In order to understand the recycling and emission processes of hydrogen atoms in HT-7, spectral profiles of the Dα(Hα) line emitted in front of the limiter have been observed with a high-resolution spectrometer and simulated by using the neutral particle transport code DEGAS 2. The results show that four processes are necessary to interpret the Dα(Hα) line shape: 1) atom desorption, 2) molecular dissociation, 3) particle reflection, and 4) charge-exchange. The products of the first two processes are cold atoms which emit photons near the peak of Dα(Hα) line shape, and those from the last two are warm atoms contributing to the blue side of the spectrum. For a typical ohmic discharge (shot 68520 ne(0) ≈ 3 × 1019 m-3), these components contribute 32%, 15%, 32% and 21%, respectively. Dα(Hα) line shapes under different plasma parameters are also discussed in this paper.
A detailed study of photon temperatures(T_(ph))of hard X-ray emission in lowerhybrid current drive(LHCD)plasmas is presented.The photon temperature increases with theincrease in plasma current and decreases with the increase in plasma density.In lower hybridpower and phase scanning experiments;there is no appreciable change in the photon temperature.The numerical results based on ray-tracing calculation and Fokker-Planck solver gives reasonableexplanation for the experimental observation.Both experimental and numerical results reveal thatthe photon temperature depends mainly on global effects of the fast electron population,synergybetween the fast electron and the loop voltage and the Coulomb slowing down.
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.
Plasmas with vertically elongated cross-sections tend to be unstable to an axis-symmetric instability.This paper studies the magnetohydrodynamic equilibria in elongated plasmas after failure of vertical feedback control by using magnetic data for EAST device.Vertical forces on the vessel due to the induced polodial and toroidal currents are evaluated.The maximum force of the F zpol in vertical displacement events for EAST designed parameters is given.
Intermittent convective transport at the edge and in the scrape-off layer(SOL)ofEAST was investigated by using fast reciprocating Langmuir probe.Holes,as part of plasmastrTictures,were detected for the first time inside the shear layer.The amplitude probabilitydistribution function of the turbulence is strongly skewed,with positive skewed events("blobs")prevailing in the SOL region and negative skewed events("holes")dominant inside the shearlayer.The statistical properties coincide with previous observations from JET.The generationmechanism of blobs and holes is also discussed.In addition burst structure and dynamics characterof them are also presented.
If βN exceeds no-wallβN, the plasma will be unstable because of external kink and resistive wall mode(RWM). In this article, the effect of the passive structure and the toroidal rotation on the RWM stability in the experimental advanced superconducting tokamak(EAST) are simulated with CHEASE and MARS codes. A model using a one-dimensional(1D)surface to present the effect of the passive plate is proved to be credible. The no wall βN limit is about 3li, and the ideal wall βN limit is about 4.5li on EAST. It is found that the rotation near the q = 2 surface and the plasma edge affects the RWM more.
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.
Growth rates of the axisymmetric mode in elongated plasmas in the experimentaladvanced superconducting tokamak (EAST) are measured with zero feedback gains and thencompared with numerically calculated growth rates for the reconstructed shapes.The comparisonis made after loss of vertical position control.The open-loop growth rates were scanned with thenumber of vessel eigenmodes,which up to 20 is enough to make the growth rates settled.Theagreement between the growth rates measured experimentally and the growth rates determinednumerically is good.The results show that a linear RZIP model is essentially good enough for thevertical position feedback control.
Reconstruction of experimental axisymmetric equilibria is an important part of toka-mak data analysis.Fourier expansion is applied to reconstruct the vessel current distribution inEFIT code.Benchmarking and testing calculations are performed to evaluate and validate thisalgorithm.Two cases for circular and non-circular plasma discharges are presented.Fourier ex-pansion used to fit the eddy current is a robust method and the real time EFIT can be introducedto the plasma control system in the coming campaign.
The effect of passive plates on vertical displacement control in the EAST tokamak is investigated by open loop experiments and numerical simulations based on a rigid displacement model.The experiments and simulations indicate that the vertical instability growth rate is reduced by a factor of about 2 in the presence of the passive plates,where the adjacent segments are not connected to each other.The simulations also show that the vertical instability growth rate is reduced by a factor of about 10 if all adjacent segments on each passive plate loop are connected to each other.The operational window is greatly enlarged with the passive plates.
