Characteristics of tongue-shaped deformations in hydrogen and deuterium plasmas in the Large Helical Device

The tongue-shaped deformation (TSD), a possible trigger for the evolution of quasi-stable magneto hydrodynamic (MHD) modes into MHD bursts, was first experimentally identified in the large helical device (LHD) (Ida et al 2016 Sci. Rep. 6 36217). Further analysis has been based on the experimental data in low density (<) and low () hydrogen plasmas Ida et al (2017 Phys. Plasmas 24 122502); Ida et al 2018 (Sci. Rep. 8 2804). However, since the mechanism causing the tongue-shaped deformation has not been clarified, it is important to study the characteristics of this nonlinear instability in a more broad spectrum of experiments to stimulate the study in simulation and theory for this event. Identification of tongue-shaped deformations in both hydrogen and deuterium plasmas is automatised and characterisation of the TSDs is done by quantifying their size, timing and location using ECE, RF and magnetic probe diagnostics. The dependence of these characteristics on plasma parameters such as density, toroidal rotation and NBI power is given.