[NortySpock]

Wouldn't it produce high-velocity charged particles?

Thus you would capture the energy with a "reverse coilgun", "regeneratively braking" the particles down to a non-relativistic speed, rather than just using them to charge a capacitor plate.

[rklaehn]

Once the capacitor plate has a voltage in the range of the kinetic energy of the particles, this is exactly what will happen. The particles convert their kinetic energy into electrical energy by rolling "uphill" against the electric potential.

[pfdietz]

And all the free electrons produced in that target are helpfully just sitting there, right?