Well, I've decided to make the leap into RF driven particle acceleration for the new beam system. Not necessarily less hassle than DC, but different in kind, and at least I don't need the full potential in any one place that way. Not sure whether I prefer shocks or RF burns, probably the former, but I'll report on that when I've had more experience.
OK, some really brown estimates here, and I know they're not real good, but you gotta start somewhere. Lets suppose I want about 90kv per beam, and about 1 ma per beam. That implies I need about 180 watts of drive if everything else is 100% efficient. OK, so I need 500 watts or thereabouts. I plan to have 3 stages of RF gaps per beam, so I need 30kv peak voltage (60kv peak to peak). I will ground the alternate drift tubes to avoid needing a two phase center-tapped output from the RF driver, and also avoid some loss and voltage standoff issues where one of the tubes is right inside the grounded tubing coupler into the vacuum system -- it'll be nice to be able to have that one grounded.
On top of the presumably mostly-resistive load from the particles, this is going to have to be well shielded and will create a capacity to ground as part of the load. I'm guestimating about 500pf total for all the tubes to shield capacity (assuming I use quartz and pyrex vs something with higher D). Add a few pf for the short coax and some tuning fudge, we need to make this 30kv peak across about 700pf.
To make the tubes decently long at these voltage levels (too short gives you other issues, like can I make it and can things stand off the volts/inch), I chose a nominal frequency of 5 mhz. Obviously I have to stay off precisely 5.0 mhz or I'll be jamming WWV's time service with any leaked RF and bring trouble on my head. But close will be good enough. It's the nature of the device that you can tune the frequency around a little as long as you tune the voltage to match, so that the transit time through a drift tube matches up with the RF phase as the ions hit the gaps.
So, in some perfect world, which I don't inhabit, what I want is a super frequency stable high power, low impedance RF supply, where the voltage wont' change with changing loads (because we all know that no ion source is perfect and all vary a bit). This, quite simply, ain't gonna happen, though it's possible. Yes, I could do a full gallon amateur radio lashup with a VFO (or xtal), 100w or so exciter, and a linear amp with ALC. I even have all that junk, though I have doubts the ALC in it is anywhere near good enough for this. What I don't have is a spare few KW of mains power, or the floor space and cooling for all that, so I propose to scratch build one for this.
Looking around the shack, and keeping simplicity in mind, I'm looking at using a 3-500z triode in a Hartley oscillator setup, much like a vacuum tube tesla coil setup. I might just be able to get it to sync to an xtal oscillator that only puts out a few watts, rather than needing 100w or so to drive it as a linear amplifier. In this case, I want class C operation for efficiency, and to run the power tube at the highest possible plate voltage so the stepup to 30kv peak is easier (far easier than stepping up from a FET for certain). Looking around, I also have a 1400W microwave oven transformer that doubled, will give the rated 4kv needed for the max plate voltage recommended for a 3-500z tube. Looks like I can either add a few turns to the existing filament winding for the 5v, 14 amp filament requirement, or I might use a 5v 20 amp switcher from Marlin for this -- price is right after all. I'd like to cram this whole mess into about a 12" rackmount box, which looks a little tight, but possible if I keep this simple, which is plan A for certain. In this kludge, the idea would be to regulate the output voltage via sensing it off the tank coil, and adjusting the effective grid-leak current to keep the output volts constant. If I'm lucky, the oscillator will do the right frequency change with output power change -- but I don't feel that lucky, so I'll probably build up a 5-10w output xtal oscillator to try and keep it synced, and then just adjust the output power via grid current in the main power tube. In theory, it should take a lot less RF to sync up another oscillator than it takes to drive the same stage as an amplifier.
Now to calculate the circulating current in the tank, given the above assumptions. My gut tells me it's going to be fierce. X = 1/(2*pi*F*C). For .7 nf, 5 mhz, this gives me 4.547284088e+1 ohms. Uh oh, that will never fly at 30kv! That works out to roughly 600 amps! Back to the drawing board! I either need to cut that capacity WAY down, or refigure other things here. So this little post is instructive if nothing else. 60 amps RF circulating current would be at least feasible, though it would require the very best parts in existence to get that level of Q. The question of course is, can I cut it down that far with a physically realizable build of the basic accelerator? Shortening the tubes by going up in frequency is a wash -- cut the capacity in half, run twice the frequency, same problem, no gain there in the tradeoff space. Same thing happens if you double voltage. Seems like cutting the voltage in half and going to twice the stages doesn't buy anything either, because now each tube gets longer to accommodate that, and therefore has more capacity to ground. Hopefully my guestimate of the capacity in the load is way off! It might be wise to build a mockup, complete with shielding and see about that, as this can't be done with reasonably gettable parts. I doubt I could get tank coil losses down below 1% of 45 ohms, after all, that'd be quite a trick with skin effect and shielding losses though not quite impossible. It now becomes apparent why the guys back in the day limited out at 40-50kv for cyclotrons! And I've seen some of their matching devices, they were huge (a couple foot tank diameter by 10 feet long), in a tank (vacuum or pressure) with a lot of distance between the fat tank coil and the tank walls to reduce skin effect losses in the shield, not to mention they were running 10's of kw and had spare power to burn in losses. While I can get about 800w out of a single 3-500z (and I have two), that's kinda pushing what I want in my room anyway.
And heck, I had a neat circuit all drawn up in my mind, but now I have to drop back and see how low I can make the drift tube capacity by clever design work. I was not wanting to have to use tiny diameters, as I figure the focus issues will be not so easy to work out here as with DC -- depending on when particles hit each gap, the effective lens focal length will vary with phase. I'd planned to put a little phosphor screen on the wobble stick I described elsewhere on the board and move it around to look at focus, but it will need a bit more finesse than that to see if the focus varies at 5 mhz! I'll need an RF probe to work with that, movable in the beam.
Sigh, but I thrive on a challenge, and heck, have already gathered the box and parts to make this thing. Those who won't be stopped, can't be stopped! More later. Now I have to rassle up a good cap checker for the range in question. The multimeters don't go that low, and neither do the war-surplus LCR bridges -- I'll have to make and calibrate a known inductor and see where it resonates with my mockup test load. Maybe I can get the load C way down and make this not so hard? Stay tuned. I won't be stopped.