Alpha particle loss and neutral beam current drive in fusion plasma with central current hole

Alpha particle loss due to toroidal magnetic field (TF) ripple in a current hole (CH) was numerically assessed. In a tokamak with a conventional aspect ratio A (=4.1), α-particles are well confined in plasma with a hole radius (ρ_hole) of ≤0.3. When ρ_hole increases to 0.5, α-particle loss can exceed 10%, resulting in an intolerable heat load on the first wall. When born in a CH, toroidally trapped α-particles are regarded as being produced near the CH boundary. This causes an increase in α-particle loss with ρ_hole. In order to reduce the loss to an acceptable level of about 3%, the TF ripple amplitude at the plasma surface must be as low as 0.3%. In contrast, a low aspect ratio tokamak has an advantage over α-particle confinement in the CH, in that the calculated α-particle loss for A = 2 was lower than that for conventional values of A. The neutral beam-driven current in the CH was also investigated numerically. The calculation indicates that, in the CH plasma, the beam-driven current is induced in the outer region, compared with the equilibrium current that sustains the CH.