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RF for new ion source

Posted: Mon Sep 01, 2008 7:15 am
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.


Re: RF for new ion source

Posted: Mon Sep 01, 2008 9:11 am
by John Futter

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

Re: RF for new ion source

Posted: Mon Sep 01, 2008 3:41 pm
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

Re: RF for new ion source

Posted: Mon Sep 01, 2008 7:58 pm
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...


Re: RF for new ion source

Posted: Tue Sep 02, 2008 3:08 am
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.

Re: RF for new ion source

Posted: Tue Sep 02, 2008 3:19 am
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.

Re: RF for new ion source

Posted: Tue Sep 02, 2008 5:54 am
by John Futter

what freq range was this unit originally made for ie 118-136Mhz or some other band of freq's

Re: RF for new ion source

Posted: Tue Sep 02, 2008 9:23 am
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.


Re: RF for new ion source

Posted: Tue Sep 02, 2008 4:00 pm
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

Re: RF for new ion source

Posted: Wed Sep 03, 2008 12:45 am
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?)