Cusped Field Thrusters (CFT)

The Divergent Cusped Field Thruster (DCF, or DCFT) is a concept belonging to the general family of Hall devices, but being clearly distinguished by the use of a cylindrical set of magnetic cusps for electron confinement, in replacement of the simple radial field of common Hall thusters. This has, among others, the effect of avoiding the use of a central pole piece, which reduces exposed surface area and facilitates miniaturization as well as diagnostic access. In addition, the confining effect on the plasma (magnetic in the case of the electrons, electrostatic in that of the ions) is very strong here, and only very small portions of the insulating casing are at all accessible to the plasma, with possibly beneficial effect on life. The divergent geometry was adopted in an attempt to further reduce ion impingement near the exit plane and to better understand the effect of magnetic topography towards mitigating erosion.

The full PIC model, developed previously for Hall thrusters, has been modified for the geometry and features of the CCFT, and has been used to support our experimental data as well as our conceptual exploration of the complex physics involved in these devices. The problems relate mostly to the extreme concentration of effects dictated by the cusped magnetic configuration, which makes it very challenging to obtain a complete resolution of the local potential profiles, and in turn, ion trajectories. The difficulty is heightened by the fact that, as uncovered in our theoretical research, ion confinement near the cusps is electrostatic rather than magnetic, and is in fact nearly independent of the relative size of ion gyro radius vs. region size.