Experimental Investigation into the Enhanced Diamagnetic Perturbations and Electric Currents Downstream of the High Power Helicon Plasma Thruster
Roberson, Brandon Race
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The high power helicon (HPH) is a compact plasma source that can generate downstream densities of 10<super>17</super>-10<super>18</super> m<super>-3</super> and directed ion energies of 50-70 eV, without the need for grids that can corrode with use or requiring a larger engine diameter. Generating a quasi-neutral plasma beam that can stay collimated and impart significant power and momentum to a distant target in space has a variety of potential applications including beamed propulsion and the remediation of space debris. In order to understand and improve the coupling mechanism between the helicon source antenna and the downstream plasma, measurements were made in the plasma plume downstream of the propagating wave magnetic field and the diamagnetic perturbation of the background magnetic field with the presence of the plasma. This magnetic field perturbation (ΔB) peaks at more than 15 gauss in magnitude downstream of the plasma source and propagates tens of centimeters downstream, cancelling the base magnetic field provided by the experiment as it propagates. Taking the curl of this measured magnetic perturbation suggests a peak current density of 20 kA m<super>-2</super>. These diamagnetic perturbations and electric currents were correlated with an increase in wave-plasma coupling and increased acceleration of the plasma particles downstream. In order to increase the energy coupled into the plasma and drive a larger diamagnetic perturbation a further distance downstream a second, larger radius antenna was added roughly one wavelength downstream co-axially with the first antenna and driven in phase with the first. This resulted in improved collimation of the plasma beam over a meter downstream, increased diamagnetic perturbation, and an increase in the ion energies downstream of more than 20 eV. This work includes the development of a high power plasma source that is capable of generating a dense, collimated plasma beam with exhaust velocities comparable to devices of similar power levels but in a compact size without the need of electric grids; as well as measuring diamagnetic plasma perturbations that are larger than in any similar plasma experiment previously published, suggesting new capabilities for studying high beta (but cold and directed) plasmas in a laboratory setting.