Thrust Generation in the Center of Wall-less Hall thrusters

Wall-less Hall thrusters contain large diverging axial magnetic fields in the center, with magnetic field gradients strong enough to confine electrons through magnetic mirroring [1,2]. Recent measurements of the plasma potential in this region of diverging magnetic field have shown potentials as high as the applied anode voltage, with electric field profiles similar to that in the acceleration region near the anode [2]. Measurements suggest an appreciable portion of the thrust is generated in the center, with thrust densities similar to that in the anode acceleration region. Given that the central electric fields are generated along the magnetic field lines and are larger than the plasma pressure gradient, the mechanism of formation of a large component of this electric field is unclear. However by utilizing electron energy anisotropy levels found in particle in cell simulations, we calculate an electric field due to magnetic mirroring comparable to this missing component. We discuss other possible electric field formation mechanisms, such as the deflection of the electric field due to the ExB rotation of electrons in the diverging magnetic field [3]. [1] K. Matyash, R. Schneider, S. Mazouffre, S. Tsikata, and L. Grimaud, Plasma Sources Sci. Technol. 28, 044002 (2019).; [2] J. Simmonds and Y. Raitses, Journal of Applied Physics (2021); [3] N. Fisch, Y. Raitses, and A. Fruchtman, Plasma Physics and Controlled Fusion 53, 124038 (2011).