Plasma motion into a transverse magnetic field and plasma

Abstract

In a transverse magnetic field a tenuous plasma beam follows a curved Lorentzian trajectory. In contrast, a collisionless dense beam propagates undeflected by collective plasma processes including diamagnetic flux exclusion and the E*B drift. In recent laboratory and space experiments the magnetic field has been observed to diffuse much more rapidly than classically predicted, even in the limit of high beta and small ion gyroradius where diamagnetic flux exclusion is normally expected. In space experiments the mechanism for rapid diffusion has been attributed to a lower hybrid drift instability. However, in laboratory experiments the instability growth time is too long to account for the observations. Solving the nonlinear magnetic diffusion equation gives a conductivity substantially reduced from its classical value by the square of the plasma collisionality parameter. The resulting diffusion time scale is more consistent with experimental observations. Rapid diffusion is also observed for high-beta beam propagation in a magnetized plasma. By varying the background plasma density the perpendicular conductivity can be increased to a value that prevents polarized E*B propagation. The measured limits for complete shorting agree with a dynamic calculation of the beam polarization and shorting time scales and has resulted in an analytic expression for the ratio of beam to plasma density.<>