Anode layer ion source. Power supply construction.

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Post Reply
Andrew Seltzman
Posts: 747
Joined: Mon Feb 02, 2004 1:02 am
Real name: Andrew Seltzman

Anode layer ion source. Power supply construction.

Post by Andrew Seltzman » Sun Nov 29, 2009 8:23 pm

Initial testing of the anode layer ion sources on my fusor

proved that little current (about 1-2mA) on a single ion source will quickly saturate the fusor with ions and overload my 40kV-5mA spellman power supply that I use for testing purposes. For fusor use with a 20-30mA supply, no more then about 2mA anode current per ion source will be required.

This power supply will contain 4 EMCO F40 power supplies, which are proportional isolated DC-DC converters that will produce 0-4kV output at 2.5mA when 0-15V is supplied on the input. Each supply will drive one of the four injectors and will be independently controllable via the PIC micro controller using filtered PWM control. Each of the 4 channels will have independent voltage and current monitoring.

Andrew Seltzman
DSC05917.JPG (22.62 KiB) Viewed 1935 times
Andrew Seltzman

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA

Re: Anode layer ion source. Power supply construction.

Post by Doug Coulter » Mon Nov 30, 2009 2:26 am


What we've found here re ion sources is kind of interesting. They really help get things going for sure, and I'd not run without one had I a choice. But! Once things get rolling, the fusor itself makes a pretty good ionizer, and with my big tank it's "too good" mostly as it gets all the gas going, even in the apparent absence of any real amount of recirculation. We set ours so that at initial light off (maybe running a couple ma on the fusor) it changes things from zero to a couple ma, about what we're putting into the ion source. Then we let in more gas, and when we're running more like full currents (5-20ma) it makes no difference anymore when we turn it on and off.

Check my other recent posts on this.

One thing of interest I've noted, and this may be slightly off topic, but once our source is started, it will run down to ridiculous low pressures (1e-6 mbar and below). At that point I can run my mass spectrometer on it and look. Boy, do a whole junkload of other lines start showing up -- things that a chemist would say are impossible as any ion will stick to about anything and get into the mass spec before it can decompose into something stable -- a big forest lines you can turn on and off with the extraction electrodes over and over, kind of cool to watch and demo to people.

The main thing I see is that it really helps start things up when bringing the gas up from below the desired running conditions, but the negative R characteristic of glow discharges seems to overwhelm the effect once things get cooking. This is especially nasty in my huge tank as there's a lot of gas in there not in the fusor volume proper that can get ionized and eat energy doing so, and then makes the fusor current limit. This is a slow time constant thing you can watch happen once it gets really going.

Kind of like trying to fly an airplane with forward swept wings -- you start a climb, they flex up, that increases the climb real fast until a stall occurs. It's twitchy indeed.
Why guess when you can know? Measure!

Post Reply