Development of a collective scattering system and its application to the measurement of multiscale fluctuations in KSTAR plasmas

A four-channel collective scattering system (CSS) has been developed to extend the current turbulence measurement capability (poloidal wavenumber <3 cm(-1)) beyond the ion gyroscale in the KSTAR tokamak. By utilizing a 300 GHz probe beam and four-channel detector array, the CSS can measure electron density fluctuations of four distinct poloidal wavenumbers from 14 to 26 cm(-1) at a high sampling rate (typically 10 MS s(-1)). In between discharges the radial measurement location can be varied from the plasma center to the outer edge with the remote control of the optical system. In this paper, the details of the major components and laboratory evaluation of the optical system are described. Several initial measurements such as (1) the broadband turbulence reduction during L-H transition, (2) the broadband turbulence increase in H-mode when the amplitude of edge-localized mode crashes is reduced by increased density, (3) the amplification of a quasi-coherent mode by additional heating in high-performance H-mode, and (4) the appearance of high-frequency magnetohydro-dynamical modes during slow L-H transition, are briefly presented. The possibility of the CSS as a hybrid system (of a scattering system and fluctuation interferometer), which complicates the interpretation of the CSS data, is also discussed