RF for new ion source

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Carl Willis
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RF for new ion source

Post by Carl Willis »

I am working on a new ion source (two identical ones, actually) to be implemented on my next fusor or other small accelerator projects. The design will use a magnetically-enhanced RF electrodeless discharge in a 3/4" Pyrex tube, and borrows heavily from Kiss and Koltay whose old paper is attached below.

There are many variants on the RF ion source, but what I favor in the Kiss version versus others, like the earlier Thonemann variant, is that the extraction is applied not across the discharge tube longitudinally, but via a negatively-charged puller electrode supported beneath the grounded mounting flange. In a fusor, such an arrangement means that particles enter the main discharge with an energy slightly lower than the main potential, and can be turned around at the walls of the fusor rather than run into them. My interpretation is centered on a 3/4" glass-to-metal adapter and a machined double-sided Conflat flange. More later on this.

Today I worked on an RF system to provide juice for the discharge at 200 MHz. I have a big old FAA amplifier, called an AM-6155, that I modified to suit the purpose. Some cathode bias resistors were removed, some grid bias components added, and I added my own shunt-feed components for the tube high voltage. Today it saw "first light," and produced 15-20 watts into a magnetically-coupled discharge in a 3/4" mercury vapor tube. Drive power into the AM-6155 was probably about half a watt. I intend to drive with 10 watts and extract at least 50 watts per each ion source in use. Unfortunately the amp has some funny behaviors that maybe the more RF-skilled folks will recognize and explain to me:

-Output power is low, but operating plate current is very high: in excess of 100 mA at a plate voltage of 2 kV. The idle plate current I have set with a zener on the grid to about 60 mA. (Another zener can be switched in at the front panel to nearly cut off the tube. All this works fine). What I don't understand is why I have so much plate current, clearly associated with drive power, and yet such a sorry power yield. I gotta figure out what's going on there.

-I can't tune this thing by watching for a plate current dip as I tune the plate resonator. Plate current is uniformly high no matter where the resonator is set. Maybe this problem is the same as the last one.

Pics show the setup causing a discharge in a mercury vapor lamp. The "antenna" is a single turn of wire, split opposite the coaxial feedpoint, with little capacitive tabs on the cut ends. Works great and a piece of cake to build. The other photo is of the amplifier per se, taken out of the power supply crate. The long pipe is the plate resonator. Tube is at left.

-Carl
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John Futter
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Re: RF for new ion source

Post by John Futter »

Carl

It appears that it is running in class A
don't expect to see a dip in plate current
if Class B/C
could be caused by grid /plate being way off tune

tube looks like 4CX125A or 4CX250A type please confirm?
I'm not familiar with the built up unit but if it is a lab type unit it could be class A to give the best stability into difficult loads

if this is the case then the max power out will be slightly less than 30% of max plate dissipation for the tube

hope this helps
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Re: RF for new ion source

Post by Frank Sanns »

Hi Carl. Nice progress.

I have a Henry 1 KW amplifier for 144 Mhz. The tuning procedure on it is very different than the conventional HF amp for example. As you said, the normal procedure is to adjust the plate and load for a marked dip in current and this is very close to maximizing output power. With the Henry, that proceedure will not work and the plate current can go through the roof with very little output power. This sounds like the situation that you are having.

It has been a while since I operated the amp so I can not remember the exact proceedure but I know for a fact that the output power had to be monitored for the "input" control setup. I think a nominal plate current is set and then the input control adjusted for maximum output into the proper load (50 ohms via an antenna tunner that was already tuned for a 1.1:1 VSWR). This control is then not touched again. The load then is adjusted for maximum output power and minium plate current. This is a very close setting on this particular amp. With less than 10% change in power, the plate current can swing from proper current load to completely off scale. The Q of the circuit must be very high with this amp for such response.

I don't know if this will help you with your amp but I know you know that this is not the "normal" way of setting a conventional amp but it was the ONLY way to properly set the Henry amp. Switching back and forth between plate and load will not work with amps like the Henrys unless you just happen to get lucky but even then you will not be optimized.

Frank Sanns
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Carl Willis
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Re: RF for new ion source

Post by Carl Willis »

Thanks John and Frank for your interest.

The tube is indeed similar to a 4CX250, but is not. The markings are worn off of it, except for a faint "EIMAC" that can still be seen on the cooler. It probably has nearly identical electrical behavior. It is configured here for linear service, Class AB1 according to the manual. As I said, I re-worked the bias circuit, which originally depended strongly on inputs from an air-band exciter that I don't have. The bias is now derived from a zener diode and trim pot.

My tuneup method is to apply a fraction of a watt of input power and tune the grid cap. I had to add capacitance across this to make the amp resonate at 200 MHz, but indeed I get a good resonance, at which point the plate current rises. The grid loading control has a minor effect on power output. I find a single resonance in the plate line, but I seem to be having a lot (hundreds) of watts going into heat in the tube and very little coming out the output line. Back to troubleshooting...

-Carl
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Re: RF for new ion source

Post by Dustinit »

I used to work on UHF 100W transmitters in the military and it looks
familiar but I'm sure I have never worked on that model.
I'm very familiar with the 4cx250B and if its similar it may have BeO.
It seems to be a PA for some exciter?
Is it possible its O/P impedance is not 50R? as some of these type
transmitters had power couplers. That may explain the low O/P pwr.
Dustin.
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Re: RF for new ion source

Post by Dustinit »

You don't say what you are driving it with
But it looks like a tunable cavity in the plate CCT,
These have a pretty good Q so you want low phase noise
in the drive cct to couple all the power out.
Hope this helps.
Dustin.
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Re: RF for new ion source

Post by John Futter »

Carl

what freq range was this unit originally made for ie 118-136Mhz or some other band of freq's
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Re: RF for new ion source

Post by Carl Willis »

Problem solved: incorrect tuning of the output resonator.

I am now getting ~80 watts out for about a watt in. The mercury-argon discharge tube in the photo lights from one end to the other and would melt if I ran it more than half a minute at a time. The wimpy RG-233 feedline gets warm. I can feel my hands heat up slightly when I bring them near the coupling coil on the discharge tube. Just wonderful.

So what was messed up? This amplifier (AM-6155) has a piston that switches the output resonator from "VHF" frequencies to "UHF" frequencies by shorting out a re-entrant part of the coaxial anode line (describing what happens mechanically is hard, but a schematic is attached...use your imagination). I figured 200 MHz would be covered by the "VHF" (extended) resonator position, especially because I found some sort of resonance in that position. The resonance I found must have been a spur or sub-harmonic coming through from the driver oscillator at low power. Anyway, the correct setup for 200 MHz is evidently to set the cavity piston to "UHF," whereupon tuning and loading are textbook easy, and I can pull out 80 watts with 130 mA plate current (this is about 30% efficiency).

To answer John's last question, these units were designed for the aviation bands: ~108-130 MHz ("VHF") and ~230-350 MHz ("UHF").

Thanks for your contributions. Anyone wanting such an amp can find them at hamfests for under $100 (sometimes $50, if like mine they've been roughed up), with the tube and everything else. Fair Radio Sales of Lima, Ohio has them for about $300. Good cheap tunable VHF powerplant.

-Carl
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Re: RF for new ion source

Post by Richard Hull »

Thanks for this fine ion opitcs exciter supply posting. I've marked it for future reference should I go to formal ion sourcing.

Richard Hull
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Re: RF for new ion source

Post by Dustinit »

Good work carl,
Im having trouble visualising your coupling to the tube
Is it capacitive across the tube or inductive loop around the tube?
Could you post a picture?
It seems to have very good coupling and its interesting that the whole tube
lights up - are you using the case ground as rf return (the tube seems to be touching the generator case?)
Dustin.
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Re: RF for new ion source

Post by Carl Willis »

I have a "real" discharge coupler put together now, and another eye-candy discharge pic. Dustin asked about the coupling, and previously it just consisted of a piece of loosely coiled hookup wire. Not particularly effective. Now it is a well-constructed piece of hardware, some kind of pi network or L network depending on how you choose to analyze it.

The coupling inductor can be a single turn or hairpin of copper sheet as shown, about 5" long; or may alternately be 1-3 turns of wire at ~1" diameter or less. The tuning capacitor typically develops very high voltage and carries high current--it needs to be made well and have some space between the plates. The input loading capacitor is not as critical. A choke is shown around the braid of the cable leading up to the coupler. I found that the coupler could be tuned to positions where a lot of sauce got coupled onto the outside of the coax, resulting in very erratic, irrepeatable performance. The choke is a low-frequency ferrite transformer core in my case. It essentially kills the problem.

In the photo is an inductively-coupled, electrodeless air discharge, probably about 0.1 torr. The inductive hairpin can be seen as well as its clear relationship to the bright inductive plasma. The SWR was 2:1 and the forward power about 60 watts.

On another note, my ion sources are almost done. Some details on those coming shortly.

-Carl
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Re: RF for new ion source

Post by Dustinit »

That looks cool.
Can you wrap the inductor around the glass
and still match to it?
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Re: RF for new ion source (NEW DRIVER AMP)

Post by Carl Willis »

My most recent post about this ion source itself was here:

viewtopic.php?f=6&t=2910#p12420

I just added a new amplifier stage to drive the AM-6155. I had been driving it with an old Mini-Circuits ZHL-2, which only managed about a watt output, max. This got me a maximum output of ~75 W from the AM-6155, which was running in Class-A, linear mode in order to enable that kind of output. The new stage is a broadband, linear amplifier good for about 10 watts out. It uses a Polyfet SQ202, a push-pull MOSFET amp, in the test circuit described in the manufacturer's application note here:

http://www.polyfet.com/tbplt/tb165.pdf

I did not make the circuit board itself--that was made by Jim Potter, who builds the RF hardware for the accelerator company I work for. These amps are a standard part of the low-level signal train for what ultimately is a ~hundred kilowatt power plant.

The amp I built, shown in the photo, had over 15 dB small-signal gain all the way from 30-500 MHz when I checked it on the network analyzer at work. At 200 MHz, it was 15.3 dB. When I got home, I tried running the AM-6155 with it. Even with the amp in "tune" mode, with the grid bias probably putting it in Class C, I can get close to 200 W out with ~6 W out of the SQ202 driver. Flipping the bias switch back to the "operate" setting for a second appears to get about 0.5 kW, at a great expense in terms of efficiency. I know some in the ham radio community like to push this amplifier, but I will be running it CW and don't need all that sauce. I'm happy with 200 W and the improved efficiency of a less linear bias point.

That's all for tonight!

-Carl
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Re: RF for new ion source

Post by steve_rb »

Hey Carl,

This looks great. I am very interested to try this. It looks very simple and exactly what I was looking for to get a sence of a plasma as starting step for building an ICP ion source myself . Could you list exact parts specifications along with key assembling instruction steps.

Steve Robinson
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Re: RF for new ion source

Post by Carl Willis »

Hi Steve,

Thanks for your interest. The most detailed information I have is probably in the top drawing shown here: http://carlwillis.wordpress.com/2009/02 ... on-source/

You'll have to adapt the idea to your own fabrication capabilities and make your own decisions about how to build it. But the entire design principle is simplicity and non-precision construction. This ion source seems to be able to do about 1 mA, tops, in its current configuration and drive level. The copper parts sputter quite a bit and copper is also known to promote the formation of molecular ions that are undesirable for nuclear reactions; if I built this again I would replace all copper parts with a non-magnetic stainless steel. You might also check out Andrew Seltzman's posts describing his ECR source or his more recent magnetically-enhanced DC ion source for different ideas.

-Carl
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Re: RF for new ion source

Post by Doug Coulter »

Carl,
Nice work! The RF design here reminds me a bit of swatting a fly with an elephant, but it will work (obviously). Of course, that's because my own source is a much simpler one based on a magnetron from an oven ;~). I run that with about 20-50 watts *input* (not counting filament power, which is 30w).

Class C operation will also cut your cooling and plate draw needs, but will need significantly more grid drive to get to.

You'll find this works much better with quartz tubing than pyrex, the loss of which is getting to be quite high by 200 mhz (much less the 2.45 ghz I run). Changing to quartz on my setup made a world of difference for the better -- it runs for hours now and barely gets warm. If you need quartz tubing, get in touch, I've got a stock in various sizes, or can direct you to where I get it.

I also had copper sputtering troubles with mine (the extraction electrode of course), and have switched to something that doesn't sputter as well. I'm now using magnesium, but Al would probably be nearly as good, based on sputter rates from the old "procedures" book. At any rate, it's easy enough to clean off -- gun cleaning supplies are cheap and they specialize in copper removal chemistry. (ammonia or acetic acid work, but the pro copper removers add some things that work even better). With stainless, it will take longer to sputter to failure, but...will be lots harder to get off to fix it too.

I find that in my ion sources, an ECR magnet tuned right is worth quite a lot. For the same everything else I can run 2 orders magnitude lower pressure, or at the same pressure, *far* less power -- which in turn helps with sputtering, heat and all the rest. I've not yet made the promised measurements on atomic vs diatomic ions, but I will, and various literature says go ahead and do the magnet thing, it's better. In my case, I made a yoke that fits outside the rest with magnets on it, and one pole piece slides to adjust the field. It's not real critical, but moderately so. I happen to have gotten a magnetometer to measure this, and found that the simulations and other online resources are a bit optimistic about the net field you're going to get where it counts (eg in the middle of the gas tube) -- they tend to be off by about 20%, and that's more error than this process tolerates. In that range, though, you can just make a thing where you can slide one pole piece back and forth, and you quickly find the sweet spot. In your case, you'll need a mag field 200/2450 of what I'm using (about 980 gauss), which will be almost too easy to get -- and stray lab fields will mess you up somewhat if you're not careful about that.
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