INTRODUCTION, SUMMARY, AND MAJOR RECOMMENDATIONS

 

The Accelerator Technical Advisory Committee (ATAC) for the J-PARC Project held its sixth meeting over the period March 1-3, 2007 at the JAEA site in Tokai, Japan. The committee heard presentations from project staff on March 1-2, held several closed sessions to discuss reactions and opinions, and presented a verbal report to project management on March 3. In addition the committee was given a comprehensive tour of the project.. The meeting agenda is attached in the Appendix.

Committee members in attendance at this meeting included: R. Garoby/CERN, D. Gurd/ORNL, I. Gardner/RAL (deputy chair), S. Holmes/Fermilab (chair), A. Noda/Kyoto, T. Roser/BNL, L. Young/LANL (retired), and J. Wei/BNL.

Y. Cho/ANL and I. Hoffman/GSI were unable to attend.

The committee heard excellent presentations covering all aspects of the project. The committee appreciates that these presentations were generally responsive to recommendations from the February 2006 meeting. Significant progress has been made over the last year on all fronts highlighted by the achievement of full energy beam commissioning of the linac and the development of a viable strategy for overcoming the problems in the RCS and 50 GeV Main Ring rf systems as described in last year’s report. The committee offers its congratulations on these achievements to the entire J-PARC team !

The J-PARC project is now nearing completion both in terms of civil construction and accelerator component fabrication. Installation is complete and beam commissioning is underway in the linac. RCS beam commissioning is scheduled to start in September of this year in the RCS and in May 2008 in the 50 GeV Ring. This schedule is unchanged from a year ago―an excellent achievement―and will lead to a startup of accelerator operations in support of the research program in the 3^rd quarter of JFY2008. Commissioning plans for the RCS and MR are also more developed and detailed than a year ago. Nonetheless there are several areas of concern that the committee feels will require particular attention to bring the project to successful completion:

• Main Ring performance
• RCS performance with the 181 MeV linac
• Performance of rf accelerating cavities for the RCS and 50 GeV MR during the operations phase

• Planning for the transition to operations

 

These items are discussed in this report. Because of the importance to the successful completion of the J-PARC Project, and the difficulties being experienced a year ago, we have devoted a single chapter to issues related to the rf accelerating cavities.

 

Finally, the ATAC wishes to express its appreciation to JAEA and KEK management and support staff for their hospitality during this meeting.

 

 

 

Summary and Major Recommendations

Linac

Excellent progress has been made since last year’s meeting. Linac installation is now complete and beam was accelerated to 181 MeV in January, 2007. The committee congratulates the Linac team on the achievement of this important milestone, ahead of the schedule presented a year ago. The long range plan remains to upgrade the linac energy to 400 MeV based on the addition of an annular coupled structure (ACS) linac following completion of Phase I. A prototype of the ACS buncher has been fabricated and tested and preparations are aimed at constructing, installing, and commissioning the ACS linac over the period JFY08-11. The committee applauds the start of preparations; however funding for the upgrade remains unsecured.

 

Comments and Recommendations

• The ion source remains a concern. The development of a non-cesiated tungsten filament source has had difficulty meeting the 33 mA current specification. In response a LaB₆ filament is now under development. This filament provides the required current at a 150 msec pulse length, but is unstable at the required 600 msec pulse length. A stabilization system has been proposed and testing will start in roughly a week.

Recommendation: The ion source requires continuing management attention. If the LaB6 source does not meet performance specifications in the next month or two, consider alternative designs for the operation phase.

• The committee was not presented with a comparison between achieved (to date) beam parameters, commissioning goals, and operations goals.

Recommendation: Prepare and maintain a table of major operational parameters containing operations goals, commissioning goals, and currently achieved performance. (This recommendation applies to all machines).

• Residual activation levels in the vicinity of the debuncher have been measured at 88 mSv/hour. This translates into a fairly significant level when extrapolated to full power (a few mSv/hour).
• Many of the beam diagnostic devices have been exercised, but are not yet in a state to support full characterization of the linac beam as will be required to support RCS beam commissioning starting in September. (For example BLMs, intensity monitors, and emittance diagnostics).

Recommendation: Diagnostics need to be providing reliable information by the time of RCS commissioning.

• The closed loop control on the rf system is still a ways from being complete and working effectively. The feedback response looks to the committee to be slower than it should. In addition the feedforward systems still need to be implemented.
• The committee remains concerned about noise from the pre-chopper. Since this device is not yet installed there is no further information relative to last year. We suggest continued attention to this item.
• The prototype ACS buncher module exhibits a shunt impedance that looks low to the committee. We suggest that the result be confirmed.
• The committee has a concern relative to non-accelerating modes in the ACS coupling cavity, and the possible impact on the structure dispersion curve.

·          The spares situation in the linac was highlighted as a concern by the linac team. However, a complete risk analysis, which would include expected failure rates and performance impacts of failures, was not available to the committee.

Recommendation: Undertake a risk analysis of the linac based on expected failure rates, recovery periods, and replacement lead times, as a means to establishing a reasonable strategy with respect to spares inventory. (This recommendation applies to all machines.)

• The committee concurs with prior years’ assessments that the linac is likely to achieve design performance goals at 181 MeV operations. However, performance of the RCS and MR will suffer from the lowered linac energy.

Recommendation: We continue to urge the identification of funding for restoration of the linac energy to 400 MeV as a high priority item.

 

 

RCS (Other than accelerating cavities)

Excellent progress has been achieved on many fronts over the last year. Very significant progress has been made on the rf structures problem identified last year. Installation is well advanced and hardware commissioning has begun. These activities are heading towards a September 2007 consistent with the schedule established a year ago. While there is little float associated with this milestone, the committee regards the September beam commissioning goal as achievable. Accelerator modeling has incorporated several improvements suggested by the committee a year ago. The modeling demonstrates, and the committee continues to believe, that the full potential of the RCS will not be realized until the injection energy is raised to 400 MeV.

 

 

Comments and Recommendations

• A problem with loosening of bolts holding quadrupole magnets together has been identified. The problem was observed during tests of magnets with the design excitation fields and appears to be due to the torquing procedures. The problem appears to the committee to be understood and should not pose a threat to performance provided that it receives continuous attention in the future.
• The RCS commissioning plan presented to the committee looked very good. However, we are concerned that the plan does not extend beyond the start of MR commissioning (at which point the RCS is operating at 10 kW).

Recommendation: Extend the RCS commissioning plan to cover the MR commissioning period and the first several years of operation (see more general recommendation under the heading Commissioning and Operations Planning).

• The problem of field flatness in the extraction kicker has been solved by offsetting (in time) kicker modules to provide cancellation of non-uniformities. The price is a longer risetime that does not quite have the specified risetime. While nominally acceptable the timing is now tight and leaves little flexibility.

Recommendation: Evaluate the relative impact of not meeting the design value for the RCS extraction kicker risetime, which is 263 ns, and pulse flatness, which is 1%.

• The committee found the RCS simulations to be much more realistic than a year ago, now including power supply tracking, BPM resolution, and magnetic interferences. However, the simulations indicate that the 4 kW loss budget cannot be met with a delivered beam power beyond 300 kW, based on a 181 MeV injection energy. The simulations extrapolate satisfactorily to 1 MW of beam power with a 400 MeV injection energy.

Recommendation: Continue to improve the integrated tracking simulation, including the longitudinal dynamics.

• In last year’s report we recommended coating the ferrite and aluminum surfaces of the extraction kicker with TiN to suppress secondary electron yield. This recommendation appears to have been misinterpreted to be aimed at improved vacuum. The recommendation is repeated here.

Recommendation: Consider coating the Ferrite and aluminum surfaces of the extraction kicker. Coating with TiN was successfully achieved for the SNS project.

• Tests of the stripping foil with H-, as suggested by the committee last year, have been initiated. However, the stripping foil still does not show the required lifetime. A new foil material, hybrid boron carbon (HBC), is under development and shows promise for being more robust than commercially available carbon foils.
• The committee believes the collimators still represent a major performance risk. The collimation design requires that the 4 kW of loss power be distributed over the ten collimator jaws in a manner than none receive more than 400-700 W of load. The simulation presented to the committee indicated that the individual jaw specification would be achieved with 20% margin at 0.6 MW of total beam power. The committee does not regard this margin as adequate.

Recommendation: Prepare an improvement plan for the RCS collimator system to provide sufficient margin to cope with realistic operations scenarios.

• Several items are on the critical path to the scheduled start of beam commissioning in September (corrector power supplies, rf stations (10 stations minimum), and septa). While our judgement is that the schedule is achievable these items represent a schedule risk and will require continuing attention.
• The transverse impedance of the extraction kicker has now been measured, as recommended in last year’s report. The result is higher than anticipated. The level is high enough to provide a potential limitation in beam intensity in the RCS.

Recommendation: Verify the impedance measurement and, if confirmed, prepare appropriate remedies and a back plan.

·          The committee concurs with prior years’ assessments relative to RCS performance: 0.33 MW beam power represents a lower limit on achievable performance in the RCS with a 181 MeV injection energy. 0.4-0.6 MW remains plausible, but will require successful solutions to the collimator and, possibly, the kicker impedance issues described above. Recovery of 1 MW performance will also require restoration of the linac energy to 400 MeV.

·          To summarize: The committee has assessed the most significant impediments to full (1 MW) performance in the RCS as follows:

–         181 MeV injection energy

–         Collimation system/loss budget

–         Potential beam instabilities due to either kicker impedance and/or electron cloud

 

 

MR (Other than Accelerating Cavities)

Good progress has been made on many fronts. Dipole, quadrupole, and sextupole installation are roughly 90% complete and very significant progress has been made on the rf structures problem identified last year. Activities are aimed at a May 2008 start of beam commissioning, consistent with last year’s schedule. Accelerator modeling has incorporated several improvements suggested by the committee a year ago. The modeling raises several concerns in our mind as described below.

 

Comments and Recommendations

• The single pulse accident, identified in the worst case as an unintended extraction kicker fire, is now recognized as potentially destructive to the extraction septum. The proposed solution, a passive mask in front of the current septum, appears to the committee as a potentially viable solution and should be pursued.
• Serious problems have been identified in the extraction kicker including failure to meet both risetime and voltage requirements. Potential solutions have been identified but will take a while to confirm and implement. A strategy has been developed for operations through the summer of 2010 including: MR operations with 6 bunches, rather than 8, in light of the lengthened risetime, and modification of the kicker magnet to increase the voltage capability. The committee notes that this mode of operation implies a 25% reduction in beam power.

Recommendation: Develop and install a kicker meeting the performance specification as a high priority item.

• Comprehensive modeling was presented which indicates that the loss budget for the MR (450 W on an individual collimator) cannot be met with a beam power above 150-200 kW. The committee regards this as a serious issue that must be overcome.

Recommendation: Reassess the operating scenario to establish a goal of 450 kW performance with the 181 MeV linac. Such a reassessment could include increasing the loss budget at 3 GeV, means for providing longer bunches, and/or possible reduction in the cycle time.

Recommendation: Depending upon the outcome of the above reassessment, reexamine the decision to operate the neutrino program at 30 GeV, rather than 40 GeV.

• The above described issue with respect to the extraction kicker, plus problems with cracked ceramics in the extraction septum, represent the primary risks to achieving the May 2008 date for start of MR beam commissioning.
• The transverse impedance measurement described above on the RCS kicker has not been completed on the MR extraction kicker..

Recommendation: Complete transverse impedance measurements of the MR kicker including the powering cables and the pulse forming network.

• The commissioning schedule and initial operations plan shows operations at 100 kW through 2010. The committee believes this is too modest a goal (especially in 2010).
• The plan for restoration of full (~750 kW) beam power with the 181 MeV linac is now based on 8 bunch (h=9) operations in the MR, coupled with h=1 operations of the RCS. There are sufficient accelerator physics uncertainties associated with such a scenario that the committee is unable to comment on viability. Among the issues that need to be studied are modifications to the RCS rf system, space charge, and beam stability issues in the MR..

• The committee concurs with last year’s assessment relative to MR performance with h=9 and 181 MeV linac: 0.45 MW should be achievable in this mode contingent on resolution of the loss management and kicker issues described above. The committee remains convinced that a necessary component of any path to 750 kW in the MR includes a 400 MeV linac

·          To summarize: The committee has assessed the most serious impediments to full (0.75 MW) performance in the MR as follows:

–         Management of losses at in the 3 GeV transfer line and during injection into the MR

–         181 MeV linac

–         Extraction kicker

–         Possible beam instabilities related to either kicker impedance or electron cloud

 

 

RF Cavities for the RCS and MR

Significant effort has been invested in the last year in overcoming the serious difficulties that had become evident in the accelerating structures for the RCS and MR. Great progress has been made on understanding the source of a variety of problems, developing a strategic approach that addresses short, long term, and, initial operations needs, and engineering innovative solutions. As a result cavity fabrication and testing is proceeding and four (of the 10 required) rf stations have been tested and are now installed in the RCS.

 

 

Comments and Recommendations

·          A very significant mobilization of effort was undertaken over the last year to address the rf issues. Two new test stands were brought on line, one at KEK and one at JAEA, supporting extended testing of a wide variety of cavity configurations.

·          The initial RCS rf system is being fabricated from uncut cores, augment by parallel inductors to provide the required cavity Q. Water cooling has been retained. Performance on the test stand has been good. All stations are subject to 300 hour high power testing before installation in the RCS.

·          The initial MR rf system is being fabricated from cut cores, with the gap subject to diamond polishing. This process eliminates the acid etch that was deemed the primary source of the problems. Because the polished surface does not allow for utilization of a sealant, deoxygenated water is used as the coolant. Two such accelerating cavities have been successfully subjected to more than 1000 hours of high power testing. Only five cavities are planned to be available for commissioning. Adequate resources remain to be allocated for modifying an existing prototype which would provide a sixth system.

Recommendation: Provide adequate resource for the sixth rf system to be installed in the MR.

·          In acknowledgement that problems could occur over the long term an R&D team has been assembled and is investigating the possibility of developing a cavity based on utilization of oil (paraffin) as the cooling medium.

Recommendation: Studies of alternative cooling schemes should be actively pursued.

·          The committee believes the strategy being pursued is technically sound and is likely to support the needs of the J-PARC complex throughout commissioning and the first year or two of operations. However, long term performance remains a risk.

Recommendation: Continue to follow the developed strategy for the RCS and MR rf systems, including the development of longer term solutions. One additional system of each type should be built to allow this effort to be pursued and to serve as potential spares in case of a major failure.

·          However, the committee is concerned that the capability for high power testing of complete accelerating cavities at JAEA will disappear shortly as the test stand is relocated into the RCS as part of the RCS rf system. It is the opinion of the committee that this capability must be retained to support the long term goals of the J-PARC complex.

Recommendation: Retain a high power test stand on the JAEA site.

 

 

Controls and Global Systems

The controls system has successfully supported the commissioning of the linac. This significant achievement demonstrates the basic viability of the system. The committee offers our congratulations to the controls group.

 

Comments and Recommendations

·          The committee is concerned about the development of high level applications programs to support the commissioning and operations of all machines. The development and implementation of HLAs requires close coordination between the controls group and the commissioning teams. Although there are regularly meetings between these two teams, it did not appear to the committee that a master list of required applications, accompanied by detailed specifications, manpower estimates, and assignment of responsibilities, had yet been developed.

Recommendation: Based upon the commissioning plan presented, specify the high-level physics applications needed to carry out this plan and begin development of these applications as soon as possible.

·          The staffing level of the controls group appears marginal to the committee. However, we do not have sufficient information to assess the overall need. We suggest that management review the staffing level of the group in parallel with the identification of needs implied by the above recommendation.

 

 

Commissioning and Initial Operations Planning

The ATAC found fairly detailed plans covering the period of final installation and commissioning of the RCS and MR.

 

Comments and Recommendations

·          The committee did not see the level of integration across machine boundaries, the identification of required resources, and specification of required HLAs that we would have expected. The committee believes that successful integration of this activity and the transition to operations can only be accomplished under the direction of a full time dedicated leader with the responsibility and authority for commissioning the accelerator complex.

Recommendation: Identify a commissioning team leader with responsibility and authority for coordinating the commissioning of the entire J-PARC complex, including the transition to operations. An integrated commissioning plan should be developed containing the roadmap towards Phase I design goals and defining technical, schedule, cost, and personnel requirements.

·          While the RCS and MR commissioning plans were well thought out and sufficiently detailed, the RCS plan did not extend beyond the start of MR beam commissioning, and the MR plan extended through first year of operations.

Recommendation: The integrated commissioning plan should include the transition to operations and should be discussed with, and made available to, the user community. The published plan should include estimates of performance, anticipated reliabilities, and the time allocation between users and accelerator physics.

·          The committee feels establishment of a downtime log, and an equipment tracking log, is an important component of optimizing efficiency of operations within the complex. Such a log should be established immediately (if it hasn’t already been).

·          The committee found it difficult to assess progress against commissioning goals because no document exists in which operational and commissioning performance goals are succinctly described and compared with current achievements.

Recommendation: Establish and maintain tables of major beam parameters containing Phase 1 design goals, commissioning goals, and currently achieved performance for each element of the accelerator complex.

·          The committee is concerned about the availability of spare parts and components as commissioning and subsequent operations are initiated. For example, the number of spare klystrons currently available is based on an expected (but unconfirmed) lifetime (30,000 hours) and the leadtime for procuring a klystron (1 year).

Recommendation: Establish a spares strategy based on a risk analysis incorporating mean time between failure (MTBF), performance impacts of failures, fabrication/procurement lead times, costs, etc. Such a strategy should be used as a basis for establishing the spares component of the operations budget.