"Multiplier" implies an output greater than the sum of the inputs, which is impossible. Perhaps it is just a misnomer.
Nope, not a misnomer. Stellar radiation, upon striking aluminium, radiates 2-3x more than input. Nasa had an engine designed, for satellites and interplanetary observatories, that used a radiation multiplier propulsion engine (which produced 8x output), through stages I believe.
That's interesting. But what happens to the aluminium? Does it degrade over time as I expect the energy must come somewhere, ie. the chemical bonds or something.