Management

Highlights for the month: (1) attainment of 300 watts of cw laser power with energy recovery of the 1.6 mA drive curent; (2) before commissioning operations ceased on Dec. 15, a total of 180 conlombs of charge was delivered from the photocathode.

A meeting of Maritime Technical Advisory Committee (MTAC) was held on Dec. 9 to review progress with the FEL and plans for a potential collaboration with Northrop Grumman on the FEL Upgrade.

Preparations continued for January's visit to the lab by Secretary Richardson (on Jan. 12) and DOE for the lab's presentations at the Jan. 19-21 BESAC panel on "Novel and Coherent Light Sources."

The Jefferson Lab FEL project received a 3-month, no-cost extension in our current Navy/DOE MOA for the commissioning milestone (from Dec. 31, 1998 to March 31, 1999).

CA 321: Upgrade Cryomodule System

Pair number AIA074/76 was vertically tested with good performance. The total gradient less any field emission was 10.3 MV. Gradient limitation for both cavities was a quench at 13.5 and 18 MV/m, respectively. The next step is to assemble tuners and instrumentation and then complete the helium vessel.

Pair number IA004/006 was vertically tested with good performance. The total gradient less any field emission was 8.8 MV. Both cavities reached a quench as their final limitation. The next step is to assemble tuners and instrumentation and then complete the helium vessel.

Pair number IA017/18 is in teststand and waiting vertical test.

Pair number IA021/23 is leak checked and waiting teststand insertion.

IA076f

IA074h

Note: Ia004/6 data was not complete and will be available in January report.

CA 521: FEL COMMISSIONING

Commissioning of the recirculation loop in the presence of cw lasing continued through 15 Dec 98 to the point of reproducibly generating ~200 W cw laser power upstairs in the Optical Control Room at cw average currents ~1.5 mA. Best performance was 211 W cw upstairs (corresponding to about 300 W cw out of the FEL proper given roughly 40% losses in the optical transmission line) with 1.6 mA current. At that point we were experiencing a trip of one of the beam-loss monitors indicating beam scraping in the reinjection region, i.e., near the end of the recirculation loop. A second limitation also emerged, namely heating of the infrared detectors in the waveguides of the accelerating cavities that led to persistent trips in one of the cryomodule cavities. Preliminary off-line measurements indicate that the detector itself is sensitive to microwaves. A copper screen with about 1 mm mesh size removes all the measurable RF response at 19.5 GHz and below, and still allows 60-65% of the IR signal to be detected. Consequently we are planning to install a screen over each IR detector in the cryomodule during the January down.

On 15 Dec 98 we lost the quantum efficiency of the cathode and decided to begin the process of replacing it with a new one. Plans are to resume running on or about 25 Jan 99. The present cathode proved to be quite resilient in that it delivered roughly 180 C of total charge during its operational lifetime.

FEL INSTALLATION

A comprehensive list of tasks for the January down was compiled and prioritized. "Must-have" tasks to aid in expeditiously achieving kW-level operation were compiled based on commissioning experience to date, and they include:

- Repair or rebuild the cesiator for the gun.

- Install a new photocathode in the gun.

- Provide 74.8 MHz operation in the Drive Laser Pulse Controller (to get ~4.5 mA cw current with 60 pC bunch charge).

- Resolve the IR detector trip problem (see FEL Commissioning above).

- Add a post-raster viewer and temperature sensors in the energy-recovery dump line (to permit quick, unambiguous setup of the dump line).

- Put a hole in the post-cryomodule viewer (to pass the accelerated beam unperturbed and thereby aid in observing the recirculated beam).

- Install two new rf cavities for path-length measurements.

- Calibrate the gradient of the buncher cavity.

By Christmas break, most of these tasks had been initiated. Numerous other tasks of lesser priority had also been initiated. One example is starting construction of a new viewer that will be installed just downstream of the scraper in the first arc. That viewer will permit quick centering of the electron beam in conjunction with changing its energy, and it will permit unambiguous observation of the effect of the scraper (which to date has not yet been used).