Minutes of the LRFF Task Force
17th meeting on Tuesday 30/10/2012 (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.
1) Comments on the last minutes + Actions
- Discussion with Eric Montesinos about some past experience he had with RF contacts and springs => Will be described in a forthcoming meeting.
- Final "resto meeting" planned for Tuesday 27/11/2012:
- From 09:00 till 10:30: Wrap-up by EM and draft of the slides to be presented at the LMC.
- At 19:30: FONDUE at the "Cafe du Soleil" (Petit-Saconnex).
2) Penetration depth in ferrite and practical recommendation for the ferrite's thickness (EM and FC): pdf
- The general formula for the penetration depth in a material was given (which corresponds for instance to the skin depth for the case of a good conductor - metal - at not extremely high frequencies).
- It was then applied to the case of the (fitted) 4A4 ferrite between 1 MHz and 2 GHz. FC reminded us that the values below ~ 1 MHz and above few GHz are doubtful and that we should be very careful in these frequency ranges.
- The penetration depth in this ferrite was plotted from 1 MHz to 2 GHz, revealing a monotonic (decreasing) behaviour with frequency: the higher the frequency the shorter the penetration depth. At 1 GHz it is ~ 7 mm.
- Practical recommendation for the ferrite's thickness (as a 1st guideline) => Ferrite thickness ~ penetration depth / 2.
- As an example with the previous (fitted) 4A4 ferrite, if one wants to damp a mode at 1 GHz, a thickness of ~ 3-4 mm is OK.
- Remarks:
- Depending on the frequency, one has to optimize the ferrite to be used.
- A lower limit for the ferrites thickness is given by mechanical considerations => Should be > few mm for ferrites tiles. For plasma sprayed ferrite, the thickness is dictated by the technology (maximum of few hundreds microns)
- FC made some comments about the frequency range of the ferrite, imaginary part of the complex permittivity, and the use of mm for the plot instead of cm (as it is like this in the workshops) => The comments were taken into account and the slides modified and updated (and we reached an agreement with FC!).
3) Overview of LHC Collimators RF system (AlessandroB): pptx
- Outlook:
- Specifications,
- Phase I,
- TCLIA & TCTVB,
- Phase II,
- TCLD,
- TCTP,
- Conclusions.
- Context => Large and sometimes misleading as there are not only 1 collimator in the LHC but many! => Focus on RF design of main variants of LHC Collimators:
- Phase I: TCP, TCSG, TCTA, TCLP, TCLIA, TCTVB ,
- Phase I Upgrade: TCTP, TCSP ,
- DS Collimators: TCLD,
- Phase II: TCSM.
- RF impedance specifications (started on 25/06/2003):
- Integral contact resistance (on each transition) < 1 mOhm.
- Phase I design baseline:
- RF Specifications led (along with other inputs) to following design decisions:
- Carbon-based jaws with lowest possible electrical resistivity for (Phase I) Primary and Secondary Collimators =>
- Jaw Material: Across Corp. (Tatsuno) AC150K Carbon/Carbon (rho ~ 7 microOhm),
- No thin metallic coating (e.g. Cu) on jaw surface.
- Separate (0.15 mm gap) Tungsten (Inermet 180) blocs for Tertiary Collimators.
- RF Contacts:
- Longitudinal Contacts (Jaw-Vacuum Chamber): Ag-coated CuBe (C17410 TH02),
- Transversal Contacts (Jaw1-Vacuum Tank-Jaw2): Ag-coated CuBe (C17200 TH02 off-the-shelf strips by Feuerherdt),
- Transition Contacts (Vacuum Chamber-Vacuum Tank).
- Nice pictures on slide 6 (the right one is just a zoom of the left one) very often used (also for the coming HiLumi workshop in Frascati) which was abandoned ~ 10 years ago due to too high contact resistance between RF fingers and Carbon directly (uncoated CuBe fingers sliding on C/C. Electrical Contact Resistance ~ 30 mOhm (whereas specification was 1 mOhm) => Redesign necessary.
- Alloy chosen after several iterations => CuBe etc.chosen => See SergioC talk few meetings ago.
- Reminder: Rhodium is used to avoid cold welding under vacuum.
- Longitudinal Fingers. Lessons learnt:
- Adopt appropriate steps to avoid stick-slipping (tolerance chain, geometry, coatings, de-burr ),
- Use stroke restraint to limit RF finger azimuthal movements.
- Slide 9 => Additional contacts, fixed, but here to limit the volume only.
- Transverse RF contacts => Not possibility to use the CuBe coated etc. => Use the 17200 CuBe with a lower ability to conserve it s elasticity at high temperature => But should be OK.
- Slide 10 => Comment from FC that the currents do not go to ground and for the RF it is a mess. FC says he sees this for the first time and it was accepted in the past and according to AlessandroB there was also a review. FC said that it is much more critical than people thinks.
=> To be checked what was said in the past.
- 2 in 1 collimators => TCLIA / TCTVB design
- More complicated.
- As opposed to PhaseI 1 beam per tank collimators, for 2 in 1 designs (TCLIA and TCTVB), ferrite was added to standard sliding contacts to damp HOM in large cavities (A. Grudiev simulations) => Ferroxcube 4S60 Ferrite.
- Ferrite was also added in transitions.
- Reminder from FC: fingers should never slide on Cu otherwise they will never slide very long.
- AG mentioned that the thickness was 6 mm as there were no reason to cut it as it was supplied from supplier like this and there was no optimization of the thickness here.
- Visual inspection of TCTVB.4L2 on 03/02/2012 and some RF fingers were found not touching.
- Reminder from FC: when we can avoid RF fingers we should avoid them (by putting ferrite).
- Phase 2 design baseline:
- Slide 14 => There emerged the idea to go to contact-less and use ferrite. Indeed, Phase II RF design was triggered by the requirement to avoid sliding contacts between RF strips and St. Steel Rail: in 2008 it was decided to replace RF contacts with Ferrite Tiles.
=> New design for Phase 2:
- Ferrite tiles => If we could have the ferrite thinner we would gain for space. FC said that we can do it thinner.
- Advantage of the plasma spray ferrite is that it has a better conductivity to remove the heat.
- Request: Not directly seen by the beam (if possible).
- Some past studies by HugoD:
- Comment from AlessandroB about a request to use ~ 0.1 or 0.2 mm of Molybdenum thickness instead of 2 mm => Being followed-up by impedance team (Nicolas Mounet).
- SiC => Comment from FC about the DC value which can lead to very strong errors => To be checked with Carlo Zannini.
- Based on these studies, SiC was abandoned.
- The reference where SiC was measured is the following: http://cdsweb.cern.ch/record/1381531/files/CERN-ATS-2011-104.pdf => To be sent to FC.
- From mechanics, SiC has good mechanical properties and was therefore an option for them for high Z material.
- DS (Warm) Collimator (TCLD) studies.
- TCTP design:
- Same extremity RF contacts of phase I collimators.
- Ferrite tiles replacing longitudinal RF strips.
- Change of jaw tapering to accommodate BPM buttons.
- Ferrite proposed for TCTP collimators: TT2-111R Trans-Tech due to higher Curie Temperature of 375 ˚C. Question: What is the best solution for the support material? Several cases studied: pure copper OFE, stainless steel, copper OFE with CrO coating. The latter is the best choice from the thermal point of view, temperature on ferrite decreased by 25-30% with respect to stainless steel (this reduction could be ~ 40% when the upper screen is also coated with CrO).
- Question raised from StefanoR or AlessandroB: Is the chrome coating on copper a potential UFO generator (as black chrome presents a dusty surface, i.e. risk of particles detachment)?
4) Electromagnetic Simulations of VMTSA Equipped with the RF Fingers and Ferrites (OK): pptx
- Outline:
- Realistic finger deformations for VMTSA equipped with the longer fingers (OK had to rotate each finger separately around the edge near the beam pipe). Time domain, eigen frequency and wire simulations.
- Simulations of the deformed shorter fingers and effect of Philips 8C11 ferrites.
- Conclusions:
- Different shapes of the finger deformations have been studied for longer and shorter fingers. Unconformities could result in ~ kW power losses => enough to melt fingers.
- Ferrites in the proposed position and amount dont help. Additional dedicated study is necessary to see if we can damp modes with ferrites.
- FC reminded us that this is thanks to the matching resistors that we can suppress the periodic oscillations in the measurements (nice past work from JL. Nougaret and FC).
- According to FC, the deep notch is a classical sampling pb and this explains why we have the different scale (between measurements and simulations).
- Reminder: doing a wire measurement is difficult but we can simulate the wire method with simulations.
- When an RF finger is not touching, this generates a lambda / 4 resonator. In fact, it is a capacitively loaded lambda / 4 with the frequency slightly decreased due to the capacity.
- EM will send info to MG about past observations of spring and RF fingers melting.
- MG will give a presentation next week about the temperatures reached with the power loss computed by OK, but he saw already that 100 deg is more or less the max before the melting of the RF fingers... To be followed up.
5) Actions to be taken for the next meeting
- Old actions.
6) Miscellaneous
- The next (18th) meeting will take place on 06/11/2012 between 09:00 and 11:00 (max.) in room 6-R-018 => Agenda:
1) Follow-up of the previous actions from 14th, 15th and 16th meetings (everybody) => Starts to become URGENT!
2) MG (Action 1),
3) OB (Actions 11 and 2),
4) FC and MB (Action 9).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 19/11/2012.