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Recent progress towards an increased understanding of detached divertor physics has been made with the highly closed divertor geometry in HL-2A. Non-intrinsic impurities were injected into the outer divertor chamber, and increased divertor neutral pressure and enhanced volumetric energy loss in the divertor were observed. Meanwhile the neutral pressure in the main chamber decreased slightly, and neutral compression between the divertor and main chamber increased greatly. This led to divertor detachment with a low upstream plasma line-averaged electron density ( ∼ 0.5 n GW ). In the H-mode, slight degradation of the core confinement, characterized by a decrease in plasma stored energy and pedestal pressure and an increase in edge-localized mode frequency, was observed, but the H-mode was still sustained well with H98 > 1. Pedestal density fluctuation was increased during detachment, implying that the enhanced pedestal transport might be responsible for the degradation. During the divertor detachment phase, the impurities were well controlled in the divertor without strong radiation near the X-point region, and the main plasma density did not increase but decreased slightly; this could be a benefit of the highly closed divertor geometry. The experimental results suggest that a closed divertor geometry has the advantages of volumetric energy loss, gas pumping and impurity control in the divertor without significant effects on the plasma confinement, thus giving a wider operating window for divertor detachment. [ABSTRACT FROM AUTHOR]

Turbulent transport of impurity ions with hollow density profiles (HDPs), which are widely observed in magnetically confined plasmas and desirable for fusion reactor, is self-consistently investigated. A full gyrokinetic description is employed for main and impurity ions. Instead of conventional ion temperature gradient (ITG, including impurity ITG) and trapped electron modes (TEMs), impurity modes (IMs), driven by impurity ion density gradient opposite to that of electrons, are considered. The impurity ion flux induced by IMs is shown to be approximately one order of magnitude higher than that induced by TEMs when both kinds of modes coexist. Main ITG and electron temperature gradient (ETG) are found to reduce influx of impurity ions significantly, resembling temperature screening effect of neoclassical transport of impurity ions. The simulation results such as peaking factor of the HDPs and the effects of main ITG are found in coincidence with the evidence observed in argon injection experiment on HL-2A tokamak. Thus, the IM turbulence is demonstrated to be a plausible mechanism for the transport of impurity ions with HDPs. A strong main ITG, ETG, and a low electron density gradient are expected to be beneficial for sustainment of HDPs of impurity ions and reduction of impurity accumulation in core plasma. [ABSTRACT FROM AUTHOR]

• A 13-channel far-infrared (FIR) laser Polarimeter/Interferometer has been designed for the HL-3 tokamak, including eight horizontally-viewing channels and five oblique channels. • The novel structure of support tower allows us to arrange multi-channel light-path on both sides. • Multichannel line-integral electron densities have been successfully measured by the interferometric measurement in 2023, with a time resolution of <1.0 microsecond. The interferometer system will be upgraded to the three-wave based Polarimeter/Interferometer. Aiming to achieve important parameters of the electron density and plasma current density profile, a multi-channel far-infrared (FIR) laser Polarimeter/Interferometer is being developed for HL-3 tokamak by using the formic-acid laser source (HCOOH, λ=432.5 μm). All the 13 probing channels were distributed in two columns, including eight horizontally-viewing channels and five oblique channels. The stainless steel support tower (7.0 m high) was specially designed, and itself novel structure allowed us to arrange the light-paths of horizontal channels and oblique channels on two sides, respectively. Copper corner-cube-reflector (CCR) was mounted in the vacuum vessel to retrace the incident wave. In 2023, multi-channel line-integral electron densities were successfully derived by the interferometric measurement, with a time resolution of 1.0μs, line-integral density resolution of ∼5.0 × 1016/cm2. Right now, the interferometer system is being upgraded to three-wave based Polarimeter/Interferometer, which is able to simultaneously measure the electron density and Faraday rotation angle. [ABSTRACT FROM AUTHOR]

• Combined analysis of interferometer and reflectometer is developed based on the projection of electron density to equilibrium magnetic surfaces. • The density profile is reconstructed by Gaussian Process (GP) within the Bayesian framework, by which the uncertainty can be quantified via the confidence interval of a posterior probability. • This combined analysis approach is proved to provide a consistent density profile with improved accuracy and reliability. In order to acquire a consistent and accurate electron density profile for HL-2A tokamak, a combined analysis of the laser interferometer and microwave reflectometer based on the mapping of density profiles to a common equilibrium magnetic coordinate has been proposed. In this approach, the edge density profile measured locally by microwave reflectometer is taken as a supplementary boundary condition in the procedure of inverting line-integrated data from interferometer diagnostic. The density profile inside that boundary is reconstructed by a Gaussian Process Tomography (GPT) method [1] within the Bayesian framework, by which the uncertainty of the result can be quantified via the confidence interval of a posterior probability. In the simulation benchmark, two representative cases of density distribution with different profile shapes, e.g. peaked and flat, are tested and the factors that may affect the accuracy of the results are investigated, providing a useful reference for practical applications. The application of this approach to actual data proves its ability to resolve the dynamic evolution of density profiles. Specifically, the effect of Electron Cyclotron Resonance Heating (ECRH) on electron particle transport observed in HL-2A experiments has been confirmed. The combined analysis approach developed is expected to provide a robust means to study the confinement performance and particle transport on HL-2A, with the focus on the core region. [ABSTRACT FROM AUTHOR]

• Analyzing the propagation characteristics of non-Gaussian beam. • Designing an in-vacuum collection optics with stable safety shutter structure. • Developing a new compact polychromator suitable for multi-point measurements. A vertical edge Thomson scattering diagnostic system on HL-2 M Tokamak has been developed recently. A Nd: YAG laser (1064 nm, 0.6–2.0 J, 30 Hz, 15 ns) is used as the probe beam. The laser beam propagates vertically through the plasma region and the scattered light is observed horizontally. The combination of a half-wave plate and a polarized beam splitter is used for stray light suppression. A group of in-vacuum collection lens is designed to image the 400 mm scattered region onto the rectangular fiber arrays. The laser beam waist and vertical spatial resolution are 2 mm and 10 mm, respectively. Scattered light is imaged on the 2.20 × 2.86 mm (10 × 13) fiber optic bundle. The development of collection optics with a special designed safety shutter ensures a reliable scattered signal, where the solid angle at central field of view (FOV) is 0.018 sr. A compact polychromator is developed to measure the scattered light. Relative calibration of the filter response (1060 nm, 1052 nm, 1029 nm and 982 nm) and absolute calibration by nitrogen Raman scattering have been completed. In the 2022 plasma campaign of HL-2M, the scattering signal is measured at E 0 = 1.2 J, confirming the capability of the system for pedestal measurement in the future H-mode campaign. [ABSTRACT FROM AUTHOR]