Oscillations and instabilities in a propulsive magnetic nozzle*

Instability-driven oscillations can enhance cross-field transport in magnetized plasmas. This can have a major impact on the operation of electrodeless plasma thrusters, both internally in the discharge chamber and externally in the accelerating magnetic nozzle, increasing plasma losses and plume divergence. The present work studies the naturally-occurring oscillations in the magnetic nozzle of a helicon plasma thruster from different attack angles.

Firstly, we employ an array of 3 time-resolved, electrically-floating probes to resolve the cross-correlated power and the azimuthal and axial components of the wavevector at each frequency in the plume of the device. We observe waves below 200 kHz with a mainly-azimuthal component, with a magnitude that peaks around 50 kHz, and presence of a first harmonic around 100 kHz. We find that the magnitude and dispersion of the oscillations depends on the location of the measurement point in the magnetic nozzle and the operating point of the device, with an abrupt transition near the plume edge at around 80 kHz. Secondly, a bicoherence analysis is used to corroborate the nature of the transfer of power from the 50kHz to the 100 kHz bands. Thirdly, following a local linear analysis we present the (fluid) dispersion relation of stable and unstable waves in a cylindrical plasma column with density and electrostatic potential gradients, including collisionality and parallel propagation, to find potential candidates that explain the observations. Finally, under the hypothesis of each of the candidate branches, a quasilinear analysis is used to estimate cross field transport due to the oscillations.

The results of this work allow confirming the presence of oscillations in the magnetic nozzle, the existence of nonlinear power coupling among frequencies, suggesting the possible dispersion relations of these waves, and quantifying the effect they have on the slow plasma dynamics.