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.

Boron fouling might be an issue for the chamber walls, electrodes and insulator. Once the decaborane is converted to a plasma, it will decompose. People observe this in methane PFs. The chemical bonds break in the high temperature, high density plasma. There is no reason for the carbon to grab back four hydrogen atoms. Decaborane will be the same. Boron could coat every surface or form a debris dust. Devices that fire many shots observe dust formation in the vacuum chamber as electrode and insulator material is converted to plasma and then solidifies from the plasma as it cools. Even if the chamber is held at temperature, the boron will still plate out unless there is chemistry that demands for the formation of a gaseous form or boron. I don’t know the answer for sure but I doubt it. If boron readily formed a gas with hydrogen the tokamek guys would already know about it. It was common practice to coat the inside of tokamek with boron for a time.

I find it hard to believe that the fuel will be evacuated between shots if the repetition rate is much greater than 1 Hz. Continuous feed and pump is done for inert gases for high repetition rate PF devices in lithography. I’d imagine a similar strategy would be implemented at the power plant level. Most of the fuel gas could be recycled. It would be interesting to fire a few shots and see how much the gas pressure drops for gases that chemically react and are likely to have solid products. Methane or acetylene seem like good choices as carbon is inert but very messy in terms of vacuum as carbon holds onto everything. Black powder all over the place isn’t very appealing either.

This is a difficult problem. It will take some creative solutions to keep everything working correctly.

Boron can be made conducting as well so same problem as carbon. It can gum up the insulator and ruin the breakdown along the insulator. Any gas that can decompose into a solid has the potential to gum up the process. This is a common problem in the semiconductor industry with silane and large molecules that contain silicon. I think you will get some shots before it becomes a problem but operating at steady state could be challenging. High temperature will only improve the bonding of boron atoms and grow dense thin films.

I would think that any material coming from a plasma would have an electrostatic charge for a short time. Maybe an electrostatic dust collector inside the chamber could collect the boron dust. It would be interesting to try it with acetylene first to see if carbon dust could be collected that way.

Maybe low pressure oxygen could be admitted to the chamber and the plasma system fired a few times. The highly reactive oxygen plasma should oxidize the boron into B2O3. Boron oxides may chemically combine with the insulator. I don’t know if that would be an improvement or not.
It would be interesting to try an oxygen plasma on a carbon coating to see the effect.

Oxygen might not be aggressive enough. The beauty of carbon is oxygen turns carbon into CO2 which is easily pumped from the chamber. Boron forms a stable solid oxide. Fluorine can form a boron gas but it is highly toxic and fluorine attacks almost everything. The key question is the etch rate vs the deposition rate. Fluorine is commonly used in semiconductor chamber cleaning so the recipe exists. An question to pose would be using a mixture of BF3 (toxic boron gas) and hydrogen as fuel. The fluorine would attack the electrodes, but it would also keep the boron in gas form. I guess it would be a trade off in which gets you first, boron build up or electrode/chamber erosion.