It sounds like they're trying to build a device with a distribution of ions in velocity space which is very far from the equilibrium Gaussian-type distribution ('Maxwellian'). This is difficult because there will be about a million collisions for every fusion reaction that happens, and collisions tend to redistribute velocities toward the equilibrium. Collisions between unlike particles (different species of ions, or ions vs electrons) will also cause diffusion in physical space. This could cause ions to leave the chamber much more quickly.