Next Meeting

Date: 25 Feb 99

Time: 1500-1600

Place: FEL Facility Break Room

Agenda for Next Meeting

• Check status of ion-implantation studies (Phillips, Biallas, 15 min).
• Discuss progress on focused cesiator design (Biallas, 15 min).

This Week's Attendees

G. Biallas, C. Bohn, J. Gubeli, K. Jordan, T. Siggins, C. Sinclair

Discussion

The gun has been running very well at 330 kV. At this writing (22 Feb 99, 1330) it has delivered over 100 C, but now needs recesiation to support continued higher-power cw lasing. Recesiation will be done this evening.

The focus of this meeting was on two items, namely, whether to modify the lip of the spare cathode ball, and choosing a design for a focused cesiator. G. Biallas had Jim Hinkson of LBL (Jock Fugitt's contact) look at the merits of tapering the lip of the ball. Hinkson found that the envisioned modification would make the peak electric field worse, not better. The question, then, is how to make a taper for lower peak field? The Committee elected to defer answering the question; no modification of the ball will be done now. The status of the spare ball is that it is out of the Machine Shop, which did some machining to fit the ball to the stalk. It needs cleaning and polishing, but is overall in good shape.

Ling-Ling has said she is ready to ion-implant the spare cathode ball. According to Fred Dylla (Wednesday PM conversation with C. Bohn), she has been asked to write up a procedure for doing so. She will not implant the ball before the Committee has reviewed and approved her procedure.

Regarding a focused cesiator, the Committee decided to adopt Larry Phillips' design, but not before the following two issues are put to rest: (1) Since the nozzle will be below the anode plate and therefore relatively far from the cathode, careful pointing is necessary. Can the plume be narrowed to provide some leeway on pointing? (2) Thermal control (RI^2 ?) needs to be worked out before cutting metal.

Concerning the aperture from which the cesium comes, what was presented used a 0.25 mm diameter aperture. The conductance of such an aperture is VERY small. Presently, with the cesium emitted from a 3 mm diameter aperture, it takes of order 30 to 60 minutes to activate a cathode. A 1/4 mm aperture would admit a flux down by a factor of 12 squared - 144 - for the same cesium pressure on the high pressure side of the aperture. One can gain some in this pressure by increasing the temperature, but there are practical limits to this. It remains to determine how long it might take to activate a cathode with the new restricted aperture cesium supply. It would be a pain if it took 5-6 hours, and exceptionally difficult if it took 2-3 times as long! On the other hand, were the hole to be enlarged, one risks illuminating undesirable areas with cesium, defeating the original purpose. We already saw that the proposed design came within 3 mm of illuminating undesirable areas even if it had perfect alignment.

Other options were discussed. The conceptually simplest solution is to make a screen for the cathode. Unfortunately it is not so simple to implement because to do so necessitates adding a port to the UHV gun chamber, i.e., it is very invasive to install. Whenever we make a second gun, we should probably include a cathode screen for cesiation. The Committee also recognized that Charlie Sinclair's proposal for a retracted cathode/channel cesiator merits further consideration, but agreed to defer it in favor of Phillips' cesiator. Finally, Larry Cardman's proposal for a "cheap" load lock was also discussed. Phillips' design was adopted over Cardman's simply because it should be much less expensive and is manifestly noninvasive to implement.

Closed Action Items

• Siggins: Identify possible sources of aluminum splatter in the gun. Status: Best guess is that it came from the beam shield, but there's no way to be sure without ripping the gun apart.
• Neil: Ask D. Bullard to get some 6-inch plates fabricated and polished, and in parallel teach Ling-Ling how to polish. Status: Closed because the new plan is to have Ling-Ling ion-implant the spare cathode ball.
• Biallas: Compose a strawman drawing of the new cesiator. Status: Done, based on suggestions by L. Cardman; however, the Committee has given higher weight to L. Phillips' design.

New Action Items

• Biallas: Resolve pointing (hole size) and thermal control issues associated with Phillips' cesiator design. (due 1 Mar 99)

Old Action Items

• Sinclair: Prepare procurement package for DLC-coated samples for Dylla's signature. Status: Unknown. (due 4 Feb 99)
• Siggins/Biallas: Specify design of experiment to measure cathode temperature. (due date TBD)
• Dylla: Ascertain feasibility of low-cost "load lock" for cathode-processing experiments in the ARC. Status: No progress due to lack of funding. (due date TBD)
• Fugitt: Follow through with Ling-Ling on having her develop a strawman table of HV experiments on candidate electrode materials. Status: Reassigned to T. Siggins in view of Fugitt's retirement. (due 4 Mar 99)
• Siggins: Begin scanning gun data and, with D. Gilchrist's help, compiling information into an electronic logbook. Status: Web site was created; progress slowed by recent FEL gun and commissioning activities. (due 4 Mar 99)