Structure-preserving Kinetic Simulations Of Lower Hybrid Wave In Magnetically Confined Plasma

Radio-frequency wave system is an important part in various magnetic confinement fusion devices today.The lower hybrid wave(LHW),whose frequency is located in the lower hybrid range,has a wide range of applications in many devices because of its relatively low frequency and simple antenna structure for launching.Since the 1970s,the utilization of LHW for plasma heating has been studied in-depth.Nowadays,in mainstream tokamak devices,it is one of the most important ways to achieve the steady-state operation by using the Landau damping mechanism of LHW to generate and maintain plasma current.Although the application of LHW in today’s fusion device has achieved great success,many problems still hold.For instance,with the successive increment of plasma parameters and the wave source power,non-linear phenomena could lead to critical problems on wave coupling at the plasma edge,propagation and absorption in plasmas,and under certain parameters that will lead to complete failure of the system.Thus,a full understanding of the interaction between LHW and plasma is the key to solve these problems.Numerical approaches provide an intuitive way to understand and study physical phenomena.In recent years,the rapid increment of computer performance and the development of structure-preserving algorithms have made it possible to simulate multiscale,long-term plasma problems based on the first-principle particle-in-cell(PIC)method.In this work,using the SymPIC,a PIC simulation framework,and based on the explicit high-order non-canonical symplectic algorithm,the full-kinetic electromagnetic simulation of the LHW is carried for two typical magnetic confinement regimes:tokamak and magnetic mirror.In the tokamak devices,LHW is mainly applied for auxiliary heating and current drive.In this work,we studied the most concerning issues of LHW,such as the coupling efficiency,the density cut-off in the high-density plasma,Landau damping absorption.Then,we present a systematic analysis of the LHW current drive and its energy deposition.Thanks to the electromagnetic modeling in our PIC method,we have given the reflection coefficient of the lower hybrid multi-junction waveguide antenna under different edge density profiles consistently.In the meantime,we show the reflection location and present the physical picture in the simulation of slow to fast mode conversion.We also performed a long-term simulation that shows the transition of linear to non-linear damping.The damping rates for different regimes are given and well agree with theoretical results.For the simulations of LHW injection and current drive in the tokamak regime,we have elucidated the importance of momentum conservation and proposed a micro-mechanism that explains the generation of macroscopic current.In addition,the energy deposition of LHW in non-uniform plasma is shown,and effects of parasitic absorption are investigated.For the magnetic mirror device,we investigated the feasibility and the effect of lower hybrid resonance heating mechanism.Theoretically,the lower hybrid resonance heating can raise the perpendicular kinetic energy of ions through various absorption mechanisms.It has a great potential application value in the linear magnetic mirror device because the raising of perpendicular energy means the raising of the pitch angle which can benefit to particle confinement.Here,we mainly studied the influence of the high axial non-uniform magnetic field on the propagation and resonance behavior of LHW in the magnetic mirror.The kinetic and non-linear effects of lower hybrid resonance itself are studied as well.In this part of the work,we present the 3D modeling of magnetic mirror device,which demonstrated the behavior of LHW propagation in the high non-uniform magnetic field,especially for the wave scattering at mirror throat and angular transmission results.Under the resonance parameters,we present the ion heating effect near the resonant region.It shows that ion temperature near magnetic throat increased significantly.Over 10%increment is achieved within several wave periods in the resonance layer.The results indicate the feasibility and advantages of lower hybrid heating in magnetic mirror devices.In general,we presented the first-principles,structure-preserving,full-kinetic,electromagnetic simulation of the multi-scale and long-term evolution of LHW in magnetically confined plasmas.Our simulations of LHW in tokamak and magnetic mirror show many meaningful phenomena in the interaction between LHW and plasmas.Based on the unified PIC simulation framework,SymPIC,we can readily shift to different issues with only a minor modification on simulation parameters.Moreover,thanks to the excellent scalability of our algorithm,we can use a variety of supercomputers to carry out device-scale simulations and would provide strong support for experiments and theories.