Collective Scattering System for high-k turbulence measurement in KSTAR

Abstract

Anomalous electron heat transport issue has been a long-standing issue and may be attributed by a small-scale turbulence such as electron temperature gradient (ETG) mode which can induce thermal energy losses even for the regime where the ion-gyroscale turbulence is suppressed in fusion grade plasmas. Magnetohydrodynamic (MHD) instabilities and turbulence of ion-gyroscale are relatively well understood while turbulence of the electron-gyroscale is not well known because of difficulty in the measurement of such small-scale turbulence. Hence, understanding and validation of the electron energy transport arisen from electron-gyroscale turbulence are essential for the magnetic confinement fusion research. To measure such a small-scale turbulence in KSTAR plasma, a collective scattering system (CSS) has been developed. This diagnostic to simultaneously measures electron density fluctuations at four discrete high poloidal wavenumbers (kθ) utilizing 300 GHz (λ = 1mm) ordinary (O-mode) probe beam. The range of detectable poloidal wavenumbers is from ~14 to ~26 cm-1, which corresponds to 0.085 ≲ kθρe ≲ 0.16 for 1.8 T KSTAR plasmas with electron temperature (T𝑒𝑒 ) of 2 keV, where ρe is the electron gyroradius. The optical system with remote control capability is able to flexibly change the measurement location from the plasma core (r/a ~ 0.2) to outer edge (r/a ~ 1) on a shot-by-shot basis, where a is the minor radius. To extract phase information as well as the scattered beam power, the CSS employs a quadrature detection system consisting of four-channel detector array and electronics. Inphase and quadrature (IQ) signals from the electronics of 4 MHz bandwidth (determined considering the detectable poloidal wavenumbers and the poloidal rotation speed of KSTAR plasmas) are recorded by a digitizer at a nominal sampling rate 10 MS/s for full discharge pulse length. The spatial resolution is ~6-10 cm in the radial direction and ~1-2 cm in the poloidal direction. Due to installation of second neutral beam injection (NBI) system, the microwave imaging reflectometer (MIR), developed for ion-gyroscale fluctuation measurement, had been moved to a narrow diagnostic port and shared with the CSS. For sharing a single port, a large-aperture strip-grid beam splitter was developed to effectively separate and combine the 300 GHz O-mode beam for the CSS and 78-96 GHz extraordinary (X-mode) beam for the MIR system mi9nimizing unavoidable power loss by the beam splitter. The CSS was first installed in the middle of 2018 KSTAR campaign, and successfully commissioned in the 2018 and 2019 KSTAR campaigns. Initial measurement results had been obtained from various plasmas such as ohmic plasmas, low-confinement (L-mode) and high-confinement (H-mode) plasmas. However, it has to be noted that the CSS can measure large-scale MHD activities since it also works as an interferometer. As the 300 GHz probe beam and four scattered beams pass through the plasma entirely or partially, the measured signals are able to contain information of fluctuations from the large-scale MHD activities. Therefore, one has to consider the interferometric contribution to the CSS data to avoid misinterpretation. This thesis work describes the theory of scattering, design characteristics of the optical systems, configuration of overall systems, their laboratory test result and initial measurement results. Also, it shows that various physical parameters were calculated and considered to develop CSS suitable for KSTAR plasma research. The initial measurement results show that the turbulence decreases after the L-H transition as well known. In order to measure ETG turbulence in KSTAR plasmas, the experiments were performed to increase the electron temperature gradient. As the temperature increased, an increase in broadband turbulence was measured. In addition, turbulence moving in different poloidal directions, which is estimated as ITG and TEM turbulence in the H-mode plasma pedestal region, was simultaneously measured. In the future, more intensive analyses of CSS data will be carried out for studies of physics associated with small scale turbulence such as characteristics of ETG or trapped electron mode (TEM) in KSTAR H-mode or hybrid mode edge and their contribution to the plasma performance.