Very interesting - I had not heard of this before. I see that the entry on the “dusty plasma” design says that the exhaust flow can be decelerated for power (presumably that applies to other designs as well.) It does not mention how the deceleration is performed (electrostatically, a bit like a reverse Van der Graaff particle accelerator?) or how efficient and compact this could be.

Wikipedia has a short list: https://en.wikipedia.org/wiki/Direct_energy_conversion