Low Pressure Plasmas in Magnetic Nozzles

Abstract

This thesis details experiments of low pressure plasmas interacting with magnetic nozzles in two different plasma reactors. In the Chi Kung reactor, a radiofrequency (RF) plasma is created in the throat of the magnetic nozzle and is allowed to expand through the diverging field. Under the experimental conditions used in this thesis (0.3-0.7 mTorr argon, 145 Gauss peak axial magnetic field, 315 W RF power), a current-free double layer is measured on axis and its characteristics are explored with in-situ diagnostic probes as the argon pressure is increased. With increasing pressure, the magnitude of the potential drop and beam velocity decrease. The axial beam velocity profiles are consistent with those previously published using an ex-situ laser technique, giving confidence in the in-situ measurements and allowing for reliable measurement of the beam density.

The axial plasma properties in Chi Kung were measured when a pyrex extension tube was inserted in the expansion region, thereby separating the geometric and magnetic expansions. The plasma density in the source region increased and the peak velocity of the ion beam decreased. With the extension tube inserted, measurements of the electron transport showed that those electrons that would normally ionise high-density conics downstream are blocked by the extension tube and increase the rate of ionisation in the source. The extension tube was then positioned further downstream, creating a window through which these electrons could pass. With increasing window sizes, the conics were seen to reform and the beam characteristics return to the standard case, thereby demonstrating the relationship between the conics and the ion beam.

The field-aligned electron transport from the plasma source to the region of high-density conics was investigated by measuring the plasma potential and density along the most radial magnetic field line to escape the source region. The results demonstrate an electric triple layer at the point where the magnetic field line passes closest to the radial wall. This is found to be due to negative charging of the radial wall and the build up of negative charges repel the electrons streaming along the field line.

In the Echidna reactor, a solenoidal magnetic field and RF plasma source are progressively separated meaning the plasma is incident on an increasingly distant converging magnetic nozzle. For 1 mTorr argon, 300 Gauss maximum axial magnetic field and 200 W RF power, axial plasma density profiles measured in the Echidna reactor showed two regimes of operation depending on the ion magnetisation under the RF antenna. When ions are magnetised, electrons heated by the antenna are effectively transported along a column defined by the converging magnetic field lines and increase in density as the cross-section decreases. The peak density is located underneath the antenna and the system appears to behave in a Boltzmann-like manner. When ions are unmagnetised under the antenna, electrons are not as effectively transported to the region of high magnetic field strength, resulting in a doubly-peaked axial density profile where one peak is under the solenoids and the other is under the RF antenna.