High-Beta Plasma Characteristics Using Helicon Sources

Helicon sources have been extensively investigated because of their efficient high-density (up tp $\sim$ 10$^{13}$ cm$^{-3}$) plasmas production in a wide range of operational parameters. Using this source with a large diameter of 74 cm [1], high-beta ($\sim$ 1) plasma characteristics have been investigated. The magnetic field less than \textless 120 G was measured with and without plasma, so that the reduction rate of the magnetic field, i.e., the diamagnetic effect due to the plasma, was evaluated. For a large magnetic field and/or a low fill pressure, the reduction of the magnetic field pressure was found to be nearly equal to the plasma pressure, confirming balance between magnetic and plasma pressures. For a low magnetic field and/or a high fill pressure, however, the magnetic field was hardly changed. We suggest that the neutral pressure associated with a neutral depletion plays a role in weakening of the diamagnetic field. Detailed results with a theoretical analysis will be presented.\\[4pt] [1] S. Shinohara and T. Tanikawa, Rev. Sci. Instrum. \textbf{75} (2004) 1941.