SNS Diagnostics Advisory Committee Report
Linac BPM Systems - Preliminary Design Review
February 27, 2001
Los Alamos National Laboratory
Committee members: R. Webber (FNAL/chairman), J.
Crisp (FNAL), R. Johnson (SLAC)
Non-LANL guests: Craig Dawson (BNL), Dave Purcell
(ORNL), Saeed Assadi
(ORNL), Ken Reece (ORNL), Tom Shea (ORNL)
The advisory committee last met during the introductory diagnostics review in December at Oak Ridge. The committee observes numerous signs of management and organizational progress, in addition to the technical progress, since that time.
· Technical requirements have been separated into categories identified as minimal, target and desirable. This should help focus designs and resources to efficient solutions.
· Two individuals, Saeed Assadi and Dave Purcell, have been added to Tom Shea's ORNL SNS Diagnostics staff to provide better coverage for gathering, documenting, and communicating beam diagnostics requirements. This should help focus and direct activities at the various collaborating laboratories.
· Plans to provide 'final-like' beam diagnostics and instrumentation systems for first beam tests at LBL early next year are progressing. This should be to the benefit of all parties; duplication of effort will be avoided, hardware and software providers have early goals to meet and early testing from which to benefit, and beam parameter measurements will be traceable from LBL to ORNL.
On the less positive side, it is still apparent and was in fact explicitly stated at this review that designs are ahead of requirements definitions in some areas. This has the potential of leading to performance and interface problems. Subsystem details seem to be largely well-in-hand, but there remains an immediate and continuing need to concentrate on system-wide issues that may impact those details. Examples of problem areas that surfaced at this review include: cable length requirements for the BPM calibration design, uncertainty about available cable chases to the tunnels, absence of alignment tolerance budgets, requirements for absolute vs. relative beam phase measurements, and requirements for signals to be provided by diagnostics to low level RF systems. The committee recommends that urgent attention be paid to these and similar system integration issues. This will require ongoing effort throughout the project and the addition of Assadi and Purcell should help address that need.
The presentation of system and component costs at these reviews is of some interest to the committee, but in fact little utility. This committee was not impaneled to address cost issues and the information presented is typically just incremental component cost excluding design, testing, installation, commissioning, and overhead. It should be recognized that for many beam diagnostics systems, the individual quantities are small and that the incremental costs represent only a small fraction of the true system costs. We are happy to comment on the incremental costs as we see fit, but project management should make no assumption or representation that these reviews are meaningful project cost reviews.
In general,
the review committee feels that the BPM/Phase measurement electronics design
looks sound and that final design and prototyping should proceed rapidly to
identify any problems that may arise.
Committee Observation - Much progress is apparent in BPM mechanical design since the December meeting, especially for the DTL BPMs.
Committee Observation - MEBT BPMs are nearly completed;
the design appears to be satisfactory.
Committee Observation - HEBT BPMs are only in preliminary
design stage, but sketches appear to describe satisfactory designs and
requirements seem straightforward. We
would expect no problems for these designs provided normal precautions are
taken to achieve vacuum and electrical connection integrity.
Committee Observation - CCL and SCL BPM design appears to be progressing satisfactorily. They are based on proven design
techniques used at BNL. If there may be
a weak link in the design, it is the pressure contact in the signal feedthrough
path; BNL has reported no problems with this feature of the design.
Suggestion - Write quality control procedures to assure electrical integrity of the pressure contact in the signal path. Such a procedure might include an electrical resistance measurement at a moderate current level (not just a DVM measurement, you're looking for milliohms!) and a TDR or swept frequency network analyzer measurement.
Committee Observation - It was noted during the review
that the SCL BPMs may undergo a vigorous
"scrubbing" at Jefferson Lab before connection to the cryo
module assemblies.
Suggestion - Establish communications soon with the Jefferson Lab collaborators to identify and document any special requirements of the SCL BPMs. These may include materials issues, bake-out requirements, cleaning operations, or assembly and operational procedures in the superconducting linac section.
Committee Observation - DTL BPMs suffer the most mechanical constraints and demanding
requirements. Excellent progress has been made since December and it appears that
a satisfactory design has been
achieved provided Kaman delivers parts as specified. The committee's concerns now center around schedule and quality
assurance to guarantee delivery of BPMs that realize the performance
promised by the design. Lifetime vacuum
and electrical connection integrity are at risk.
Suggestion - Keep on top of Kaman's progress with the feedthroughs.
Suggestion -
Establish fall-back plans in case
vacuum leaks develop in the feedthroughs either initially or after installation
of BPMs in drift tubes.
Committee Observation - DTL BPMs must go through numerous brazing and welding steps during fabrication and assembly into
drift tube.
Suggestion - Establish a written vacuum integrity quality assurance plan to be followed during BPM fabrication. This could be as straightforward as a list of tests to be performed and qualifications to be met at appropriate stages of fabrication. It should include vacuum leak tests to assure that cable connection to the right angle feedthrough does not ruin the vacuum integrity of the feedthrough.
Committee Observation - DTL BPM feedthroughs and cables are captured and inaccessible for repair after drift tube
assembly. It is crucial to assure electrical integrity during drift tube
assembly, storage, installation into DTL tank, and for long term
operation. There will be stages in the
life of these BPMs that the electrical cables and connections will not be in
the loving care of the diagnostics engineer. Design features and quality tests
must be in place to guarantee
electrical integrity.
Suggestion - Incorporate design features to minimize vibration of the buried SMA connections and protect exposed cables and connectors during handling, storage, and operation.
Suggestion - Establish written electrical integrity quality assurance plan for DTL BPM cabling and feedthrough components. This plan should include electrical tests, inspections to be performed, and qualifications to be met at appropriate stages of fabrication and assembly. Cable assembly tests, before installation, might include electrical resistance measurements of center conductor and shield under vibration conditions at a moderate current levels (you're looking for milliohms!), a 'hipot' up to the voltage rating of the cable to assure no soft shorts at connectors, and a TDR or swept frequency network analyzer measurement. A sensitive cable resistance measurement setup is easily constructed that will permit monitoring cable voltage drop on an oscilloscope while cable is wiggled and vibrated. Sufficient test current turns milliohms into millivolts without an amplifier. Similar resistance measurements and TDR/swept frequency tests should be performed just before drift tube is welded closed and again just before drift tube installation into DTL tank.
Suggestion - In general, for all BPM pickups and before fabrication begins, prepare a set of QC steps to be taken throughout the fabrication process to assure required properties including vacuum integrity, electrical integrity, cleanliness, ability to withstand bake-out, etc. as appropriate for each fabrication.
Committee Observation - The expected BPM pick-up electrical response appears
to be well understood and acceptable for anticipated system requirements.
Committee Observation - Given the incomplete state of requirements and system integration issues, a direction for BPM electronics design seems to be well along. The scope of the task, potential problems, and planned solution for individual signal channels appears to be well in hand. The committee believes the plan described is feasible to meet known requirements.
Committee Observation - System requirements not yet defined include interface to low level RF
system, scope of required absolute vs. relative beam phase measurements
(exactly which BPM locations will be involved), and cable plant
flexibility. These impact signal
handling at the very front end of the BPM/Phase electronics, cable phase
matching requirements, potential electrical interference, temperature effects,
and hardware layout options affecting phase measurement accuracy.
Suggestion - Strive to reach resolution on these and other outstanding requirements issues as early as possible.
Committee Observation - Requirement for and range of adjustable gain stage(s) required in
BPM/Phase measurement front-ends seems as yet unresolved. Does one requirement apply to all BPM
locations?
Suggestion - Proceed to resolve this uncertainty.
Committee Observation - Phase measurement accuracy and resolution depend on the stability
of the 2.5Mhz reference signal and on higher frequency signals derived from
that.
Suggestion - Establish quantitative specifications for amplitude stability and phase noise characteristics of the 2.5Mhz signal received by the BPM/Phase electronics.
Committee Observation - BPM/Phase measurement design
depends critically on the analog front-end
boards to be supplied by BERGOZ.
Suggestion - Plan a thorough qualification testing program for the first boards to be received and begin testing as soon as possible. Testing should be performed with test signals of the same spectral energy density as anticipated beam signals and over the full signal level dynamic range. Mate to digitizer and PCI board as soon as possible so as to do testing in final system environment.
Committee Observation - BPM/Phase measurement accuracy and resolution depends critically on the performance of the built-in calibration system. There are many potential problems. Analog RF switches on front-end electronics are a critical part of calibration system. The calibration system described depends on a minimum cable length between electronics and pick-up, yet MEBT apparently does not satisfy this minimum length. Cable plant specifications have yet to be determined as civil construction plans become finalized.
Suggestion - Proceed at full speed with quantitative testing of calibration scheme as soon as electronics assemblies become available.
Suggestion - Don't base design of critical element of BPM/Phase measurement system on unknown, perhaps uncontrollable, parameters.
Suggestion - Electronics packaging issues should begin to be addressed with the goal of a design that will facilitate performance, trouble-shooting, maintenance, and replacement. Available rack space and cable entry options should be considered in packaging design.
Committee Observation - It appears that there is already a commitment to a PCI based data acquisition path and that considerable work has been completed to assure the feasibility of this design. The committee agrees that the plan seems acceptable and sounds promising.
Suggestion - At this point, we can only recommend getting a complete prototype acquisition system up and running coupled with a real analog front end as soon as possible. This will allow maximum time to resolve any problems that may be encountered. Good Luck.
Committee Observation - The issue of software version and repository control for all levels of firmware and software was raised.
Suggestion - In this regard the committee can make no specific recommendation; only to urge the management to recognize the need and support efforts to establish a satisfactory method of control.