Numerical Simulation Of The Effect Of Edge Density Fluctuation On Lower Hybrid Wave In EAST

Lower hybrid current drive(LHCD)is one of the main auxiliary heating and driving modes of Tokamak devices,which is widely used in major devices.With the continuous improvement of the operating parameters of the device,higher plasma density and temperature will have a great impact on the LHCD efficiency.Especially in the scrapoff layer(SOL),the nonlinear effects of lower hybrid(LH)wave and plasma,including parameter decay,collisional absorption and wave scattering caused by low-frequency density fluctuation,will obviously change the LHCD efficiency.At present,the researches of parametric decay and collisional absorption show that parametric decay will greatly broaden the spectrum of LH wave and reduce the LHCD efficiency.The higher electron temperature in SOL can significantly reduce the broadening of LH spectrum.When the boundary density is high and the temperature is low,the share of boundary collisional absorption will increase significantly.Therefore,reducing the boundary density and increasing the boundary temperature can effectively reduce the loss of collisional absorption.However,the influence of wave scattering caused by lowfrequency density fluctuation on LHCD and the dominant factors are not clear.The possible main reason is that the density fluctuation at the edge causes the spectral change in SOL,which changes the LH power deposition and the distribution of driving current.Therefore,this paper will focus on the numerical simulation of the impact of wave scattering caused by edge low-frequency density fluctuation on LH wave.And further analyze the current driving characteristics to provide a powerful theoretical tool for carrying out and analyzing LHCD experiments at high density,and lay a foundation for further exploring the application of LH wave in fusion reactors in the future.Based on the eikonal method and Bonoli-Ott theory,combined with Runge-Kutta method and Monte-Carlo method,LH_scattering program is independently developed with verified Wentzel-Kramers-Brillouin(WKB)approximation,ray trajectory and scattering model.LH_scattering program focuses on the effect of wave scattering caused by low-frequency density fluctuation on LH spectrum at last closed flflux surface(LCFS).According to the calculated LH spectrum at LCFS,the influence of wave scattering on LHCD is analyzed.Results show that wave scattering can significantly change the distribution of LH parallel refractive index(N‖)and the poloidal refractive index(Nθ)at LCFS.Since the frequency of low-frequency density fluctuation is much less than the LH frequency,and the wave number of density fluctuation in the parallel magnetic field direction is much smaller than the wave number of the LH in the same direction,and the wave number in the vertical magnetic field direction is equivalent to the vertical wave number of the LH(k⊥),the frequency and N‖ of LH wave will not be changed in the single scattering process,but the LH wave number in the vertical magnetic field direction will be changed sharply.According to the wave-wave interaction,the k⊥ of LH wave incident from the antenna port will deflect at a certain angle,due to the density fluctuation,which will lead to the change of the LH ray trajectory,consequently,further change the N‖ and Nθ of LH wave.Results show that the value of Nθ has a great influence on the LH power deposition position and LHCD.Using LH_scattering program,the scattering probability and scattering angle probability distribution of different parameters on the local position are studied,and the difference and relationship between them and the probability distribution of LH N‖and Nθ at LCFS are analyzed.The analysis shows that the scattering angle probability distribution of local position is affected by the geometric approximation term and E×B drift term.Under the conditions of low frequency,small initial N‖,high LCFS density,high density fluctuation level and small density fluctuation coherence length,the amplitude and proportion of geometric approximation term and E×B drift term can significantly affect the scattering probability and scattering angle probability distribution of single scattering process.In addition,the probability distribution of LH N‖ and Nθ at LCFS are mainly broadened by LH parameters and plasma parameters.The frequency,polar injection position,initial LH N‖,density parameters at LCFS and the density fluctuation coherence length have a great influence on the probability distribution of LH Nθ at LCFS,while the change of density parameters at the antenna port more affects the change of N‖ at LCFS.