Multi-chord IR-visible two-color interferometer on KSTAR

Major parts of an IR-visible two-color interferometer (TCI) on KSTAR have been upgraded for the multi-chord operation: (1) a diode-pumped-solid-state (DPSS) laser (660 nm) replacing the former HeNe laser (633 nm), (2) vacuum-compatible vibration isolator with titanium retro-reflectors, and (3) full digital phase comparator for multi-chord real-time density signals. The commercial compact DPSS laser suits the multiple chord configuration with its strong beam power (500 mW) and long coherent length (>100 m). Ti retro-reflectors are mounted on vacuum-compatible vibration isolators. The isolators are essential for the visible beams to avoid any fringe skips due to their short wavelength, considering the speed of the mechanical vibration (up to hundreds of μm). Field-programmable-gate-array (FPGA) modules count the entire fringes fast enough with a signal output rate up to 1.25 MHz, solving the fringe skip issues. The FPGA module enables the full digital processing of the phase comparator with a CORDIC algorithm after the sampling rate of 160 MS/s for the 40 MHz intermediate frequency of each beam. The full digital signals are transferred to the main plasma control system in real-time. Stable single-input-single-output operation of the KSTAR density control was demonstrated with the TCI. The real-time density profile control is also promising in the near future, with multiple actuators such as pellets and gas puffings.

Fringe jump compensation techniques for the time-averaging zero-crossing phase measurement in the KSTAR millimeter-wave interferometer

A fringe jump compensation algorithm has been developed for a phase measurement that measures the phase within a single fringe. The algorithm is extremely useful in the case of the time-averaging zero-crossing phase detector on noisy environments. When the noise level on the measurements is not sufficiently suppressed, the signals near the fringe jump show a negative slope instead of a sharp drop. The slope brings an ambiguity over the compensation process. An algorithm with an additional channel that measures the phase of a half fringe shift has been applied in the millimeter-wave interferometer on the Korea Superconducting Tokamak Advanced Research device. These techniques removed the ambiguity in most cases. The algorithm can provide a most simple, robust, and cost-effective solution for the phase measurement system in various fields.

Operation results of the KSTAR far infrared interferometer

The 2015 KSTAR experimental campaign was the first year of routine measurement with a far infrared interferometer (FIRI) utilizing 118.87 μm CH₃OH lasers at maximum 200 mW CW beam power. By using rtEFIT reconstruction, the path lengths of interferometers can be calculated and so the line-averaged electron densities n¯_e from the FIRI and a millimeter-wave interferometer were in excellent agreement. In this way, the number of successfully diagnosed discharges is counted: 1003 shots or 83.7% of sustained discharges, defined as shots of plasma current I_P ≥ 0.3 MA with pulse lengths t_f ≥ 2.0 s, have good-quality FIRI data within a few fringe jump errors. In addition, real-time H-mode density feedback control based on the FIRI was also successfully achieved with supersonic molecular beam injection as an actuator. Both constant density and controlled linear increment with a ramp-up rate of 1.0 × 10¹⁹ m^-3 s^-1 were achieved.

Design of Stark-tuned far-infrared laser with circular hollow waveguide

A novel design of a Stark-tuned far-infrared laser using a circular hollow dielectric waveguide is proposed, and its characteristics are studied. Supplementary electrodes are inserted inside the circular hollow dielectric tube to suppress charge accumulation while keeping field uniformity. In what is believed to be a new design, the mode property is found to be improved, and the angular dependency of the attenuation loss according to the beam polarization is estimated to be much smaller than that of the conventional rectangular hybrid waveguide design. In this new design, DeltaM=0 far-infrared (FIR) transition as well as DeltaM=+/-1 transition can be observed, and the power enhancement for the DeltaM=0 FIR transition is expected.