Study Of The Effects Of Magnetic Field Direction And Impurity On ELM Behavior In The EAST Tokamak

Energy confinement is essential for the application of fusion energy in future.Although the tokamak energy confinement time in high confinement mode(H-mode)is 2-3 times of that in low confinement mode(L-mode),the ITER98(y,2)scaling suggests that the energy confinement time decreases with heating power,which is undesired for the high power operation of future fusion reactor.Therefore,it is still necessary to further improve global energy confinement of H-mode operation.Significant improvement of plasma performance in H-mode discharges with favourable toroidal field Bt(i.e.anti-clockwise direction viewing from the top in upper single null configuration)has been widely observed in the EAST tokamak with pure radio-frequency(RF)heating in contrast to that with the unfavourable Bt(i.e.clockwise direction viewing from the top in upper single null configuration).Studying the physical mechanism behind the performance improvement in the favourable Bt would facilitate the high performance operation of future fusion reactors.In tokamak high confinement operation,divertor targets and first wall would tolerate the erosion of excessive stationary heat flux transported from the main plasma.More seriously,H-mode plasma is generally accompanied by the periodic edge localized modes(ELMs),and the transient heat flux induced by ELMs would lead to a serious erosion of wall materials.The mitigation of stationary and transient heat loads is a great challenge for tokamak high confinement operation.The radiative divertor technology with external impurity injection is an effective method to control the stationary heat load.Many experiments show that the external impurity injection not only reduces the heat load on divertor targets,but also has an important influence on plasma confinement and ELM behavior.The study of the physical mechanism behind the impurity effects on energy confinement and ELM behavior could provide an experimental reference and theoretical support for the application of radiative divertor regime in future reactors.In this thesis,a database of H-mode plasmas under pure RF heating is established,both statistical analysis and typical discharge comparison are utilized to study performance improvement under pure RF heating in favourable Bt.It is found that the improvement in plasma performance is mainly contributed by the increase of core electron temperature in the favourable Bt.The significant increase in the plasma performance has been observed with high lower hybrid wave(LHW)power and on-axis heating of electron cyclotron resonance heating(ECRH)in the favourable Bt.This performance improvement in favourable Bt demonstrates to be more evident with LHW power increasing while other operational parameters are similar.The scrape-off layer(SOL)density and recycling in the favourable Bt are siginificantly lower than that in the unfavourable Bt.A underlying physical mechanism is proposed in this thesis that,the lower density and reduced neutral particle recycling in the SOL region can mitigate the nonlinear parametric instability(PI)of LHW,improve the LHW power penetration into the core plasma without excessive power dissipation in the edge region,and thus enhance the LHW heating effect,increase the core electron temperature in the favourable Bt.For ELM behavior,it is found that the most evident difference of grassy ELM behavior in different magnetic field directions is that a type of ELM named "clustered grassy ELM" occurs more frequently in the favourable Bt.The impurity injection experiment under the background plasma with type-III ELM has been analyzed in detail in this thesis.It is observed in experiment that,with impurity injection,the transition from type-III ELMs to low-frequency large ELMs occurs,plasma density siginificantly increases,and the energy confinement shows no significant degradation.In the pedestal region,pedestal density and its gradient strongly increase,electron temperature decreases,there is little change in the pedestal top pressure and edge bootstrap current,and the pedestal pressure gradient moderately increases.The particle confinement improvement induced by impurity may be responsible for the increase of plasma density.More experiments show that the occurrence of large ELM is closely correlated with the pedestal density gradient.Linear stability analysis shows that the large ELM equilibrium is more unstable than the type-III ELM equilibrium with respect to the peeling-ballooning(PB)instabilities.Nonlinear simulation has successfully reproduced the ELM crash processes of type-?ELM and large ELM.A scan of pedestal density gradient in the linear stability analysis demonstrates that the steep pedestal density gradient could directly destabilize the low-n and intermediate-n peeling-ballooning modes(PBMs)instead of through the modifications of pedestal pressure gradient and edge current density,which may be the main reason for the triggering of large ELMs in this impurity injection experiment.The impurity injection experiment under the background plasma with mixed large and relatively small ELMs is analyzed in the thesis.It is observed in experiment that,with impurity injection,the mixed ELMs are mostly suppressed which is compatible with divertor detachment.Simultaneously plasma density increases and energy confinement is improved due to higher core pressure.In the pedestal region,density pedestal width increases,density gradient is significantly reduced,and edge temperature reduces.As a results,both pedestal pressure gradient and edge current density remarkably decrease.Further analysis indicates that the main physical mechanism of ELM suppression in this experiment is as follows:with Ne seeding the edge collisionality significantly increases,enhancing the edge coherent mode(ECM)amplitude and thus outward particle transport in the pedestal region,therefore a wide density pedestal with low density gradient is obtained.As a result of the combination of flattening of pedestal density,reduction in pedestal electron temperature and increase in Zeff and edge collisionality,the pedestal pressure gradient and edge bootstrap current,i.e.ballooning drive and peeling drive,decrease remarkably.consequently the pedestal becomes much more stable and mixed ELMs are suppressed.With EAST experimental data and the numerical calculation of NIMROD two-fluid model,it is demonstrated that the enhanced resistivity due to increased effective charge number Zeff(i.e.increased impurity level)can provide a stabilizing effect on low-n and intermediate-n PBMs in EAST,and the existence of the stabilizing effect does not depend on whether the Zeff profile is uniform or not.However,this resistivity stabilizing effect on PBMs in EAST appears to be weaker than that observed in NSTX.This result could facilitate our deeper understanding of the physics of impurity effects on energy confinement improvement and ELM mitigation.