Diverging Cusped-Field Thruster (DCF)
Dan Courtney
Masters Candidate

Objectives
- Design, build and test a plasma thruster using cusped magnetic fields from permanent magnets.
- Test the thruster performance over a range (~500-1000W) of input powers.
- Establish the tradeoffs between the cusped approach and traditional Hall thrusters.
- Evaluate the ability of the cusped fields to contain the plasma within the discharge channel and reduce wall erosion.
- Confirm the impact of introducing a diverging discharge channel on the plume divergence outside the thruster.
- Experiment with different anode and fuel inlet configurations to determine the most effective approach.

Finite element model of the magnetic field in the DCF thruster.

Approach
The thruster being built has origins from both the Thales Electron Devices, High Efficiency Multistage Plasma (HEMP) DM7 and Princeton University Cylindrical Hall Thruster (CHT) thrusters but includes several design variations. Our thruster uses permanent SmCo ring magnets arranged to create cusped fields with a diverging feature. Furthermore we are experimenting with alternative methods of diffusing the fuel inflow so as to produce the most uniform plasma within the channel. The magnetic field has been modelled using finite element models to confirm the presence of strong radial fields at each cusp, as shown below. In order to obtain uniform fuel flow into the discharge channel, a porous steel filter has been employed at the thruster base. The thruster will be tested in the MIT Space Propulsion Laboratory (SPL) vacuum chamber.

Background/Motivation
Hall thrusters provide high Isp and low thrust, making them ideal for many satellite station-keeping manoeuvres. The general magnetic field design for Hall thrusters has remained unchanged for several decades; although mainstream applications on commercial satellites are still relatively new. Several research groups (eg. Princeton, Thales Electron Devices GmbH) have experimented with creating the radial fields desired in Hall thrusters by using permanent magnets to create cusped fields. The result is both a strong radial field at the cusp apex and also large gradients in the field which suggests a mirroring effect. Previous works have shown that the cusped approach results in a thruster with similar thrust and total efficiencies to Hall thrusters of comparable power (~500W) but with a distinct reduction in wall erosion. One drawback of the field pattern is the presence of very strong field strengths which could magnetize the ions. The magnetic topology is such that magnetized ions could be deflected as they traverse the discharge channel, resulting in a divergent beam. This project uses a conical magnet arrangement to discourage this divergence.

Full view of the DCF thruster.