Ding Siye

A Dip Structure in the Intrinsic Toroidal Rotation Near the Edge of the Ohmic Plasmas in EAST

Ions toroidal velocity, vt, in both the outermost 4 cm of the confined region and the scrap-off layer of Ohmic L-mode plasmas in EAST was measured using Mach probes. At about 1 cm inside the separatrix a local minimum in vt was observed, from which a cocurrent rotation increased both inwards and outwards. The radial width of the vt dip was 1 cm to 2 cm, and both the density and electron temperature profiles exhibited steep gradients at this dip position. It was observed in both divertor and limiter configurations. To find out its origin, the toroidal torques induced by neutral friction, neoclassical viscosity, collisional perpendicular shear viscosity, ion orbit loss and turbulent Reynolds stress were estimated using the measured parameters. Our results indicate that in this particular parameter regime the neutral friction was the dominant damping force. The calculated cocurrent toroidal torque by the neoclassical viscosity dominates over those from the collisional perpendicular shear viscosity, ion orbit loss and turbulent Reynolds stress. These results are potentially important for the understanding of boundary conditions for the intrinsic toroidal momentum in tokamak plasmas.

Simulations of Neutral Beam Ion Ripple Loss on EAST

Predictions on the ripple loss of neutral beam fast ions on EAST are investigated with a guiding center code, including both ripple and collisional effects. A 6% to 16% loss of neutral beam ions is predicted for typical EAST experiments, and a synergistic enhancement of fast ion loss is found for toroidal field (TF) ripples with collisions. The lost ions are strongly localized and will cause a maximum heat load of ~ 0.05 MW/m2 on the first wall.

Estimation of Neutral Density in Edge Plasma with Double Null Configuration in EAST

In this work, population coefficients of hydrogens n = 3 excited state from the hydrogen collisional-radiative (CR) model, from the data file of DEGAS 2, are used to calculate the photon emissivity coefficients (PECs) of hydrogen Balmer-α (n = 3 → n = 2) (Hα). The results are compared with the PECs from Atomic Data and Analysis Structure (ADAS) database, and a good agreement is found. A magnetic surface-averaged neutral density profile of typical double-null (DN) plasma in EAST is obtained by using FRANTIC, the 1.5-D fluid transport code. It is found that the sum of integral Dα and Hα emission intensity calculated via the neutral density agrees with the measured results obtained by using the absolutely calibrated multi-channel poloidal photodiode array systems viewing the lower divertor at the last closed flux surface (LCFS). It is revealed that the typical magnetic surface-averaged neutral density at LCFS is about 3.5 × 1016 m−3.

Neutron Yields Based on Transport Calculation in EAST ICRF Minority Heating Plasmas

Neutron diagnostics, including flux and energy spectrum measurements, have been applied on the experimental advanced superconducting tokamak (EAST). The absolute calibration of neutron yields has been achieved by a calculation method using the Monte Carlo automatic modeling (MCAM) system and the Monte Carlo N-Particles (MCNP) code. Since the neutron yield is closely related with the ion density and temperature, it is a good measure of plasma performance, especially the wave heating effect. In ion cyclotron range of frequencies (ICRF) experiments, the increase in the ion temperature derived by the neutron yield indicates an effective plasma heating. Minority protons damp a large fraction of the total wave power, and then transfer part of the energy to deuterium by collisions. Neutron spectrum measurements also indicate that no tail is created by high energy deuterons during ICRF heating. However, the ion temperature derived by the neutron yield is consistent with the result by using a poloidal X-ray imaging crystal spectrometer (PXCS), showing a reliable transport calculation.

Reciprocating Probe Measurements of L-H Transition in LHCD H-Mode on EAST

As the power available in the initial phase of the ITER operation will be limited, accessing the high confinement mode (H-mode) with low heating power will be a critical issue. In the recent experiment on EAST, the H-mode was obtained for the first time with lower hybrid current drive (LHCD) wave only. Reciprocating Langmuir probe measurements at the outer midplane showed that the electron density ne and electron temperature Te in the scrape-off layer (SOL) were significantly reduced in the ELM-free phase, resulting in the increase of lower-hybrid wave (LHW) reflection. It was found that the power loss Ploss was comparable during the L-H transition, by comparing the adjacent L-mode and H-mode discharge. The Dα emission, Te and ne decreased rapidly in the time scale of about 1 ms, and the radial electric field Er turned positive in this process near the last closed flux surface. Multiple L-H-L transitions were observed during a single shot when the applied LHW power was marginal to the threshold. The floating potential (Vf) had negative spikes corresponding with the Dα signal, and Er oscillation evolved into several intermittent negative spikes just before the L-H transition. In some shots, dithering was observed just before the L-H transition.

Study of Scrape-Off-Layer Width in Ohmic and Lower Hybrid Wave Heated Double-Null Divertor Plasma in EAST

Edge profiles in Ohmic and lower hybrid (LH) wave heated discharges in EAST are presented. A comparison of the measured profiles is made with those from the theoretical prediction for the scrape-off-layer (SOL) width. The edge plasma parameters are diagnosed by a triple probe divertor diagnostic system and fast reciprocating probes at the outer mid-plane. The experimental results show that the SOL width of double-null (DN) divertor plasmas in EAST appears to exhibit a negative dependence on the power crossing the separatrix, which is consistent with the collisional SOL scalings of JET and Alcator C-Mod. This will provide useful information for extrapolation to the ITER SOL width scaling for power deposition.

Observation of Electron Energy Pinch in HT-7 ICRF Heated Plasmas

Inward energy transport (pinch phenomenon) in the electron channel is observed in HT-7 plasmas using off-axis ion cyclotron resonance frequency (ICRF) heating. Experimental results and power balance transport analysis by TRANSP code are presented in this article. With the aids of GLF23 and Chang-Hinton transport models, which predict energy diffusivity in experimental conditions, the estimated electron pinch velocity is obtained by experimental data and is found reasonably comparable to the results in the previous study, such as Song on Tore Supra. Density scanning shows that the energy convective velocity in the electron channel has a close relation to density scale length, which is qualitatively in agreement with Wangs theoretical prediction. The parametric dependence of electron energy convective velocity on plasma current is still ambiguous and is worthy of future research on EAST.

Numerical study of fast ion behavior in the presence of magnetic islands and toroidal field ripple in the EAST tokamak

A peculiar first orbit loss type was found apart from the normal ones when we use ORBIT code to simulate fast ion orbits in the EAST tokamak. Fast ion orbits were studied in the presence of toroidal field (TF) ripple and magnetohydrodynamic (MHD) perturbations. We analyzed the properties of the drifted orbits in detail and compared their differences, finding that the combined effects of ripple and magnetic islands are much greater than the effects of either one of them alone. Then we investigated the orbit deviations as a function of pitch angle in different radial positions. The modeling results demonstrate that the loss of trapped particles is mainly caused by the ripple, while MHD perturbation mainly plays an important role in the passing particles. Furthermore we modeled the loss rate using different equilibriums. Results prove that a higher beta can indeed improve the confinement of fast ions, while a little change in the q profile can make the topologies of magnetic islands become quite different and results in quite different total particle losses.

Alfvén Instabilities Excited by Energetic Particles in a Parameter Regime Similar to EAST Operation

The kinetic excitation of ideal magnetohydrodynamic (MHD) Alfven instabilities is investigated for operations at the EAST tokamak. The instabilities include α-induced toroidal Alfven eigenmodes (αTAE; here, α = −q2Rdβ/dr, with q being the safety factor, β the ratio between the plasma and magnetic pressures, R the major radius, and r the minor radius), toroidicity-induced Alfven eigenmodes (TAE), and the energetic particle continuum mode (EPM). The αTAE, trapped by α-induced potential wells along the magnetic field line, can be readily destabilized by energetic particles due to negligible continuum damping via wave energy tunneling. It is shown for the geometry and the parameters similar to those of the EAST equilibrium that αTAE is different not only from the EPM by the potential-well determined frequency, but also from the TAE by the broad frequency spectrum outside the toroidal frequency gap.

Intermittent Bursts in the Boundary Plasma of HT-7

Intermittent characteristics of turbulence induced by coherent structures (blobs) was clearly observed in the ion saturation current signal, density fluctuation, particle flux and heat flux in HT-7. It is obvious that ion saturation current signal has deviated from Gaussian distribution and the skewness (S) and flatness (K) of signal increase radially outwards in the scrape-off layer (SOL). Using conditional analysis (CA), asymmetric character of the intermittent bursts are demonstrated. Owing to the radial propagation of the coherent structures, the particle density profile in SOL is non-exponential and flat outwards from the last close flux surface (LCFS). It is found around LCFS that the large burst fluctuations (above 2.5 rms) are responsible for about 50% of the total particle transport. Burst events move radially outwards with Eθ×B velocity, and the blob size can be calculated as δr ~ Vrτc. Our experiment shows that the blob size, life time and drift velocity experienced a pronounced decorrelation in the shear layer. The electrostatic Reynolds stress components become very strong and show a radially steep gradient in the proximity of the shear layer. These experimental findings may imply that the coherent structures are titled by the developed shear flow in the E × B shear layer.