To: J. Cook, D. Helms, W. Skinner

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IR Demo Project Weekly Report, November 2-6, 1998

Date: November 6, 1998

Management

Highlights for the week include: achievement of quiet electrodes in the photocathode gun at 350 kV, achievement of sufficient cathode quantum efficiency to run electron beam, and checkout of the machine with electron beam (proceeding today).

The group had several presentations at external meetings this week. At the Denton Conference on Applications of Small Accelerators, Steve Benson presented a talk on the IR Demo FEL and Jim Boyce gave a talk on the use of the IR Demo Linac for Electron Beam exposure of materials. Jim also presented a second talk on the production of x-rays by Compton scattering. Fred Dylla presented a talk on the Design and Conditioning of the demo vacuum system at the AVS symposium in Baltimore and a FEL program update for the SURA Board of Trustees fall meeting at George Washington University.

FEL Installation/Maintenance Activities

Apart from the gun activities summarized in the next section, this week's maintenance/installation activities resulted in the following accomplishments:

The cathode-photoresponse scanner was made operational and used this week after the last heat treatment. Both hardware and software are working.

The beam-scraper hardware was commissioned without electron beam this week and appears to be functioning per design.

The 1497 MHz RF noise on the drive-laser fast photodiode was finally eliminated. The problem was found to be a poorly shielded cable from the patch panel to the spectrum analyzer. Installation of a new 1/4" heliax cable solved the problem

FEL Commissioning Activities

At this writing (0945 Friday, 6 November 1998), we are running the FEL. Translated, that means the gun is presently operational. However, we may need to replace the cathode wafer starting early next week. A detailed summary follows:

At week's beginning, after finishing the rework of the cesiator, we still had achieved neither a good cathode photoresponse nor a gun that behaved quietly at 350 kV. Consequently, we did two things. First, we high-voltage processed the gun to 500 kV in an effort to suppress what appeared to have been two field-emitting sites (a soft statement, given the grossly coarse resolution of the geiger-tube array, though the emission was observed unambiguously to be bimodal). Then we heat-cleaned the cathode at a high temperature (675 C) for a long time (8 hours), an action that was motivated by vacuum data from previous heat cleanings. Finally, we recesiated the cathode yesterday morning. The resultant photoresponse appeared to be about a factor three better than previously with this cathode, but still poor. Basically, the cathode exhibited a photoresponse like this summer's cathodes after they had been well used and were in need of replacement. We then did a high-voltage check with the now-cesiated electrodes and found a quiet gun (no field emission) at 350 kV -- finally achieving at least one success.

Given the state of the gun yesterday afternoon, we decided to try pulling pulsed electron beam off of the cathode, and we found we could do it. We are presently unsure of the calibration of our beam-current-monitoring (BCM) cavity and will need to check it, but taken at face value, it indicates we are achieving 60 pC bunch charge with modest power (a 25% polarizer setting) from the drive laser. Consequently, we embarked on a 32-hour program (through Swing Shift tonight) of commissioning with the fundamental goal of taking pulsed beam through the machine and putting together a punch list of hardware/software problems; one would expect problems given the gun has been down for 12 weeks and considerable work has been done on the machine in the interim. At this writing, we have already put the beam into the straight-ahead dump and were making some steering adjustments in the wiggler region in preparation for pulsed lasing to check the FEL systems. The electron beam looks quite reasonable, again given the machine has sat dormant for so long. Plans are to take the beam all the way around, check the new diagnostics in the recirculation loop, and then go back to the straight-ahead dump for a brief cw run to calibrate the BCM cavity. Presently the conditioning resistor is in place, meaning the running resistor still needs to be put in prior to going cw. The new resistor-change hardware should make it possible to do this in just a few minutes, versus the two hours it used to take.

Presently we cannot know whether the cathode will need changing next week. The photoresponse measurement suggests that we should expect a very short lifetime. We shall find out soon, perhaps by trying to turn on again Monday morning and seeing what is left of the cathode. We will run the machine as long as is sensible to do so before replacing the wafer.

Another way to view the developments is as follows: The exorcism our gun owner, Tim Siggins, undertook on Halloween night turned out to be a protracted one. Relics of the exorcism are still present and remain to be purged from the FEL vault. We gave Tim the day off today. He has a new twitch, but we don't think he needs the straight jacket yet.

The two new M_56 cavities are ready, and they will be installed in parallel with the next cathode replacement. These cavities, along with the available software, will provide data on path length and phase transfer. An off-line calculation will be needed to translate the data into M_56 values because the software has not yet been upgraded to do this. That should not pose a big problem since the calculation is straightforward.