Probably wouldn't make much difference. Since the plasma is not in contact with the material, but is confined by a magnetic field, the drag coeficient of the construction material is not much of a concern. Buckyballs would be more resistant to damage, but I think the idea of a magnetically confined liquid would be better for that purpose.
There is some information about that at http://www.pppl.gov/projects/pages/cdxu.html [Broken] if you look in the middle of the page.
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