Detection of high energy electrons in a cusp-field magnetic nozzle radio frequency plasma thruster

In a cusp-field magnetic nozzle radio frequency (rf) plasma thruster, a large negative floating potential approaching −100 V is detected at the upstream peripheral region. The negative floating potential suggests the presence of high energy electrons generated by the rf antenna, while the heating region is observed to be extended to the back wall region across the magnetic field lines. The magnitude of the negative floating potential is observed to increase with an increase in the upstream solenoid current, implying that the geometry of the confinement region and the magnetic field play a major role in the behavior of the high energy electrons. Here, the extended heating region is discussed in terms of inductive coupling by considering the longer skin depth in the low-density region. Furthermore, high rf voltages are detected at the antenna terminal, which induces a negative “self-bias” on the inner radial wall. As the radial asymmetries are observed for the floating potential, the results suggest that the self-bias has some contribution to the generation of high energy electrons. The two-dimensional measurement reveals the presence of high temperature electrons in the peripheral region, where the electrons are confined by the magnetic field lines and the floating physical walls.