Assessment of cross-field electron transport in a magnetic nozzle

A magnetic nozzle is a key element for a high-power electric propulsion device called a helicon plasma thruster. Since the magnetic field lines are closed and turn back to the thruster, the plasma has to be detached from the field lines. The plasma detachment has been a challenging problem, especially when electrons have a gyro-radius smaller than the system's scale. Here we experimentally demonstrate that a cross-field transport of the electrons toward the main nozzle axis is induced by the spontaneously excited wave having the frequency considerably higher than the ion cyclotron frequency and driving an ExB drift that only effects the electrons. The density and electric field fluctuations are simultaneously measured, and the direction and the magnitude of the cross-field electron flux are assessed and compared with the cross-field ion flux induced by the electrostatic field. Wave-induced transport and loss have been one of many important issues in plasma physics over the past several decades. Conversely, the presently observed electron inward transport has a beneficial effect on the detachment by neutralizing the ions deviating from the field lines.