Study On The Effect Of Downshift Resonant Absorption On Electron Cyclotron Current Drive Efficiency

Electron cyclotron resonance heating(ECRH)and electron cyclotron current drive(ECCD)are one of the most important auxiliary heating and non-inductive current drive methods in tokamak.Megawatt ECRH systems have been implemented on many fusion devices over the past decades.With increasing electron cyclotron(EC)heating power,the electron temperature of plasma also increases.EAST achieved a discharge of electron temperature of 10 keV in the experimental campaign of 2018,and even achieved a discharge of electron temperature of 14 keV with pulse width of 20 s in 2021.In the latest conceptual design of the China Fusion Engineering Test Reactor(CFETR),the electron temperature of the hybrid scenario plasmas reaches 30.48 keV.Therefore,it is necessary to study the ECCD configuration optimization in high-electron-temperature plasma in tokamaks.By numerical simulation of ECCD performance in high electron temperature plasma of EAST tokamak,it is found that the ECCD efficiency decreases rapidly with the increase of electron temperature.The ECCD efficiency is reduced by about 50%when the electron temperature increases from 10 to 15 keV,at a specific magnetic field and toroidal angle.As the electron temperature increases to 20 keV,the X3 absorption increases to 96.4%,and the ECCD efficiency is only 16%of that of 10 keV.The physical mechanism of the ECCD efficiency degradation was revealed.When the electron temperature is increased to 10 keV,the third harmonic extraordinary(X3)mode appears at the low field side(LFS)due to the downshift resonant absorption effect,even the cold resonance of X3 mode is located outside the plasma.The trapping electron effect at the LFS produces a reverse Ohkawa current.The competition between the Fisch-Boozer current drive and the Ohkawa current drive results in a decrease in ECCD efficiency.ECCD efficiency optimization is achieved through three methods.One is to increase the toroidal angle,leading to X2 mode predominating again over X3 mode and the electron resonance domain of X2 mode moving far from the trapped/passing boundary.The second one is to increase the magnetic field to move away the X3 resonance layer from the plasma,hence less EC power absorbed by X3 mode.The third one is to inject EC power with the first harmonic oridinary(O1)mode into plasma instead of the X2 mode,which moves the O2 resonance layer farther away from the plasma under the same parameter.Based on the above research,the EC wave absorption and current drive in CFETR hybrid scenario plasma is investigated and optimized.The ECRH and ECCD performance of 4 wave frequencies and 9 candidate launching points of EC antenna are systematically studied by scanning the poloidal and toroidal angles.The results show that the ECCD efficiency for 170 GHz EC system is quite low,it is due to the wave-particle interactions is located at the LFS,where the impact of trapped particles is larger.Since the electrons on the high field side(HFS)have higher energy,less collision and less impact of trapped particles,the wave frequency is increased up to 221～250 GHz,which lead to the power deposited at the HFS.The off-axis ECCD efficiency can be significantly enhanced by launching EC waves from the top window and injecting toward the HFS.The optimized ECCD efficiency at ρ=0.32 and at ρ=0.4 is 2.9 and 2.2 times that of 170 GHz,respectively.Considering the balance between ECCD efficiency and engineering constraints,the EC wave frequency,the position of EC antenna and the antenna steering angle are given,which provides the design basis for the next engineering design.In addition,Electron heat transport physics experiments in improvement energy confinement mode(I-mode)were carried out on EAST tokamak using ECRH modulation.I-mode discharge platform without low hybrid wave has been established on the EAST.The electron temperature modulation is achieved by injecting 5 Hz modulated ECRH power into the plasma.The experimental data of the amplitude and phase of the Fourier transform of the modulated temperature shows that there may be outward convection in the I-mode plasma.