Observation of localized oscillations at m/n = 2/1 rational surface during counter neutral beam injection in the Large Helical Device

In the Large Helical Device, the structure of localized oscillations at the m/n = 2/1 rational surface during neutral beam (NB) injection in the counter direction (counter-NB injection) has been investigated in detail using an electron cyclotron emission (ECE) diagnostic. Here m and n are poloidal and toroidal mode numbers, respectively. Under the condition of increasing plasma current in the counter direction and nearly constant pressure, the oscillations are observed at larger magnitudes of plasma current (≲-20 kA T^-1). In the NB reversal experiments, where the direction of the NB is changed from the co- to the counter-direction and vice versa, the oscillations were observed only for the former case. These results suggest that the profile and magnitude of the plasma current play an important role in the mode onset. Analysis based on the Mercier criteria shows that for a flat or a hollow current profile an interchange mode becomes more unstable at higher negative plasma current, showing a tendency similar to the experimental observations. In addition, radial displacement due to the localized oscillations evaluated from electron temperature perturbation profiles with the ECE diagnostic agrees well with that predicted by a three-dimensional ideal magnetohydrodynamic stability code.