Impact of Magnetic Field Profile on Loss Mechanisms in a Rotating Magnetic Field Thruster

The Rotating Magnetic Field (RMF) thruster employs a rotating magnetic field to entrain electrons about

the axis of a conical plasma discharge to induce azimuthal currents, which then interact via the Lorenz

force with radial magnetic fields, chiefly belonging to an applied diverging magnetic bias field, to

produce thrust. In the case of balanced induced and applied magnetic fields, a separatrix can form,

producing a field-reversed configuration (FRC) plasmoid, which is speculated to reduce wall losses owing

to its confining nature. In recent work, we have shown poor performance for the RMF thruster (<1%),

substantiated by plasma probing which suggest that a major loss mechanism is electron thermal losses

to the walls. Indeed, subsequent inductive probe measurements show the FRC separatrix to intersect

the walls of the device. In this study we parametrically vary the strength and shape of the applied

magnetic bias field, in turn changing the shape of the FRC separatrix during device operation. At each

operating condition, we measure the efficiency breakdown of the device including assessment of loss

mechanisms such as electron wall losses. While we find that moving the separatrix radius inside the

walls does reduce wall losses, these improvements to performance can be balanced out for certain field

shapes by increased excitation radiative losses owing to increased plasma density close to centerline

and by increased divergence losses.