Management

Highlights of the week include: (1) recirculating 3.3 mA of 47 MeV beam with energy recovery, and (2) lasing pulsed and cw at 3.3 µm and at up to 2.2 mA current, though at low average power, but nevertheless establishing the broadband capability of the FEL. The cathode wafer delivered a total charge of 685 C.

Last Thursday, 18 Mar 99, C. Bohn presented a seminar on the FEL to the Fermilab Beams Division. It was well received. This week F. Dylla presented an invited talk at the American Physical Society Meeting in Atlanta entitled "Applications of Free Electron Lasers for Materials Science and Materials Processing."

LCDR John Holmes of the CNO Strategic Studies Group visited Jefferson Lab Thursday to meet with A. Todd of AES, G. Neil, and C. Bohn and discuss potential uses of FELs for Navy applications. LCDR Holmes is participating in a study that is intended to provide guidance to the CNO on good technology R&D investments.

FEL Installation/Maintenance Activities

The valve that regulates the cooling water for the buncher cavity failed and was replaced. On Wednesday, the outcoupling mirror was replaced with a sapphire mirror of mediocre quality to support an attempt at lasing near 3 µm.

FEL Commissioning Activities

The week was spent running the machine for three purposes: setting up 47 MeV beam for lasing near 3 µm, doing lasing tests in User Lab 1 as part of laser-materials interaction and thermoreflectivity studies, and working on CSR-related diagnostics. During the first half of the week, several tests were done in support of the Naval Postgraduate School that involved lasing at ~100 W and ~10^4-10^5 W/cm^2. The laser ran stably for many hours at a time during these tests. One general finding is that it is easy to bore sub-mm-sized holes in samples having thicknesses of interest in a manufacturing environment.

Having succeeded in running a little over 3 mA cw at 47 MeV, on Wednesday evening we decided to replace the outcoupling mirror with a mediocre sapphire mirror that we had on hand so we could attempt to lase near 3 µm (we are awaiting delivery of a high quality mirror from the vendor). We did not expect to achieve good power output with this mirror, and thought that establishing lasing could be very difficult. It turned out, however, to be extremely easy to establish both pulsed and cw lasing at ~3.3 µm. But, both mirrors comprising the optical cavity did indeed prove to be relatively poor reflectors at this wavelength, and the highest cw power we could achieve in the Optical Control Room upstairs was 12 W.

Regarding CSR-related diagnostics, P. Piot worked on the technique for doing tomography in the wiggler region. The general idea is to measure bunch properties both at the wiggler (to provide the initial conditions for numerical simulation) and in the back leg (to compare with output from the simulation) in as much detail as possible. Tomography will provide details about the transverse phase space.

Having exhausted all we can sensibly do with the cathode wafer, we turned off the machine late Friday morning. During the next two weeks we will replace the cathode wafer with a new one and try to bring the gun back up with improved cathode quantum efficiency. The present cathode delivered 685 C total charge; may it rest in peace. Having developed the save/restore files for setting up the machine with a 330 kV gun, we plan to continue running the gun at that voltage when we come back up. During the two-week down, we also plan to receive and install two high-quality sapphire mirrors. Upon turning on, we will boost the beam energy to 48 MeV for lasing at 3.0 µm (for high mirror reflectivity). If we can run at 60 pC and 74.8 MHz, then all else being equal, we would naively expect an average power output of (710 W)(48/38)(60/48) = 1.1 kW. We shall see.