Minutes of the CCinS working group

1^st meeting on Wednesday 11/11/2009 (09h00, 6/2/004)

Present: Rama Calaga (RC), Nicolas Delruelle (ND), Nicolas Gilbert (NG), Erk Jensen (EJ), Elias Metral (EM), Joachim Tuckmantel (JT), Frank Zimmermann (FZ).

Excused: Olivier Brunner (OB), Giovanna Vandoni (GV), Jorg Wenninger (JW).

1)  Welcome and introduction (EM)

 - The working group to "Study the feasibility of installing a crab cavity in the SPS" (CCinS) is composed of 9 people:

Rama Calaga (Crab cavity expert and possible measurements) - BNL&USLARP,

Nicolas Delruelle (Cryogenics) - TE/CRG,

Nicolas Gilbert (Space and integration) - EN/MEF,   

Elias Metral (Chairman, beam dynamics issues and SPS availability) - BE/ABP,

Joachim Tuckmantel (Crab cavity expert and RF) - BE/RF,

Frank Zimmermann (Crab cavity expert, possible measurements and linkman with KEK) - BE/ABP,

Olivier Brunner (Klystrons and superconducting cavities) - BE/RF,

Giovanna Vandoni (Vacuum) - TE/VSC,

Jorg Wenninger (Machine protection) - BE/OP.

- In case of specific questions, we could contact:

Ralph Steinhagen (BE/BI and in particular for the HeadTail monitor, tbc),

Elena Shaposhnikova (BE/RF).

- Erk Jensen (present at this first meeting) is not a member of the WG, but he will follow closely our activity and will be kept informed of the information exchange.

- The activity of the working group is foreseen to last for few weeks (until Christmas) with recommendations (feasibility result) by the end of 2009.

- It was decided during the meeting that, from now on, the working group will try and meet each week on Fridays at 11:00. After the meeting, the room 6/2/004 has been reserved for the following meetings:

- 2nd meeting: Friday 20/11/09 between 11:00 and 13:00.

- 3rd meeting: Friday 27/11/09 between 11:00 and 13:00.

- 4th meeting: Friday 04/12/09 between 11:00 and 13:00.

- 5th meeting: Exact date and place still to be defined as Elias will be absent in the morning.

- 6th meeting: Friday 18/12/09 between 11:00 and 13:00.

- A CCinS web link is already provided as central storage platform for all relevant documentation, minutes, etc. at http://emetral.web.cern.ch/emetral/CCinS/CCinS.htm (it is on Elias' web page as for only few weeks of activity no dedicated web site was asked for).

2) Discussion after presentations by EM (and RC) and FZ: Slides from EM and Slides from FZ

- Elias explained the composition of the WG, the goal of the WG and where we can find all the information linked to this working group. The official statements on crab cavities form CERN were then reviewed, emphasizing that points 7 and 8 are relevant for this WG (as discussed with FZ). A list of main topics of interest (in decreasing order of priority) for the WG was then presented.

- Erk Jensen, who participated to meetings leading to the creation of this WG, also reminded us on what should be done during the next weeks and what is expected from us. The list of main topics seems to be OK for him but Erk mentioned that the last point on cost estimates should not be addressed here. Elias answered that it is the last point of the list and that it is just "first estimation".

- Some slides prepared by RC for the last LARP meeting were then discussed. A strong possibility is to use one of the KEKB crab cavities (and all ancillary equipments) but most probably not the klystron, which raises a first issue as we need one and with the correct frequency. Possibilities to use one from DESY were discussed, but this has to be followed up by OB (JT will contact him, as he could not come to this meeting, see Action 1 below). Answering a question from FZ, Erk replied that it should be possible to change the frequency of a klystron within few %. The issue of the cavity frequency was first discussed as the KEK frequency is 508.9 MHz (resonance frequency of the magnetic field of the TM110 mode used to kick the bunch horizontally) and in the SPS the ideal would be to have a multiple of 40 MHz (for 25ns bunch spacing operation), i.e. 520 MHz. However, this will not be possible to achieve. Instead a frequency of ~ 511 MHz is proposed for SPS operation, and this static frequency change of 2 MHz is considered to be achievable (this was indeed a central question to be answered). The KEK people will need to open the cryostat, perhaps without breaking the cavity vacuum etc.. We could then start our investigations assuming a frequency of 511 MHz. FZ reminded us that KEK would also like to have an answer for the use of their cavity (or not) by the end of this year.

- The RF power of the cavity is 120 kW at top gradient. But it seems the klystron is over-dimensioned as it could deliver much more.

- A dynamic tuning is also possible but the speed of 1 kHz/s mentioned by RC is too slow (FZ mentioned another figure of ~ 10 kHz/s). The exact figure should be checked (see Action 2 below)

- ND asked which kind of head load we have. A value of ~ 100 W was mentioned and ND said that a small refrigerator exists in LSS4 with a capacity of 100 W. In this case, it might not be enough as we have other power considerations. FZ said that we can reduce the heat load (static and dynamic). ND also mentioned that there might be also another (very old) refrigerator with a capacity of 400 W (to be checked). FZ&RC will check with KEK for a more precise value of the heat load (see Action 3 below) and ND will check which kind of refrigerators are available for the next meeting (see Action 4 below).

- Joachim mentioned that we need a gas (helium) storage or lines etc. not too far, which could fix the location of the crab cavity.

- In his slides, RC wrote that the total length of the equipments to be installed (from flange to flange) is 5 m and that the mass of the cryomodule is ~ 5 tons. . Furthermore, a preliminary check of the apertures revealed that it should be compatible with the SPS aperture as the cavity aperture is always larger than 94 mm diameter (and a limit of 84 mm for the SPS was given to him). NG can start thinking where these equipments could be installed (see Action 5 below). In his slides, RC said that the radius of the cryomodule is ~ 0.5 - 1 m. If it is close to 1 m, then NG concluded that these equipments cannot be installed in LSS5 (a place where many other equipments are installed such as BBLR, ecloud monitors or the UA9 crystal collimation experiment) because there is a maximum possible distance between the beam and the outside dimensions for the transport. If it is too big, the only possibility is LSS4 because there is space there (as there is a special cavern).  

- I mentioned that in the past there was the COLDEX experiment in LSS4 and as it seems it is not used anymore we could perhaps remove it and install the crab cavity instead. EM (with NG and ND) will contact Miguel Jimenez and Giovanna Vandoni (Vacuum) to check what the status with COLDEX is and if the previous assumption is sensible (see Action 6 below). A visit to LSS4 should also be organised (see Action 7 below).

- FZ said that COLDEX was made movable and that we could/should make the crab cavity experiment movable too.

- Joachim mentioned that he would like the RF noise effects to be clarified.

- The issue of the control and precision of the orbit in the crab cavity was discussed and this should be followed up with the machine protection on how precise it has to be.

- I mentioned that I had some discussion few days ago with RalphA who suggested me to put the crab cavity such that there is a multiple of Pi/2 between the crab cavity and the collimator to be able to perform some measurements of possible blow-up.

- Question (from Erk): Do we need to have a location at high beta? RC replied that the higher the better but it is not so important. Some flexibility on beta would be fine. NG mentioned that bumpers will be installed next year before and after the BBLR in LSS5, which could help in creating some local bumps. NG sent an email after the meeting to make a clarification concerning the "bumpers" which could be installed around the BBLR equipmentss: NG confirms that it has been requested to install 2 magnet "bumpers" type MDPV during the previous shutdown for the BBLR. However, due to lack of time, this request was delayed. Therefore, these 2 MDPV will certainly be installed during the next long shutdown, but this still has to be confirmed by J.P. Koutchouk.

- Erk mentioned that in the list of Rama one will certainly not do all. I reminded that the list of topics was presented by Rama at the last LARP meeting and that it does not apply totally for the SPS experiment.

- Discussing the frequency of 511 MHz, it was mentioned that several bunch spacings could be tested and in particular the 100 ns bunch spacing which is discussed these days for ALICE, because in this case we could crab every bunch, which is not the case for the other bunch spacings (25 ns, 50 ns and 75 ns), where interesting observations could nevertheless be performed (anticrab intermediate bunches at 50 ns, and kick in opposite direction every second 25 ns bunch). The 25 ns case would however be irrelevant for stability studies.

- Important question: Can we perform these tests without perturbing "too much" the SPS operation (and in particular the high-intensity operation for CNGS)?

- Concerning the impedance of the crab cavity, FZ mentioned that KEK could not measure the impedance contribution. RC said that he can have all the data for the impedances (longitudinal and transverse) and that we could then study the beam dynamics starting with that. RC will send Elias the impedance data (see Action 8 below). 

- Concerning the WG, FZ asked whether the members should write some chapters. I replied that I have been asked to provide only a feasibility result (i.e. a list of answers).

- The SPS conditions were also discussed and the idea would be to use a coast beam at 55 or 120 or 270 GeV/c, with a preference for the lower energies as in this case we have more margin in the cavity voltage. One needs at least to stay ~ 1/2 h with a good lifetime. The idea would be that once in coast, one slowly ramps the crab cavity and perform our measurements.

- A very important point came into the discussion, revealing that we have to do something more compared to what is done in KEK (is it really true?): We should have an RF feedback to keep the 0 V what ever happens. Do we have to build it? Without this feedback, it would be very dangerous with CNGS operation.

- Joachim reminded us that the RF noise is part of the LLRF.

- Another (active) feedback (acting on the beam) was discussed and it seems there could be some confusion between the different feedbacks. It was proposed first to check all the feedbacks used at KEK with their crab cavity (see Action 9 below).

- Concerning the instrumentation needed, FZ replied that the usual one would be fine: BWS, BPM, Qmeas, Schottky monitor (is it still operational? Elias will contact FC to check, see Action 10 below), HeadTail monitor (would be good to have Pi/2 phase advance between the crab cavity and the HeadTail monitor), LHC BPM.. Concerning all the possible measurements which could be performed (don't we need a streak camera?), it would be good to know exactly what we want to observe (precisely) and how we will observe it (see Action 11 below).

- The cavity trips rate is a major concern and it depends on the RF voltage. RC informed us that the main reason for the high cavity trips rate observed at KEK could be a problem with the coaxial coupler. This specific trip is not relevant for the LHC (even if the trip rate is of course relevant for LHC).

- The 1-turn failure was discussed as it would be a killer for the LHC. This can happen if the cavity has a totally wrong phase (we need a feedback for that and there is no reason to assume that it will not work!). A 10-turn failure mode would be a safe one for LHC.

- FZ mentioned a proposition from Stephane Fartoukh to use crab cavities for off-momentum cleaning. But this is a study for the future and it has big implications on beta*.

- 2 proposals were discussed: schedule possible tests during the LHC shutdown or have an experiment of only few weeks (is it really realistic?).

- FZ reminded us that the stop of KEK (to recuperate the crab cavity) could happen at the end of this year (at the earliest) or next year.

- NG reminded us that at first sight the installation could be done during a normal (few months) shutdown.

- It should be possible to make the crab cavity transparent or one should be able to move it as it was the case (to be checked) with COLDEX. Joachim reminded us that if we go up with Qext we can go down with the power (An antenna type input coupler is connected horizontally to the large beam pipe to excite crab mode. The external Qext of input coupler is set to about 1E5 to tolerate about 1mm offset of beam position during operation and higher than about 100kW RF power must be handled).

- Erk then made the assumption that we can perform tests in the SPS between 2011 and 2013. He then asked the question: What do we need to know to take the decision to install  crab cavities in the LHC? In other words, will all the questions to take the decision to install a crab cavity in the LHC be answered with these SPS tests? To be thought about...

- The important issue of the cavity trips is not easy to answer as it seems the main reason for the trips in KEK is specific and we don't know which kind of trips we will have in the LHC with our cavity. RC mentioned that by the time we perform the measurements in the SPS one might have a crab cavity built through LARP.

- RC mentioned some collimation studies which were performed, as collimation is also a critical item in the LHC and the crab cavity should not perturb the collimation scheme (or at least should be included in it).

3) Actions to be taken for the next meeting

- Action 1: JT will contact OB to discuss possibilities with klystrons (where can we found one as it seems KEK will not give us their klystron? Can we have the good frequency? If not, can we modify the frequency? By how much? etc.).

- Action 2: FZ&RC will contact KEK to check the figure of the speed for the dynamic tuning (1 kHz/s or 10 kHz/s?).

- Action 3: FZ&RC will contact KEK to check the heat load (static + dynamic).

- Action 4: ND will check which kind of refrigerators could be available (power capacity?, fixed or mobile? etc.).

- Action 5: NG will start checking where 5 m of equipments could be installed (with a distance between beam and outside dimensions of ~ 0.5 - 1 m).

- Action 6: EM (with NG and ND) will contact Miguel Jimenez and Giovanna Vandoni (Vacuum) to check what the status with the old experiment COLDEX in LSS4 is and see if we could remove it and install the crab cavity. In fact Giovanna could also make a summary at one of the next meetings.

- Action 7: EM&NG&ND (and any other people) would like to go and visit LSS5 and LSS4 (in particular COLDEX) => NG will add us in the SPS access requests. We could do this access in case of SPS stop or after December 16th (i.e. on 17/12/09 or 18/12/09). 

- Action 8: RC will send all the crab cavity impedance data to Elias.

- Action 9: RC&FZ will contact KEK to have a description of all the feedbacks used with a crab cavity operation. 

- Action 10: EM will contact FC to check if the Schottky monitor can still be used.

- Action 11: RC&FZ will provide a detailed list of what we want to measure (precisely) and how, with (if possible) numbers and pictures without and with the effect of the crab cavity (for instance coming from KEK measurements).

4)  Miscellaneous

- Next meeting on Friday 20/11/09 between 11:00 and 13:00 (max), in room 6/2/004.

Minutes by E. Metral, 12/11/09