IR Demo Project Weekly Report for March 8-12, 1999

Highlights of the week include lasing cw at up to 710 W with the new silicon downstream optical-cavity mirror, lasing onto assorted targets in User Lab 1, and recirculating 47 MeV beam (desired for 3 µm lasing) at cw currents up to 0.6 mA. Thus far the cathode wafer has delivered over 520 C total.

On Wednesday, March 10, the Commonwealth's Secretary of Technology, Don Upson made a

presentation to the entire Virginia Congressional Delegation concerning the top R&D priorities for the state with regard to FY 2000 federal funding. The proposed $15M Upgrade to the IR Demo FEL was prominent during the presentation.

A close-out meeting with the Navy for review of the present Navy/DOE MOA which ends on March 31,1999 will be scheduled sometime during the last week of April or the first week of May.

This week's "installation" activities were predominantly associated with setting up tests in User Lab 1. The machine was up and running nearly the entire week, save for two recesiations of the cathode in the electron gun.

On Tuesday we lased up to 487 W cw upstairs into the Optical Control Room. Based on the best

measurements to date, we had inferred that the losses in the optical transport line were about 30%. Consequently we inferred the corresponding FEL output was 695 W. On Thursday we put the power head downstairs close to the outcoupling mirror, and lased cw up to 710 W as measured there. The cathode performance (~3.6 mA cw) was close to the same in both cases. Consequently our estimate of optical transport losses appears good. Moreover, the machine ran stably at powers in excess of ~680 W, meaning it just sat and delivered steady power until interrupted for another attempt at raising the power level.

We cannot generate more charge with the cathode wafer now in use. Its peak output right after

recesiation is about 3.9 mA (52 pC bunch charge). Consequently we will not try pushing the power higher until we install a new wafer (installation should take place during 29 Mar - 9 Apr 99). We took a wealth of data from which to infer the scaling of power versus average current with the present machine setup, and it extrapolates to a ceiling of roughly 800 W at 5 mA at 5 microns with the presently installed optics. However, it is conceivable that modest improvement of electron-beam transport would raise the ceiling. For now, all we can say with confidence is that the achievable power output is cathode-limited.

We can say one other thing with confidence. The machine lased at 710 W with a sub-standard

cathode and with mirrors that were specified for first light, i.e., ~400 W maximum. Therefore, we are pleased to report that good fortune has been with us and Mr. Murphy has taken a vacation.

During Swing Shifts we began commissioning tests in User Lab 1, delivering 5 µm laser light to

various targets at ~100 W with <1 mA recirculated current (while, incidentally, running for typically many hours uninterrupted in the process and merely opening the User Lab shutter when light wasdesired at the target). Targets have thus far included aluminum and a carbon composite with which data was taken in support of one of Bill Colson's students at the Naval Postgraduate School, as well as a gold foil as part of a study of picosecond transients in its thermoreflectivity using the pump-probe technique.

During Owl Shifts P. Piot continued refining diagnostic techniques for emittance measurements at the wiggler and in the back leg. It is now possible to take emittance data efficiently, and the process of doing systematic studies of beam quality versus machine settings has begun. It includes exploratory measurements to decipher the presence and effects of coherent synchrotron radiation. As time passes, there should be much to say on this subject.

This morning (Friday, 12 Mar 99), we made a first attempt to set up recirculating 47 MeV beam as part of preparing for eventual lasing at 3 µm. Before noon, the machine was delivering ~600 µA cw in this mode, at which time we reset the machine to support lasing studies in User Lab 1. Improving the 47 MeV setup will entail cleaning up the beam transport, just as was done for the 39 MeV setup. The sapphire "3 µm" mirrors are due in soon, perhaps within two weeks. After the scheduled replacement of the cathode wafer, we may be in position to do 3 µm lasing. The sapphire mirrors are explicitly specified for >1 kW lasing, and the higher electron-beam energy (47 MeV vs. 39 MeV) will generate proportionately higher-power light.

Especially to support beam-quality studies at high bunch charge (as high as we can generate), we will recesiate the cathode once again on Monday. As mentioned, we are not planning to do more

"high-power" runs until we have a completely new cathode wafer. Instead, we will work more on

beam diagnostics, tests in User Lab 1, and improving the 47 MeV setup. We also will be working on presentations and papers for the fast-approaching Particle Accelerator Conference, to be held during the last week of March (hence the dates for the wafer change).