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

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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.