Plasma production experiments using a folded waveguide antenna on LHD

A folded waveguide (FWG) antenna was used in the ion cyclotron range of frequency (ICRF) on the large helical device (LHD) in the National Institute for Fusion Science. The FWG antenna is a waveguide antenna folded several times in order to make its size smaller. The FWG antenna in the LHD is designed so that slow waves are excited preferentially. It was used in the fourth experimental campaign in 2000-2001 for the purpose of plasma production and plasmas with an average electron density up to 3.0 × 10¹⁸ m^-3 were obtained. This will be a high enough density for initial plasmas of neutral beam injection or ICRF to obtain plasmas with higher densities and temperatures. This is the first demonstration of the utility of an FWG antenna in magnetic confinement devices. Further investigations were made in order to understand the mechanism of plasma production. The maximum achievable plasma density increased with injection power and gas-puffing rate, and became saturated. The density became higher as the magnetic field strength was increased. Such experimental observations were explained by the wave accessibility conditions of a shear Alfvén wave.