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Double-peaked distribution (DPD) of particle flux has only been observed on the outer divertor target in electron cyclotron resonance heating deuterium plasmas with B × ∇ B toward the X-point in the HL-2A tokamak using high spatiotemporal Langmuir probe arrays. The experimental results demonstrate that the formation of the DPD is mainly due to the enhanced poloidal E × B drift flow stimulated in the divertor region, which is dependent on the plasma density, heating power, and divertor structure. The experimental results are qualitatively consistent with the SOLPS simulation. The experiment also shows that the formation of the DPD might be related to the enhanced cross field transport in the far scrape-off layer. This experimental findings presented here reveal the crucial role played by the synergistic effect of poloidal E × B drift flow and the closed divertor structure in the redistribution of the particle flux, which provides a potential way for the control of high heat flux in future fusion devices. [ABSTRACT FROM AUTHOR]

$Z_{\mathrm {eff}}$ is measured by two visible bremsstrahlung diagnostics on HL-2A tokamak, one of which uses a high spectral resolution spectrometer coupled with an electron multiplied CCD (temporal resolution of 100 Hz), and the other uses a filterscope system (temporal resolution of 1 MHz). The bremsstrahlung brightness measured by them coincides with each other very well. $\bar {Z}_{\mathbf {eff}}$ is systematically measured under different discharge conditions, such as divertor and limiter configurations, ohmic and auxiliary heating phases, before and after siliconization, and $L$ – $H$ transition. Under ohmic phase, $\bar {Z}_{\mathbf {eff}}$ is higher in the limiter configuration than that in the divertor configuration by a factor of ~1.5 under high-density regime. $\bar {Z}_{\mathbf {eff}}$ increases in all cases of auxiliary heating scenarios due to enhanced plasma–wall interaction, accompanied by influx increases of low ionization states of impurities such as C2+ and Fe1+ in plasma edge. Siliconization for wall conditioning is prominently effective in lowering the main impurity influxes in plasma edge and thus the impurity level, and a concentration of ~4% for carbon and ~0.2% for iron is roughly evaluated for a typical nonsiliconized plasma. A simple relationship that $Z_{\mathrm {eff}}$ is positively correlated with $\Phi _{\mathrm {in}}$ and $\tau _{\mathrm {p}}$ is verified from the L- and $H$ -mode features, where $\Phi _{\mathrm {in}}$ is the impurity influx and $\tau _{\mathrm {p}}$ the particle confinement time. A nearly constant $\bar {Z}_{\mathbf {eff}}$ evolution is observed during $L$ – $H$ and $H$ – $L$ transition and it seems that $\bar {Z}_{\mathbf {eff}}$ is insensitive to $L$ – $H$ and $H$ – $L$ transition under some $Z_{\mathrm {eff}}$ (0) threshold. The $Z_{\mathrm {eff}}$ profiles are flat/weakly hollow in plasma core region (rho < 0.7) in case of low $Z_{\mathrm {eff}}$ (0) and tend to be peaked when $Z_{\mathrm {eff}}$ (0) is larger than 3, which agrees with the neoclassical theory prediction. [ABSTRACT FROM AUTHOR]

The inward transport flux accompanied by the long-lived mode (LLM) has been observed by the edge Langmuir probe array during the L-H transition in the HL-2A tokamak. The cross phase between the poloidal electric field fluctuation and the electron density fluctuation in the LLM frequency changed greatly during the L-H transition, which led to the radial transport flux in the LLM frequency to change its transport direction in a given radial region near the pedestal barrier. By using the long distance correlation method, the phase shift of poloidal electric field fluctuation proved to be the key reason, which led to the cross-phase term to change in the LLM frequency. Furthermore, the shear effect has a mode-selecting characteristic on the LLM and turbulence because the direction of turbulent transport still remains radially outward during the H-mode. In addition, when the inward conducted heat flux accompanied by LLM occurs, the poloidal electric field fluctuation and electron temperature fluctuation both have a positive gradient in the LLM frequency; this may indicate that the positive gradient of radial velocity and electron temperature in the LLM frequency play an important role during the entire transport direction change process. [ABSTRACT FROM AUTHOR]

Three sets of triple probe arrays (four-tip) are applied to study the transport properties at the edge of a HL-2A tokamak. The Geodesic Acoustic Mode (GAM) exhibits intermittent characteristics during the discharge. The radial particle flux has been studied in different phases corresponding to the variable GAM intensity. The experimental results reveal that the radial particle flux contributed by the ambient turbulence ( 20 kHz – 100 kHz ) has been suppressed by 13% during the GAM bursts, contrasting weak GAM cases. Power of density fluctuations and coherence between density and potential fluctuations contribute to most reduction of the particle flux, while changing the cross phase between the density and potential fluctuations and the suppression of power of potential fluctuations hardly contribute to it. These results suggest that the GAM can regulate the turbulent transport mainly by changing the amplitude of ambient turbulence, rather than the cross phase between density and potential fluctuations. [ABSTRACT FROM AUTHOR]