Full-orbit simulations of fast-ion charge-exchange losses induced by neutral particles outside the last-closed flux surface

The interaction between neutrals outside tokamak plasmas and fast ions that are generated by neutral beam injection and pass outside the plasma is studied. It is shown that at beam energies up to ∼100 keV fast-ion losses are significant, up to 20% in the studied case, because of charge exchange reactions between the neutrals and the fast ions. About half of the fast ions that neutralize outside the plasma get lost to the wall while the other half re-enters the plasma and ionizes again, often on better confined orbits, which is visible as an enhanced beam power deposition in the plasma core (ρ ≲ 0.7). As a result, the plasma performance as measured from beam-plasma fusion reactions is hardly changed despite the significant fast-ion losses toward the plasma edge.