As we've done our other research, we find that we do better the lower the pressure we can run at, down to the point of needing an ion source to make it go at all -- well off the Pashen's law light off point, and at a point where you pretty much need to provide all the ions some other way, not just a trickle. Nothing has yet beat my other ion source design (adapted from RevSciIns), but it's on the wimpy side for a main source. So, I've been thinking on how to do all of the above. Since this is primarily an RF design exercise, it landed here -- we already know that we want an RF cyclotron source, that discussion is elsewhere.
For this sort of thing, what you want is an E field that a lot of volts/meter (concentrated) for the amount of power input, crossed with a static H field at the cyclotron strength for electrons at the frequency in question. For 2.45 ghz that's in the 980 gauss range, not too difficult for modern permanent magnets, even over a fairly large space. But getting a high E field to power ratio needs something more than a waveguide. In the previous design we used a quarter wave tuned line to get high voltage at one end. In this design, I'm going to use a re-entrant cavity for that purpose, and build the thing right into the ISO nipple between pump and cylindrical tank, leaving the tank innards as pristine as possible. The idea here is to have it close, straight shot for the ions to get into the tank easily, and simple (at least in appearance
)Here's a rough drawing.
- Sketch -- graph blocks are 1/4", nearly at scale.
You can think of a cavity of this type as being tuned by the capacity between the screens in the middle, across the inductor that is a torus around the outside. Energy is coupled in with a loop from a microwave source (in this case, an oven magnetron at 2.45 ghz). The E field is concentrated between the screens here, vertically. A static magnetic field on the order of 980 gauss will be applied horizontally with permanent magnets to get electron cyclotron resonance over some of the area between the screens -- it's that trick that makes this work down to very low pressures, otherwise known as long mean free paths, as the electrons just go round and round until they finally hit something and ionize it.
Tuning is a sensitive function of inter-screen distance (eg capacity). So at least one of them has to be a sliding fit and adjustable. If it's mounted in the end of a slightly smaller piece of Cu tubing that is insulated from the 1" tubing by mica, it's at the right potential for RF, yet can have DC applied to it to "push" the ions out into the main tank area. Various other schemes could work as as well, or be added to promote ions into the tank through the final, grounded screen that keeps the in-fusor DC field shape uniform (eg no big dip where the hole would be).
Here's some code I wrote that does the equations from Terman. The book claims the original equation will be off by factor between 1.25 and 1.75, so I built a 1.5 factor in -- 2 pi becomes 3 pi.
Code: Select all
#!/usr/bin/perl
use strict;
#use warnings; # can't becuse of a slungy trick to have defaults
my $minorrad = 1.27; # assume one inch inner pipe
my $majorrad = 3.81; # assume 3 inch outer pipe
my $majorheight;
my $minorheight;
my $temp;
my $twopi = 3.1415927 * 3; # fudge factor of 1.5 added in already
my $wavelength; # the answer
# get inputs
print ("Supply me with some info on the cavity- all in cm.\n");
print "\nInput minor radius ($minorrad) -->";
chomp ($temp = <STDIN>);
if ($temp + 0 > 0){$minorrad = $temp};
print "Input major radius ($majorrad) -->";
chomp ($temp = <STDIN>);
if ($temp + 0 > 0){$majorrad = $temp};
print "input minor height -->";
chomp ($minorheight = <STDIN>);
print "Input major height -->";
chomp ($majorheight = <STDIN>);
$majorheight /= 2.0;
$wavelength = (($majorheight * $minorrad*$minorrad)/$minorheight) * log($majorrad/$minorrad);
$wavelength = sqrt($wavelength);
$wavelength *= $twopi;
print "\nResulting wavelength = $wavelength\n";
# done, exit
Here's how I see this when editing it in linux/gedit, a lot more clearly.
- Screen shot of simple program and its action.
I have some defaults you can override in the program, but the assumptions are using 3" Cu pipe for the outer part of the re-entrant, and 1" for the inner size, though it's possible to make this tune with 1.5" inner size too -- just pickier. I believe that a 1" hole will let enough pumping happen for a small (6" diameter by about 1 foot long) fusor, if it has no leaks and few extra surface area contributing "warts" welded into the tank. I plan to use glass ends, covered by screen wire, one drilled to let the HV feedthrough be glued in. Viton o rings for the glass to metal seal to handle the tempco force monster there. Might even use an ignition system to run it pulsed mode, there's some good stuff out there for hot-rods that makes 50kv and more at decent currents.
This should let me toss it in the back of the truck, even run it off the truck's AC inverter....or put it someplace far from me and run unattended for long periods to activate stuff. It will obviously be a lot easier to shield the X rays from than the big tank in the lab as well.
We will feed this with a telescoping hardline I'll make in the shop. We'll use a retractable antenna for the center conductor (if I can find one still! Kind of obsolete these days) and sliding Cu telescoping outer pipe for the ground part, held with a clamp. I will taper this line (have to anyway to make it telescope) so that it also acts like an impedance stepup from the nominal 75 ohm output of the magnetron to a few hundred ohms Z at the coupling loop in the cavity. Hpefully, I can also get some fine tuning that way by coupling some reactance into the loop when the match isn't perfect, and still not have it arc or fry the maggie tube via SWR. Something tells me this might just need more RF power than we got in the last one, we may not be able to go CW with this, but for a demo, it may not matter (and we might use one of my HF switchers and get close enough to CW anyway -- this will ring for awhile when the power goes away between pulses and the ions coming out will also time-smear some). With the little turbo, I can use the same gas control I use in the big system, and let in D in the foreline while the turbo is spinning slowly. So all the "warts" will be on that one ISO nipple, or between the pumps.