Basic characterization of TORPEX electrostatic modes in closed field line configurations
Electrostatic coherent modes are studied in the TORPEX device [Fasoli et al., Plasma Phys. Controlled Fusion 52, 124020 (2010)], in closed flux surfaces. The accessibility to this magnetic geometry is provided by a current-carrying in-vessel toroidal conductor developed to generate a poloidal magnetic field [Avino et al., Rev. Sci. Instrum. 85, 033506 (2014)]. The background plasma parameters are measured, and the ion saturation current fluctuations are characterized in terms of power spectral density to identify the dominant coherent modes and their spatial localization. A statistical approach is implemented to determine the mode spectral properties by computing the statistical dispersion relation. The poloidal wave number k(theta) and the toroidal wave number k(phi) are obtained, as well as the corresponding mode numbers. A three-dimensional linear code based on the drift-reduced Braginskii equations is used to investigate the nature of the instabilities. The linear analysis suggests a dominant ballooning character of the modes. (C) 2014 AIP Publishing LLC.