Any gap is fatal! And therefore one should avoid any gap!
- What is the maximum longitudinal gap? It is better to remove 1 finger instead of having one not touching.
- Discussion on the melting temperature of some components (and info sent after the meeting by MG):
- For 316L spring: ~ 1350°C,
- For CuBe: 865°C / 1025°C (C17200 and C17410 respectively).
=> This goes towards the hypothesis that failure of the spring cannot be related to heating coming from the ‘lower-T-melting’ fingers but, as suggested by FritzC, rather to some inner current induced heat generation.
3) Beam impedance of 63mm VM with unshielded Bellows (OB): pptx
- Conclusions from CST simulations:
- The longitudinal impedance depends on how much the RF fingers are stretched. The impedance is close to zero in either completely compressed state or completely stretched state. The maximum normalized longitudinal impedance (imaginary part) is Zlongitudinal / n ~ 1.5E-4 Ohm.
- A normalized impedance of 1.5E-4 Ohm can not be used for all the PIM modules, but only for selected equipments.
- For the transverse impedance studies, the values given in Ohm should be divided by the transverse displacement to have the transverse impedance in Ohm/m. The imaginary part of the transverse dipolar impedance is thus 2.2 / 0.005 ~ 450 Ohm / m.
4) Aspects of heat transfer from ferrite to metal in ultra-high vacuum (FC and MB): pptx
- The 1st slide (on near-field effects) is very very important as one can see nicely the smooth transition for heat transfer in vacuo from “far-field radiation” to" near-field radiation”= contact. Near-field radiation can also be seen as heat transfer including evanescent waves or some people call it tunnelling.
- For a gap larger than ~ 10 microns, we are in the "far-field regime" and for a gap closer than ~ 10 microns, we are in the "near-field regime" => Evanescent waves + classical travelling waves. Another name of it is "tunnelling effect". If we put more pressure the heat transfer is better but it is due to a smooth transition to classical radiation. Reminder: real contact only happens on 3 points. So we see that they don't need to touch but it has to be sufficiently close. Everything can be described in fact by waves and very rarely contact.
- Blackbody is not true for a metal in the infra-red range.
- 2nd slide: application on a MKI kicker magnet. Metal at 10 micron is not black and is in fact shiny and it has a low emissivity. The ferrite of course is black (with an emissivity of ~ 0.8-0.9). We are here exactly in the transition regime for the ferrite on the metal as they both have not flat surfaces etc. => We have to consider the air gaps (which are not air gaps in vacuum of course...).
- Reminder from FC about the painting used for radiators: this painting is white in visible but black in infra-red.
- The emissivity of the MKI tank could be increased from 0.1 to 0.7 (with an ion bombardment treatment ) but it could not be done uniformly. The output nevertheless is that we can considerably increase the emissivity to a value of ~ 0.6.
- How to improve heat transfer as we cannot braze it and cannot apply big pressure on it?
- Project with KT and university of Aachen.
- Increase emissivity of surface of “heat source” and “heat sink”:
- We cannot easily braze to a metallic surface – because of differential expansion coefficients: only feasible with small volumes and mechanical pre-stress (delicate and risky).
- Alternative solutions for heat transfer include:
- Ion bombardment of metallic surface to increase emissivity (see MKI studies).
- Vacuum evaluation required.
- Bake-out evaluation required.
- Plasma spray technology of thin ferrite layers (avoids expansion problems, but cannot be cut mechanically). Emissivity measured to be ~ 0.85. Thickness ~ 100 microns reached. We should be able to reach several 100 microns (~ 0.3 - 0.4 mm) but not more to avoid expansion problems => This is the equivalent of brazing without brazing... This is a new technology. In fact, FC reminded us that this is the same as at home with our frying pans with ceramic and aluminum coating.
- Vacuum evaluation required.
- Where applicable: direct liquid cooling of heat source (not necessarily applicable where there is high voltage).
5) Actions to be taken for the next meeting
- Old actions.
6) Miscellaneous
- The next (19th) meeting will take place on 20/11/2012 between 09:00 and 11:00 (max.) in room 6-R-018 => Agenda:
1) Last working meeting!: Follow-up of the previous actions from 14th, 15th and 16th meetings (everybody) => Starts to become URGENT!
2) AlessandroB (Actions 15 and 16)
3) Some RF contacts (with power transmission lines) issues in 2012 in the SPS (Eric Montesinos),
4) Some pictures of the TCDD of sector A4L2 with some twisted RF fingers (VB),
5) Abstracts for papers for IPAC13.
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 19/11/2012.