Management

Highlights (in this case, lowlights) of the week include finding that the cathode wafer cracked during processing and is unable to hold voltage, so another round of wafer changeout has begun. All other systems, including the newly installed kilowatt class optics for operation at 3 microns are ready for operation.

Members of the FEL team who participated in the design and engineering of the IR Demo have been working for the past several weeks preparing detailed cost estimates and design trades for the FEL Upgrade. The first announcement for the next Laser Processing Consortium workshop was distributed and posted on the FEL WEB site. The workshop will be held on June 17-18th at the CEBAF Center Auditorium.

FEL Installation/Maintenance Activities

The Machine Protection System was audited and found to be working properly. Several unlogged buggers were found and were logged. The gun's high-voltage power supply was locked out during the audit, but it will be audited next week. Developments with the gun are described under FEL Commissioning Activities below.

A representative from Coherent, our vendor for the drive laser, thoroughly checked out the drive laser and was able to increase the maximum optical power it can deliver to the photocathode. This will permit generating higher bunch charge and thereby extend the time between cathode cesiations.

The photocathode scanner was realigned and is now able to be operated remotely. The patch-panel system was reworked and reorganized. The BNC's in the Optical Control Room were repaired.

FEL Commissioning Activities

The week ended with a disappointment. After readying the machine for high-power lasing at 3 microns, we discovered Wednesday afternoon that the cathode wafer cracked in two places, and yesterday we found that the cracked, cesiated cathode could not hold 330 kV voltage without field emitting (as suspected). It may have been operable for some unknown amount of time at 320 kV, but we felt the best use of peoples' time was to replace the defective wafer rather than try to set up the machine with a "suboptimal" lower-voltage gun with uncertain prognosis of success. So today a new cathode is being deuterium-cleaned, and we anticipate turning on the machine with beam around 28 Apr. 99. The FEL will have to be down from 29 May 99 through 7 Jul. 99 because the Central Helium Liquifier is scheduled for maintenance during that time. Consequently we are striving to have a solid, productive 1-month run before then. Our main goals are to achieve a full kilowatt, get a set of good, comprehensive CSR data, and do more tests in the user labs. Though we have not generally had problems with cracking cathodes, we did see this a couple of times in the old Injector Test Stand days. Mounting the wafer is touchy, and if it slips, it can catch during insertion after, e.g., a heat clean, and the stress can crack it. We hypothesize that this is what happened. Following recent practice of the CEBAF Polarized Source Group, we will use an indium solder with the next wafer to keep it in place and buffer any stress on the wafer. One important piece of data gleaned from the last round of "gun refurbishment" is that quantum efficiency degrades during high-voltage processing. A check of preprocessing QE indicated the installed wafer was quite good (at least 2.4% QE with red/white ratio of 2:1).

During processing it degraded to the low levels seen on the last few cathodes (well less than 1% QE with red/white ratio of about 23:1). We conjecture that ion back-bombardment is causing the degradation. Regardless, we now know with confidence that the cathode needs to be protected during processing, and our plan for the next turn-around is to do the processing with the cathode retracted into the stalk. We also are looking into designing and installing a shield during the June down.