Antenna Design for the Lebanese Linear Plasma Device

Abstract

Plasma, the fourth state of matter comprising almost 99% of the world, grabbed the attention of many researchers whose work referred basically to analyzing and understanding the characteristics and behavior of this state of matter. Due to their high efficiency in depositing electromagnetic power and producing dense plasma with high ionization degree, Helicon plasma sources have become extensively applied in basic research and for several applications. One of the major parts of a Helicon plasma device is the antenna responsible for coupling RF power into the plasma and driving the discharge. Thus, it is of high importance to investigate, analyze, and assess the performance of the employed RF antenna in order to maximize or optimize the power being deposited into the plasma. In our work, we conduct a comparative analysis among different antennas that are characterized by different properties such as; directivity, gain, input impedance, and others. This allows establishing a relationship between the properties of an antenna and its power deposition capabilities, and deducing what parameter enhancement generates denser plasma. This was followed by proposing a new antenna design with enhanced characteristics, capable of coupling energy more efficiently and generating denser plasma.

A comparative analysis is conducted among four different antenna structures that are commonly used in helicon plasma sources; namely, the anti-parallel double loop, the parallel double loop, the fractional helix, and the parallel fractional helix. These antennas are symmetrically wrapped around the plasma column of AUB’s Lebanese Linear Plasma Device (Polaris), and tested in order to investigate and analyze the overall plasma profile of this plasma source. The investigative, theoretical and experimental work have shown that the parallel double loop antenna, which is the antenna with the highest gain value, generated plasma with the highest electron density. Hence, in order to correlate the gain enhancement with the improved energy coupling and higher density plasma generation, a new antenna design is proposed. The proposed design is a modified helix antenna that was designed and experimental validated. Once installed on Polaris, it attained an average electron density that is much greater than the average electron density value achieved by the parallel double loop antenna. This reassures the conclusion drawn in our comparative analysis, which states that antennas with higher realized gain values are the antennas capable of generating denser plasma.