First results of CO₂ dispersion interferometer on EAST tokamak

Abstract

A dispersion interferometer (DI) has been installed and operates on the Experimental Advanced Superconducting Tokamak (EAST). This DI system utilizes a continuous-wave 9.3 μm CO2 laser source to measure line-averaged electron densities accurately. In contrast to conventional interferometers, the DI does not require substantial vibration isolations or compensating systems to reduce the impact of vibrations in the optical path. It also employs a ratio of modulation amplitudes, ensuring it remains immune to the variations in detected intensities. Without a variation compensation system, the DI system on EAST reaches a density resolution of less than 1.8 × 10-2 π rad and a temporal resolution of 20 μs. The measurements made by the POlarimeter-INTerferometer (POINT) system and the far-infrared hydrogen cyanide (HCN) interferometer are remarkably consistent with the DI’s results. The possibility of fringe jumps and the impact of refraction in high-density discharge can be significantly decreased using a shorter wavelength laser source. A rapid density change of 3 × 1019 m-3 during 0.15 s has been measured accurately in shot No. 114755 of EAST. Additionally, the DI system demonstrates dependability and stability under 305 s long-pulse discharges in shot No. 122054.

Real-time data processing method for CO2 dispersion interferometer on EAST

“…As for electron density diagnostics on EAST, the verticalviewing 0.65 THz SSI has been installed for line-integrated density measurements on EAST [28] and the POINT system [29] operates at a wavelength of 432 μm and a modulation frequency of 850 kHz for density measurement. They have consistently delivered density feedback for experiments conducted on EAST.…”

“…Therefore, density obtained through them can be used to confirm the accuracy of CO 2 -DI measurements. , SSI [28], and POINT [29] systems during a stable plasma discharge (#120531) in the EAST experiment. The difference between the measurement results of three systems in this figure is about , which is reasonable due to their setup differences on EAST.…”

Progress of CO2 Dispersion Interferometer on EAST

“…In principle, they can overcome the variation in the optical path owing to mechanical vibrations. Simultaneously, the use of shorter-wavelength laser sources (9.3 μm of DI on Experimental Advanced Superconducting Tokamak (EAST) [4,5], for example) contributes to an elevated integration density of 2π. Recent years have witnessed notable enhancements in DI performance, as evidenced by application and refinement in projects such as LHD [6], DIII-D [7], HL-2A [8], and KSTAR [9].…”

“…The principle of the DI is shown in figure 1(a). Previous studies have elaborated on this in detail [4,5]; here, we present only the essentials. The dispersion interferometer uses two nonlinear crystals to generate two second-harmonic beams.…”