Constantly being ionized and exposed to repeated detonations, confined only by a magfield, wouldn't it still leak out into space and need to be continuously replenished?

Also, what is the spectrum of the energy released by the nuclear explosions? Only a small amount of the total energy would go into EMP, the rest (electromagnetic radiation, reaction products, unconsumed fuel, etc), would be lost, unavailable for momentum transfer).

In the Orion "pusher" concept, you have an analogue of a combustion chamber, like a rocket engine, nothing leaks out except normal exhaust. Not so here.

I'm not giving you a hard time, I am genuinely interested in this idea and woul like to know more about it.