Effect of ion acceleration on a plasma potential profile formed in the expander of a mirror trap

We consider a flow of completely ionized magnetized plasma streaming along divergent magnetic field lines from an open magnetic trap. An analytical model is proposed that takes into account a transition from a collisional (fluid) regime to a collisionless kinetic regime as the flow expands. This allows for study of the formation of a plasma potential along the magnetic field and the acceleration of ions in a self-consistent way. We show that ion acceleration may influence the potential profile; in particular, it significantly reduces the potential gap inside the Debye sheath near the plasma collector. This positive effect predicts low arcing in plasma absorbing units, which is important for all large-scale fusion-aimed experiments in mirror machines. Applications of the theory to smaller table-top machines are also considered.