Inverter Microwave for Ion Source RF

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Kade
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Re: Inverter Microwave for Ion Source RF

Post by Kade » Tue Jan 05, 2010 6:15 pm

Doug
I have been studying your ion source, and am a bit puzzled by the apparent direction of the magnetic field and would like some clarification please.
It seems from the photographs and the arrangement of the yoke, that the flux is running parallel to the direction of ion flow, is this interpretation correct?
Thanks in advance and Regards:
-Kevin

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Doug Coulter
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Re: Inverter Microwave for Ion Source RF

Post by Doug Coulter » Tue Jan 05, 2010 7:22 pm

Yes, kind of. The gas and ions flow down the tube in general, but may execute some spirals along the way. That's not what's important. What is important is that the E field of the RF and the H field be 90 degrees to one another. The E field is across the tube diameter (loosely speaking) and the H is along it.

The idea is we make a little cyclotron for any electrons that are free (lots, once it's lit off) and whirl them around to bash gas molecules to break them up and knock off further electrons to get monatomic ions. A DC E field along the tube length then pushes/pulls the ions into the tank proper. This is why it's called an ECR type source, which stands for Electron Cyclotron Resonance. Due to the electrons circling around, they have very long paths and can be effective even at pressures where the mean free path is much larger than the tubing size -- because going in a circle, the electron paths can be very long indeed -- basically till they hit something. So this source works to pressures far lower than an oversimplified interpretation of Paschens's law would at first indicate.
Why guess when you can know? Measure!

Kade
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Re: Inverter Microwave for Ion Source RF

Post by Kade » Tue Jan 05, 2010 7:31 pm

Thanks Doug.
I did some calcs and found the cyclotron diameter to be about 4.3 mm at 2.45 ghz.Edit: this calc needs to be re-checked!.
Nice improvement to the ion sources given in the paper you referenced previously.
Thanks again.
Kevin.

Edit: ( still fumbling in the dark!) Clearly this calc only applies if there were no collisions with other particles, the radius inside the wave guide will presumably always be less than this because of collisions and a forced rotational frequency.

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