vansig wrote:

Let’s say you design your system to go after particles of 65Kev and were able to capture all the energy from them. Any particles of energy less than that would be decelerated, stopped and reversed by the induced magnetic field. Accelerating them back into the reaction chamber and taking some energy with them. Any particles faster than that will still have their residual velocity (the amount in excess of 65Kev ) when they reach the helium catchment container (Which thus may need cooling.)

Wont the speediest ones come out, first?
Let’s design the system to allow the particle velocities to stratify.

The effective coil length would then change through the duration of the exit beam.

What I wrote is, of course, a bit of a simplification. The field strength of the coil is not constant, but will increase as particles deposit their energy there. And some slower particles will likely come before some faster particles. But the ideal particle speed distribution is exactly the opposite of what stratification would create. You would like the slowest particles first with a gradual increase in speed as the coil’s field strength increases. Unless we are extremely fortunate nature will not be so cooperative.