Birdcage Antenna for Helicon Plasma Thruster

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This thesis is based on the study of a birdcage antenna for a helicon plasma thruster. Helicon plasma thrusters (HPT) are a new development in the field of electric propulsion and it uses helicon waves to ionize and heat the plasma. Because of this, the antenna to excite these waves is a crucial part of the technology. Furthermore, birdcage antennas are an attractive way of generating the plasma in HPTs since the field pattern created by the antenna matches the helicon wave pattern leading to higher and better power coupling. The work consists of the review of the physics of helicon plasma and birdcage antennas, a comparison of the existing analytic models for birdcage antennas considering both shielded and un-shielded configurations, through simulations and experiments and finally, the design of a high power birdcage antenna. The main focus of the research is a high-pass birdcage antenna for exciting the m=+1 helicon mode at a frequency of 13.56 MHz. Three different approaches to calculate the self- and mutual inductances are studied. For each of them different relevant plots including the inductance, current distributions and magnetic fields are presented. For all three methods the calculated inductances agreed very well. The model that did not take into account the mutual inductance yielded different resonant frequencies (for the same value of the capacitance) showing that the mutual inductance cannot be neglected. The results obtained from the shield analysis agreed well with results found in the literature The experiments were performed with a Vector Network Analyzer for different shielded configurations, varying shield radius and the relative position to the antenna. The results obtained were consistent with those reported in the relevant papers and the location of the antenna in the shield resulted in being equivalent to changing the shield radius for certain positions. To conclude, a high power antenna design is proposed, which includes an analysis of materials, geometry and capacitors, as well as a detailed budget.
Birdcage antennas, Helicon waves, HPT, Plasma
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