First Wall Conference

[MSimon]

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First Wall Conference

This is supposed to be the permalink:

http://link.aip.org/link/?PHPAEN/15/058303/1

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Interactions between plasmas and their surrounding materials (plasma-facing components) are of great interest to present and future magnetic fusion experiments, and ITER [ITER Physics Basis Editors, ITER Physics Expert Group Chairs, ITER Joint Central Team, and Physics Integration Unit, Nucl. Fusion 39, 2137 (1999)] in particular. This interest is the result of concerns with the survivability of these materials, as well as the impact of these interactions back on the plasma. These interactions begin on the surface, but can have consequences a few microns into the material. This mini-conference on these “first microns” was designed to bring to the Division of Plasma Physics Meeting experts on these topics who would otherwise not attend. At the same time, the mini-conference was intended to expose the broader fusion community to these issues. The mini-conference covered in three half-day sessions the topics of lithium coatings and surfaces, mixed materials characteristics, and issues associated with graphite. ©2008 American Institute of Physics

[TallDave]

Yeah, that's going to be useful to everyone.

[jmc]

That's one thing that concerns me about cusp machines, their are lines heading from the central reaction region directly into material surfaces, this means unless you are very careful, the ions incident on materials will be of energies 100's of Kev rather than 100's of eV

[MSimon]

jmc,

If the ions are generated inside the reaction zone their energies should be somewhat less than the drive voltage available.

You are then left with stalk losses. Those might be reduced with focusing electrodes/grids.

[jmc]

The stalk losses, will still be at full core fusion energy won't they? This would present a sputtering problem, it is concieveable you could do clever things to reduce them but it won't be easy, because electrons are coming out of the stalks aswell, change the stakls positive to repel the ions: you attract more electrons, charge them negative to rel the electrons and you attract more ions.

[MSimon]

In theory the ions should be confined to the reaction zone.

Electrons should then be the only thing you would have to worry about re: stalks.

[jmc]

But the potential along any fieldline is roughly constant due to the fact that electrons are highly mobile, therefore the cusps will not just be cusps of magnetic field, they will also be cusps of low potential, this should allow some ions to penetrate to them. I think this is what Dr. Bussard was refering to when he spoke of the 'ion holes' in WB5

[MSimon]

Do you have a reference to Dr. B's discussion of ion holes? Or if it is in the Google Video an approximate time.

I'll review it and get back to you.

In theory ions from the reaction space that get near the cusps should be at low energies (assuming they have not been sufficiently upscattered).

[jmc]

http://www.askmar.com/ConferenceNotes/2 ... 0Paper.pdf

page 11

"These losses were extensive, and attempts to reduce
them by use of floating ceramic repellers placed along about
1/2 of the seam lines reduced e-losses by 2.5x but only at the
price of opening up huge loss areas for trapped ions."

I think the actual phrase 'ion holes' might be in the video, but I can’t get the pdf.

Here is a reference for a fusion technology paper which says that field lines have a strong tendency to be regions of equipotential:

Ion defocusing in multicusp plasma confinement systems
Author Dolan T J

Fusion Technology, vol.24 no.1 August 1993 p.127. Journal Article

[MSimon]

jmc,

Thanks!

I think Dr. B. makes the point that the high losses were specific to a closed box machine. My description was related to an open box (electron recirculation) machine.