FAST DECAYS OF PLASMA RETURN CURRENTS DUE TO WHISTLER WAVES

The evolution of the return current induced by a charged particle beam in a magnetized plasma is studied. The beam current is perpendicular to the background magnetic field. The return current is shown to depar t from the beam along the background magnetic field with a whistler ra ther than a diffusion or an Alfven velocity. In a plasma bounded by tw o conductors the return current oscillates with the whistler period. A nalytical expressions for the evolution of the magnetic field and of t he plasma return current are derived for a beam with a finite width an d with various rise time dependences. When the whistler time is shorte r than the rise time of the beam current, the plasma return current do es not grow beyond the whistler time.