Theoretical Study Of Nonlinear Saturation Of Reversed Shear Alfvén Eigenmode And Core Plasma Heating In Fusion Reactors

With the continuous progress of human society,the energy problem has gradually become an important factor limiting the future development prospects of human beings.Controlled nuclear fusion is theoretically a way to solve the energy problem once and for all.Among the mainstream methods currently being explored to achieve controlled nuclear fusion,the tokamak using a magnetic confinement mechanism to maintain continuous fusion of fuel ions is most likely to succeed in the future.In tokamak,the self-heating of the plasma by energetic particles(EPs),including fusion alpha particles,is important to achieve steady state operation and sustained fusion burning.But at the same time,EPs may resonately excite collective modes such as Alfvén instability due to wave-particle interactions;the Alfvén instability in turn affects the confinement of EPs.Therefore,the research of Alfven instability,especially the Alfvén eigenmodes(AEs),is of great significance for the realization of controllable fusion in the future.However,future large-scale advanced tokamak devices such as the International Thermonuclear Experimental Reactor(ITER)and the China Fusion Engineering Test Reactor(CFETR)often operate under reversed shear scenario,as the safety factor,q-profile,in the core is not monotonous and has an extremum.So that in the core of such devices,the reversed shear Alfvén eigenmode(RSAE)is most likely to be excited first by EPs.Thus,the generation,evolution,and saturation of RSAE in the system is likely to be very important for future large-scale experimental devices and fusion reactors.This is also themotivation of this study.The linear physics of RSAE has been thoroughly investigated,but the research on its nonlinear physics is still very insufficient.In this paper,the possible nonlinear evolution channels of RSAE are investigated using nonlinear gyrokinetic theory.The main results are as follows:1.General nonlinear equations describing RSAE self-modulation via zero frequency zonal structure(ZFZS)generation are derived,which are then applied to study the spontaneous ZFZS excitation as well as RSAE nonlinear saturation.It is found that both electrostatic zonal flow(ZF)and electromagnetic zonal current(ZC)can be preferentially excited by finite amplitude RSAE,depending on specific plasma parameters.The modification to local shear Alfvén wave continuum is evaluated using the derived saturation level of ZC,which is shown to play a comparable role in saturating RS AE with the ZFZS scattering.2.Equations describing the spontaneous decay of RSAE into low frequency Alfven modes(LFAM)are derived.And this process is a potential channel for core plasma heating because that LFAM can heat fuel ions via collisionless ion Landau damping.The conditions for RSAE spontaneous decay are investigated,and the saturation level and the consequent fuel ion heating rate are also derived.The channel is expected to be crucial for future reactors operating under reversed shear configurations,where fusion alpha particles are generated in the tokamak core where the magnetic shear is typically reversed.3.Nonlinear mode coupling processes of RSAE with toroidal Alfvén eigenmode(TAE)generating secondary modes are proposed and investigated.Specifically,there are two different channels.In the first channel,RSAE nonlinearly couples to a co-propagating TAE with the same toroidal and poloidal mode numbers,and generates a geodesic acoustic mode(GAM).In the second channel,RSAE couples to a counter-propagating TAE and generates an ion acoustic wave quasi-mode(IAW).The condition for the two processes to occur is favored during current ramp.Both processes contribute to effectively saturate the Alfvenic instabilities,as well as nonlinearly transfer of energy from energetic fusion alpha particles to fuel ions in burning plasmas.