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

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IR Demo Project Weekly Report, October 5-9, 1998

Date: October 9, 1998

Management

This week was spent diagnosing and high voltage conditioning the photogun. Highlights include: (1) successful high voltage conditioning of gun to 480kV, however the cathode photo response was minimal requiring replacement of cathode. (2) numerous interactions with industry groups.

Three industrial groups visited the laboratory this week to discuss on-going and new interactions with the FEL project and ARC: (1) Siemens Automotive (Newport News); (2) Neocera (College Park, MD) and Advanced Energy Systems (Princeton, NJ). The group from Siemens is discussing joint ARC/FEL studies on laser surface modifications and laser micromachining of metals. Neocera is a company involved in the development of laser pulse deposition systems. Advanced Energy Systems is a new spin-off from Northrop Grumman that will continue the former Northrop Grumman collaboration on FEL and SRF technology development.

F. Dylla met with representatives of the LLNL laser processing group and the program manager of DARPA's Precision Laser Machining Consortium at this week's OSA conference in Baltimore. The LLNL group has recently delivered an ultra short pulse (170 fs), Ti-sapphire laser machining system to the ORNL Y-12 plant. Because of the short pulse, reasonable micromachining rates can be achieved with the system's modest average power (18 watts). The developer of this system cannot envision this laser technology evolving above 100 watts.

DARPA's 5 years of support of the Precision Laser Machining consortium comes to an end by next September. To date the prime contractor TRW has delivered a 500 watt Nd-YAG laser processing system for industrial tests. The consortium will continue its activities next year without further DARPA support. According to the DARPA program manager (L. Durvasula), DARPA will not be funding any new activities in laser processing. Laser work will concentrate on detection of chemical and biological agents.

We heard from the UVA chemistry department that their preproposal for establishing a chemical physics lab at the FEL was approved by the NSF.

FEL Installation/Maintenance Activities

The totality of the Machine Protection System (MPS) was satisfactorily tested this week. A formal audit of the MPS is scheduled to be conducted in parallel with the next high-voltage processing of the gun.

The alignments of the wiggler and its adjacent quadrupole magnets were checked. Some adjustments appear to be needed, and they will be made next week.

The nitrogen line to the gun and its high-voltage-power-supply tank was modified.

Preventive maintenance was done on the Instrument Air compressor.

Administrative locks and tags were placed on the two large magnet power supplies while we are down. Covers for their connections to the large dipole magnets are being designed.

Progress was made in releasing the instrumentation-and-control documentation. Complete CAD packages for two printed circuit boards were released, and the boards were ordered. The wiring diagrams and assembly drawings for five different chassis were also finished.

FEL Commissioning Activities

Concerning the photocathode gun, last week's helium processing turned out to have no discernable influence on either the cathode's photoresponse or the field emission. Consequently this week was spent doing high-voltage processing, ultimately up to 480 kV, prior to opening the gun. During the processing, a new site of field emission was generated, and as it processed (up to 480 kV), a strong nitrogen component prevailed in the emission. Based on this observation, a conjecture is that the site may have been associated with the coating on the stalk.

In view of the ultimately successful outcome of high-voltage processing, we are in the process of replacing the cathode wafer today, after which we will do a vacuum bake, heat clean and cesiate the new cathode, and then bring the gun on-line. Once the bad cathode is removed, it will be subjected to Auger analysis in the hope of gaining information about the cause of the gun problems. At this writing (1115, 9 Oct 98) the estimated date for gun turn-on is 20 Oct 98. If successful (meaning delivery of 60 pC pulses at 350 kV), commissioning activities will resume per the plans already in place.

Regarding lasing, it is not known how much cw power the installed CaF mirrors can sustain. The power may peak at ~400 W due to thermal distortion, but the actual limit awaits empirical determination by way of successful commissioning of the full machine. Presuming the limit is well below a kilowatt, and recognizing the need to lase at wavelengths other than 5 µm, it is of interest to start considering in detail other lasing configurations, and we have begun to do so.

In this light, we have developed a set of requirements for lasing at 3 µm with sapphire mirrors. The strategy is to follow the philosophy of the "High Power Setup Procedure", i.e., first lase straight ahead, then lase with recirculation using 60 pC bunch charge and the electron-beam quality that has already been achieved. The fundamental implication is that a total beam energy of 47.5±0.5 MeV will be needed at the wiggler. A 4 mA average current (e.g., 60 pC at 75 MHz) should be sufficient to provide 1 kW light near 3 µm. These numbers are to be compared to the original design parameters of 42 MeV with 135 pC bunch charge at 37.4 MHz (5 mA

average current).

A concern about the higher beam energy is whether the acceptance of the injection/extraction chicanes is sufficient to accommodate the energy in view of unknowns about the transverse beam halo. We plan try it, and if problems show up, then perhaps increasing the energy of the injected beam will help.

Analysis of the sapphire mirrors at China Lake has shown that the vendor has yet to succeed in applying high-reflectivity coatings of sufficiently good quality. Their coatings delaminated, and they contain scratches and pits. Consequently they cannot sustain the high-average-power light associated with kilowatt lasing. Last week M. Shinn met with the vendor to discuss the problem and determine remediary measures. Our intent will be to work with the vendor to expedite delivery of acceptable optics.

Another potential avenue for kilowatt lasing is to do it at 6 µm with ZnSe optics. We are due imminently to receive a set of these optics, after which we will mount them and subject them to acceptance tests. The option for using them, of course, depends on the outcome of their tests.

In view of all of these considerations, we plan to stay the course of commissioning at 5 µm with the installed CaF optics and extract as much power from them as possible. Afterward, at an opportune time, we will conduct a machine-development program to explore the feasibility of providing 47.5 MeV beam at high cw currents. In parallel, work on the sapphire and ZnSe optics will continue, and we will have some basis for deciding the timing of their installation.

Duke University continues to plan for irradiation of stents in the straight-ahead machine to be done in conjunction with the next (presently unscheduled) preparation of GEn targets for the Lab's nuclear physics program. The motivation for the irradiated stents is to use them in conjunction with angioplasty. By inserting stents during angioplasty, there is the prospect of substantially increasing the time interval between successive treatments. Duke is fabricating the hardware necessary for installing stents in our straight-ahead dump line.