So I started to think about a very simple penning (pig ion gun or pigatron) type ion gun:
The idea is to ionize the stream of gas flowing through the gun from left to right by the mess of electrons accelerated by the electric field and following spiral paths due to the axial magnetic field. This is cold cathode gun and operates on single low current HV supply. The gas inlet and extractor electrodes are grounded, making it very easy connect do the chamber. I decided to build a simple proof of concept prototype:
The parts of the PIGatron. The insulator is a plexiglas tube with a hole for exit of the anode wire. The extractor is made form a random KF25 fitting.
The assembled gas distributor. The small screw is for grounding connection.
Assembled core of the PIGatron. The end pieces and the anode wire are glued with low vapor pressure Loctite epoxy.
The complete assembly with the ring magnets.
PIGatron mounted on the vacuum chamber.
It’s alive! The bare clip is for grounding the gas distributor, the one with the insulating tube is for anode HV.
The view into the eye of the PIGatron. The bright core with diameter smaller than the aperture of the extractor is clearly visible.
To my surprise the thing worked from the first shot. I’ve not made any measurements yet, and for sure there are needed some improvements:
- The clearance between anode and grounded electrodes is too small. Very small variation of gas flow or voltage causes the discharge to stop or to arc between electrodes. But maybe after some time under vacuum it will improve, as the epoxy will outgas.
- The gas distributor holes are way to big (0.5mm), and probably to widely spaced. The flow adjustment would be much easier with smaller holes.
- The extractor hole should be also significantly smaller. The ions hitting the extractor will generate secondary electrons and improve ionization. And the extracted beam should be also less diffused.
- It will be nicer to have this device dismantlable (with o-rings and longitude bolts for example) instead epoxied.