Progress was also made in preparations of User Lab 1 for the DuPont experiments: the output mirror cassette assembly was completed and installed in the optical transport line. Several additional equipment items from DuPont were uncrated and moved into the User Lab, including the large web coater from DuPont.

Commissioning Activities

Enormous progress was made this week on commissioning the FEL accelerator driver. The gradients of the cryounit cavities were calibrated to an accuracy sufficient to show that the entrance cavity was operating at about 20% lower than indicated on our control screens. We raised its gradient and thereby boosted the beam momentum in the injection line from about 9.1 MeV/c to about 9.5 MeV/c as inferred from the dipole-magnet fields, an act that should enhance machine performance by giving a momentum closer to the design value. The Happek bunch-length monitor installed in the wiggler location is up and running. We found a very simple procedure for using it to establish a minimum bunch length, and that is to adjust the gang phase of the cryomodule cavities to maximize the signal from the Happek device. We also measured the beam quality in the wiggler region: transverse emittance, spot sizes, bunch length, and energy spread. All of these meet requirements for first-light lasing, and within the accuracy of the measurements they seem generally to agree with simulations (PARMELA predictions). We ran 1.1 mA cw average current; it took just ten minutes to establish this current, then the machine ran stably at 1.1 mA for a half hour, after which we terminated the high-current run simply to preserve the cathode. In short, almost all aspects of the machine fit within lasing requirements, and therefore we decided to install the wiggler tomorrow (13 Jun 98) and go for lasing next week.

Two areas that we would have liked to improve are the drifts in the drive-laser phase, which seem to be slow, i.e., hour-long time scale, and peak-to-peak noise in the current. Concerning the former, we may be able to use a detector to look at coherent synchrotron radiation at one of the light ports, the amplitude of which will change as the drive-laser RF phase changes, but we will learn more about its consequences in the process of trying to lase. Concerning the latter, the current jitter is of order 5% to the best we can determine after working on drive-laser stability, whereas we wanted 2% peak-to-peak. Given the measured beam parameters, however, the laser gain is estimated to be about 60%, at which we should be able to tolerate the measured current fluctuations.

The program for next week is to lase, first at 10 W (order of magnitude) using the already-installed high-reflectivity CaF mirrors, then at 100 W (order of magnitude) after replacing one of the mirrors with a higher-transmission mirror for more outcoupled power. During the first run, we will narrow the electron-beam phase space for lasing since lasing would be easier. This is an interim step for the higher-power lasing of the second run. Owl Shifts will be devoted to continued electron-beam diagnostics. Plans for the week after next will be determined based on progress next week.