Research Of Current Density Profile In EAST High Poloidal Beta Operation

Current density profile has been considered as a key issue in the realization of high confinement steady-state tokamak operation.In the advanced steady-state operation scenario,plasma is driven by fully non-inductive current including bootstrap current and external driven current.Through proper current density profile tailoring,detrimental instabilities can be effectively avoided and internal transport can be reduced,which would consequently improve plasma confinement.On the other hand,external driven current is supplied by radio-frequency(RF)current drives and neutral beam current drive(NBCD),whose cost and current drive efficiency severely limit the economy of fusion reactors in the future.Fortunately,bootstrap current originated from plasma itself could serve as a compensation.Researches have shown that the bootstrap fraction(fbs)is proportional to the poloidal beta(βp),indicating that higher fbs could be achieved with higher βp,thus decreasing the dependence on external driven current and improving the economy of fusion reactors.EAST(Experimental Advanced Superconducting Tokamak)is the first superconducting tokamak in the world and one of the few devices similar to ITER(International Thermal Experimental Reactor).Exploring current density profile tailoring in EAST high βp scenario and understanding the physics underneath could lay the foundation for high confinement,high boostrap fraction long pulse plamsa operation in EAST.At the same time,it also provides strong support for the steady-state operation for ITER device in the future.The dissertation focuses on the current density profile broadening utilizing both external current drives and bootstrap current,exporing current density profile control methods through delicate adjustment of both plasma parameters and external current drives in EAST high βp operation.In this dissertation,full kinetic equilibrium reconstruction using EFIT is conducted with current density profile constraint of either POINT(Polarimeter-INTerferometer)diagnostics or MSE(Motional Stark Effect)diagnostics in EAST.Calculations of current densities of different external current drives are conducted by simulation codes,including GENRAY,TORAY,CQL3D and NUBEAM along with Sauter model for bootstrap current.Characteristics of current density profile under various parameters are presented and the mechanism are analysed in the dissertation as follows.In high βp plasmas with pure RF heating and current drive,it is found that higher plasma density is benefit for current density profile broadening through plasma density scan experiments.Simulation analyses show that higher βp can be achieved in higher density,thus increasing fbs,which would help broaden current profiles.On the other hand,current density of lower hybrid current drive(LHCD)broadens with increasing plamsa density as well,which also benefits in the current profile broadening.Moreover,the LHCD efficiency drop in higher density could be compensated by higher fbs and higher input power of LHCD,hence the non-inductive current fraction is maintained by proper power arrangement.Experiments of different injection time of RF power show LHCD current profiles are quite sensitive to core electron temperature.It is found LHCD current profiles peak with higher core electron temperature,whereas lower core electron temperature results in broader LHCD current profiles.Moreover,off-axis electron cyclotron current drive(ECCD)is found to achieve current profile broadening as well.However,the gradient of both density and temperature profiles deceases under off-axis ECCD,lowering fbs.To maintain high fbs and non-inductive operation,higher power input is required.Based on experiments of both RF and NBCD,it is found that injection of neutral beam could further increase βp,consequently increasing fbs,which could further broaden current density profiles.However,limitations pertain to the application of NBCD in current profile broadening.Analyses show that fast ion pressure induced by the injection of neutral beam accounts in large part for the βp increasment,whereas it does not contribute to the bootstrap current.Therefore,current density profiles can not be further broadened in some high βp experiments.The optimization of fast ion pressure is critical in next high βp experiments with NBCD.The other limitation concerns the integration of RF and NBCD and the heat load in plasma facing components,especially in the divertor region considering high input power of both RF and NBCD and fast ion loss induced by NBCD.Therefore,a dynamic foveated imager has been installed in EAST with increased spatial resolution by a factor of 2.It is helpful to monitor local heat load with its high resolution and mobility,providing safety ensurance when utilizing high power input in experiments exploring weak or reversed shear in EAST.Researches in the thesis provide significant physics understanding and experimental references for the establishment of weak or reversed shear and the improvement of confinement in EAST by dedicated tailoring of current density profiles.