To: J. Cook, D. Helms, W. Skinner

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IR Demo Project Weekly Report, October 12-16, 1998

Date: October 16, 1998

Management

This week was spent baking the photogun after replacement of the cathode last Friday. High voltage conditioning and reforming of the cathode are planned for early next week followed by start-up of operations if the cathode is responsive.

An internal review of the Laser Saftey System was held this week.

Next week, a DOE/SURA review of the the laboratory's "institutional management" will be held on Oct. 19-20. Presentations to the committee will include the FEL program and the associated activities in the Applied Research Center.

Plans were made to hold two planning sessions next week to brainstorm on the subject of the 20kW IR FEL upgrade. System requirements, gun designs, FEL design issues and beam transport lattice issues will be examined.

A draft announcement was prepared for a workshop on UV and x-ray FEL applications that will be held on Nov. 20th at SURA headquarters.

IRFEL Installation and Maintenance

The positions of the wiggler and its associated quadrupole magnets were adjusted based on last week's alignment check.

A weld on the tank surrounding the gun was redone, and the tank was successfully pressure tested. Also, an additional valve was added to the SF6 control panel for this tank.

The Personnel Safety System (PSS) was recertified following some hardware improvements. One of the drive-laser shutters that is an important indicator for the PSS was found to stick in the open position. It was therefore modified and is now being tested.

The Thomson backscattering X-ray detection chamber has successfully passed vacuum tests and is now being assembled for calibration tests prior to installation in the beam line. After calibration, the chamber will undergo a final cleaning, assembly, and installation into the beam line.

IRFEL Commissioning

Regarding the photocathode gun, the cathode wafer was replaced early this week and verified to yield a photoresponse. The vacuum bake was readied and executed; tear-down of the bake is slated for this afternoon. The whole process has thus far progressed well. Plans are to do careful high-voltage processing of the gun starting Monday morning and ending Wednesday morning, followed by heat cleaning the cathode wafer, and then cesiating it after it has cooled down. We should be able to pull electron beam off of the gun starting Thursday morning, 22 Oct 98. As a related aside: the previous cathode wafer is undergoing a barrage of surface studies at William & Mary which thus far has yielded no surprises. The cathode has a crack that may have been the source of field emission during the last attempt to refurbish the gun.

The internal Laser Safety System Review was held 16 Oct 98, and it was well received by the review committee. The committee is submitting its findings and recommendations this afternoon, and they will be used to strengthen the LSS as well as the documentation being prepared for the external reviewer, Anteon Corporation, that is slated to be sent by the end of next week.

Efforts to organize a set of beam-breakup (BBU) experiments on the FEL continued. The fundamental motivation is to validate the code TDBBU that calculates BBU in a recirculating accelerator. Once benchmarked, the code can be used with more confidence to establish BBU threshold currents in FEL upgrades or alternative designs. It also could be used to establish with more confidence whether HOM couplers are required on the new 7-cell cavities envisioned for the 12 GeV upgrade of CEBAF. Considerable money and effort would be saved were the answer to be "No".

TDBBU predicts the threshold current in the IR Demo to be 27 mA, well below its attainable current. Thus, the strategy is to force the BBU instability in the FEL by, for example, driving the dominant deflecting mode with an external rf source and possibly lowering the electron-beam energy. Stripline kicker and pickup cavities are envisioned. Those used in a previous energy-recovery experiment done parasitically on the CEBAF accelerator have been found and examined. Their apertures don't match those of the FEL vacuum chambers, so new designs are presently under consideration. The kickers will go in the injection line (one horizontal and one vertical). There is an issue concerning the lowest attainable electron-beam energy, particularly if it involves lowering the energy of the injected beam since the "injector aperture" is presently undetermined.

A final plan for the BBU experiments is anticipated to be available by 15 Nov 98. If the plan were implemented, the experiments would likely be conducted next summer.

Plans to support the "stent irradiation" for Duke continued to be developed. As it stands now, Duke would build the holders to mount the stents on the straight-ahead dump line, and Jefferson Laboratory would build the target and shielding. The purpose of the target is to generate gamma-rays from the (nominally 2 kW) electron beam; the gamma-rays serve as the radiation for the stents.