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

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IR Demo Project Weekly Report, January 10-15, 1999

Date: January 15, 1999

Management

This week is a continuation of the January shutdown of the FEL for planned maintenance and installation activities. A summary of these activities is given below.

Final preparations were made for JLab presentations to the BESAC Panel Meeting on Novel and Coherent Light Sources next week in Gaithersburg, MD. In addition to a summary of the JLab FEL, supporting presentations will be given by Len Feldman (Vanderbilt) Materials Science, Bill Cooke (CWM) in Atomic Physics, and Kevin Lehmann (Princeton) in Chemical Physics.

C. Bohn gave a presentation on JLab's FEL to the Directed Energy Directorate of the Air Force Research Laboratory last Friday, 8 Jan 99.

Secretary of Energy Bill Richardson visited JLab this week and voiced his strong support for the FEL program.

FEL Installation/Maintenance Activities

The following maintenance items were completed this week:

The new cesiator for the photocathode gun was installed and activated. The photocathode was also installed in the gun, and the vacuum bake is underway. In addition, a new residual gas analyzer for monitoring the constituents of the gun vacuum was received, installed, and is now operating. The metering circuits in the gun's high-voltage power supply (HVPS) were modified to read 5 mA as the maximum current, rather than the previous 4 mA. A test of the HVPS is planned for next week to determine its ability to produce 5 mA current into a low-impedance load.

After modifying the 3-way cross at the insertable dump in the reinjection region into a 4-way cross, we installed a new viewer there. It will aid in configuring and steering the recirculated electron beam back into the cryomodule. In addition, the viewer foil at the exit of the cryomodule was replaced with one having a 5.75 mm-dia hole in the center. This will allow inserting the viewer without disturbing the injected beam, thereby providing a pristine view of the recirculated beam after deceleration through the cryomodule.

New beam-current-monitor cavities were installed at the exit of the first recirculation arc and in the reinjection region. They will be used primarily as aids in setting the path length of the electron beam around the machine.

A mechanical support was installed for the beam-position monitor in the energy-recovery (ER) dump line. It had been omitted from the original design of the beam line. Klixon temperature switches were installed and wired on the lower part of the ER dump. They are designed to open at 88 C and are for additional protection from beam-induced heating.

FEL Commissioning Activities

The activities scheduled for the next two weeks are designed to relieve stress on the people doing installation/maintenance while providing for restoration of the "Dec 98" machine by 29 Jan 99. The vault will be open all next week to permit installation/maintenance. Gun processing will proceed early the following week, and injector/accelerator/FEL restoration will proceed after that. The schedule is only a projection; it will be modified, if need be, as turn-on gets closer.

A series of benchtop measurements were made to provide data by which to formulate plans for circumventing the waveguide IR detector faults that surfaced in December. The faults are very likely caused by higher-order modes. Bottom line: A copper screen of ~1 mm mesh size removes all measurable rf response at 19.5 GHz and transmits ~60% of the IR. Screens are being installed over all eight of the detectors in the cryomodule, and three gate valves are also being installed to permit testing alternative solutions and measurement methods with rapid turn-around. The trip points will need to be reset during the time-frame of system checks (likely early the week after next).

R. Li has first results from her coherent-synchrotron-radiation simulation. She finds that the emittance growth through the first recirculation arc is sensitive to the energy spread at the wiggler location (which corresponds to the starting point of her runs). For an assumed gaussian longitudinal bunch profile with rms parameters previously measured at the wiggler, she predicts a 34% CSR-induced emittance growth from wiggler to exit of the first arc. She has benchmarked her code against analytic calculations pertaining to the longitudinal CSR force, and is presently doing likewise for the radial CSR force (the latter being more difficult analytically than the former). She will also do runs to check for sensitivity of emittance growth to longitudinal bunch profile to provide guidance on the steps needed for a careful empirical study.

Fundamental limitations on pushing the FEL power to higher levels have been further investigated theoretically. The essential concern is ultimately reaching an rf instability in the energy-recovery mode that cannot appear in the straight-ahead mode. We have found empirically that the power output and the gain are extremely and nonlinearly sensitive to the optical-cavity length in the vicinity of saturation (which occurs at ~311 W in the straight-ahead mode), a property that will adversely drive the rf system. This means it is possible to excite an rf instability whereby the FEL rapidly switches on and off, i.e., with a frequency ~10 kHz. By contrast, as one backs the optical-cavity length away from saturation (to, e.g., ~210 W in the straight-ahead mode), both the gain and power output become much less sensitive to cavity length and the rf instability will not appear. The bottom line is that with ~5 mA beam we should be able to get close to 1 kW (nominally 900 W) with the 5 µm optics presently installed, and have some hope of pushing to the full kW, without getting too close to saturation and exciting the rf instability.

The prediction of 1 kW with 5 mA assumes that there are no mirror limitations. The laser is less sensitive to mirror distortion if we operate in the low-slope region where the small signal gain is high so we may get lucky, especially if we have one silicon mirror. If we are close to 1 kW, and the mirrors are not limiting the performance, we can try to increase the power by (1) getting the bunch tails incorporated into the main bunch using injector settings, and (2) increasing the acceptance of the recirculator beyond its specification and lengthening the cavity until 1 kW is reached. Finally, it is worth noting that in December, with limited time to optimize, we achieved the power one predicts theoretically for a 1.6 mA beam. The most productive path to 1 kW still seems to be to increase the current and continue optimizing, which is what we plan to do upon resumption of commissioning.

Regarding the FEL Accelerator Readiness Review (ARR) process, we cannot formally close out the ARR until we are ready to do routine user experiments, for which we are unfunded. Presently we are exclusively doing commissioning tasks, ultimately to include test experiments, for which we are following established EH&S policies and procedures. Today we sent our DOE site office a memo to that effect, namely, stating that lack of operating funds prevents us from reaching the point of being able to support routine user experiments.