[en] An analytical algorithm for fast calculation of the Abel inversion for density profile measurement in tokamak is developed. Based upon the assumptions that the particle source is negligibly small in the plasma core region, density profiles can be approximated by an asymmetrical Gaussian distribution controlled only by one parameter V₀/D and V₀/D is constant along the radial direction, the analytical algorithm is presented and examined against a testing profile. The validity is confirmed by benchmark with the standard Abel inversion method and the theoretical profile. The scope of application as well as the error analysis is also discussed in detail

[en] A naturally occurring velocity shear layer was observed at the plasma edge of HT-7 tokamak in regular ohmic heated discharges. One fast reciprocating Langmuir probe was used to measure all quantities in the radial force balance equation for main ion, which enables us to present the first report about the radial force balance in the boundary region of the HT-7 tokamak. The sharp gradient of radial electric field and the reduced fluctuation correlation and turbulent particle flux characterized the edge velocity shear layer. It was found that the shear of turbulence poloidal velocity was dominated by the E x B flow shear and the poloidal rotation determined the structure of radial electric field profile and as a result the E x B flows

[en] Reynolds stress component due to magnetic turbulence was first measured in the plasma edge region of the HT-7 superconducting tokamak using an insertable magnetic probe. A radial gradient of magnetic Reynolds stress was observed to be close to the velocity shear layer location; however, in this experiment its contribution to driving the poloidal flows is small compared to the electrostatic component. The electron heat transport driven by magnetic turbulence is quite small and cannot account for the total energy transport at the plasma edge

[en] Measurements of boundary fluctuations and fluctuation driven electron fluxes have been performed in ohmic and lower hybrid current drive enhanced confinement plasma using a graphite Langmuir probe array on HT-7 tokamak. The fluctuations are significantly suppressed and the turbulent fluxes are remarkably depressed in the enhanced plasma. We characterized the statistical properties of fluctuations and the particle flux and found a non-Gaussian character in the whole scrape-off layer with minimum deviations from Gaussian in the proximity of the velocity shear layer in ohmic plasma. In the enhanced plasma the deviations in the boundary region are all reduces obviously. The fluctuations and induced electron fluxes show sporadic bursts asymmetric in time and the asymmetry is remarkably weakened in the lower hybrid current driving (LHCD) phase. The results suggest a coupling between the statistical behaviour of fluctuations and the turbulent flow

[en] This paper reviews the current state of understanding of the L–H transition phenomenon in tokamak plasmas with a focus on two central issues: (a) the mechanism for turbulence quick suppression at the L–H transition; (b) the mechanism for subsequent generation of sheared flow. We briefly review recent advances in the understanding of the fast suppression of edge turbulence across the L–H transition. We uncover a comprehensive physical picture of the L–H transition by piecing together a number of recent experimental observations and insights obtained from 1D and 2D simulation models. Different roles played by diamagnetic mean flow, neoclassical-driven mean flow, turbulence-driven mean flow, and turbulence-driven zonal flows are discussed and clarified. It is found that the L–H transition occurs spontaneously mediated by a shift in the radial wavenumber spectrum of edge turbulence, which provides a critical evidence for the theory of turbulence quench by the flow shear. Remaining questions and some key directions for future investigations are proposed. (review)

[en] The statistical properties of plasma fluctuations are characterized in the boundary region of HT-7 tokamak. A non-Gaussian feature is observed in fluctuations of ion saturation current and floating potential in most of the scrape-off layer regions. The statistical properties of fluctuations have a clear radial dependence, showing a near-Gaussian character in the proximity of the velocity shear layer location and another region where the poloidal velocity has a tend to zero. Fluctuations show a burst character with pulses asymmetric in time and the time asymmetry reaches the minimum around the shear layer. From the results, we can see an obvious coupling of the pulses and the poloidal flow

[en] The phenomena of improved edge confinement due to ion cyclotron radio frequency boronization were observed with a fast reciprocating Langmuir probe in the HT-7 tokamak. A strong shear layer of radial electric field was produced in the plasma edge region, which resulted in the formation of an edge transport barrier. Consequently, the edge profiles of electron density were steepened, and both the absolute and relative fluctuation levels were suppressed by the shearing E x B flows. Concomitant reduction of the coherence between electron density and poloidal electric field fluctuations and the change of their cross-phase resulted in turbulent particle flux dropping by more than a half at the plasma edge. This demonstrates the de-correlation effect of turbulence and its contribution to edge transport. The results presented here suggest a link between wall conditions and boundary plasma physics, especially an interaction between atomic processes and turbulence through the formation of radial electric field shear at the plasma edge

[en] Measurements of boundary parameters and their fluctuations have been performed in ohmic (OH) plasma and Ion Bernstein Wave (IBW) and Lower Hybrid Current Drive (LHCD) enhanced confinement plasma using a pneumatically driven Langmuir probe array on HT-7 tokamak. In the enhanced confinement plasma, the gradients of electron density and temperature become higher and a transport barrier comes into being in the vicinity of the limiter. The boundary potential shows a clear modification in the same region. The fluctuation levels are significantly depressed and the coherences between fluctuations are reduced evidently in the enhanced plasma. Meanwhile, we obtained the spectral features and the poloidal phase velocity of fluctuations using a two-point correlation technique and found obvious modifications of the turbulence and the poloidal flow. The results suggest that the improved confinement in the IBM and LHCD enhanced plasma is at least partially due to the modification of the boundary parameters and the suppression of the boundary fluctuations and fluctuation induced fluxes

[en] High time resolution measurements of the electrostatic fluctuations, radial electric field E_r and turbulence-induced electron flux Γ_e have been performed across the transition of lower hybrid current drive improved confinement with a graphite Langmuir probe array at the last closed flux surface of the HT-7 tokamak. The decrease of Γ_e is dominated by the suppression of fluctuation levels, which follows the change of E_r. A reversal of the poloidal propagation direction of turbulence demonstrates that the poloidal propagation is dominated by E_r x B_φ drift. The enhancement of poloidal coherence accompanies the fluctuation suppression, which suggests the subtle variation of turbulence features

[en] The neutral density plays an important role in the low to high (L-H) confinement mode transition. In this work, population coefficients of hydrogen's n=3 excited state from hydrogen collisional-radiative (CR) model (from DEGAS 2 data file) are used to calculate the photon emissivity coefficients (PECs) of hydrogen Balmer-α (n=3 → 2) (H_-alpha). The results are compared with the PECs from Atomic Data and Analysis Structure (ADAS) database and fit well. 1.5-D fluid transport code FRANTIC gives magnetic surface-averaged neutral density profile of typical double null (DN) plasma of EAST. It is found that the sum of integral D_-alpha and H_-alpha emission intensity calculated via the neutral density agrees with the measurement from the absolutely calibrated multi-channel poloidal photodiode array systems viewing the lower divertor at the last closed flux surface (LCFS). This fact indicates that the typical magnetic surface-averaged neutral density at LCFS is about 3.5x10¹⁶ m^-3. (author)