Next Meeting
Agenda for Next Meeting
This Week's Attendees
G. Biallas, C. Bohn, F. Dylla, R. Evans, J. Gubeli, L. Phillips, T. Siggins, C. Sinclair, R. Walker
Discussion
We must change the cathode wafer again because the present one is cracked and will not hold voltage. The cause of the cracking is not really known; there are several conjectures. Following C. Sinclair's suggestion based on current practice in the Polarized Source Group, we will incorporate an indium solder to "glue" the cathode wafer to the molybdenum cup so it cannot slip and so it will be better "shock-protected". The only other change we will make to the turn-around procedure will be to do high-voltage processing with the cathode retracted into the stalk. The field at the lip of the ball will be enhanced, but that should not cause undue problems. It seems most likely that ion back-bombardment is the cause of the QE degradation during HV processing. Retracting the cathode should reduce back-bombardment, but it won't be completely eliminated. Having a data point will be helpful.
A sure-fire way to defeat ion back-bombardment is to install a shield. Options were briefly considered, including devising a mechanism to set up the shield that involves going through the stalk, and buying an off-the-shelf shutter. Phillips and Biallas will look into the options and propose a design. Plans are to install the selected mechanism during the June down.
Concerning the apertured cesiator, all parts are in (save for a 1-inch tube), and the detail drawings are complete. G. Biallas just picked up the drawings today; he and T. Siggins still need to check them. Plans are to install a new, pretested cesiator during the June down.
Concerning 6-inch plates for ion-implantation studies, we now have six (4 new plus the 2 originals). One of the originals has been polished to 9 microns, and the other to 15 microns. Unfortunately the material seems to be poor, making it difficult and time-consuming to polish to a finer surface finish. It is believed that the 4 new plates are made of the same class of material (Biallas will check). So, in the interest of getting timely data, Biallas will check into procuring four plates fabricated from high-quality (cross-forged) steel, from which we can quickly make and polish test plates. Two of them would then be polished to 9 microns, tested, then ion-implanted, and retested. The fate of the other two will ultimately be determined from the test results. To expedite polishing (in the face of the present gun work), Biallas will check with B. Chronis to see if Tina can be put on polishing.
It is unlikely we will have an ion-implanted ball to install in the gun during the June down. Instead, what will be done is to remove the present ball, clean it up, repolish it, and reinstall it. So, in summary, gun work during the June down will encompass: (1) repolishing the existing cathode ball, (2) installing a shield/shutter, and (3) installing a pretested apertured cesiator.
The specifications for an off-line test chamber cannot be conclusively established without the additional information that we will get by processing with the cathode retracted and by using the apertured cesiator. The general purpose of the test chamber would be to try new cesiating and HV processing procedures if it turns out alternative procedures are needed. Presently we do not know what to require of a load lock. For example, if ion back-bombardment during beam operations turned out to be the lifetime limiter, then a load lock would be of little value compared to improving the UHV. Therefore, the Committee agreed to table discussions of a test chamber until we see what benefit the planned improvements have.
Sinclair mentioned that the Polarized Source Group is seeing chlorine in the UHV of one of their systems. That's very bad for cathodes, and the source is uncertain. Perhaps it is traceable to the cathode-cleaning procedure (?). Hopefully the matter will be put to rest soon.
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