Minutes of the LRFF Task Force

1st meeting on Tuesday 20/03/2012 (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), Rhodri Jones (RJ), Roberto Losito (RL), 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, RJ, RL, SR, VB, VP, WW.

 

 1) Welcome on board! => LRFF (LHC RF Fingers) Task Force: Kick-off meeting! (EM)

- EM presented some slides about this new Task Force, going first through the web site (http://emetral.web.cern.ch/emetral/LRFF/LRFF.htm) to review the mandate, members, proposed modus operandi, some useful documentation and presentations, and the future meetings.

- EM then gave few explanations about the RF heating due to a high-Q trapped mode (more detail can be found for instance in EM's Chamonix2012 talk).

- After this, EM discussed in some detail the case of the VMTSA (double-bellow) module, mentioning the bench impedance measurements which have been performed with 1 wire (to simulate the beam) and the resulting beam power loss in case of "good" and "bad" contact. No impedance issue is foreseen in case of good contact but the problem is that the mechanical design was weak and therefore for some conditions, e.g. misalignment etc., a bad contact for several RF fingers could easily be achieved with as a consequence an important resonance which would then lead to an important heating which can then lead to even worse contacts which will then lead to an even larger RF heating and so forth and so on (this is the current interpretation as VB reminded everybody).

- Default => Defect.

- It was mentioned that in May all the VMTSA were OK as some Xrays were done (by chance) => See VB's LMC talk and other slides from VB. In fact, in VB's talk an X-ray campaign (of the 10th and 12th of May) is mentioned for LSS2 only as there were no Xrays for LSS8.

- Note that the VMTSA modules should be able to move laterally by few mm (for the nearby collimators) but according to the collimation team the VMTSA modules were not moved in 2011.

- The spring (keeping all the RF fingers together) is in SS 316L.

- In a crash program during the Christmas break it was decided to have a new design for 2012, with shorter RF fingers, ferrite plates and hopefully better contacts. In fact it is seen that in case of bad contact with this new design the resonance at ~ 200 MHz is still there and therefore what is important is really to minimize the possible gaps at the top and/or bottom (between the RF fingers and the insert), which should be the case with the modules installed in the LHC for the 2012 run. The exact explanation for the ~ 200 MHz resonance is still under investigation by the impedance team. Action 1: Impedance team to try and fully understand the ~ 200 MHz resonance in the case of bad top/bottom contacts (results will be presented in a forthcoming meeting).

- Before installing the new VMTSA in the LHC for the 2012 run VB analyzed the ferrite FC gave him and after some vacuum tests it was found that there was no heavy mass and no strong outgassing and that therefore it was OK from the vacuum point of view.

- Length of the RF fingers:

-  21 cm for the old (2011) design => Picture 1.

- 9.25 cm for the new (2012) design => Picture 2.

- There is a groove near the tip of the RF fingers to keep the spring in place. If it is correctly in place even if the VMTSA is moved laterally the spring should not jumped back as we saw during our impedance measurements (where the spring was maybe not perfectly in place).

- AlessandroB proposed to make the spring stiffer. To be followed up later as new designs could even remove completely the spring.

- JMJ reminded us the different steps of the crash program during the shutdown: after some studies we were convinced that the resonance frequency was linked to the length of the RF fingers and therefore an easy solution (with the very limited time) was to use new shorter RF fingers to move the resonance frequency to a much higher one where the power spectrum is much smaller and therefore the associated power loss also much smaller. But then it was discovered that the shorter RF fingers were not the solution if the contact was not good. JMJ reminded us that in the future one should work not only in static perfect situation but also in a non-perfect moving one as it can change completely the picture.

- FC reminded us that with the PIMs there was no impedance issue in the past but only a buckling effect (see also Discussion on the LHC PIMs (22/09/2010) between EM and Francesco Bertinelli) => Action 2: EM to contact Pierre Strubin and summarize the past work on the PIM and PIM working group. Some pictures of the crisis of the PIMs: 1, 2 and 3.

- Past work on the RF contacts => Action 3: EM to contact Sergio Calatroni to summarize the past work on RF contacts.

- AlessandroB mentioned several notes about mechanical and heat treatments etc. => Action 4: AlessandroB to send these notes to EM to put them on the web site.

- Email and question from WW on a problematic bellow system: Doc 1 and 2. These large deflection bellows (+/- 20mm) are used for the TCDQ. Inside the bellow we also have the racetrack-to-round transition. These bellows require a too large force during movement, pulling on the TCDQ tank. Furthermore, the contact fingers often buldge out (see figure 4 in Doc 2). We hope to install an improved design in LS1. To be seen if this could/should be discussed in this working group => To be presented at some point.

 

2) Actions to be taken for the next meeting(s)

- Action 1 (Impedance team): Try and fully understand the ~ 200 MHz resonance in the case of bad top/bottom contacts (results will be presented in a forthcoming meeting).

- Action 2 (EM): Contact Pierre Strubin and summarize the past work on the PIM and PIM working group.

- Action 3 (EM): Contact Sergio Calatroni to summarize the past work on RF contacts.

- Action 4 (AlessandroB): Send mentioned notes about mechanical and heat tretments etc. to EM to put them on the web site.

 

3)  Miscellaneous

- The next (2nd) meeting will take place on 27/03/2012 between 09:00 and 11:00 (max.) in room 6-R-018 => Agenda:

1) (Possible) summary of the past work on the PIMs, PIMs' crisis and PIMs Working Group (EM. EM will discuss with Pierre Strubin on Monday afternoon).

2) Start of the equipment review:

- Some equipments from VSC (VB).

- Some equipments from BI (RJ).

=> REMINDER ON REQUESTED INFO: (1) name of the equipment, (2) number of such equipments, (3) exact locations of these equipments, (4) some real pictures with clear explanations (on some slides) of each relevant pieces etc.: with materials used (coating's parameters in case of coating, i.e. thickness and resistivity), contact force, how it should work in practice (elongated vs. contracted, static vs. dynamic etc.), (5) functional specifications (if any) and impedance contact person (if any), (6) real 3D drawings for further electromagnetic simulations to compute the impedances, and (7) if there are (and if yes, where? etc) some spare equipments on which bench impedance measurements could be performed (and with which time scale).

- See preliminary agendas for the next meetings => In particular, the meeting after (3rd one) on 03-04-2012 will be devoted to "Past development work on RF contacts (Sergio Calatroni).

 

Minutes by E. Metral, 22/03/2012.