Date: June 10, 1996
To: Accelerator Division Technical Staff
From: H. F. Dylla
Subject: Report of the FEL Accelerator Technical Review Committee
Attached is a copy of the report of the FEL Accelerator Technical Review Committee pertaining to the Review held on 23-25 April 1996. You will see that the report is favorable overall, but it also points to the need for better communications and coordination internal to Jefferson Lab, and better overall integration of the IRFEL system.
Several steps toward this end have been initiated during the past two months. The current focus is on development of the commissioning plan and operations manual, and this activity is driving the detailed machine design. The Operations Department has agreed to commission and operate the IRFEL in a manner that takes full advantage of in-house staff having expertise with wigglers and optical systems. Participants in the planning process meet every Tuesday afternoon at 3:00 in the Test Lab Conference Room. This is an open meeting; anyone interested in participating or learning about the present state of the project is invited to attend. Furthermore, we are planning to hold an FEL Commissioning Workshop, to include outside invitees, in late July. Minutes of the IRFEL Commissioning Meeting are posted on the FEL Home Page on the Web. An FEL Documentation Tree was put on the Web last week and is now being implemented as a "file cabinet" open to all.
I encourage full, open communication and involvement with the FEL Program. Anyone interested in participating or learning about the Program should feel free to talk to me, C. Bohn, or G. Neil. We will gladly work with interested groups and individuals to coordinate participa-tion in the FEL with other Accelerator Division initiatives, and we want to do so. The FEL is an exciting project at the frontier of both the physics and technology development of high-brightness electron beams, and the FEL community views its capacity for efficient generation of high-power laser light as a new innovation. We welcome and need your enthusiastic involvement.
IR FEL Accelerator Technical Review Agenda
Review Committee -- external membership: M. Cornacchia, A. Garren, S. Y. Lee, P. Morton; internal membership: J. Bisognano (Chairman), J. Delayen, L. Harwood, G. Krafft, C. Leemann
Location: CEBAF Center, Room L104
Tuesday, April 23, 1996
8:00 Executive Session Review Committee
8:15 Welcome F. Dylla
- FEL Program Status
- Programmatic Requirements
8:30 FEL Accelerator Overview C. Bohn
- Technical Requirements
- Strategy for Meeting the Requirements
- Survey of Accelerator Design and Commissioning
9:30 FEL System Overview G. Neil
- Commissioning Options/Turn-On
10:15 Accelerator Transport Lattice D. Douglas
- Design Strategy for High Brightness
- Lattice Options
-- Generic Options (e.g., parallel vs. antiparallel beams)
-- Considerations for Emittance Preservation
-- Considerations for Energy Recovery (e.g., choice of M56)
11:00 - System Performance
--- Tracking results
--- Error sensitivities
1:30 - System Performance - cont'd
--- Tracking results
--- Error sensitivities
3:15 -- Wakefield Effects (B. Yunn)
--- Impedance estimates
--- Beam breakup
4:15 - Allowance/Strategy for Beam Diagnostics
4:45 - Strategy for Commissioning
5:15 Executive Session Review Committee
Wednesday, April 24, 1996
8:30 Beamline Simulations H. Liu
- Toward Low Transverse Emittance
- Toward Low Impedances
10:00 Injector/Tour D. Kehne
- Status of Photocathode Gun
- Status of 10 MeV Cryounit
- Synopsis of Planned Experiments
11:30 Emittance Preservation in Bends R. Li
- Bend-Induced Emittance Growth
- Theory of Transient Coherent Synchrotron Radiation
- Implications for IRFEL Design
1:30 Energy Stability/SRF Control for Energy Recovery L. Merminga
- Stability Analysis
-- Perturbation theory with feedback
-- Numerical simulations
-- Beam start-up and transient effects
3:00 Summary/Wrap-up F. Dylla
3:45 Executive Session Review Committee
Thursday, April 25, 1996
8:30 Review Committee Meeting -- closed session
11:00 Feedback from Review Committee -- open session
June 6, 1996
Report of the FEL Accelerator Technical Review Committee
Review Committee: external membership: M. Cornacchia, A. Garren, S.Y. Lee, P. Morton; internal membership: J. Bisognano (chair), J. Delayen, L. Harwood, G. Krafft, C. Leemann
At the request of the management of the CEBAF FEL team, the above review committee met on April 23-25, 1996 to critique the design of the driver accelerator for the proposed IR FEL driver and to evaluate its readiness for construction.
Summary and Key Findings
The committee was impressed by the high standard of the work presented and believes that the project is overall in good shape. In addition to being a worthwhile facility in itself, the IR FEL provides a reasonable stepping stone to the development of a high average power UV FEL and addresses essentially all the critical issues involved in such a machine. The lattice design is sensible, lean, and straightforward and offers a good probability of success. The construction and commissioning schedule is tight, and the management is urged to keep its eye on the principal goals that must be achieved. Commissioning will not be easy and calls for deliberate and energetic planning now; the commissioning plan has come a long way, but has a long way to go. There has been much progress since last fall's review on understanding coherent synchrotron radiation effects. For this design, care has been taken that it will not be a show stopper, and allowance has been made to measure emittance degradation should it occur. Since design of high-peak-current recirculating drivers and bunch compressors depends absolutely on the magnitude of this effect, these experiments are the next priority after achieving baseline IR FEL operation. The committee realizes that the FEL itself was not subject of this review, but recommends that it receive attention soon and that it be included in future reviews. Finally, it is recommended that more collaboration and exchange of expertise occur between the FEL and accelerator personnel. Commissioning will strain the small staff, and it is time to broaden the skills of all members of the team to support the upcoming intense activities.
Concerns and Recommendations
500 kV Gun
The most critical element is the electron photogun, which must meet ambitious performance specifications for the project to succeed. The gun is being built with a high priority and will be tested soon. The computer modeling of the gun, buncher, and 10 MV accelerator section has been done, benchmarked with available experimental data, and shows that performance goals are sound. This modeling should, of course, continue, but what is really needed is to finish building the gun and to operate it. Only then will we find out whether it will hold 500 kV, how robust the cathode is, and what other unforeseen problems arise. The importance of getting the injector system up and running cannot be overestimated.
RF Control Sensitivities and FEL interaction
The analysis of the RF control system has progressed well and now includes a first estimate of the effect of the FEL on the RF. The quoted numbers for the microphonics and the sensitivities to deviations from nominal operation are a little worrisome. Work should proceed on improved optimization to determine if the tolerance to microphonics and deviations (path length, phases, etc.) can be improved.
Although some work has been done to estimate the effects of FEL startup on the RF, a more complete model including the effects of the RF on the FEL is necessary to close the loop on the RF/FEL interaction. Only then will there be a clear picture of the overall stability of the integrated system. This work should be pursued in a timely fashion. Also of concern is the sensitivity of the energy recovery to the electron path length. It was stated that a shift of 3 degrees (and a corresponding pathlength change of 1 mm) between incoming and recirculated beams was sufficient to compromise control. This tolerance appears rather tight and raises the issue of a possible energy modulation and its impact on the FEL performance. Pathlength feedback may be needed, and magnetic adjustment should be considered.
There was little discussion of the performance of the FEL and how it depends on beam characteristics and on possible perturbations of the accelerating and transport system. The emphasis of this review was on the accelerator driver, and there were no detailed presentations of the FEL performance. It is recommended that, in a future review more specifically directed towards the FEL design, the interaction of the physics and operational issues between the driver and the FEL be brought into focus. Such a review might benefit from the presence of experts in both fields. On a similar vein, the committee encourages a close interaction between the accelerator and FEL design teams. This interaction could be strengthened by lectures, joint discussions, etc.
Specifications and requirements for the diagnostics need to be finalized and must be well thought out and closely tied to the commissioning plan. Diagnostics are an area where apparent savings can easily lead to schedule delays and commissioning difficulty. For example, the use of the higher performance switched electrode electronics with the BPMs in critical areas will be well worth the additional cost. Clearly, BPM structures should be evaluated for coupling impedance.
Since understanding beam loss and halo formation will be critical in scaling machines of this kind to production facilities, plans must be made to measure beam loss in the operation of this machine. There has been substantial progress in understanding halo formation elsewhere, and this work should be evaluated for applicability to this project.
Much work has been done on a commissioning plan since the last review. One still gets the impression, though, that commissioning has received less priority than other activities that have taken place and is still perceived as an afterthought, following the design activity instead of being an integral part of the design. The planning and organization of a structured and integrated approach to commissioning must be seen as one of the top priorities in the near future. As with CEBAF, the IR FEL has many unique characteristics and full use should be made of the commissioning experience and expertise which exists at CEBAF for the commissioning the FEL.
Localization and control of loss will be important, and beam scrapers should be integrated now in the machine design. Questions such as "what will they look like, where should they go, and is there room for them?" should be answered immediately.
In the previous review the team appeared focussed and working together well. In spite of the certainty of the funding and the short time scale of this project, in this review the committee felt that communication did not seem to be as direct and easy between different members of the team. Even in the design phase, full system integration must be addressed. Pathlength, M56, and deceleration phase require coordination between lattice design, RF control, and collective effects. Expensive smoothing of vacuum chambers should be done only with close scrutiny for performance cost effectiveness. Everyone should be confident that upgrade plans are realizable. All this requires smooth and comfortable communication.
The realities of building and commissioning this accelerator in the context of a small laboratory with a larger accelerator just coming online require sharing of personnel and time. However, there must be a clear commitment from the staff to this project. Weekly meetings are needed so the project members become familiar with each others problems. Care should be taken that in this matrixed situation project leaders have direct interaction with all team members.
The wiggler specifications have been set and a document describing them was distributed. The effect of field imperfections, as defined by tolerances, on the electron beam dynamics has been computed and found to be acceptable. The effect of the intrinsic non-linearities, and in particular the second order octupole-like term, has not yet been estimated, and it is recommended that this should be done.
The transport system design seems to be in very good shape. The wiggler follows the cryomodule, and precedes the recirculation arcs. This sequence favors FEL performance compared to energy recovery, and lessens concerns that the CSR effect could prevent operation of the facility.
This part of the system consists of two telescopes, one before and one after the wiggler. These telescopes match the beam from the linac to the wiggler, and from the wiggler to the arc. Each telescope contains a chicane placed between two quadrupole triplets. The triplets are quite strong and have high beta-function values and chromaticities. Possibly the optics can be made more gentle by lengthening the telescopes.
The arc is a Bates-type design, with two improvements. First, the small positive and negative dipoles at the ends of the arc, instead of being rectangular and wedge, respectively, here are identical (except for the change of sign), and have intermediate edge focusing. Second, two quadrupole families have been added to control M56. In both cases there are also two sextupole families for chromaticity control.
One might also consider another modification: to replace the 180 degree bend with two 90 degree bends, and to locate a sextupole-quadrupole-sextupole triplet between them. The flexibility gained could be used to increase the differences between betax and betay in the sextupoles and consequently to reduce their strengths.
The backleg straight section transports the beam from one arc to the next. It can be adjusted to duplicate the beta functions of the first arc in the second arc and to provide phase intervals of 3¼ between the two arcs in order to give cancellations between their sextupoles. Moreover, the chromaticities in the backleg should be minimized. Currently, the design calls for 13 independent quadrupoles in the backleg; this number is certainly sufficient and could possibly be reduced. In any case, tuning flexibility should be retained with separately powering the quadrupoles.
The team should take care to think through the wavelength tunability and upgrades that they wish to address. This activity should not detract from achieving the baseline goals, but if it found essential to discuss, for example, 1 µ or 6 µ or UV FELs, the background work needs to be done.
The placement of the wiggler after the cryomodule and before the recirculation arcs favors FEL performance at the possible expense of energy recovery. This is surely the right priority--it removes worries about CSR from the primary function of the machine.
The theory of CSR has developed considerably since the past review, and the team is in good shape to interpret observations. The next effects worth considering are those of finite beam size, which may lessen the effect, and bend spacing, which may point the way to lattice designs that minimize possible emittance degradation.
It is unclear what drives the vacuum specifications, which appear to be early in formulation. In light of the higher gradients demanded, proceeding without differential pumping and precipitators should not be done without careful scrutiny. There should be a critical review to get this all worked out.
Designed to explore critical developmental issues, the IR FEL will likely push some components to or beyond the state of the art, and performance upgrades may be needed. Being prepared can go a long way in recovering from a difficult situation, and it might be advisable to think about fall-back positions and invest effort in "what-if" analysis.