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 Wang’s theoretical prediction.The parametric dependence of electron energy convective velocity on plasma current is still ambiguous and is worthy of future research on EAST.
Ultraviolet (UV) and visible impurity spectra (200～750 nm) are commonly used to study plasma and wall interactions in magnetic fusion plasmas. Two optical multi-channel analysis (OMA) systems have been installed for the UV-visible spectrum measurement on EAST. These two OMA systems are both equipped with the Czerny-Turner (C-T) type spectrometer. The upper vacuum vessel and inner divertor baffle can be viewed simultaneously through two optical lenses. The OMA1 system is mainly used for multi-impurity lines radiation measurement. A 280 nm wavelength range can be covered by a 300 mm focal length spectrometer equipped with a 300 grooves/mm grating. The Dα/Hα line shapes can be resolved by the OMA2 system. The focal length is 750 mm. The spectral resolution can be up to 0.01 nm using a 1800 grooves/mm grating. The impurity behaviour and hydrogen ratio evolution after boroniztion, lithium coating, and siliconization are compared. Lithium coating has shown beneficial effects on the reduction of edge recycling and low Z impurity (C,O) influx. The impurity expelling effect of the divertor configuration is also briefly discussed through multi-channels observation of OMA1 system.
FU JiaLI YingyingSHI YuejiangWANG FudiZHANG WeiLV BoHUANG JuanWAN BaonianZHOU Qian
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 n e and electron temperature T e 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 P loss was comparable during the L-H transition, by comparing the adjacent L-mode and H-mode discharge. The Dα emission, T e and n e decreased rapidly in the time scale of about 1 ms, and the radial electric field E r 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 E r 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.
In the discharge of EAST tokamak,it is observed that(2,1) neoclassical tearing mode(NTM) is triggered by mode coupling with a(1,1) internal mode.Using singular value decomposition(SVD) method for soft X-ray emission and for electron cyclotron emission(ECE),the coupling spatial structures and coupling process between these two modes are analyzed in detail.The results of SVD for ECE reveal that the phase difference between these two modes equals to zero.This is consistent with the perfect coupling condition.Finally,performing statistical analysis of γ_(1/1),ζ_(1/1) and ω_(2/1),we find that γ_(1/1) more accurately represents the coupling strength than ζ_(1/1),and γ_(1/1) is also strongly related to the(2,1) NTM triggering,where γ_(1/1) is the width of(1,1) internal mode,ζ_(1/1) is the perturbed amplitude of(1,1) internal mode,and ω_(2/1) denotes the magnetic island width of(2,1) NTM.
Properties of the geodesic acoustic mode (GAM) density fluctuations are studiedusing two toroidally separated Langmuir triple-probe arrays on the top of HT-7 tokamak. TheGAM scenario is identified in the potential fluctuations with the toroidally symmetric structure(n= 0) and satisfying the temperature scaling of GAM mode frequency. Some theoretical predictionsabout the mode features of GAM density fluctuations are verified in our experiments:the toroidal mode number of GAM density fluctuations is n=0; its amplitude is consistent withthe theoretical prediction in a factor of 2; the density and potential fluctuations of GAM is inanti-phase at the top of plasma cross-section. Strong nonlinear interactions are found betweenGAM density fluctuations and ambient turbulence (AT). The results support the conclusions thatthe envelope modulation of potential fluctuations is dominantly caused by the direct regulation ofGAM in the generation processing, and the envelope modulation of density fluctuation is due tothe GAM shear effect.
The spectra of HeI(587.6 nm),H_α(656.28 nm) and Dα(656.1 nm) of the helium discharges as well as the normal deuterium discharges have been measured with two optical spectroscopic multi-channel analysis(OMA) systems on the experimental advanced superconducting tokamak(EAST).The influx ratio of the sum of H and D to He spectral lines and the influx ratio of H to D are given.In this way the ratio of hydrogen/deuterium ion(S/X B)H/D to(S/X B)He as well as(S/X B)H/D is not very sensitive to the variation in the edge density and temperature. The low-density helium discharges are operated in order to reduce the recycling hydrogen fluxes; however,the effect is not obvious.The possible reason is that the number of helium discharges is not enough and the content of hydrogen in the wall is still very abundant,which is caused by frequent wall conditionings and the vacuum leakage.The H/(H+D) ratio decreases quickly after one lithium coating and reduces to less than 10%using several accumulated lithium wall conditioning.It is found that the deposited He atoms on the carbon wall will remain at a low level after several D2 discharges.
Ion’s toroidal velocity,v_t,in both the outermost 4 cm of the confined region and thescrap-off layer of Ohmic L-mode plasmas in EAST was measured using Mach probes.At about1 cm inside the separatrix a local minimum in v_t was observed,from which a cocurrent rotationincreased both inwards and outwards.The radial width of the v_t dip was 1 cm to 2 cm,andboth 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 toroidaltorques induced by neutral friction,neoclassical viscosity,collisional perpendicular shear viscosity,ion orbit loss and turbulent Reynolds stress were estimated using the measured parameters.Ourresults indicate that in this particular parameter regime the neutral friction was the dominantdamping force.The calculated cocurrent toroidal torque by the neoclassical viscosity dominatesover those from the collisional perpendicular shear viscosity,ion orbit loss and turbulent Reynoldsstress.These results are potentially important for the understanding of boundary conditions forthe intrinsic toroidal momentum in tokamak plasmas.
