FEL Gun Committee Meeting

Thursday, 14 Jan 99
Recorder: C. Bohn

Next Meeting


Date: 21 Jan 99
Time: 1500-1600
Place: FEL Facility Break Room

Agenda for Next Meeting


This Week's Attendees


G. Biallas, C. Bohn, F. Dylla, J. Gubeli, K. Jordan, G. Neil, L. Phillips, T. Siggins, C. Sinclair, R. Walker

Discussion


The nature and source of the metal spatter on the previous cathode entered as a topic of general discussion. The discussion was skewed and came to no closure because there is as yet no data. However, C. Sinclair pointed out, and all realized that, it is important to identify the source, or at least potential sources, of the impurity before we can know how to correct the problem. For example, if the problem were to persist, including a load lock would be fruitless in that the wafer would need to be removed, not just reprocessed. T. Siggins and L. Phillips will work together prior to next meeting to gather at least some cursory data. One comment: Two earlier FEL cathodes failed due to spatter; one got copper on it, and the other got silver on it, meaning their failure mechanisms apparently differed.

Sinclair proceeded with a presentation of his ideas concerning a "simple" load lock (indicating that perhaps there is no such thing). His charts are attached. The bottom line is that Charlie recommends adding a high-voltage standoff to bring the load lock to ground. Perhaps the simplest implementation would be to do a rear-mounted load lock with a new ceramic supplying the voltage stand-off. A large ring would replace the present hemispherical structure that receives the HV from the power supply and provide a link between the new ceramic and the existing HV stack. Disadvantages are that it would necessitate a long feed for the cathode and a long load-lock chamber. The key advantage is that no new electrostatic problems would ensue. By contrast, if one would go for a side-mounted load lock, one would need to bore a hole in the side of the ball, or modify the ball in some other pronounced fashion, thereby introducing risky electrostatics.

Charlie also pointed out that the availability of a load lock would then motivate looking at other cathode types, perhaps the most attractive of which would be K_2CsSb, a material that exhibits high quantum efficiency in the green, possibly long lifetimes, and only needs to be reprocessed through a load lock for rejuvenation. A study of any new cathode material would necessitate going through a learning curve, but a load lock should significantly reduce the e-folding time of the curve.

One manifest implication of Sinclair's presentation is that a load lock cannot be viewed as an incremental upgrade. Most likely its development would best transpire by way of constructing and testing an altogether new gun in the Injector Test Stand. People are invited to come up with simpler and cheaper alternatives, but to do so would appear (at least to Bohn) to be difficult.

F. Dylla advised that the definition of "incremental upgrade" is one that involves no more than ~50 k$ and a few man-weeks of labor. A load lock is ruled out at once, meaning it cannot be developed in the absence of a new funding source for the FEL. G. Biallas had previously tabulated low-cost options for incremental upgrades; he will update it and send it to Committee members for review and consideration. Further discussion will take place at next week's meeting per the agenda above.

Closed Action Items


New Action Items


Old Action Items




****Charts from C. Sinclair Presentation****

1) LOAD LOCK ISSUES FOR THE FEL GUN

I have not been a proponent of load locks for polarized guns, because they do not solve the underlying problems, but rather "work around" them. They are very costly in $$, and involve a great deal of skilled staff time. However, in some situations, the realities require "work arounds" rather than "solutions" or "understanding". This may be one of these situations. However, IT IS ESSENTIAL TO CLEARLY UNDERSTAND WHAT PROBLEM(S) A LOAD LOCK IS SUPPOSED TO GET AROUND OR "SOLVE"

2) WHAT PROBLEMS SHOULD A LOAD-LOCK "RESOLVE"?

3)THINGS TO WATCH OUT FOR

4)LOAD-LOCK STYLES 5) SHOULD WE CHANGE CATHODE TYPE?