Radio-Frequency Plasma Expansion in Different Magnetic Nozzle Configurations

This thesis presents the first measurements conducted in the novel radio-frequency plasma device Moa. The research focuses on the effects of magnetic field strength and orientation on plasma expansion and ion acceleration when the location of plasma excitation is decoupled from the magnetic nozzle (MN) throat with the aim to improve the efficiency of radio-frequency plasma thrusters and explore the feasibility of magnetic thrust vectoring. Spatial measurements of the ion saturation current and floating potential in the plume revealed a negative potential region at the location of maximum ion current along the field lines intersecting the antenna upstream. In the symmetric field case, a high-density conics structure formed downstream of the MN throat for the field strengths studied. When the magnetic nozzle was deflected, the ion current profile transitioned from a double-peaked to a single peak centred around the steered field line reconnecting to the antenna. It was observed how, independent of magnetic f ield strength and orientation, the field lines interacting with the antenna upstream, and transporting high-energetic electrons, dictated the plasma profile downstream of the MN. A supersonic ion beam formed in the expansion regardless of the magnetic field configuration analysed. With the symmetric magnetic nozzle, the ion beam extended over longer axial distances with increasing field strength, and its density increased as a result of a reduced plasma diffusion. The ion beam was measured even for low plasma confinement conditions, suggesting the importance of ion magnetisation at the MN throat for ion acceleration. The simultaneous detection of the supersonic beam centred on axis and the off-axis peak of the background ions further proves the peripheral ionisation occurring far from the antenna. Measurements of the ion beam revealed that its trajectory did not follow the magnetic streamlines. In the symmetric case, most of the beam detached inwards towards the axis and travelled on a linear path. Conversely, the ion beam was deflected in the direction opposite to the steered magnetic nozzle. Finally, a low-frequency ion instability was detected in the expanding plasma solely in the symmetric magnetic nozzle configuration, which is believed to be an electrostatic ion cyclotron wave.

Name thesis:degree_name

PhD

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Physics

Grantor dc:publisher

ResearchSpace@Auckland

Year dc:date.issued

2024

Author dc:creator

• Caldarelli, Antonella

Advisors dc:contributor.advisor

• Rattenbury, Nicholas
• Cater, John

Statement dc:rights

• Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

Licence dc:rights.uri

• https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm

Handle dc:identifier.uri

https://hdl.handle.net/2292/67855

OAI identifier oai:identifier

oai:researchspace.auckland.ac.nz:2292/67855