I am temporarily moving away from fusors and into beam devices so I can more easily learn some things in a simpler system (for the particles, the gear is more complex). Thus, I've been acquiring knowledge and parts to build a beam device, with target energies per beam in the 100kv region. Being that voltages much over 50kv are a real pain to handle in air, and that I want to be able to achieve decent energy spread and focus along with good bunching, I selected the not-very-versatile drift tube design for this project. This will let me get there in decent size with the still formidable 12kv or so RF peak voltage for D ions, and with some changes, for the other ions of interest. The sharp will note that 100kv is right around the peak cross section for DT, in beam on target applications. The plan might be to make two of these and do beam on beam, but I think it wise to make one, solve it's problems and unexpected issues first, then add another, which might be optimized for some different charge/mass ratio and output energy anyway. I wrote some software to help calculate the tube (or really, gap to gap) lengths, which is here. Fairly rough stuff, but it's not going to be used too many times - you get the answer, you're done with that tool. So I didn't bother with minor things there, like a nice GUI, and I'll need to subtract gap lengths manually, oh my.
Since this isn't new science, nor is it really new tech, I'll let Haliday say most of the words. Note the page on focusing has the only actual error I've detected in the whole book - a particle spends LESS time in the second half of the accelerating gap, not more (and see, the sentence makes more sense that way).
Per Terman, only the first gap between the ion source, and whatever goes on inside the ion source itself really affect total focus. With tubular elements, you only get a measureably small focal length when you're adding a lot more energy to a particle - the velocity difference between one end and the other end of the lens gap has to be large to get a lot of focusing action. One can improve this, as Haliday notes (as does Terman, but with math and equations) by using a smaller aperture on the inlet side of the gap, going downstream, which I plan to do. This also cleans up the beam, as things that miss the aperture at least don't get down the tube for more energy to be wasted on accelerating them further. Thus, I should be able to use pretty short inter tube gaps once past that first one, which makes things smaller, and simpler to work with, but I'll have to be able to adjust that first gap, and whatever extractor/lens I put in the ion source. I plan to use the microwave one we've kind of pioneered here, it works great for things like this, and works down to way low pressures like you want in a beam accelerator, no differential pumping should be needed at all.
But that first gap, going from ground (the ion source output tube will be ground, it will have a 20kv "pusher" at positive polarity), to varying RF voltage is certainly going to be interesting. With 20kv or so DC on the ions at that point, seeing a voltage from -11kv to + 11kv means the speed and focus at that point are going to be crazy, and time-varying. This should actually help with initial ion bunching, but finding a happy place for that lens design is going to have to be cut and try -- too hard to model, I bet even SIMION would have issues trying. But we have an existence-proof - we know these are made and that they work fine, so we'll just follow the footsteps of the giants we stand on the shoulders of, like usual. The fact that there are a good number of downstream gaps for both time and space bunching should give us a nice beam quality. In the mechanical design, based on the cross I think I've pictured elsewhere here (I'll link from here when I find it again), we'll be set to do beam on target, or beam on beam, with decent diagnostic capability -- I've been thinking on this one awhile. Since the most off ground voltage near the ion source is a mere 20kv for "pushing" the ions out, I can use that unmodified, the insulation of the ion source beam tube will take that easily, and nothing else has to get off DC ground at all. I note that the tubes should be at least 3 diameters long for the middle to not see too much field from the ends, and with a 1" beam tube, I satisfy that pretty easily, I can go to almost 3/4" diameter drift tubes and fit inside that easily -- and 1/2" will fit with a lot of quartz "rigging and jigging" as is used in say, a Tektronix CRT, where the same issues of alignment apply, so perhaps we copy how they built those. Now that I know rough dimensions (from the software) I can put one of these together and put it on a capacity tester. A big goal will be to keep the capacity to ground and from phase to phase of the RF absolutely minimal, as the circulating currents required in the tank circuit will be fierce no matter what (and those make losses). I figure perhaps 100 watts max into the beam, but this could easily require several hundred watts losses in tank circuits due to that circulating current and parasitic resistance in the tank coil....fun! I'm planning to use a 3-500z power triode for the RF output amplifier, gain modulated (volts are a picky part of tuning) to support this, and use the 13.56 mhz ISM band for the RF, since it's legal to accidentally radiate some in that band without a license. Onward and upward, probably in fits and starts, but here we really have a start - I have all the pieces I didn't plan to make anyway, and a rough design now.