Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: New Vacuum Chamber
Date: Aug 29, 9:44 am
Poster: Jim Lux

On Aug 29, 9:44 am, Jim Lux wrote:

Currently it is a 15 inch length of 9 inch diameter mild steel pipe, I have 8 hole flanges welded on

refined into a MK-IV fusor unit, also I am interested in fitting a magnetron from a microwave oven into the fusor and would like to hear any practical advice on the subject like suitable aiming angles, rf protection for the grid supplies (rf blocking inductors?) and whether it will have any effect as it is tuned for water frequencies not Hydrogen frequencies. I plan to have extensive shielding for the microwave unit but I understand the transformers in such units have one secondary leg connected to ground and am concerned as to the safety aspects of such an installation.

Iron and steel are quite lossy for RF, so you'll burn up a lot of your RF power heating the chamber walls. Perhaps if you copper or silver plate them? (watch out for outgassing from trapped plating chemicals).

You can couple the power in just about any old way if you just cut a waveguide port in the wall that matches that on the magnetron. Then, you'll need some stubs or irises in the cavity to tune it. Somewhat non trivial, but it is possible to calculate about what it should be, then you can twiddle screws to optimize it.

It's good that one leg of the transformer is grounded... makes them cheaper. The body of the tube is the anode and typically grounded. The average microwave oven uses a half wave doubler and puts a few kV on the cathode of the tube every half cycle. There are a number of microwave oven FAQs around that describe this in gory detail.

From a safety point, make sure that you aren't radiating into the big outdoors, and that your tube's waveguide is properly coupled to the chamber. If the windows in the chamber have wire screens properly bonded to the chamber, and you have bypassing/chokes on the wire feedthroughs, no appreciable RF will leak out. I would monitor the temperature of the magnetron. Too hot, and it will die...

Also a simple way of measuring approximate SWR into the chamber would be nice too

Measure the temperature of the magnetron. Reflected power will be absorbed in the magnetron heating it up. Compare the running temp of the magnetron into a good load and into your chamber.

A not so easy way would be to hook up a directional coupler, which at S band for a kilowatt is non-trivial, although certainly within the reach of some machining. You might be able to find surplus.

And, no discussion of microwave matching would be complete without the mention of a "slotted line" which also allows you to measure the VSWR.

> Regards
> Mark Harriss