broean01 said:
To clarify, the focus I'm asking about modifies the electric field of the acceleration column in some way to affect beam spread directly from the accelerator. We use magnets to analyze, steer and focus further down the beamline, but I know how those work.
As far as I know, the only thing that can happen in the accel column is to constrict the beam to a smaller diameter like a shutter in a camera. I don't think there is any effect on the actual beam width since each accel ring just gives a kick to the velocity of the beam, it does not effect focus. The only problem with using the accel column to clamp down the beam size is the beam would eat into the accel ring and destroy it eventually. That's why in the analysis section of the ion implanter, it is lined with graphite because the other ions get turned at various angles by the analyzer field, the graphite gets eaten out and has to be replaced because the beam contaminants drill holes over time in the waveguide if it wasn't there and that would do wonders to the beamline vacuum system:) In the accel columns I use, the discs have holes that are narrower at the beginning of the accel column and are somewhat larger in diameter at the end, the last ring having a diameter maybe 50% larger than the beginning, precisely because if they were all the same size, since there is no effect on focus, beam spread would eat into the ring which would heat up that ring, perhaps causing the epoxy holding the assembly together to split apart, again ruining the vacuum. As far as I know, accel columns can only effect the velocity of the beam not the focus. Perhaps you could incorporate small permanent magnets around the opening of each ring to combine the action of the beam focus area downwind and do focus at the same time but that would complicate the construction of the accel column and increase the weight, in ours, the accel column has lead power in the epoxy to reduce the radiation coming off the column for safety reasons so the columns we use are already pretty darn heavy. One of the high voltage machines I worked on, a one megavolt job, had an accel column that was about 6 feet long, surprised the heck out of me when once I saw someone working on it where the column was supported only at one end and it was hanging out in midair, I couldn't believe it would hold together so well since there must have been 50 ring sections all just epoxied together, so maybe incorporating magnetic structures in the accel rings would work. I think that is the only way you are going to be able to get focus AND accel out of the accel column. We had to replace a lot of accel columns on our implanters because of beamspread from one end of the column to the other, the rings got really pitted with beam products attacking them and we did not have any way of dealing with that since the only thing we had to prevent that was the graphite beam slit just before the accel rings. We had one we called at Varian the 'Callahan tunnel' because our chief scientist, Ray Callahan, who I believe started at High Voltage Engineering in the late 50's, made one beam slit that was designed to allow the separation of one of the higher AMU beams where the isotopes had to be separated but the actual mass difference was very small. Trying to remember what isotope that was, not Arsenic at 75, it didn't have any rivals there, maybe antimony at 130 v 131? Something like that, one isotope would be very close physically after mass analysis because the masses were such a small percentage difference so he made this quite long graphite slit, maybe 5 cm deep with a very narrow slit width which seems to have done the job but part of the reason was accelerating such high AMU isotopes together made them bore out the accel rings because of beamspread and the contamination of mixed isotopes in the beam when it came to the scanning electrodes or magnets (depending on what series of implanter it was in, the high current machines used magnetic scanning in the X-Y direction and vertical scanning via a spinning wafer wheel).
The "Callahan tunnel" did a good job of saving the accel column rings from being eaten out by poorly contained isotopes. Of course that meant the slit itself had to be replaced periodically but that was a lot cheaper and a lot less time consuming than replacing the whole accel column which meant the whole frigging beamline had to be re adjusted, a pain in the butt if you asked me:) But we never tried to focus and accelerate in the same mechanism and I suspect it was because of the weight problem of having at least four magnets near the inner disc opening would be needed on several rings at least. I don't know for certain but I do know it would certainly complicate the construction of the individual rings. Maybe you could make each ring electrode magnetic with slits separating each n/s pole to make the electrode do double duty without having to install a separate electrode, but that would be a tricky thing in itself, if the inner ring got hot enough from continuous use, the magnetic material of the rings could go past the curie point and lose its magnetic field and then the beam spread would go to hell in a handbasket. I am just speculating here, that would entail the development of a whole new kind of accel column for sure, but it might be possible. The only thing I know fore sure is all the beam focus assemblies WE used were magnetic in nature not using any form of electric field since magnets bend, electric fields deflect only, like a mirror, so it would be very tricky to turn the deflection of scanning electrodes, for instance, into a focus effect. So for whatever it's worth, you could theoretically incorporate magnetic structures into an accel column, it would at least be an engineering challenge and not something you could just slap together with spit and polish:)
