Plasma in a typically elongated cross-section tokamak(for example, EAST) is inherently unstable against vertical displacement. When plasma loses the vertical position control, it moves downward or upward, leading to disruption, and a large halo current is generated helically in EAST typically in the scrape-off layer. When flowing into the vacuum vessel through in-vessel components, the halo current will give rise to a large J × B force acting on the vessel and the in-vessel components. In EAST VDE experiment, part of the eddy current is measured in halo sensors, due to the large loop voltage.Primary experimental data demonstrate that the halo current first lands on the outer plate and then flows clockwise, and the analysis of the information indicates that the maximum halo current estimated in EAST is about 0.4 times the plasma current and the maximum value of TPF × Ih/IP0is 0.65, furthermore Ih/Ip0and TPF × Ih/Ip0tend to increase with the increase of Ip0. The test of the strong gas injection system shows good success in increasing the radiated power, which may be effective in reducing the halo current.
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.
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.
A preliminary analysis of plasma current quenching is presented in this paper based on the disruption database.It demonstrates that 26.8%of discharges have been disrupted in the last 2012 campaign,in addition,the plasma disruptive rate grows with the increase of plasma current.The best-fit linear and instantaneous plasma current quench rate is extracted from the recent EAST disruptions,showing that an 80%-30%interval of the maximum plasma current is well fit for the EAST device.The lowest area-normalized current quench time is 3.33 ms/m^2 with the estimated plasma electron temperature being 7.3 eV~ 9.5 eV.In the disruption case the maximum eddy current goes up to 400 kA,and a fraction of currents are respectively driven on the upper and lower outer plate with nearly 100 MPa-200 MPa stress in the leg.
The method of plasma current profile reconstruction using the polarimeter/interferometer(POINT) data from a simulated equilibrium is explored and validated.It is shown that the safety factor(q) profile can be generally reconstructed from the external magnetic and POINT data.The reconstructed q profile is found to reasonably agree with the initial equilibriums.Comparisons of reconstructed q and density profiles using the magnetic data and the POINT data with 3%,5%and 10%random errors are investigated.The result shows that the POINT data could be used to a reasonably accurate determination of the q profile.
