Development of a helicon plasma source and study of the helicon plasma characteristics at very high radio frequency

Abstract

Helicon plasmas are known to offer higher plasma density and better input power efficiency than other types of low temperature plasmas such as capacitively- or inductively-coupled plasma, or microwave-based electron cyclotron resonance plasma. Aiming at the application to an efficient ion source for high current accelerators, a novel high-density, compact plasma source was successfully developed at very high radio-frequency (98MHz) which is commonly used for radio communication. It was designed and fabricated to be modular for accommodating easy connection to other applicable devices. A detailed study of the characteristics of the generated helicon plasma was also performed. It includes the identification of the operation parameter window such as radio-frequency (RF) power, applied magnetic field, and neutral gas pressure, and the observation of multiple wave-mode transitions inside the helicon plasma. As in the plasmas generated at conventional frequency such as 13.56 MHz, E-H-W mode transitions were observed. In the helicon mode, a bright column was produced at the central part of the cylindrical plasma. Experiments using Nagoya type III antennas with different leg length showed that the antenna having the shortest legs produced the highest density, which suggests the importance of the antenna ring section. A study on the operation window of the helicon plasma at 98MHz demonstrated that the H-W mode transition occurs at lower magnetic field at higher gas pressure with the the RF power fixed. It was also observed that the transition arises at lower magnetic field at higher RF power with the gas pressure fixed, indicating the correlation between the three important operating parameters on helicon plasma generation and sustainment. By wavenumber measurements using the heterodyne double B-dot probe, additional wave-mode changes inside the helicon plasma were observed as increasing the external magnetic field. It was also observed that the wavelength of the ...