Interactions Between Electron Cyclotron Wave,lower Hybrid Wave And Neutral Beam Injection On EAST

Electron cyclotron wave(ECW),low hybrid wave(LHW)and neutral beam injection(NBI)are widely used in plasma discharge heating and current driving,and are the main heating means to achieve and maintain steady-state high-parameter long-pulse operation in EAST.Each of these heating methods has different characteristics,ECW has highly localized power deposition location and easy control of plasma current,but the driving current efficiency is much lower than that of LHW;LHW has the highest driving current efficiency among frequency(RF)waves and can provide main non-inductive current for EAST steady-state operation;NBI is a very effective ion heating approach on EAST and its physical mechanism of heating is relatively simpler.These three heating methods are often used simultaneously on EAST to achieve higher plasma operation regimes.In this case,there are interactions between these heating means that affect each other’s heating efficiency as well as the drive current efficiency.Previous studies on focused on synergy effects between LHW and ECW within relatively lower temperature or without including relativistic effects.However,in recent EAST experiments,the electron temperature has been raised to 9.7 keV with simultaneous injection of LHW and ECW.In this case,it is expected that relativistic effects could play a very important role in the interactions between RF waves and plasma.In addition,interactions between different heating approaches in previous works are mainly focused on the aspect that the plasma distributions in phase space are modified during heating process and in turn affects the resonant interactions between plasmas and waves.However,in a longer time scale,for example in the transport time scale,the interaction between different heating methods can also be induced due to the changes of the background plasma profiles during heating processes,and this has been observed in EAST experiments when the ECW,LHW and neutral beam are injected simultaneously.At the moment,the mechanism is not clear and the theoretical and simulation studies are desired.The purpose of this paper is to explore and study the interaction between several available future plasma heating and current driving methods with the aim of studying the actual heating and current driving of EAST,and to provide a theoretical analysis basis for improving the EAST operating parameters and improving the heating and current driving efficiency.The first part of the paper focuses on the effect of relativistic effects on the synergy between ECW and LHW.In experiments on EAST for exploring high core electron temperatures,electron temperature at the center can be raised up to 9.7 keV by injecting ECW and LHW simultaneously.With such strong core electron heating,the relativistic effect could play an important role in the interactions between the plasma and waves.In order to explore relativistic effect on synergy between the ECW and LHW on EAST,the ray-tracing/Fokker-Planck simulations are conducted to investigate electron heating for a typical discharge with the center electron temperature being 9.7 keV.It is found that the relativistic effect can lead to the ECW to deposit its power deeper into plasma core,where the synergy between the ECW and LHW occurs and enhances the absorption of the LHW.It is found that the modification of the ECW resonance layers cannot be ignored as the electron temperature is much higher than 6 keV.On the other hand,the relativistic effect on the damping of the LHW components with high N| is more significant than with lower N|,which leads to a greater difference in the power deposition of the LHW in the range of 0.6<p<0.8.The analysis shows that the influence of the relativistic effect on LHW mainly relies on N| as well as the electron temperature.As a result,a high center electron temperature can be achieved.It was also investigated that changes in the temperature profile at times longer than the collision time also affect the background plasma as well as the power deposition of LHW.The second part of the work in this paper is in recent experiments on EAST with NBI and RF waves(LHW and ECW)heating,it was observed that an increase in the line-averaged electron density and a decrease in the plasma internal inductance occurred in the duration of NBI.Experiments have shown that NBI may alter plasma transport,which in turn affects the low clutter current drive profile.To explain the experimental observation,the simulations are conducted by using the equilibrium code EFIT and the transport codes ONETWO and TGYRO.It is shown that L-H transition is triggered right after neutral beam is injected,leading to the formation of density pedestal,the plasma density gradient in the outer region(0.4<p<0.75)is thus changed,which leads to an increase in ηi(ηi=Lni/LTi)(where p is the normalized square root of the toroidal magnetic flux,Lni and LTi are the normalized density and temperature gradient,respectively).A transition from a trapped electron modes(TEM)to an ion temperature gradient(ITG)modes dominant turbulence regime then happens due to the increase ofηi in the outer region(0.4<p<0.75),which results in an inward particle pinch and the rise of the plasma density.As a result of the rise of plasma density,the parallel refractive index and ray trajectories of the LHW are affected and more power is deposited outside p>0.6,which drives more off-axis current and therefore the internal inductance is reduced.