=
Subject Inverter Microwave for I=
on Source RF
Posted by lutzhoffma=
n on 2009-21-12 14:49
|
=
Hello:
In the interest of building a microwave ion source, =
I have been looking for an old microwave to gut for RF parts. Doug Coult=
er is currently building exactly such an ion source, and his work in thi=
s area is brilliant to say the least. He is much more knowledgeable on t=
he subject than I am, and he is kind enough to share his work. Between C=
arl, and Doug's work, quality ion sources will soon be a simple buil=
d for the amateur. In my opinion this is a fairly profound event, which =
may lead to many great discoveries in the future. As far as nuclear phys=
ics on an amateur level goes, ion sources ARE a very big deal, and perha=
ps the most difficult single component home build.
Originally=
I planned on using Carl's design, but I soon discovered that it is =
very difficult to obtain the required set of RF components, which are sm=
all enough to fit inside of the HV terminal of my project, even with the=
aid of a small amplifier, like the ones from Communications Concepts. <=
br /> This is where Doug's idea of using microwaves comes in. W=
ith a microwave based ion source the nearly complete set of RF component=
s, can be had at the local Walmart for under $50! Better yet they fit in=
side of a small space, with simple control circuits. If my ion source we=
re at ground potential then I think I would go with Carl's design si=
mply because it is the proven, and traditional way to go, with few unkno=
wns, or potential issues. So in a nutshell I like both approaches very m=
uch, but I want to try Doug's design on this project.
I r=
ecently scored a working Panasonic Inverter Microwave oven, rated at135=
0W, at the local dump. To my delight it is in perfect working order, the=
interior has some burn marks, and a broken glass plate, so the owners m=
ust have "assumed" it was toast, or that it was to expensive t=
o fix.
I very carefully removed all of the innards, being car=
eful not to mess anything up. This went very well, I was even able to re=
move the control panel leaving it attached. After a short time I was abl=
e to get the magnetron to work outside of the oven (While observing RF s=
afety measures) just fine. By leaving the factory control circuit attach=
ed, I have full control over the output, both in terms of power, and tim=
e. My feeling here is that instead of trying to engineer a new control c=
ircuit, it would be very easy to just wire an extension from the control=
board to the magnetron. I will just have to make a translation table li=
ke: 1 Pound Auto Defrost =3D 100uA Beam x 10min, this could be handy : )=
What I cannot figure out is the properties of the microwave=
output from this Panasonic assembly. Traditional tranny based microwave=
s produce a pulsed output, via the HV diode / capacitor circuit. Looking=
at the Panasonic inverter based design, I notice that the input 120V 60=
Hz, goes into a FW rectifier and is first converted into DC. The DC then=
goes into a circuit with what looks like a fairly solid MOSFET, or IGBT=
, then to an inductor / small capacitors, and to the HF ferrite transfor=
mer, then finally the magnetron. It could be a resonant system. The circ=
uit however lacks anything like a substantial HV capacitor which has me =
wondering about the nature of the RF output?
Some promotional=
literature on the net seems to imply that the output is CW? By referenc=
ing to a traditional microwaves "pulsed output" being inferior=
. My question is: Is the output of this inverter microwave pulsed, or is=
it CW? Or if it is pulsed, is it pulsed at very fast rates like in the=
KHz range? I do not own a scope so I cannot just figure it out this way=
I could not find a clear answer about this question when I r=
esearched on the net, which led me to writing this post. For an ion sour=
ce CW is normally preferred, but when I think about it the efficiency of=
ion sources normally goes up with increased RF power. Thus a fast enoug=
h pulsed RF output could be an ideal situation. The higher peak RF energ=
y increasing efficiency, while the HF pulsing rate is fast enough to ena=
ble easy power measurement, and monitoring. Personally I do not want to =
have a 60-120Hz ion source output, but 10 or 20 KHz would be just fine, =
and maybe even preferred to CW : )
Can anyone shed some light=
on this, by explaining the nature of the RF output from an inverter mic=
rowave?
Thank You.....Lutz |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Doug Coulter=
a> on 2009-21-12 16:01
|
=
Well, you speak of the devil, and here I am.
I own two of tho=
se ovens myself, they are fantastic for cooking.
No, they are=
n't CW, they pulse at 30khz or so. But to reduce power, they reduce=
the pulse height/width at that frequency. This looks enough like CW to=
the food to make no difference, compared to the old on for a few second=
s, off for a few seconds older machines used to control power.
Getting a maggie to go really CW is kind of a trick, and one I guess I=
should start writing up for others. My basic idea is far from new,=
I copied the basic RF design from a rev-sci-ins article from way back t=
here, where they used a medical diathermy source (so they didn't hav=
e to figure that part out at all) and were mainly interested in cavity d=
esign. I just copied theirs more or less with some slight improvements =
of my own, and added the ECR magnets they forgot to put in there for bet=
ter performance, and indeed this is better than they claimed by a large =
factor. If I can believe my mass spectrometer it makes mostly atomic io=
ns, not D2+. I even see that with the mass spec fairly far away from it=
in the tank -- I see some D2+ too, but that would be about the amount f=
rom some hitting the walls and recombining on the way to the spectromete=
r.
I am debating whether to make a real long thread entry her=
e on "how to make a magnetron that was never meant to run CW do it =
anyway" but I'm having second thoughts as it would be pretty lo=
ng. You're not going to get there with either the panasonic mag=
gie (far too big) or that power supply (completely unsuited for this wor=
k on several levels). I am not sure either of the power supplies I'=
ve done this with would easily float up to HV, mine are operating near g=
round. They wouldn't go into a HV terminal easily at HV unless ther=
e's one heck of a fine isolation transformer, or a shaft drive/gener=
ator kind of thing around, these things take some real power input, abou=
t 80 or 90 watts to run.
Lutz, if you wanna email me, I can s=
end the sloppy version, I want the one I put up here to be pretty good f=
or beginners to follow.
But the key is this:
Find =
a magnetron tube about the *lowest* power you can find -- the panasonic =
one is a monster. We just picked a small one from dumpster microwave ov=
ens, but they're not expensive at the microwave repair stores either=
. You may need to try more than one anyway, as they mode-hop at low=
/medium powers, in and out of the resonance of the high Q cavity sometim=
es - a small one does this less as there's a lot of stored energy in=
the cavity to help keep it from doing that. You must run them very low=
power CW, as these have no facility to handle electron buildups and eve=
ntually you burn out the ends when the electrons spiral up and down to t=
hem, or in some, just build up in the tube cavity and eat RF power and s=
top all oscillations. A little tilt on the mag field stops that one, ea=
sy, and usually is there by "made sloppy and cheap" for free.<=
br /> You then need a "constant current" well filtered DC=
supply, at pretty low current, 10-20ma max. I've made two, one wit=
h standard parts from an oven (but I used a transformer at higher volts =
than required for this tube so I could afford to lose some in the filter=
ing) and one that uses my H bridge switcher. They both work fine. You =
will need a fan on the maggie, the filament power alone will get them ab=
ove max spec temperature without one. That's around 33 watts there.=
Then you put in maybe another 20-50 watts DC. Above that, things fail=
quick, and it's more than you need anyway.
It's a =
balancing act running this thing right at were it will oscillate at all,=
not mode hop, and not burn up, but once you find that point, it works e=
very time you flip the switch, very nice -- I've run it for many hou=
rs (maybe 100 or more, and many off on cycles), no problems. I am using=
a variac in mine with the standard xfrmr, set at about 89 volts....high=
er and it mode hops and becomes useless (and burns up) and lower it just=
won't oscillate.
To make a supply with a standard MOT yo=
u need to use a small series cap, on the order of 1/10 the one they norm=
ally supply (I am using a .1uf and it's still too big, about half th=
at would be better), a series R and another diode/cap (2 uf used here) o=
n the output to get decent DC. I am using about 2k series R there and h=
ave a neon bulb with it's own ballast across that as a simple curren=
t monitor to see if the thing is drawing some current and a rough indica=
tion how much. On peaks the drop across 2k/10w R is plenty to light it.=
Kind of surprised the board let me put up this 807kb file, b=
ut I copied and improved their cavity number 5 -- just a few helpful cha=
nges and builder tweaks to keep the arcing down and make it out of easy =
to get parts, mainly. Like I said, not many watts input are needed. I =
made a yoke and with magnets from amazing magnets to do the ECR part on =
this. I adjusted the field with a hall effect meter for the rignt numbe=
r in the middle, it goes up on both sides of that, but seems fine and wo=
rks fantastic -- I get the thing lit off, and it stays lit to e-6 mbar l=
evels -- very nice if you want to run low pressures and not need differe=
ntial pumping to keep the rest of your apparatus down there for long mea=
n free path.
Quartz tubing is a must for this. It works, bar=
ely, with pyrex, but needs higher powers, and basically it's the pyr=
ex getting very hot and needing a fan. The cavity (1" ID Cu pipe f=
rom the hardware store) gets warm, but only warm. I use thin wall 1/2&q=
uot; OD quartz for this. I let the gas in via a capillarly tubing in th=
e fitting on the outside the tank end.
First picture is the t=
hing in action, you can see it lighting up the gas. The picture isn'=
;t good enough to show that it's kind of pinched and expands away fr=
om the cavity. See fan on maggie tube, that's absolutely required. =
This is the business end of the tank, and I put that plexiglass shield =
on there to keep arcs from the main feedthrough down.
The nex=
t pic is the power supply I'm using. It's a standard MOT, with =
a wall wart glued on top for fan power. I have kept the original series=
cap on there in case I someday want a lot of power, but for now, it'=
;s not hooked up. You can see the end of the white series cap I am usin=
g, near the to-220 diodes I series-ed up to get high enough voltage for =
this (they are 1.5kv 6a diodes, takes 5 for this use). The big cap on t=
he back is the main filter cap, on which the charge is lethal, which is =
why it's buried in my rack -- safety, and no I'm not pulling it =
out of a working system for pictures!
Hope this helps. =
Don't tell everyone about the copyright violation that paper repres=
ents, just get it quick before we have to take it down.
Oh, f=
urther issues. I made the endcap on my lathe, and charge that with a po=
sitive voltage, and a aluminum tube was turned to fit into the quartz on=
the tank end, and extend some beyond this so any sputtering there doesn=
't make the quartz conductive. These are fed from a CCFL inverter w=
ith two half wave volt doublers for + and - polarity output, which is co=
ntrolled by an adjustable LM 317 on the input of the CCFL inverter. Wor=
ks like a champ, and lets you control the amount of ions injected very n=
icely and very fast.
In my recent record runs, I had to turn =
this off. The lm317 puts out enough with the adj terminal grounded to p=
ut in too many ions (about +/- 300 volts), and enough just drifted in to=
make that mode work. For other modes I use about +/- 3 kv or so, with =
about a 33k resistor on both leads for arc protection issues. Those bur=
n up before the rest does, kind of a fuse. |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by lutzho=
ffman on 2009-21-12 19:11
|
=
Great Reply, Thanks Doug
Ok, There is one thing that I jus=
t am not getting, sorry, I am not trying to be difficult:
If=
you feed the microwave ion source with 30Khz pulsed microwaves from an =
inverter microwave unit, instead of the normal CW microwave RF, would it=
not still function? Instead simply producing an ion beam which pulses a=
t 30KHz?
I am asking because Sulfur lamps for example are f=
ed with pulsed microwaves, and the discharge is not extinguished in betw=
een pulses or anything. This has me thinking that if a Sulfur lamp; Whic=
h is nothing but a hollow quartz ball filled with low pressure Argon wit=
h some elemental Sulfur, works just fine on pulsed microwaves. An ion so=
urce is basically the same thing, just with an added magnetic field, and=
an extraction electrode. Maybe a RF driven sulfur lamp could even be c=
onverted into an ion source, by drilling a gas/extraction hole, and add=
ing some magnets? The trouble is the sulfur lamps are hard to find in th=
e first place : ) This brings me to another idea, just for fun fill som=
e quartz balls with various elements, and energize with RF : ) A Rubid=
ium fill focusing on a Nd:YAG rod would be interesting. Anyway back to t=
he topic....
My thinking here is why not power the ion source=
with pulsed microwave RF, if it is fast enough to sustain the discharge=
? If an ion beam pulses at say 10-30KHz, then the beam current could be=
averaged out via a capacitor in the faraday cup circuit for an accurate=
reading. In other words it would not matter to me if my particle beam p=
ulses at 30KHz, as long as I get an high enough average beam current. Maybe the ion source could be used both ways: With CW RF for p=
recision work, and with pulsed microwave RF for brute force high beam cu=
rrent work?
Thank you for your thoughts, and for explaining t=
he concepts involved. Aloha.....Lutz : ) |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Doug Coulter=
a> on 2009-21-12 21:35
|
=
Yes, you'll get a pulsed beam with a 30khz component to it. It won&=
#39;t quite match the drive, as some ions get going fast enough to make =
more by collisions, the electrons will continue their cyclotron motion f=
or awhile, so there's a tail, the length of which depends on a lot o=
f things -- how much gas, volume, and so forth. If you use enough extra=
ction field, that may do some keep-alive as well.
In my case,=
I couldn't tolerate the resulting noises as I am studying time-dyna=
mics in the fusor just now, and didn't need an additional signal to =
have to deal with, or have affect the thing I'm looking at. All=
my phenomena don't mind the 2.45 ghz, that's too fast for ions =
to do much. And this thing btw makes so little uwave leakage a microwav=
e leakage detector that does register the oven I have, just sits on zero=
anywhere around it. Nice.
Sulfur lamps are designed to be e=
asy to light off (most lamps are ;~). This means that they can light of=
f on every cycle easy -- and they are dumping real significant power int=
o those anyway. They'd be using in the few torr region of gas to pr=
omote this, just like merc vapor bulbs do and for the same reason. Low =
pressure is a relative term, they are not low in terms of what you need =
for accelerator runs, but *many* orders of magnitude higher than that. For just making ions, you don't need and can't use hig=
h power -- things melt and so on. Note the fan they put on the one =
in the paper, for a 75 watt RF source. Mine doesn't need that and m=
akes plenty of ions, orders of magnitude more than you'll have the p=
ower to accelerate. Or I do, and I have kilowatts for that at much lowe=
r volts than your accelerator will need to be interesting. My sourc=
e puts out at least 10 ma as configured. That's more than plenty fo=
r an accelerator that would typically run in the small uA region at most=
.
If you try this at the half power minimum the panasonic get=
s in near CW, before it reverts to the same old on-off for a couple seco=
nds each, your stuff is going to go up in smoke in a rather spectacular =
fashion. Even the quartz will melt. That's ~5 times the max power =
used in the paper, btw.
The beauty of CW operation is that on=
ce lit, it will stay lit down to ridiculous low pressures, where over-si=
mple armchair theory says "no way, mean free path far too long"=
;. If you let the thing drop, it's hard to start again. Mine seems=
to take about 3-4 e-5 mbar or more to start, but will run way below tha=
t, but only CW or very fast at any rate -- I tried 70khz and that wasn=
39;t fast enough to keep the ions hot until the next pulse. It had to b=
e in a gas condition allowing restart every time, more pressure than I t=
hink you can tolerate. For a fusor, it's probably fine to pulse if =
it's fast -- it will start every cycle at those pressures, I am just=
looking at something special and don't want the distraction just no=
w.
You are going to find that to get high average beam curren=
t you'll need something more like DC, not a high peak to average rat=
io of pulses. The focus will change with instantaneous current due =
to space charge effects, and there will be a maximum you can keep focuse=
d to your specification, whatever that is. You don't want high ener=
gy particles to almost make it to the target, but hit the tube walls, fo=
r example. Ideal focus potential depends on current in ANY beam device,=
from CRT's on up. I've acquired a heck of a lot of literature =
on all this, and tested a lot of it -- they are not telling lies. Too b=
ad I can't upload the nearly 10 gb we've gotten hold of in fusor=
/fusion/beam technology papers and related stuff, it's quite the nic=
e library.
Remember, during off times, gas is still flowing i=
nto your accelerator, just not ionized. You're going to have enough=
troubles keeping that pressure down low enough to get a mean free path =
you need without letting in gas when it's not even getting ionized, =
and that's going to be kinda hard to pulse too at those rates. The =
big boys all use another vacuum system (called differential pumping whic=
h has nothing to do with diffusion pumps) pumping out a space between or=
ifices between the source and the accelerator -- the ions go right throu=
gh the holes when in focus, the other gas bounces around and gets remove=
d, mostly. If you're worried about what you'll need to put in t=
he HV terminal....well, a complete additional vac system sounds kind of =
troublesome to me. And no, a long quartz tube from ground won't do =
it as the gas in there will be highly easy to ionize at that pressure...=
.which is why it was AT that pressure to begin with.
In fact,=
I did all this due to my Scottish repugnance to the idea of another vac=
system being needed, and found a way to avoid that need, even for prett=
y low tank pressures. At that, it's quite cheap by comparison.
To be honest, it looks like the source Seltzmann is using might b=
e the better one for most fusors, it's certainly simpler to make. I=
doubt it lasts as long without maintenance as this one, just because th=
is one is pretty phenomenal in that regard -- it's just never failed=
in many many runtime hours, and no hint it's degrading in any way a=
t all. THAT is cool indeed. |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Doug Coulter=
a> on 2009-22-12 11:08
|
=
In answer to a question Roman asked about the transfer of power from the=
tube to the cavity:
It's not a wave guide, it's a sh=
ort (about 1/4 wave) piece of homebrew hardline coax. I don't want =
to tear this down just now for pictures, but I can describe it fairly we=
ll. A wave guide would have been quite large for this frequency (a coup=
le inches across) and had too low an E field in it, with a quarter wave =
cavity we can concentrate the E field far higher right where we want it.=
So this is all kind of a standard low frequency RF design, just real s=
mall. The cavity itself amounts to a 1/4 wave line with one end shorted=
and is on the large side for this frequency. Trying to go larger diame=
ter would allow other monkey modes to occur in it, rather than the longi=
tudinal one we want.
The top of the magnetron is the usual an=
tenna post, surrounded by the RF ground gasket weaving. I found a C=
u pipe adapter that fit that gasket, and was big enough to hold the ante=
nna post without clearances being too close. This adapter is brazed int=
o a piece of sheet metal so the big end protrudes about 1/8" inch, =
and bolting the sheet metal down presses it into the grounded gasket.
Into the top is inserted a piece of 1/2" Cu tubing. Here =
in the US we call it pipe if we're talking about specifying the insi=
de diameter and tubing if outside. Go figure, it's not a system I i=
nvented, just have to live with it.. This is all stuff you can get at L=
owes or the local equivalent. I get most things like this at McMaster. =
Sad that I had to buy a 25 foot roll of 1/2" tubing to get 2 inche=
s, but them's the breaks. A stock of various tubing sizes is su=
re handy around the lab anyway.
The inner conductor is a piec=
e of 1/8" tubing with about a 1/16" ID. One end has a washer =
brazed on it, which is then soldered to the end of the "antenna&quo=
t; on the tube, using a special flux for SS and nickel from McMaster aga=
in. The other end is drilled and tapped for a 4-40 screw, which is the =
adjustable length (for coupling to the cavity) part. This wound up more=
or less flush with the cavity wall, and the end was "rounded"=
or "melted" to eliminate sharp corners that caused arcing. T=
rying to tune this while in operation was a joke -- anything you put in =
the cavity burns up and mis tunes it, so we had to tune with power off, =
try, retune, etc. This coupling adjustment doesn't seem very critica=
l at all, since we're running *way* below the tube rated power, some=
reflected power is fine as long as nothing arcs and there's enough =
forward power coupled. This enters the cavity sort of tangentially like=
in the paper (the picture there is all I had to go by myself). Temptin=
g to try on the next one to place it closer to the "ground" en=
d of the cavity for lower impedance there, but works as it is, near the =
hot end just above the quartz tubing hole so it's coupling almost to=
the tip of the tuning 1/4 wave stub.
This tuning process was=
helped a lot by having a DC discharge in the ioniser quartz tubing -- w=
hich eventually became the pushme-pullyou ion extractor stuff. You then=
always have some light and can tune for "most smoke" more eas=
ily. Once it's all tuned, it stays that way really well, I've n=
ever re adjusted that parameter for any set of conditions -- it works fi=
ne, it ain't broke, and I ain't fixin it no more.
The=
cavity is about 2" long inside. It's a piece of 1" pipe =
(eg one inch ID). I made the top piece which is threaded for a 1/4-20 b=
rass tuning stub screw, and that's pretty thick in and outside the c=
avity so as to have plenty of thread length, it's about 1/2" or=
a little thicker, turned out of 1.25" Cu billet stock. I had to g=
rind off the threads and round the end of the screw near the quartz tube=
to cut down arcing there. I also put a spring under the screw head, as=
this tuning is very twitchy indeed -- the entire range might be 20 degr=
ees of turn on that screw.
Again, resonance was found by tuni=
ng this while there was already a discharge going on in the quartz tubin=
g -- it's easy to tell when you get it right that way. Every now an=
d then, a tiny jiggle on this screw helps the thing get going in high va=
cuum, other than that I never touch it anymore. That's good as in u=
se it's very near the HV and I'd rather not "go there"=
when in operation.
The cavity bottom is a pipe cap for that =
size, that I shortened before grooving it and the cavity tubing for the =
quartz to pass through -- you can take this off without breaking vacuum.=
I drilled and tapped both that cap for 4-40 and the cavity tubing at t=
he top for the same screw size so I can take all this apart. I should p=
robably just solder the top on now, as that joint is a high current node=
in this design and more conductivity is better, it's probably the m=
ain loss I now have.
Allow me to stress again that you NEED q=
uartz tubing. Yes, it will kind of work with pyrex, at about double the=
input RF power -- dicey due to arcs, and all that additional power goes=
into heating the pyrex, not good. As is, the cavity doesn't need a=
fan at all, barely gets hot, never hot enough to burn you. Yes, it'=
;s very clean inside, I did the usual anal prep there, and may silver pl=
ate it for fun. It works fine as is, however -- I'll plate it if it=
starts corroding in there.
A key is finding the "right&=
quot; magnetron. this is a smaller one from a junkyard oven. Some mag=
netrons do a thing we RF guys call mode hopping (I guess laser guys use =
the same term, and it's the same thing). There are usually 8 caviti=
es in a magnetron, and never are they all just the same, so sometimes on=
e or another will "take control" and it will then go at that f=
requency. If that's not the frequency this very high Q cavity happe=
ns to be tuned for, it stops working. Some of this is controlled by the=
amount of power input, so finding one that doesn't mode hop much at=
the desired power level is key. Having it be a smaller tube helps here=
too as the amount of stored energy in the cavity can help keep it from =
mode hopping better -- we're adding a ninth resonator, a better one =
than those in the tube, even under load.
It seems to be a luc=
k factor, but luckily magnetrons are cheap even bought new. The key, ag=
ain is getting a *small* one. We don't need the big power here at a=
ll, too much of a good thing is bad for this. Do remember if the power =
supply is remoted, even a couple feet, that these are 3.3v at 10 amp fil=
aments, you'll need some really fat wire for that. The tube I happe=
ned to pull off the shelf is a samsung OM52 series for what it's wor=
th -- a very used one. Since it worked fine, I've not run through a=
bunch of other types to test them for this, but since there's deman=
d, I'll probably wind up making a few more for other people. The quartz tubing enters the chamber through a viton o ring coupler =
I bought from Lesker, welded into a 1.33" CF flange (which was a tr=
ick indeed, but that's what I had on that end flange). That end=
has a 1.5" long piece of Al pipe in it, most of that turned down t=
o fit into the quartz, with about 1/4" extending from the end, so a=
ny sputtering that happens doesn't land on the tubing and make it co=
nductive. Al is good for not sputtering BTW as things go.
At=
the other end of the quartz is a fitting I made of brass that holds an =
O ring for sealing, and is bored for a soldered in piece of SS capillary=
tubing that is 1.5" long and .007" ID, to control gas flow. =
This one's a little short and fat for that, but one I made on anothe=
r source was 2" of .005" tubing and it was a little too restri=
ctive, to give an idea of the range that works. You have to low temp so=
lder this stuff as getting it hot enough to braze will cause some flakey=
oxide to form inside the tubing and jam it up, and it's a darn hard=
place to clean. The reason to have the gas flow orifice here is so tha=
t all the rest of the plumbing can run at or near atmospheric pressure, =
so all leaks are "out" rather than dirty shop air leaking &quo=
t;in". I have not noticed any sputtering off the end of the SS cap=
illary tubing, but hey, it's at the positive polarity side of things=
, so shouldn't sputter anyway. Theory does work now and then. =
Picture is a closeup of the homebrew hard line. As you can see, I=
had to make an adapter to make the 1/2" tubing fit into the pipe a=
dapter, and slotted the latter and put a clamp on it to hold things toge=
ther. I've never adjusted that length, it's fine just as it was=
put together. Drilling that hole in the side of the main cavity wa=
s actually the hardest part of all this, not that turning the endcap of =
OFHC copper was a breeze, at least that part was thick and sturdy. Pipe=
is another issue altogether, and needed a good jig to hold it and caref=
ul milling machine work with very light cuts with a 1/2" end mill t=
o get that off center oval hole made. I turned a piece of wood down to =
keep the vise from crushing the pipe while doing that. The end of the h=
ard line that has to be curved to match the cavity curve was done in sim=
ilar fashion on the mill, but really you could do that with a round file=
good enough without too much fuss.
Oh, there's nothing b=
ut air in there -- no dielectric at all other than 1.000000x air. &=
nbsp; |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by lutzho=
ffman on 2009-22-12 16:15
|
=
Thank you so much for taking the time to explain everything, now I am be=
gining to understand, at least enough to be willing to take a "wack=
at it".
Initialy I plan to run my accelerator at about=
150-200KV, this is without any SF-6 insulating gas, with the tube in de=
humidified air. The tube should handle this voltage in air at STP. At h=
is low energy level I would like to be able to crank up the beam current=
big time, up to a maximum of 5-10ma for short times. For most experimen=
ts 100-500uA should be fine.
The next step will be to ramp up=
the KV to the 400-500KV range, this could be done with an SF-6 gas mixt=
ure at atmospheric - 2 bar pressure, now the max. beam current would be =
about 2-5ma pulsed even for experiments requiring high power. For only 2=
-bar, the pressure the vessel can even be PVC irrigation pipe, with an e=
quipotential metal rings on the inside surface as well.
So in=
a nutshell the microwave ion source should work for me "as is"=
; just like Doug designed it. I was thinking about a "hybrid" =
design which I call the CAD Ion Source (Carl and Doug). I am wondering i=
f Carl's bottom end, and extractor, could be mated to Doug's mic=
rowave ion source. At this point I am not familiar with the bottom end o=
f Doug's ion source so it is to early to really consider anything li=
ke this.
One thing I did wonder about is Vicor high silica gl=
ass? I know quartz is probably the best as Doug pointed out, but if Vico=
r comes close enough, and if it is much superior to Pyrex, then it may b=
e worth consideration for some applications, since Vicor can be worked j=
ust like normal glass with oxy-propane. But then again it would probabl=
y be just as easy to buy a quartz test tube to begin with. Thanks......L=
utz |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Doug Coulter=
a> on 2009-22-12 16:44
|
=
Lutz, you may have focus troubles unless your volt gradient is WAY=
steep trying to run that current. You need about 60x the gradient as f=
or electrons for D ions for the same amount of beam blooming vs current.=
Depending, as always, what you call "focus". TV CRT tubes r=
un a few ma at 30kv or so.....you can work it out from there. This is b=
ecause the D ions are going slower, so for a given current and volts, th=
ere's more in flight next to one another than for electrons at the s=
ame volts and amps, and they have more time to repel one another on the =
way to the target.
Glass "alloys" are just about as=
diverse as for steel, it's astonishing how many there are, all char=
acterized in the Kohl book on "materials and techniques for vacuum =
devices". Or maybe even not all, but it's a heck of a loooonng=
chapter.
Vycor is one of "those", there are "=
flavors". The top grade is quartz, plain and simple -- no differen=
ce. Some is kind of porous, and I'd not use that for this, but =
you have to ask the source supplier.
The reason for this is q=
uartz is far lower loss than pyrex glass (DC and AC both), and more resi=
stant to being taken apart chemically by hot hydrogen ions. Alumina isn=
't even in the running for this, too lossy by far.
Here i=
s where we get ours, a small outfit, and nice to deal with too -- they l=
ike big orders, though, so people may want to get together and do a grou=
p purchase (I will do one for this area if people want me to -- I am usi=
ng the stuff up and want more myself).
http://www.quartz.com/
I =
also use their stuff to make my own HV feedthroughs, having gotten disgu=
sted with the commercial versions.
I work this with either a =
small oxy propane torch I made using a wire welder tip as a gas tip (wor=
ks great!) or an oxy-acetylene welder for bigger stuff. You need danger=
-sensing sunglasses as it has to be white hot indeed and is blinding to =
work on. The regular "didyium" glass blower's glasses are=
n't the thing here, they only block the Na lines and you're stil=
l blinded. Welding goggles are about right.
This design need=
s *none* of that, I guess I wasn't clear enough on that yet -- more =
pix to come. It's just a cut piece of tubing, easy to cut another s=
hould replacement be needed, on the lathe with a tool post grinder and d=
iamond wheel. Quartz you can't just score and break like glass. It=
comes apart at random if you try, and you might get cut while that expe=
nsive stuff gets ruined. I don't even flame polish the ends for thi=
s. |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by lutzho=
ffman on 2009-23-12 15:36
|
=
Hello:
Thank you so much for all your info on the ion source,=
I think I have enough to get going on it when the time comes. I checked=
with NEC reference the ion current which the tube can handle, and they =
put the max at around 25-50ma with the main limitation being the ion sou=
rce flooding the vacuum. Thank you so much for your help : ) I am curiou=
s about one remaining thing which is the extraction end of the microwave=
ion source? Is it the same as Carl's, or do you just have a small h=
ole at the other end of the quartz tube, with an intermediate extraction=
electrode close by at - potential? If I understand correctly this is w=
here the CFL inverter comes in?
An interesting feature of the=
tube is built in intermediate focusing electrodes which are located bet=
ween each accelerating section. Basicaly they are the same as the first =
elecrostatic focusing lens which accepts the ion beam entering the tube.=
This gives some control of the ion beam between sections. The entire as=
sembly contains 28 electrodes including the entrance, and the other 2 fo=
cusing electrodes. My plan is to drive the tube with a 14 stage FWCW mul=
tiplier, with 13 equipotential rings, the terminal being connected to th=
e 14th stage . I am considering two options on how to connect the tube t=
o the VM, the first is:
Every other electrode would be conne=
cted to the 14 stage VM directly , via a HV resistor. Then the electrode=
s in between these would be connected to the driven electrode below it, =
also via a different value HV resistor. The question here was do I conne=
ct the 14 intermediate electrodes to the VM connected electrode; Above, =
or Below it?
The second method being considered is to simply =
build a 28 stage resistive divider, connected at 28 points to the tube, =
and at 14 points to the VM stack?
The first method would seem=
to be more efficient, but the second would seem like it would have a be=
tter potential distibution. My plan is to build the first 6 stages, and =
then run the stack at a reduced voltage, and do a bunch of testing : ) The actual VM stack is fairly well protected with series resis=
tors (About 5-10K ohm combined value) between each stage capacitor, in e=
ach stack. This is in addition to all of the normal protective measures =
like spark gaps, input resistor ect. I am also using 3-4 times the nomal=
number of HV diodes, in the first, and in last stages of the VM (In pa=
rallel for higher surge current rating) since these diodes will see the =
full surge current if a discharge were to occur, which would only be lim=
ited by the series resistance of all of the 5-10K ohm protective resisto=
rs.
Thanks.....Lutz |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Doug Coulter=
a> on 2009-22-12 16:24
|
=
Here's the simpler of the two power supplies I've built to run t=
his. I did use a nominal 4.3 kv transformer to run a nominal 4.1kv tube=
, but this may not matter too much. This could be done a little cheaper=
than I did; this was all parts I had on hand anyway.
I didn&=
#39;t modify the transformer, knock out the saturation inducing cores, o=
r anything, just used it. In the actual build, I added a wall wart =
and 3 terminal regulator for fan power uses. The Variac is adjusted to =
about 89 to 91 volts rms output, or about the place where the transforme=
r itself begins to saturate with no load. You may be able to substitute=
a large incandescent lamp in series instead, but since I haven't tr=
ied that yet, I don't know what the wattage would be, probably on th=
e order of 250 w.
You might be able to make the 2k resistor s=
maller (as in 1k), I made it big so I could light that big neon bulb cur=
rent draw "analog meter" I used to monitor operation. In that=
huge neon bulb (got it at the last HEAS) you can see how much current b=
y how much of the big electrode lights up, it's quick to glance at a=
nd verify that things are on the bogey.
In use, this results =
in somewhat lowered filament power and terminal voltage. That's nee=
ded to not burn up the tube which was never designed to run CW -- normal=
ly the pulse mode lets extra electrons get out of the gap area naturally=
and not bombardment-heat the filament too much, in CW they can build up=
and make troubles -- a reason no one uses magnetrons for CW work, or ne=
arly no one. I have a 12 volt computer case fan strapped to the tube fo=
r cooling, and run another one across the general area to keep my HV fee=
dthrough cool as it's designed to couple heat from the grid out of t=
he tank (this will be another thread later). You *may* be able to run f=
ull line volts, but the transformer will then get hot, if you use a smal=
ler than .1uf series cap here to limit tube current. But these MOTs run=
in saturation (as a voltage regulation feature in a normal oven) and ge=
t quite hot if run for long times without a fan, or really even with one=
. Here, we just don't need all that. In this design, nothing gets =
hot.
I also made PS on a ferrite core, using my H bridge desi=
gn, but though it works, it's too complex for this, you just don'=
;t need it -- over engineering is as bad as not enough. And by the way,=
since those ferrites run at ~8v a turn, you have to then make a stepdow=
n transformer and actually rectify the filament voltage, as otherwise th=
e crummy lossy chokes inside the magnetron will burn up! Too much for s=
omething that is so simple, so I dropped that project. There's just=
no need for the complexity.
I will try to get cavity details=
posted soon, so post any questions you want answered, as that will be t=
he last post I make on this -- I will include the ECR magnet design in t=
hat one. My next version will have the cavity inside the tank on a 2.75=
" CF flange/pipe so the ions can get in there a lot easier, as only=
RF and tuning need to go through the wall, both of which should be fair=
ly easy (that is, if the type N weld in RF vacuum connectors I got from =
Lesker don't give arc troubles). In that design, there won't be=
the quartz tube, and the cavity bottom will be Cu or SS screen wire for=
the ions to come out, and the RF stay in. I am still pondering how I c=
an do adjustable ion extraction field on that one with least fuss. =
In my particular version of this, running up the input power makes=
it mode hop and go in and out of operation. This isn't a problem, =
as it has plenty of RF output well below that point. And of course ther=
e's that nice visual indication it's working. You have to see t=
his with neon, it's blindingly bright.
I seriously doubt =
that you can modify an inverter type oven supply to do this -- they only=
go to half power on inverter PWM, then revert to on-off operation for l=
ower powers -- I think the issue there is that the filament is run off t=
he same ferrite transformer and it gets flaky at low filament voltages. =
That's 5-10 times the needed RF for this, and too much is BAD for t=
his. You'd have to come up with some filament power some way, and f=
ool all the fancy feedback loops in that inverter -- I've torn one d=
own to see, then put it back together and sold the oven, it's just n=
ot worth it. They *are* great for cooking though. |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Doug Coulter=
a> on 2009-22-12 18:09
|
=
OK, here are the rest of the details needed to replicate this, on a virt=
ual silver platter.
The cavity is a piece of 1" ID coppe=
r pipe. It is 2" long, but the effective length is reduced a littl=
e by the tuning cap depth of .2365" sticking inside. The tuning st=
ub is a brass screw, 1/4-20 threaded into this to stick in .9860" f=
or resonance in my setup -- that should get you within one turn if you=
39;re using quartz tubing for the gas. The top cap is held in by ti=
ght fit and by 3 4-40 screws threaded into the pipe. There is a spring =
under the tuning screw head to get rid of backlash and so on. You will =
note the screw has it's threads taken off the end, and the end round=
ed to cut down arcing issues.
The bottom is a pipe cap, short=
ened and held on by tight fit and 3 4-40 screws threaded into it. <=
br />The gas inlet is a piece of 1/16 od, .007" id SS cap tubing, i=
n a brass fitting I turned down to fit, which seals to the quartz by a v=
iton o ring. The other O ring you see in the picture is just a cushion =
so I don't stress the quartz (it's rough end, I did not flame po=
lish it or even grind it flat) -- this makes it trivially easy to replac=
e if needed, but this one has run over 100 hours and is still pristine. =
This is charged to a positive voltage for ion extraction, via a CCFL in=
verter that has variable DC input to control the extraction voltage. Th=
is supply is bi-polar (two half wave volt doublers off the same source, =
opposite polarity), eg if it's making 1kv positive, it also makes 1k=
v negative for the other electrode. I use about a 20k 2w resistor for p=
rotecting this from arcs from the nearby HV tank input.
Not s=
hown in these pictures is a polycarbonate 1/4" thick shield I also =
use to keep arcs away from this.
The magnets are NdFeB from A=
mazing Magnets, with a half inch center hole. I used 1/2" by 1&quo=
t; to stick them to, drilled for a little over half inch to let the tubi=
ng pass, and 1/8" by 1" iron for the connecting yoke. You can=
see in the picture the shim I taped in there after measuring the field =
with a hall effect DC magnetometer to 980 gauss or thereabouts. The fie=
ld isn't very uniform, but seems fine, and really makes a *huge* dif=
ference in how well this works at very low pressures. Once lit off, I=
39;ve seen it still run down to 1.4 e-6 mbar, which happened to be the b=
ase pressure of the system at that time, due to air infiltration through=
the necessary insulating silicone tubing on the gas input. I may try a=
nother flavor for that piece, but it has to be there to run the gas inle=
t at high positive voltage to repel ions into the tank proper. With ion=
extraction volts at max, about 2.5 kv + and -, and a pressure anywhere =
above e-3 millibar, this lights off everytime just by turning it on. No=
adjustments needed.
Not shown, as the tank is under vacuum j=
ust now, is the other end of the tubing. At that end, there is a piece =
of 1/2' OD aluminum pipe that is turned down to fit inside, all but =
about 1/4" of it, and the end is about flush with the magnet closer=
to the tank. The other end projects about 1/4" into the tank past=
the end of the quartz tubing, so if it sputters, it won't sputter o=
nto the quartz. This is hooked to the negative polarity of the ion extr=
action supply through the same value protection ressistor, with a wire r=
un inside the tank and insulated with alumina beads -- you can see it in=
other pictures I've posted here on fusor operation, glowing red in =
X rays.
The hole you see in the cavity in the last picture is=
for tuning the coupling rod length. It's useless more or less, eve=
n a teflon tweaker mis tunes this so bad it's not funny, and burns u=
p in the bargain in the field. So that part was tuned with power off an=
d try a little this way and that until it seemed fine.
So, gr=
ab that paper linked above, and this thread, and doing this will get you=
a very versatile and reliable ion source of mostly monatomic ions for y=
our fusor or accelerator. As I said above, the next step after this is =
for the cavity to be in the tank itself so we maybe don't even need =
any explicit ion extraction field at all -- the grid field will probably=
do fine at that point.
So, that's it, any questions? =
 =
; |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Kevi=
n Dooley on 2010-05-01 12:15
|
=
Doug I have been studying your ion source, and am a bit puzzled by =
the apparent direction of the magnetic field and would like some clarifi=
cation please. It seems from the photographs and the arrangement of=
the yoke, that the flux is running parallel to the direction of ion flo=
w, is this interpretation correct? Thanks in advance and Regards:<=
br />-Kevin |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Doug Coulter=
a> on 2010-05-01 13:22
|
=
Yes, kind of. The gas and ions flow down the tube in general, but may e=
xecute some spirals along the way. That's not what's important.=
What is important is that the E field of the RF and the H field be 90 =
degrees to one another. The E field is across the tube diameter (loosel=
y speaking) and the H is along it.
The idea is we make a litt=
le cyclotron for any electrons that are free (lots, once it's lit of=
f) and whirl them around to bash gas molecules to break them up and knoc=
k off further electrons to get monatomic ions. A DC E field along the t=
ube length then pushes/pulls the ions into the tank proper. This is why=
it's called an ECR type source, which stands for Electron Cyclotron=
Resonance. Due to the electrons circling around, they have very long p=
aths and can be effective even at pressures where the mean free path is =
much larger than the tubing size -- because going in a circle, the elect=
ron paths can be very long indeed -- basically till they hit something. =
So this source works to pressures far lower than an oversimplified inte=
rpretation of Paschens's law would at first indicate. |
|
=
Subject Re: Inverter Microwave f=
or Ion Source RF
Posted by Kevi=
n Dooley on 2010-05-01 13:31
|
=
Thanks Doug. I did some calcs and found the cyclotron diameter to b=
e about 4.3 mm at 2.45 ghz.Edit: this calc needs to be re-checked!. Nice improvement to the ion sources given in the paper you referenced p=
reviously. Thanks again. Kevin.
Edit: ( still fumbl=
ing in the dark!) Clearly this calc only applies if there were no collis=
ions with other particles, the radius inside the wave guide will presuma=
bly always be less than this because of collisions and a forced rotation=
al frequency. |
|
|
|