We are talking about different things. The walls of the MHD throat have to be kept as clean and clear as possible. Thus the film-belt has to interfere with the E and M fields as little as possible, and yet remove the gunk that gets blown sideways onto the walls, as well as remove some of the heat from the throat.
This is sort of what my body is going thru now : ulcers(erosians in esophagus)and congestive heart failure(fluid in lungs) : a week in the hospital.
I’m sorry to hear of your illness. I hope it’s just temporary.......As for the gunk build-up, is there no coating that can withstand the pressures and temps? Diamond coatings are now used for high temp substrate in military ICs, and I believe it’s an INSULATOR (kinda has to be for ICs).........