Workshops, Meetings & Summary Minutes

Workshops & Conferences
Meetings & Minutes


Workshops

* LHC-CC11 (Nov 14-16, 2010): 4th Joint CERN, US-LARP, EUCARD-ACCNET, DL/CI, and KEK, CERN, Geneva, Switzerland
* LHC-CC10 (Dec 15-17, 2010): 3rd Joint CERN, US-LARP, EUCARD-ACCNET, DL/CI, and KEK, CERN, Geneva, Switzerland
* ICFA Beam Dynamics Mini-Workshop on Deflecting/Crabbing Cavity Applications in Accelerators (Aug 31 - Sep 3, 2010): Daresbury Lab, Warrington, UK.
* LHC-CC09 (Sep 16-18, 2009): 3rd Joint CERN, US-LARP, EUCARD-ACCNET, DL/CI, and KEK, CERN, Geneva, Switzerland
* Crab Cavity Status, CM11 (Oct 27, 2008), FNAL, IL
* LHC Crab Cavity Validation, CARE-HHH Mini workshop (Aug 21, 2008), CERN, Geneva
* Crab Cavity Coordination Meeting, LARP-CM10 (Apr 23-25, 2008), Port Jefferson, NY
* Mini-Workshop on Deflecting/Crabbing Cavity Applications in Accelerators (April 23-25, 2008): Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China
* LHC-CC08 (Feb 25-27, 2008): 1st Joint BNL/LARP/CARE-HHH Mini Workshop, Brookhaven National Lab, Upton, NY


Meetings & Minutes

* 1st LHC-CC Webex Meeting: Dec 5, 2007
* 2nd LHC-CC Webex Meeting: Jan 30, 2008
* 3rd LHC-CC Webex Meeting: April 7, 2008
* 4th LHC-CC Webex Meeting: July 16, 2008
* 5th LHC-CC Webex Meeting: September 25, 2008
* 6th LHC-CC Webex Meeting: December 9, 2008
* 7th LHC-CC Webex Meeting: February 24, 2009
* Mini LARP-Cryomodule Meeting: May 7, 2009
* Collimation Meeting 2: June 12, 2009
* 8th LHC-CC Webex Meeting: July 1, 2009
* 3rd CERN-KEK Meeting: Oct 6, 2009
* 11th LHC-CC Webex Meeting: June 17, 2010
* 2nd Machine protection meeting: June 16, 2011

1st LHC-CC Webex Meeting: Dec 5, 2007

Summary: The goal of the meeting was to start a collaboration and establish regular phone meetings between the LHC & ILC crab cavity teams
To identify the synergy between the LHC & ILC crab cavity program, exchange of information about the design challenges, to identify where the contribution is most urgently needed, and develop a global parameter list to start the design work for the LHC crab cavities.

At the meeting, R. Calaga gave an overview of the LHC crab cavities, motivation and possible scenarios in the view of future LHC IR upgrades, described some options of the cavity design and discussed some beam dynamics issues. The R&D path of the crab cavity development was briefly outlined which consists of a 800 MHz prototype cavity to demonstrate the feasibility of crab scheme at the LHC. Several interesting questions ranging from cavity design, aperture & collimation requirements, cavity components such as couplers and some exotic concepts to meet tight space requirements were addressed. From the extensive discussion it is clear that there is considerable synergy with ILC crab cavity expertise, and also a significant desire of the collaboration working on ILC crab cavities to contribute to design of LHC crab cavities. Some of the urgent tasks for the global collaboration are:

  1. To produce a more optimized design of the 800MHz prototype.
  2. Manufacture, which upon approval may start in a half a year to a year time scale (via SBIR).
  3. Detailed studies of the requirements to the crab cavity parameters and effects on the beam dynamics and collimation efficiency are also needed.

The next steps were suggested to enable the global collaboration to start work and hopefully come up with results that can discussed at the February 2008 Workshop at BNL are:

  1. Create a set of working assumptions on all needed parameters for the crab cavity (such as the needed kick, limit on transverse size, mode damping characteristics, etc). Provide reference to the source of information and indicate how reliable is the particular assumption.
  2. Share information about the present status of work, in form of some table, and identify the working different groups responsible for the several tasks at hand and focus on the efforts needed more urgently.
  3. Create Wiki page where all the above mentioned information, as well as existing drawings, geometries, minutes, talks, and discussions will be posted to aid the different working groups and enhance communication.

Next phone meeting of the global collaboration: sometime mid Jan, 2008.
-- Prepared by R. Calaga and A. Seryi

2nd LHC-CC Webex Meeting: Jan 30, 2008

Participants of January 30 2008 meeting (Please forward to those who we have missed):

Leo Bellantoni, Ilan Ben-Zvi, Graeme Burt, Rama Calaga, John Cary, Amos Dexter, Valery Dolgashev, Philip Goudket, Alex Kalinin, Zenghai Li, Peter McIntosh, Cho-Kuen Ng, Serguei Ovtchinnikov, Bob Rimmer, Andrei Seryi, Nikolai Solyak, Rogelio Tomas, Haipeng Wang, Genfa Wu, Liling Xiao, Slava Yakovlev

Summary: Second crab cavity synergy meeting was held on January 30, in preparation for the LHC-CC08 workshop in February. Meeting was focused on mostly two topics:
1 Discussion of various specifications and constraints for the crab cavity. 1 Discussion of optimization of the cavity shape for 800MHz.

R. Calaga discussed the specifications and constraints, referring to the his proceeding of the IR07 workshop at Frascati (Nov 2007). Discussion during the meeting in particular was focused on the geometrical and space constraints. For the local compensation, when cavities would be placed in vicinity of IR5 and IR1, the transverse separation available for nominal LHC between the beams is about 19cm. It "may" be increased somewhat a few cm depending on optics of the phase I upgrade since it is only anticipated to have minimal IR modifications (replacement the triplet and D1 magnets). Assuming the 19cm beam-beam separation, a solution proposed by R. Tomas for the local scheme is to consider vertical crossing for both IR's (the nominal is HV crossing, hybrid scheme to cancel beam-beam effects). For the global scheme, a place in IR4 where the beam separation is 40 cm could be used for cavities. The longitudinal space available is about 5-10m. Rama also have shown a tentative version of the Wiki page, which will be used by the collaboration for efficient exchange of information.

Liling Xiao presented results of optimization of the 800MHz cavity shape, starting from Rama's design, aiming to minimize the peak fields by varying the cavity parameters (angle, cell dome radius, iris aspect ratio). The squash ration was also varied to optimize the mode separation. For the 5MV kick for two cell cavity, the peak E and B field were found to be ~25MV/m and ~83mT correspondingly. A few people noted that flat wall on the crab cavity will make it difficult to process the cavities and drain the chemicals efficiently. A slightly positive wall in this respect is generally preferred. R. Calaga suggested that the HOM spectra should be calculated to fix the beam-pipe radius for efficient HOM damping after which a fine tuning of the geometrical parameters can take place to decrease the peak fields to the minimum level while maximizing the shunt impedance. Next step would be design of the couplers of various kind (Input/LOM/HOM). For a preliminary assumption of damping requirements, Rama suggested to assume 1E3 or better 1E2 Qext based on KEK-B specs as a tentative requirement. It was pointed out that the higher energy of the LHC may require less stringent damping. The real and imaginary parts of the impedance will be calculated and thresholds for single and couple bunch instabilities will be calculated soon by R. Calaga and CERN impedance group to have a concrete impedance budget.

For the cavity size reduction, Nikolai Solyak mentioned that FNAL colleagues are considering to look at a mushroom shaped cavity, which could potentially reduce the transverse size of the cavity by 20%. The difficulties for production and treatment of such cavity were noted, and would need to be studied.

A question was raised by L. Bellatoni about program management and leadership. BNL along with LARP (R. Calaga, I. Ben-Zvi) are taking the lead role to coordinate the crab cavity R&D. The proposed 800 MHz prototype is a joint effort of BNL-AES, but with extensive design input from various participating labs. Close collaboration among all the participating labs (US, Europe & Asia) on various components and different funding sources is envisioned for the success of project.

These and the following developments will be further discussed at the LHC-CC08 workshop at BNL on February 25-26.
--Prepared by R. Calaga and A. Seryi

3rd LHC-CC Webex Meeting: April 7, 2008

Rama presented updated results for parametric scan of the cavity geometric parameters of 800MHz cavity to obtain the optimum RF characteristics for the inner and outer half-cell of the two-cell cavity. This scan is valid for a cavity wall angle 6 deg or larger since the behavior is different with wall angle = 0 (SLAC Design). Plots are posted at wiki.

Liling presented updated studies for crab cavity coupler design, focusing on coax-to-coax coupler which was showing most promising damping for LOM and SOM. To accommodate the coupler the beampipe was enlarged. It affected R/Q and other parameters but not significantly. The achieved Qext are 1723 for LOM and 136 for SOM. There are some high field regions in the coupler that need to be investigated. Another version of coupler -- coax to waveguide was studied. It so far give not as good damping as the coax-to-coax. Matching will be optimized using door-knob structure. Multipacting study of TESLA cavity were shown. In discussion it was askedif delta grooves or modulation of radius could be used to suppress it.

In discussion it was asked which team could make copper model of the copper. Graeme said that maybe Daresbury could do that, provided positive funding situation.

Zenghai presented ideas for cavities of different shapes. First is the spoke cavity. So far only single cell periodic solution was calculated but it looks it could allow reduction of R_outer to < 170mm for 800MHz. Shunt impedance seems acceptable. Field enhancement behind the spoke need to be optimized -- perhaps by lengthening the cavity or increase the number of cells, as there is not much constraints in length. Spoke cavity could also allow damping ports in each cell or opening at the equator, for damping. Zenghai also presented a coaxial cavity which is a combination of coax and mushroom cavity, with thicker coax. In discussion it was noted that cannot use coax diameter larger than ~100mm otherwise the operating mode can radiate into the coupler. Zenghai noted that this could be prevented by a notch filter in the coupler. A question is if this notch filter may be too sensitive and not easy to tune.

Nikolai Solyak presented updates on mushroom cavity design and coupler optimization. Increased aperture and diameter of coax to ensure that the coupler will couple to SOM mode that will have larger frequency (due to squashing?). Squashing the cavity is not yet done -- one of next steps. Multipacting study started to be done with Microwave studio and Analyst. See multipacting in three regions: in the reentrant part of the cavity, in coaxial line and in the pocket. Number of secondaries does not decrease with E field. Question was asked how to condition such cavity. Solyak said that FNAL has just made monopole spoke cavity and there are simulation and experimental results for multipacting of this cavity that could be applied to crab cavity design. Also checked multipacting of SLAC crab cavity design (without couplers) -- found that number of secondaries decrease with E field.

Rama have shown estimations for the required impedance of the crab cavity. Based on analytical estimates:
1. R_sh (Inj) < 137 kOhms, R_sh (Store) < 196 kOhms
2. Im{Z/n} (Inj) < 0.24 Ohms, Im{Z/n} (Store) < 0.15 Ohms
3. R_sh (Transverse) << 2 MOhms/m

Need to resolve question if "impedance" was R/Q or Rsh. The exact definition of Rsh needs to specified for the values calculated. Damping with Qext of a few hundreds is required to meet sepecifications. The growth rate for individual modes of interest as a function of Qext needs to be calculated to specify detailed damping requirements for the most important modes.

For CC10, plans and budgets from collaborators are requested.
--Prepared by R. Calaga and A. Seryi

4th LHC-CC Webex Meeting: July 16, 2008

5th LHC-CC Webex Meeting: September 25, 2008

6th LHC-CC Webex Meeting: December 9, 2008

Rama presented in brief the conclusions of the CERN workshop from Aug 21, 2008 and LARP, CM11 from October 2008. The key conclusions being that any demonstration of crab cavities will need consider the safety and performance of the LHC and by no means be an obstacle to the operation. There is a window during the 2012-13 shutdown during which the crab cavities can be installed and be tested for RF, beam studies and substantial luminosity gain measurable by the experiments. This can will require a special configuration for the prototype tests to enhance the effect in the phase I upgrade. A brief layout near the IR4 region where the crab cavities are anticipated to be installed were shown. More tunnel visits are needed for measurements to verify the engineering drawing and the actual transverse space available for both beams. It was mentioned by P. McIntosh and G. Burt that mechanical tolerances of all cavity designs and couplers (especially those with notch filters) need evaluation.

Y. Morita presented a combination of superKEK-B type concept for the LHC crab cavities and some beam studies planned at KEK-B later this week. The cavities consist of waveguide and coaxial type dampers on the cavity body longitudinally placed with respect to the cavity. The cavity is strongly damped giving Qext in the range of 50 for the TM010 and similar number for SOM and HOMs. The loss factor is reduced compared to the existing KEK-B design but with slightly increased peak fields near the iris region and the near thee waveguide mounting points. It was noted that the frequencies of the TE modes are higher than in the regular KEK-B design. Morita-san mentioned the need of 3D multipacting and showed results from experimental measurements from the current KEK-B cavity near 12MV/m and 18 MV/m. Also multipacting observed at 5 MV/m near the coaxial regions which were non-issue after sufficient conditioning. For the LHC design, the concern is mainly from the region near the coaxial couplers and any regions with high magnetic fields. They require LARP experts to analyze their structure for possible multipacting that can signal possible problems with the design. Two beam experiments are proposed at KEK-B later this week which are related to LHC crab crossing. Controlled noise near the betatron frequency and perform a scan while measuring beam sizes during the noise injection. The second experiment uses normal conducting cavities to modulate (kick) the beam longitudinally and therefore measure the Qs sidebands from the longitudinal oscillations. The idea was proposed by R. Tomas at an earlier meeting this year.

G. Burt presented the current status of the UK design with waveguides attached to the equator of the cavity body. He pointed out that using waveguide on the cavity was self polarizing and didn't need additional squashing which alleviates some fabrication issues. Also, the use of ridged waveguides help lowering the cutoff frequencies of some the modes which need to be propagated in the waveguides and in general achieve better damping. The Qext numbers are quite high compared to the requirements but with some optimization the Qext numbers maybe possible.Some simulation results from the rod type structures were also presented and look quite promising. The transverse size of 400 and 200 MHz are close to 200 mm which are close to the requirements of phase II upgrade. H. Wang noted that one needs to take care of the optimization of the rod type structures so that the deflecting mode is the lowest mode. It was also noted that all designs considered for prototype were also quite aggressive and a more simpler and conventional design like the one considered for Argonne light source will be pursued by UK as a backup.

Z. Li presented the latest status of the LARP baseline design and some issues relating the cross coupling of fundamental power coupler and the LOM/SOM couplers used for damping. A modified design to separate the two cells to weakly couple them and introducing a waveguide/coaxial couplers in the center gets rid of the cross coupling but some more optimization is needed to achieve damping requirements for the pi-mode. In this configuration the 0-mode instead of the pi-mode is used for the deflection. It would also have two LOM and two HOM couplers on either side for damping. As a general concept, he noted that symmetry in all coupler configurations should be maintained to avoid modes to polarized to one side of the two cells. Also a small tilt in the LOM coupler is needed to achieve a proper notch and therefore not extract the main deflecting mode.It was noted by P. McIntosh and G. Burt that error analysis for such coupler configurations maybe be needed to establish robust operations. Also cleaning and assembly may not be trivial for the coaxial insert envisioned for the strong damping. As in the KEK-B coaxial coupler, some cooling at 4K for the coaxial insert will be needed handle any losses very close to the cavity. H. Wang mentioned that a two cell cavity design was evaluated for Argonne cavity which resulted in the separation of the 2 cells to reduce peak field and increase damping.

S. Yakovlev presented the current status of the cryostat design and the different cryo circuits for 4.6K and 2K respectively. Both cryo supply systems are present in the LHC tunnel while the 2K is significantly more difficult. Also the helium available via the QRL line is at 1bar pressure which is extremely high for cavities. It was noted by R. Calaga that the 4K system already used for main RF circuit can be extended for the LHC-CCs which will pose the least impact to the cryogenic system and incorporate all the safety factors already in pace for the main RF.

Discussions followed on down selection and R. Calaga will as a first step compile the different RF parameters for the deflecting mode and few modes of interest with their corresponding damping numbers for the three different designs. The different structures will be sent to FNAL & SLAC where they will perform multipacting simulations. Other issues like thermal, mechanical, fabrication and cleaning of each structures will be discussed in the following meetings.

7th LHC-CC Webex Meeting: February 24, 2009

R. Calaga presented the latest status of the project which is reported in a detailed CARE-HHH publication from Jan 2009. The recent tunnel visit and analysis of the space in IR4 was detailed in the presentation. The space between the beam pipes was confirmed to be ~42cm from the engineering drawings. However, a new issue of the space between the inner beam pipe and the cryogenic line (QRL) was raised. The cryostat for the outer beam pipe can share the inner beam pipe within the shell. However, due to the presence of the QRL line at ~44 cm from the inner beam pipe needs the cryostat of the inner beam pipe to fit within the 44 cm space. This poses significant challenge from the point of view of engineering such a cryostat at 4.6 K within such a constrained space. Solyak mentioned that Peterson who was formally assigned for the cryostat/cryogenics is unavailable for this task and a new person will be appointed within FNAL by March. A brief view on the schedule was also presented where the next meeting is dedicated mainly for beam dynamics simulations and detailed meeting will take place during LARP collaboration meeting, CM12 during April within a small crab cavity working group. Following this meeting, PAC09 will cover several papers on different components of the cryomodule and simulations.

G.Burt presented in the waveguide on cavity design proposed in collaboration with UK-JLab-TechX. A fairly advanced design is in place and damping of different modes were presented. Some optimization of the damping still is in progress. The advantage of this design is that one doesn't have to polarize the cavities as the presence of the waveguides automatically polarize the the dipole mode by 10-20 MHz. If more polarization is needed the cavity body can be squashed to enhance the effect. Preliminary multipacting analysis has been intitiated in collaboration with Tech-X and observation of electron trajectories near 5 MV/m surface fields were observed. A detailed analysis and scan of the impact energies and corresponding SEY for the electrons needs to calculated to evaluate the structure. Calaga raised a comment about the mechanical stability with long and large waveguides was put forth. Mechanical analysis will be needed to confirm the sensitivity of mode frequencies due to waveguide deformations. F. Caspers mentioned that multipacting in waveguides is a real issue but it was pointed out that Cornell has been successfully operating waveguides in SRF cavity ~500 kW. Haipeng mentioned that JLab type waveguide on the cavity design is built for the Argonne deflecting cavity and simulations on multipacting are being bench marked using measurements on real cavity at 3.9 GHz in collaboration with Tech-X. G.Burt mentioned about using Solenoids but the use of magnetic fields very close to SRF cavities may pose a significant challenge. However, it was mentioned that field needed maybe small, but still challenging at extremely high peak fields.

L. Xiao presented the latest simulations of the LARP design for the LHC crab cavities which meets strict damping requirements of Qext in the range of ~100. A notch filter in the LOM/HOM damper will be placed to reject the operating mode. The design of the cavity-couplers also allows easy adjustment of the coupling the the operating mode. A detailed multipating analysis was carried out for this design starting with the cavity body. SLAC 3D code is being used for this analysis. The analysis points to most of the multipacting near the iris region where the magnetic and electric fields have their peak values. The impact energies were calculated as a function of the peak field on the surface which seem to have a continuous band near the operating fields range of 6.6 MV kick. Since the region of the multipacting was concentrated mainly in the iris region, the iris ellipse radius was used to tune for better multipacting performance. The peak fields tend to like a larger radius while multipacting requiring smaller radius may require a compromise. thus increasing the surface fields significantly for better multipacting performance and therefore more cavities for the same voltage. Calaga asked if changing the wall angle of the cavity would affect the multipacting but Z. Li pointed out that the two-point multipacting happens at points on the azimuthal axis and therefore independent of the cavity wall angle. R. Calaga pointed out that one could separate the two-cells by lambda/2 to weakly couple the cavities to reduce the effect of multipacting as it is mainly dominant in between the two-cells of the cavity.

N. Solyak presented multipacting analysis of the KEK design of the crab cavities. No significant multipacting was observed in the cavity body except for a localized region of the coaxial-coupler inserted fairly to the cavity iris. The field decay from the iris to the coaxial line which is below the cutoff of the transmission line causing a very localized region to multipact at ~24 MV/m. R. Calaga asked why no significant multipacting was observed in that field range for the bare cavity near the iris region similar to the SLAC analysis. A bare cavity analysis and comparison between the SLAC and FNAL codes were needed to benchmark the results of the mulipacting. KEK measurement results maybe available to benchmark the simulations with the measurements. However, KEK-B cavities were able to process through these bands and operating successfully. Therefore it is important to identify the strength of these bands to see if one can process through such multipacting which maybe important for the final cavity design to reduce the surface fields. R. Calaga mentioned that all the cavity designs will be available in either SAT or STL formats for both bare cavity and cavity-coupler structures to be able to run benchmarks on the same structures for different designs. It is expected to have more detailed results by CM12 and PAC09.

Mini LARP-Cryomodule Meeting (PAC09): May 7, 2009

Participants: R. Calaga, D. Holmes, D. Li, Z. Li, Y. Yakovlev

These are summary notes from a short meeting that took place during PAC09 in Vancouver between AES-BNL-FNAL-LBL-SLAC groups working on crab cavity cryomodule.

The cryostat designers would like the SLAC 2 cell cavity design to be fixed. It was decided that we proceed with the original two-cell design and abandon the separated two-cell design proposed in the 7th webex meeting. The problem of cross coupling between the fundamental and same-order-mode will be solved from the RF standpoint. This will avoid any waveguides couplers protruding from the mid region of the two-cells which can pose some difficulties for the cryostat.

Additionally, Z. Li and SLAC team will pursue a T-type design for the fundamental coupler as suggested by Y. Yakovlev. The design and optimization of this T-type coupler is of immediate importance as it will determine some of the constraints on the cryostat design. This T-type coupler is required because horizontally oriented couplers are impossible due to adjacent beam-pipe or QRL. 0-mode suppression needs evaluation.

An important study that needs immediate attention is the cavity field distribution due to imperfections. L. Xiao is carrying out such studies which will be presented in the June webex meeting. This study will also determine the coupler performance due to imperfections and give tolerances for alignment of the cavity and couplers as an input to the cryostat and tuner designers. Sensitivity studies for all modes other than the fundamental mode will follow.

The optimized shape and properties of the Helium vessel will be studied by I. Gonin and the FNAL team. ANSYS studies will be performed on the latest SLAC-type cavity-coupler design. The study will determine the tuning range from mechanical deformation and best fixed points which is required for the tuner design. Due to vertically placed couplers on either side of the two-cell and co-axial geometry of the beam-pipe, the CERN tuner design needs to be modified to allow for tuning the cavity body without deteriorating the couplers performances. Due to strict ASME pressure vessel code imposed by the DOE, the LHC Helium vessel and cryostat needs to adhere to these regulations to allow for testing in the U.S. The cryostat team will also begin evaluation of the KEK & UK designs for the placement of the cavity-couplers inside the cryomodule and associated issues relating to Helium vessel, tuning, interfaces etc...

Collimation Meeting 2: June 12, 2009

Participants: R. Assmann, R. Calaga, Y. Sun, R. Tomas, F. Zimmermann

Y. Sun presented studies performed for two conventions and goal was to identify any differences observed between the nominal LHC.

Using convention I, tracking was performed for on-momentum particles within approx 5-million particles and the halo was adjusted to have impact parameter on the primary collimators to be roughly 5.9sigma. No significant effect due to the crab cavity was observed as expected (on-momentum). Only collimators may change the momentum after impact. There is a small effect on the impact parameter after the first turn and percent particles absorbed but not significant. Ralph pointed out that orbit changes due to crossing angles etc.. can easily change the impact parameter and is expected even without a crab cavity. A question was raised about the particles absorbed and why it is only ~70%. Ralph pointed out that by tracking more turns one can get larger absorbed particles until one reaches a plateau (for ref: nominal LHC, the particles absorbed in 200 turns - 70%). A scan on the impact parameter as a function of turn number yields the largest impact parameter around 20 turns. This number is consistent with Chiara Bracco's simulations for the nominal LHC. Ralph commented that the large impact parameter could be from particles that have impacted from the collimator on the first turn. Need error bars on the average impact parameters to understand the sensitivity. Within the statistical error, the loss map looks very similar to the nominal case. The global cleaning efficiency looks almost identical and the local loss maps are also very similar. Ralph pointed out that the local quench maps are most important to judge the exact performance. Yipeng concluded that in convention I has no significant or negligible effect on the impact parameter and loss maps both in the collimators and cold regions from simulations.

In convention 2, the impact of z-dependent crab dispersion on the cleaning efficiency is studied. The global crab cavity is treated as a dipole corrector for a given sigma_z particle and synchrotron oscillations are essentially frozen. The initial halo is populated to around 5.5sigma (on-momentum) with 5 million particles and then tracked. The relevant parameters are then calculated for particles at different sigma_z. The initial halo sigma is chosen so that the 1st turn impact parameter is approx 1 micron. No significant difference in the global cleaning efficiencies and the local loss maps are observed. The phase space cuts are calculated due to off-momentum beta-beat and dispersion beat with crab cavities (including both dispersions). The separation between the primaries and secondaries collimators are found to be roughly 1-sigma which is consistent with nominal. It appears as though that the dispersion component from the momentum part and crab part cancel to allow for the separation between the primaries and secondaries to be rougly 1-sigma or slightly smaller. Ralph wants to know if there is a case where the cancellation doesn't happen and the 1-sigma difference is not respected.

These simulations indicate that safety margin is slightly reduced According to Ralph, we may loose some safety margin and leave at least ~0.7sigma will prove to be no show stopper. The critical requirement is that the hierarchy of the collimator is respected and preserve the cleaning efficiency. We should now contact Brennan/Wenninger et al. to help us understand machine protection issues and determine the operational requirements.

8th LHC-CC Webex Meeting: July 1, 2009

R. Calaga started the meeting with the announcement of the LHC-CC09 and the format of the workshop that will take place on Sept 16-18, 2009. This workshop will address several key issues from crab cavity cryomodule, integration, cryogenics, bench testing, prototype beam tests and finally a strategy towards phase II upgrade of the LHC. A detailed set of guidelines to the RF designers were laid out to be addressed at the workshop. This will set the scene to determine the complexity of the various designs and help down select towards a baseline design. It is also essential that a baseline design is picked so that the SBIR can move forward to the phase I. Finally, an idea to propose the crab cavities as a separate DOE project will be tried at the upcoming LARP DOE review where LARP and other CC collaborators will play a supporting role.

M. Cole showed the status of the SBIRs submitted in 2008. He first presented the SBIR format where the phase I is small with limited resources ($100k). If preliminary mechanical and thermal analysis to show no show stoppers, the phase I will move forward to more rigorous phase II. September level time line is well in sync the phase I plans for AES to start with their mechanical/thermal analysis to prepare for phase II. Then move to phase II for complete thermal/mechanical design. Once phase II is approved, hardware can be pursued at the level of $750k. The 4 SBIRs were reviewed together and reviews were identical and the support was only given to crab cavity SBIR and other 3. He noted that new DOE pressure vessel code makes it a quite difficult process. And any testing in the U.S. mandates the pressure vessel code and the associated effort to comply with it. The cost of the Nb can be very good supplement and perhaps can help achieving more that just the cavity with the given SBIR money. Perhaps coupler ports and couplers could be added given there are enough resources. The exact extent of the fabrication will depend on the final available resources from all sides and SBIR is quite flexible to accommodate the different contributions. The testing of the cavity in an existing cryostat is possible if it is only testing on the bench and doesn't have to go into the tunnel.

RF simulations from G. Burt shows some discrepancies between VORPAL and CST. For example the operating mode is different by ~ 4MHz but some other modes seem ok. Also additional features relating to boundary conditions in CST are not fully understood which could lead to the discrepancies and VORPAL will be used from now on to continue the studies. For multipacting, the main goal has been benchmarking of the structure. Two point high order multipacting is observed at increased power through the waveguide couplers and has a feature of continuous growth. Preliminary conclusion was that the SOM is unlikely to multipact, LOMs and other modes will be followed up next. Also to address the issue of multipacting on the iris, the curvature was increase but cannot increase much more as the magnetic fields will get worse and limit the gradients. A simple estimate on the polarization of the cavities show very tight tolerances based on the power output in the SOM of the deflecting mode. This corresponds to "nm" level alignment errors which needs to be confirmed. The non-polarized can possibly improve the tolerances by 2-3 orders of magnitude but this has to also be confirmed. Rod type cavities further being studied and results will be presented in subsequent meetings.

Zenghai discussed the outcome from CM12 and studies following that which focused on multipacting and mechanical tolerances. One of the main problems was the coupling between operating mode and SOM coupler. This coupling was significant and had to be remedied. A new FPC design with vertical waveguide placed opposite to the SOM coaxial coupler with a coupling slot eliminates this coupling between the modes. The waveguide coupler itself can have large static heat losses, so it was made small (size of the input coupler) and therefore the operating mode is cutoff in the waveguide block. The mode is captured by a coaxial probe from the rectangular block is added at 90 deg and connected to power supply. Mechanical limitations of this design will follow. Artificial imbalance in the cavity length was introduced to test some mechanical tolerances. Roughly ~10% field imbalance in the operating mode doesn't significantly alter the profile and therefore not extremely sensitive. No significant changes in the damping was found with imperfections. More tolerance studies will follow.

3rd CERN-KEK Meeting: Oct 6, 2009

The meeting topics were focused on:
1. Feasibility of KEK-B cavities in the SPS for testing
2. Future KEK-B experiments during Fall 2009 run.

Nakanishi-san presented an overview on the tuning capabilities of the KEK-B cavities as designed and observed. The nominal extent of the tuning is approx 100 kHz during operation but the mechanical range of the tuner is capable of going up to 200 kHz. The speed of the tuning is limited to 1 kHz/sec which maybe too slow for the SPS ramp.

The static frequency adjustment from the KEK-B frequency (509 MHz) to 511 MHz to be usable with 100 ns bunch spacing in the SPS is possible. However, the large frequency adjustment (~2 MHz) requires disassembly of the cryostat and mechanical deformation of the cavity and the load. The cavity vacuum can perhaps be preserved for this deformation and thus saving additional treatment procedures which can take long. Some assembly at CERN will be maybe necessary which also makes transport easier to avoid cryostat support vibrations. Realistic estimates of such changes can take between 3-6 months if all procedures go smoothly.

The heat load to liquid helium is about 100W. But to cooling circuit to the coaxial coupler uses a separate volume which flows through the coax to provide active cooling. Circulation of liquid helium was avoided, as a part of liquid helium flowing back to room temperature port was making ice balls (see picture). Need to verify with cryogenic group if this is acceptable. A detailed plan of installation of cryogenics and related infrastructure is necessary at CERN. Nakanishi-san also noted the availability of prototype coupler which is slightly shorter and consequently can be operated with a larger Q-ext if desired for SPS operation. This will reduce the power requirements and a wideband IOT at 511 MHz can be used which is relatively cheap and commercially available up to 20 kW in solid state form. A question was raised if the Qext will be determined by the available amplifier which has to be determined on what is available for SPS tests at CERN.

The proposals for future experiments were shown based on discussions at LHC-CC09 and CERN meetings. Several proposals were mentioned and after feedback from KEK, three of the experiments seem feasible for this fall experiments:
1. Noise: observation of coherent oscillations close last year's experiments
2. Ramp studies (loss/no loss/ other phenomena)
3. Re-start the cavity after trip with no beam dump

Option 2, ramp studies are important both for adiabaticity of the crab cavities and luminosity leveling. Option 3 was some reason discarded in last year's experiments due to differences between protons and electrons but seems relevant as the problem of re-starting with beam is critical for LHC due to the long turn-around-time. Consequently, the effects of the trip on the beam stability (orbits, aperture, losses etc..) and behaviour of the RF control loops will be important information. It was mentioned that an attempt maybe made on performing luminosity leveling but with limited beam current if time is available. Variation of Qext is not possible in the KEK-B cavities, therefore a study of RF trip effects as a function of Qext is not possible. This topic maybe interesting for the LHC due to machine protection issues which need a few turns of reaction time to safely compensate for crab cavity failures.

11th LHC-CC Webex Meeting: June 17, 2010

The meeting started with N. Kota giving an account on the analysis of RF signals of the KEK-B crab cavity during a cavity failure or trip. Different data taking systems like the abort monitor, beam current, RF amplitude and phase were used in to study the behavior of the trip and its effect on the beam. Several different cases were shown which correspond to very different behavior on the RF signals. However, the signals of the RF and the corresponding beam behavior were consistent and in some cases associated with beam loss. Vaccuum during the trip was not degraded significantly. Phase shifts of +/- 50 deg (HER) were observed during a time period of 50 mu.sec and the beam was typically aborted intentionally. Also corresponding orbits shifts of 5mm was also observed on the beam in certain cases. The phase signals were oscillating rapidly in some cases. Signals from LER look rather different and more clean voltage decay during a trip or a failure. Beam loading in the LER cavity was observed, feedback was always on but not fast enough to compensate for it.

G. Burt gave a summary of the developments on the 4-rod compact cavity development at CI-LU. Damping HOMs using waveguides were tried including ridged waveguides. However, the size of the waveguide and the cavity were rather incompatible and hence changed to CERN type demountable loop coupler for damping. The couplers were placed on the cell to achieve maximum coupling and studies are underway to establish the strong damping required. Multipacting with VORPAL is also underway to characterize the cavity and the couplers. It is anticipated that a Ph.D. student from CI-LU could be sent to CERN for some time to perform studies related to LHC crabs.

Z. Li followed SLAC cavity developments which were also shown at the LARP CM14 meeting earlier in April 2010. The SLAC concept is a lambda/2 coaxial resonator, using the TE11 type of mode. Damping concepts developed for the elliptical TM110 type cavity was applied to achieve the strong damping and results are within specifications dereived from the LHC-CC09 impedance budget discussions. Due to cross coupling issues between the FPC and LOM a short waveguide stub (below cut-off) was used to couple to the fundamental mode and then tranformed to a coxial-line from the waveguide. Surface fields are within accepatable limits for kick voltage of 4 MV. A phase I SBIR submitted based on this concept has finished mechanical and thermal analysis and awaiting decision on phase 2 for complete engineering design and fabrication of the cavity.

2nd Machine Protection Meeting: June 16, 2011

Joachim gave a short outline of the potential failure scenarios where he highlighted two main error sources: 1. operator error: rely cavity speed, Q0 2. quench: Cavity Q0 reaches the order of magnitude of Qext

For quench studies, the temperature (T=Tcritical) approximately moves from the source point approximately at a constant velocity, therefore the normal conducting area increases as the square of time. The propagation speed is approximately 30m/s. These quenches can be caused for example due to multipacting, field emission. Erratic movements of cavity components are possible and therefore triggering a quench. Some ideas of fast quench detection was discussed but there is no system or detector at the present moment which can act in a fraction of a turn and coherently switch off the cavities across the IRs. Erk: change of phase by 90 degrees in a single turn is unphysical, and described the analogy of a pendulum which in such a case would have to go from an oscillating configuration to rest instantaneously. However stressed that operator requests by accident can happen and has to be avoided with appropriate interlocks. A fast interlock and fast feedback for crab cavities requires some careful thought.

Following the failure scenarios Rama presented Sixtrack simulations for nominal LHC using modified collimation tracking tools for abrupt voltage and phase failures. A beam size enlarged by a x2-3 of the nominal size was used to calculate absorbed particles in the collimators and lost elsewhere in the ring. The losses were in the order of 10^-4 of the total population which is consistent with Gaussian distribution of the tail distribution. Ralph mentioned that this distribution must be revised with a double Gaussian to include 2% of the particles in the 4-6sigma of the transverse distribution. Ralph mentioned that one should consider the affect of on the beam after a crab trip. This will be important for an asynchronous dump which may limits on the max allowed emittance blow up before beam is extracted.

Tobias talked about MADX simulations with upgrade optics and 15cm beta*. A comment was raised about the final beta* for the crab cavity as there is a strong dependence on the beta* of the particle losses. Therefore, it was mentioned that a crab scenario both from optimal optics and machine protection should be deviced as a reasonable solution. For the 15cm solution provided by Stephane/Riccardo, the particle losses can reach well beyond 2% for a voltage failure and 10's of % for a 90 degree phase failure in one turn. The non-physical nature of this failure is still under discussion. There is an additional frequency dependence which preferred a higher frequency of 800 MHz. It was asked if the frequency is frozen. From LHC-CC10 it was decided to use a baseline frequency of 400 MHz with compact cavities. However, if quantitative studies of machine protection show a strong preference for a higher frequency, this subject can be revisited.

The action items were to follow up on the simulations already performed but focus on the upgrade parameters.


R. Calaga - Jul 22, 2010
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Topic revision: r22 - 2012-05-11 - RamaCalaga
 
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