Macroscale particle simulation of relativistic electron beam injection into a magnetized plasma channel

Abstract

A large-scale particle simulation of an intense relativistic electron beam injection into a longitudinally periodic and magnetized, three-dimensional plasma channel is performed with application to the steady-state current drive of tokamaks. It is found that the electromagnetically beam-induced electric field uniformly decelerates the beam electrons and generates a plasma return current as far as the injection continues. The beam electrons are decelerated also by the scaler potential field especially before the beam path becomes closed. A net longitudinal current, and hence the azimuthal magnetic field, is formed in the vicinity of the beam path after the return current has reached a steady force–balance equilibrium with the electric field and anomalous friction. The net saturation current is independent of the injection beam current and is scaled as Inet∼(γ2−1)1/2, where γ=(1−v2b/c2)−1/2 and vb is the velocity of the electron beam. For γ≫1, a macroscale helical instability develops and prevents the net current from reaching the level given by the aforementioned scaling. This instability is suppressed by application of a strong longitudinal magnetic field, which recovers the net current of the previous scaling.