Converting Absorbed Radiation to Electricity

X-rays and microwaves that land on the wall become heat; a thermal or photovoltaic backstop can reclaim a slice of that as power.

The last recovery stage

Radiation that is not reflected back into the plasma is absorbed on cooled walls, where it becomes heat. Rather than reject all of it, the burner can place a recovery backstop on those walls: a thermionic or thermal converter that skims electricity from the elevated surface temperature, extracting a final slice before the remainder goes to cooling. This is the bottom of the cascade — lowest energy quality, smallest contribution, but still worth having.

Options for the backstop

• Thermionic: if the wall runs hot enough, the same emitter-collector physics used elsewhere applies here.
• Thermal: a small closed thermal loop could raise a working fluid — but this reintroduces cooling water, so it is used sparingly if at all.
• Photovoltaic-like: for specific radiation bands, direct photon-to-current conversion on a tuned absorber is conceivable.

Keeping the water story intact

The reason radiation recovery leans on thermionic or dry backstops rather than a steam loop is the water premise. Reintroducing a wet thermal cycle to chase a small slice of radiation energy would undo the whole siting advantage. So the backstop is chosen to add power without adding water demand, and whatever it cannot recover is dry-cooled as part of the small residual.

Honest scope

This is a minor contributor — radiation is a small and low-grade stream compared with the charged-particle energy the main train converts. It is included for completeness and to show that even the residual is treated deliberately. Like the rest of the train, it is a design-and-simulation concept pending the burner test program.