Intense whistler-frequency emissions at the pedestal collapse in KSTAR H-mode plasmas

Abstract

The edge confinement barrier of high-confinement mode (H-mode) plasma involves a variety of plasma waves alongside with fluid instabilities and collective particle transport during the barrier collapse. We demonstrate a new method of resolving the plasma waves by measuring the modulations embedded in the second harmonic electron cyclotron emission (ECE). Utilizing mm-wave heterodyne detection and fast digitization technologies on the KSTAR tokamak, we resolve not only the frequency spectrum but the wavenumber as well. At the plasma boundary during the barrier collapse, we observe multiple bursts of broadband whistler-frequency (<8 GHz) waves, together with intense narrowband emissions. The narrowband emissions exhibit rapid rising and falling tones within a few microseconds. Bispectral analyses reveal the existence of nonlinear interactions between the broadband and the narrowband waves, confirming their concurrence in the barrier zone. We estimate that the vertical wavenumber of the narrowband waves is comparable to that of a whistler wave, utilizing multiple mm-wave mixer channels. Our work opens a new experimental way to study wave-wave and wave-particle interactions in magnetically confined plasmas.