Microwave Saturation ???

[guest]

I read in an article recently that in one of the larger Tokamek systems someone saturated the plasma with microwave energy from a common oven magnetron. Per the article, this increased the poisser output to a much higher level. I have played around with the MW plasmoid effect but never gave much thought about how it would react in a fusion chamber.

Can anyone expand on this phenomenon ? Has anyone tried this ?

[guest]

Mark,

The reason microwaves are used in the tokamak is to increase temperature. The method they use relies on generating more and more heat to eventually cause fusion. You see that this causes tremendous loses in the system which they hope can be made up for with a sustained reaction. Which of course to date hasn't happened beyond a few seconds.

As for using microwaves to increase the reaction in the fusor.... I'd don't see how increasing heat would benefit the reaction. If anything it would probably be detremental to ion circulation in my opinion.

Lee

[teslapark]

There is a short thread in the "Fusion History" forum titled "Aussie Fusor" which briefly touches using RF excitation in a fusor. There is also a link in there to a fusor that uses a 13GHz RF source to create a plasma in region between the inner and outer grid.

They are using a three ring inner grid and did some rather nifty field plots that predict the potential over a 2D area. They have one or two papers on-line. I haven't read the whole thing, but it looks like they are specifically trying to make plasma near the outer grid, which is where is really needs to be. Ions that form near the outer grid fall through a full potential well and collide in the poissor with maximum energy. This, of course, is different from a tokamak which bulk heats.

With moderate effort, one could probably get an oven magnetron to help ionize the entire fusor volume, but concentrating it near the outer grid would be a little tricky. So far no amateurs that I know of have attempted anything like this.

Adam Parker

[guest]

Back when I had my fusor running I did attempt to use an old microwave oven magnetron as a source of chamber ionisation. The only noticable improvement was that the glow could be maintained at lower pressures. As far as I could tell my setup was way too simple to do anything more- it was also way to dangerous to play with for an extended period of time!

[guest]

Ben, in reference to the "glow", would this include the star mode ?

Also, how did you inject the microwaves into the chamber ?

[jim-frank]

One way to use microwaves that might be beneficial would be to mount an oven magnetron directly through the wall of the vacuum chamber. A waveguide could be constructed of mesh material (such as the screen on an oven door) which would wrap an appropriate number of wavelengths around the inside of the chamber. The ions formed by the microwaves inside the waveguide would then be able to move through the perforations on the inner facing side and be accelerated to the poissor region. An appropriate inside dimension for such a waveguide would be 86 by 43 mm, the size of WR-340. It might be informative to make such a device and see what happens.

[guest]

You guys be carefull out there oven magnetron's microwaves can cook your butt and you would not know it till you dropped dead.
Only the coroner would be able to figure it out.
Microwaves at short range are especially nasty.
I worked with very high power microwave jammers in the service. They had the ability to cook a frozen pot pie at 200 yards. It takes much less exposure to cause tissue damage way under 1 watt! The Navy's had pretty bad accidents with high power search radar.
Your microwave ovens run at 600 watts up to 1500 watts.... that is a peace time application of military hardware. Search radars run about 1500 watts normally.
Microwaves take no prisoners.

Larry Leins
EX USAF PMEL TECH
Physics Teacher

[guest]

Jim, The system that you describe is what I am currently working on. I am hoping that by summer I will be able to post some test results.

I am still in the design phase of the MW injection system. I cant decide between a modified glass viewport w/wave guide or putting the entire magnetron in the vacuum chamber. The latter idea allows for a much more efficient saturation effect. I am just a bit worried about two things. 1) Cooling of the 1100 watt magnetron. #2) How the magnetron will react in a 10 micron or lower vacuum. Chamber size is really not a problem as I currently have two to choose from(one with ample space).

Anyway, I guess time and money will tell.....

[guest]

Larry's words of caution are timely and appropriate. Any magnetron stuff that I experiment with is tested with a simple microwave field strength indicator composed of a couple of fast diodes and a small mica capacitor on a 50 uA meter. This simple device is sensitive enough to indicate an unsafe condition, and will show hazardous areas around microwave transmitters. With all the voltages, x-rays, implosion hazards and such that this group deals with routinely, I sort of figured that microwave safety would be a given...
The idea of putting the entire magnetron inside the fusor would not probably be too good- cooling is a major issue, and the magnets on the beastie will play hob with ion and electron trajectories. It might be better to couple the energy into, say, a flexible waveguide, then use an antenna type coupler between outside and inside the fusor. Look into the feed horn of an LNA on a satellite dish to see what I'm thinking.

Jim Frank

[Richard Hull]

Indeed, Microwave ionization is a fantastic idea, but requires a bit of expertize in application. I like the idea of the screen at the end of the waveguide flush with the anode shell. (enclosed, no leak cavity).

I am concerned though that there will be little electrostatic impetus for the ions to leave though. (perhaps a kinetic one though) A magnetron on a waveguide opening freely into the chamber might be best, but tuning the line to match the fusor cavity might be a bear. Too much reflected energy and you could heat the guide or smoke the maggey. Just remember, what you are playing with is not far removed from black magic and is very dangerous. Any view ports could be death ports without special, nearly opaque screening. Lots of gotcha's to consider.

Richard Hull

[guest]

Hmm- looks like you're right about little E-field impetus. I ran a simulation with Quickfield, using the biggest holes that will still feasibly contain the microwave field, and there is very little electrical field penetration into the waveguide. An approach that would work would be to make the waveguide into a multi-slot antenna, which would couple energy into the plasma and match to the magnetron, but then you'd have to isolate all of the feedthroughs for microwaves, shield the viewports, etc. Might still be worth trying, but not as easy as I first imagined. It would be no mean feat to machine accurate slots in the inside curvature of a waveguide bent into a circle.
Jim Frank

[guest]

This is all good stuff and I plan on doing it. However, I still have to build a neutron producing fusor first. After that, its time for some serious experimentation.

I really hope people arent just building these things and saying, "Ahhh, I built a fusor....Now Im done."

It is my firm belief that these things need to poked, prodded, tweeked, overpowered, underpowered, zapped with pulse discharge caps, microwaved, x-rayed, lased, heated up, cooled down, frozen and anything else one can imagine(keeping all safety precautions in mind). Sooner or later the right combination will be discovered. That is ONLY if post fusor construction experimentation takes place.

Mark Rowley
Garage Scientist (Unlicensed)

[guest]

Like all fields of endeaver you will find someone has been there before you. It seems that the German Tech Types allready have the answer you seek. It can be found in any ion source book printed after 1998 under rf and microwave ion sources. It seems that the Droids at CERN use good old German Know How when it comes to feeding its collossal accelerator. It seems that the German Technical Institute did fusion research on its own hook. The small scale effort sought to acheive fusion by electron cyclotronic resonance using "cooking magnitrons as the power source".(Protos I) Their 1500 watt device was a dud as fusion machine but a great ion source...... go figure.

[guest]

Microwaves for ion generation have been quite well researched for sputtering and CVD applications. Those were some of the resources I had in mind when I posted my ideas.
JIm Frank

[guest]

I wanted the people who feel like I do...
when you ski down a waterfall have a first aid kit ready.
But I felt that after warning them give them data.
The beauty of living in interconnected times is the interplay among all fields of technology at play here.

Larry Leins
Physics Teacher