Two-fluid Simulations For Tearing Mode And Error Field Penetration In Tokamak Plasmas

In the advanced tokamak,a high fraction of the bootstrap current and hollow current profile is needed for steady-state operation.For example,the goal of China Fusion Engineering Test Reactor（CFETR）is to replace 80%—90%of the total plasma current by bootstrap current,and the core of plasma exits strong reversed magnetic shear.Although the high fraction of the bootstrap current offers the opportunity for non-inductive tokamak operation,reversed magnetic shear configuration with high bootstrap current can trigger the neoclassical tearing mode instability.Magnetic reconnection deriving from tearing modes leads to the formation of magnetic islands,which can degrade the confined energy levels,result in violent disruptions,and even damage the fusion device.Besides,the error field penetration in experiment can induce the tearing mode instability,and lead to the subsequent island growth.The two-fluid equations about drift-tearing mode is far more convincing than single-fluid theory in the regime relevant to present-day researches of high temperature plasmas.In this dissertation,we numerically studied the drift-tearing mode and double tearing mode（DTM）instability in tokamak by a four-field two-fluid model.The major researches include the effects of diamagnetic drift on tearing mode bursts in reversed magnetic shear configuration,the suppressive effects of electron diamagnetic velocity on neoclassical double tearing modes and the density scaling of error field penetration in EAST tokamak experiments.The main content of this thesis contains the following parts:In chapter 1,a brief review of nuclear fusion energy,magnetic confined plasma physics theory and Magnetohydrodynamic（MHD）instabilities.In chapter 2,fast magnetic reconnections,resulted from DTM,are numerically investigated through a reduced two-fluid model.It is found that diamagnetic drift plays a destabilizing role in the DTM’s Rutherford phase,but can effectively stabilize the following explosive fast magnetic reconnection.The plasma viscosityhas an important effect on the critical value₍（8） for completely suppressing the DTM’s explosive bursts,and the relevant mechanism in low and high viscosity regime are analyzed.In addition,the relationship of critical viscosity₍（8） on viscosity and resistivity are studied.In chapter 3,based on the research of chapter 2,the explosive behaviors of neoclassical double tearing mode（NDTM）are numerically studied with the bootstrap current effect.It is observed that fast magnetic reconnection of NDTM can also be absolutely suppressed by diamagnetic drift flow.The dependence of the critical value of diamagnetic drift flow(1_{0*（8）avoided the explosion on typical parameters,such as rational surface separationΔand bootstrap current fraction1(}（7）,is investigated in detail.Furthermore,the particle perpendicular transport and parallel transport coefficient have a significant effect on(1₍0*（8）.In chapter 4,the density scaling of error field penetration has been analyzed under radio-frequency（RF）dominant heated in the EAST tokamak.The density scaling of resonant magnetic perturbation penetration threshold is about(7∝9)₀)^0.4,where(7represents the error field amplitude and9)₀)is the line averaged electron density.The results show a weaker density dependence compared to previous ohmic discharge((7∝9)₀)^0.5).We convert the key discharge parameters into theoretical penetration region,and infer the error field penetration lies in Waelbroeck regime([(7/]_（84）∝9)₀)^7/16-7/16₀)^9/321)₀^5/8).The observed scaling is consistent with the estimated theoretical result by taking into account the density determination with electron temperature(₀)),viscosity diffusion time()and plasma mode frequency（1）₀)in experiments.The empirical expression obtained by scanning heating power can also conform to theoretical values[(7/]_（84）∝9)₀)^7/16-7/16₀)^9/321)₀^5/8（Waelbroeck regime）.Considering that the heating effect of Lower hybrid wave（LHW）depends on electron density,the strong relationship₀)∝9)₀)^-0.65 results in weaker density scaling in auxiliary heated experiments.Moreover,Using realistic parameters under auxiliary and ohmic heating as input respectively,the numerical scaling based on reduced four-field two-fluid model agrees well with the experimental results.This provides an excellent validation of MHD theory on error field penetration in the RF dominant heated L mode discharges.At last,the summary and outlook of the paper are presented.