Highlights of the week include attainment of almost 4 mA cw recirculated current while lasing. The cw laser power was apparently saturating at ~500 W, most likely (but not certainly) due to a thermal limit of the optical cavity. We therefore replaced the CaF reflecting mirror in the optical cavity with a more thermally resistant silicon mirror.

The FEL project, along with the entire Laboratory, was reviewed this week (1-4 Mar 99) by a DOE sponsored national review team on the subject of safety management. The Laboratory staff and management were given kudos by the review team for instilling a safety-conscious culture within all laboratory activities.

This week we refurbished two viewer cameras in the wiggler region, we added heater tape in an

effort to improve thermal control of the buncher cavity, we added screens across the waveguides of cryomodule cavities 1 and 2 to see if that eliminates the IR detector interlock trips at high cw

currents, we converted one of the corrector magnets into a skew quadrupole for reasons explained under "FEL Commissioning Activities" below, and today (Friday, 5 Mar 99) we are completing a cycle in an effort to refurbish the quantum efficiency of the cathode.

FEL Commissioning Activities

Experience to date is such that, given a good cathode, it is now "easy" to restore cw lasing at

400-500 W (~280-350 W upstairs in the Optical Control Room) by loading a "golden file" of

machine settings, and then lase at that level for several hours. For example, last Friday we ran 400 W, 2.5 mA cw for six hours while monitoring various diagnostics, such as vacuum, IR detectors, etc. We recesiated the cathode Monday, and on Tuesday the machine was lasing cw (at roughly 500 W) with a recirculated average current of nearly 4 mA. However, we observed the laser power to saturate at ~500 W; raising the current did not appreciably affect the laser power. Therefore, with the educated guess that mirror heating was limiting the power output, on Tuesday evening we replaced the CaF reflecting mirror with a more thermally resistant silicon mirror.

Early Wednesday morning, in the process of turning on the machine, the gun voltage was

accidentally raised too high by ~ 10 kV, and an arc occurred that eradicated the cathode quantum

efficiency. Since then, the gun was heat cleaned (8 hours at 675 C), high-voltage processed to 516 kV (field- emission-quiet at 512 kV), and heat cleaned again (1/2 hour at 675 C). At this writing (1100 Friday, 5 Mar 99) we are making a cathode and doing a cathode scan. There is a

photoresponse, but it is too early to tell how the cathode will come up. Hopefully we can bring the machine back on early this afternoon -- we'll see. If so, we will do a high-power-lasing run today, which will possibly continue into Swing Shift.

As a consequence of events that led to the gun going down, we prepared a software interlock that

will limit the highest gun voltage accessible to an operator. The interlock is being installed this

morning.

Of course, in establishing a "good" 5 µm high-power setup, minimization of beam scraping is a

common goal. A radiation survey immediately following the Tuesday run showed two relatively hot spots (not all that hot -- ~0.5 mrad/hr versus much less everywhere else around the machine). One was near the center of the cryomodule, and the other was at the flange just after the wiggler.

Consequently, Thursday we moved two of the diagnostic beam-loss monitors, putting one at each of these two locations, and we will monitor them during future high-current runs. We also know that the present setup dumps ~100 W beam power into the cryomodule at cw currents in the range 3-4 mA, and of course we want to reduce this number.

As we continue to push machine performance in the common activity, the present status of the

machine is capable of supporting some very interesting tests in User Lab 1, and (given a working

cathode) we plan to be running at least two tests during the coming week.

We are planning to run the "traditional" commissioning pattern through next week (i.e., high- power runs during Day Shift, lasing studies during Swing Shift, and diagnostics during Owl Shift). By the end of next week we should have reached the currently perceived limit. Consequently, after next week we will go into two-shift operation (machine on hot standby during Owls), and will continue that way until it again becomes prudent to do otherwise (e.g., after the sapphire mirrors are installed and we go into 3 µm lasing). So, the week after next will be CSR experiments during Day Shift, lasing studies during Swing Shift, and hot standby during Owl Shift.

Known issues that could potentially limit the high-power capability of the present setup include the list below; there surely may be other issues/limits, and we will learn about them as we proceed: