The Role of Instabilities in Electron Thermodynamics of a Magnetic Nozzle*

Magnetic nozzles heat a plasma and expand it through a magnetic field to convert thermal energy into thrust. They are theorized to exhibit high lifetimes and propellant ambivalence, thus being ideal candidates for long missions requiring in-situ resource utilization. However, the physics behind how these devices operate is still not fully understood.

One question regarding magnetic nozzle operation that remains open is that of electron heat conduction. The electrons exhibit a downstream decay in temperature as they expand. However, a classical Fourier law has been shown to predict a heat flux that exceeds the power levels typically put into these devices. This work explores the theory that a Fourier law may apply if the collision frequency used is an effective wave-induced (or "anomalous") collision frequency.

Previous work has identified a lower hybrid drift instability that exhibits significant propagation parallel to the magnetic field. This field-aligned propagation induces a collision frequency in this direction that inhibits heat conduction. In this work, we use a suite of electrostatic probes to measure these waves and to determine the heat flux from a Fourier law with the anomalous collision frequency. We quantify the relation to experiment by predicting a polytropic index from the waves and the measured value, finding a reasonable agreement.