Simulation studies on alpha-particle-driven current in tokamak reactors

An asymmetry particle trapping caused by the finite banana width of alpha particles and a current produced by the resulting imbalance between transit particles with positive and negative velocities parallel to the magnetic field line (simply referred to as transit current) are numerically demonstrated by using an orbit-following Monte-Carlo code. The transit current is considerably enhanced by a collaborative effect of the finite-banana-width and a non-uniformity of the alpha particle source. The pitch-angle scattering is also very important for it. Banana-trapped particles drive a substantially negative current in the central region. Consequently, the positive transit current is degraded by the negative banana current and the net alpha current is almost flat near the plasma centre. The net alpha-driven total currents in an ITER-like normal aspect-ratio system (aspect ratio = 3.17) and in a low aspect-ratio system VECTOR (aspect ratio 1.98) estimated with a conventional electron screening effect are about 0.4 MA and 1 MA, respectively. A low-aspect-ratio tokamak has an advantage over a normal or a large-aspect-ratio system in driving current by fusion-produced alpha particles