Permanent-Magnet Halbach Array for an Argon AF-MPD Thruster

Organisation: Theta Systems  |  Role: Electric Propulsion Research Intern
Deliverables: Technical Note TN-02 and a 10-sheet A3 manufacturing drawing set

Summary

Applied-field magnetoplasmadynamic thrusters (AF-MPDTs) combine high thrust density and high specific impulse, and can run on cheap argon. An electric arc between a central cathode and an outer cylindrical anode ionises the argon. An applied axial magnetic field acts on the radial discharge current, and the resulting Lorentz force spins the plasma up and accelerates it out of the thruster.

Argon is much cheaper than xenon, but it needs a strong magnetic field to ionise and accelerate efficiently. That field usually comes from solenoid coils, which need their own high-current power supply, draw hundreds of watts and need active liquid cooling. A small spacecraft cannot support any of that.

In this project I replaced the coils with a passive permanent-magnet Halbach array, for a 1–5 kW breadboard thruster intended to fire continuously during orbit raising. The array is an 8-segment, 5-ring Halbach cylinder of NdFeB N48UH magnets inside a low-carbon steel casing, which carries the flux through the plasma channel.

Results

The design was settled through a coupled study of 143 magnetostatic configurations and 84 thermal cases, and meets the magnetic, thermal and mass requirements:

• The array gives 0.631 T on axis at the cathode tip at steady-state operating temperature, 57.7% above the 0.40 T requirement.
• To keep the magnets cool during continuous firing, the 500 W arc heat load is conducted rearwards through an OFHC copper anode and an aluminium nitride (AlN) insulator into the spacecraft cold plate. 488 W of the 500 W (97.6%) leaves through the rear interface, which holds the magnet tiles at 72°C (their limit is 180°C) and the anode at 288°C.
• The complete thruster weighs 7.17 kg within a 114 mm diameter envelope, under the 7.50 kg limit.
• Current-streamline modelling and an empirical fit to 667 published argon thruster firings predict 186 mN of thrust at 100 A discharge current.
• The coercivity margin is at least 23% in every magnet tile, so the magnets do not demagnetise irreversibly up to 130°C.
• I produced 10 dimensioned A3 manufacturing drawings, with geometric dimensioning and tolerancing (GD&T) for every component.

Field plots and poster

The two COMSOL 2D axisymmetric solutions behind the magnetic and thermal design, and the project poster:

Figure 1: Magnetic flux density from COMSOL (2D axisymmetric). The flux circulates through the 1010 steel casing and concentrates on axis in the bore, giving 0.631 T at the cathode tip at operating temperature.

Figure 2: Steady-state temperature field from COMSOL under a continuous 500 W arc load. Conduction to the rear keeps the magnet tiles at 72°C, below their 180°C rating.

Figure 3: Project poster covering the magnetic circuit, the thermal resistance network and the predicted thrust scaling.

Drawings

Two of the ten A3 sheets: the general arrangement of the assembly and one part drawing. The remaining part drawings are available on request.

Figure 4: General arrangement, AFMPDT3-001.

Figure 5: Anode barrel, AFMPDT3-002, with fits, surface finish and flatness called out for the rear conduction joint.

Full report

Technical Note TN-02 (Permanent Magnet Architecture for an AF-MPD Thruster: Settled Design Configuration for Continuous Operation, with the Magnetic, Thermal and Discharge Analysis Behind It) is below, with the general arrangement drawing. The part drawings are not included.