AFAIK, the fuel in the “production model” will be decaborane gas. It will decompose into 14 H and 10 B in the plasmoid. Decaborane liquifies at about 216°C, solidifies at ~97°C. IIRC, the chamber will hold a mild vacuum (plasma) at about 500°C.
Fouling should not be a problem as long as the temp is kept up, I’d guess.
