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Re: Microwave TC

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 02:08 PM 6/26/2005, you wrote: 
Original poster: Steve Conner <steve@xxxxxxxxxxxx>


if you forget to put a resistive load
inside a microwave oven, then you've got a high-Q, high frequency, high
voltage resonator driven by about a kilowatt RF source at the same
frequency.   It's 2.5GHz Tesla coil.

I always wondered about this. Could you make a 2.5GHz VTTC by fixing a breakout wire to the antenna on a MO magnetron, choosing the wire length for a voltage maximum at the tip? Of course for safety you would have to enclose it all in a faraday cage (such as the oven it came with)

But then according to this poster, you can make one by just leaving the glass plate out and putting a fork in the oven ;)


There's a bit of a difference here. In the TC case, the electrode would need to be the appropriate length so that it is resonant. In the fork case, it's more likely just a field concentration with a small radius of curvature.

Either way, high power breakdown at microwave is a fairly well studied phenomenon. The folks that design that magnetron, for instance, need to make sure that they don't have a standing wave.

We do testing at work on just this sort of thing, for radio equipment that's going to be flying to Mars (which has an atmosphere that's largely CO2 at a few torr, about where the breakdown field is minimum, and just to make things more exciting, there's a goodly component of Argon in the Martian atmosphere). We put the test article in a vacuum chamber, pump it down, and run RF through it and see if it breaks down (i.e. sparks). You can detect the breakdown either by reflected power, or by optical means (e.g. the sparks give off light), or by looking at it after the test for burned/melted/pitted components.

JPL has one of the larger clear bell jars (6 feet in diameter) in a chamber lined with RF absorber for this sort of test, so we can put a whole antenna into the bell jar and test it at full power (or, more likely, test it at 4 times the expected full power).

At higher frequencies, in vacuum, there's another breakdown phenomenon called multipaction that you have to worry about too.

For instance, the Mars rovers have a UHF antenna (at about 400 MHz) that potentially has breakdown problems at only 15W of power, so we're not talking about multimegawatt radars here. When you start looking at things like bandpass filters with reasonably high Qs, you also run into problems that any VTTC or SSTC afficionado would be familiar with, because, with resonant rise, and small parts, you can get pretty high field strengths.

Hey folks, designing high power RF to operate on Mars, or in a vacuum, is still a tricky thing. It's hardly a cookbook operation. The whole physics of breakdown in low pressures or vacuum, especially with odd mixtures of gases is not very well understood, nor is there good empirical test data that's applicable to arbitrary configurations. Paschen tested CO2, so we've got that data, but, darn it, he didn't also test mixtures of CO2 and Argon. Other gas mixtures also have data: CO2 and Nitrogen, for instance, and all manner of combinations of Nitrogen and freons, and combinations with SF6.

If someone wants a science project, senior project, master's thesis topic, etc. that has real scientific value, they could do some characterization of the breakdown characteristics of low pressure mixtures of CO2 and Argon in varying pressures and compositions. Most useful would be to have data for DC and for RF at various frequencies... So really, running tests with a TC in a bell jar full of "Mars Gas" would be quite useful, and would be worthy of publication.





Steve Conner


Anyone who's interested in doing tests like this, feel free to email me or call. (Or, if you really want to run a TC, Terry will probably let the discussion go on the list). Heck, a good proposal might get you a job here at the lab. (Although that's not something I can do anything about.)

James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875