Edge harmonic oscillations at the density pedestal in the H-mode discharges in CHS Heliotron measured using beam emission spectroscopy and magnetic probe

Edge harmonic oscillations (EHO) offer the potential to relax the H-mode pedestal in a tokamak, thus avoiding edge localised modes (ELM). The mode structure of the EHO in CHS was investigated using a poloidal array of beam emission spectroscopy (BES) and a magnetic probe array. The EHO exhibited a peculiar characteristic in which the first, second and third harmonics show the same wavenumber, suggesting that the propagation velocities are different. Change in the phase of higher harmonics at the time when that of the first harmonic is zero can be described as a variation along the (m, n) = (−2, 1) mode structure, though the EHO lies on the ι = 1 surface. This behavior leads to an oscillation that exhibits periodic dependence of shape on spatial position.