Experimental Investigation On Excitation And Modulation Of Beta-induced Alfvén Eigenmode In The J-TEXT Tokamak

In the future tokamak plasmas,fast particles can excite various magnetohydrodynamic instabilities,such as toroidal Alfvén eigenmode(TAE),beta-induced Alfvén eigenmode(BAE).These instabilities can cause the re-distribution and significant loss of fast particles,which is unfavorable for self-heating.Therefore,the study of excitation and modulation of these instablities is of great significance.In this thesis,using Electron Cyclotron Resonance Heating(ECRH)and resonant magnetic perturbation(RMP),we have investigated the characteristics and excitation mechanisms of the BAE on J-TEXT,and further studied the modulation of the BAE via ECRH,Supersonic Molecular Beam Injection(SMBI)and Electrode Biasing(EB).The characteristics and excitation mechanisms of the BAE are studied on J-TEXT.The fast electron induced BAE(e-BAE)of m/n=7/2 and 5/1(m is the poloidal mode number,and n is the toroidal mode number)with frequency being 22-35 k Hz and law proportional to the Alfvén speed,is successfully excited by ECRH.It is found that the frequency of e-BAE is comparable to the precession frequency of trapped fast electrons.This verifies the mechanisms of trapped fast electrons driven e-BAE through wave-particle resonance.Moreover,the m/n=3/1 magnetic island induced BAE(m-BAE)is excited by applying the 3/1 RMP,and the dispersion relationship between the frequency of m-BAE and the Alfvén speed is verified.By bi-spectrum analysis,it is found that there is nonlinear coupling between magnetic island,electrostatic geodesic acoustic mode(GAM)and m-BAE.Moreover,the amplitude of GAM is gradually weakened,while the amplitude of the m-BAE is gradually enhanced with the increase of magnetic island width.Therefore,we conclude that after the magnetic island width exceeds certain critical value,the energy redistributed between the GAM and the m-BAE,then the mBAE is excited via the couple between GAM and tearing mode.Furthermore,we also investigate the modulation of m-BAE by ECRH,SMBI and EB based on the above understanding of the excitation mechanisms.The m-BAE excited by the static magnetic island(sm-BAE)is suppressed by increasing the ECRH power.The amplitude of the sm-BAE is inversely proportional to the ECRH power and there is a power threshold.While,the m-BAE excited by the rotating magnetic island(rm-BAE)can be enhanced by increasing the ECRH power.We can explain the experimental observations on the modulation of m-BAE by ECRH via the interaction between ECRH and magnetic island.We conclude that ECRH can stabilize the static magnetic island,and then stabilize the sm-BAE.In contrast,ECRH can destabilize the rotating magnetic island,which will increase the local temperature and the number of fast electrons,thereby destabilizes the rm-BAE.The amplitude of sm-BAE as well as its standing wave node is modulated by SMBI,and the mechanism for the phase locking between the standing wave node of m-BAE and the magnetic island is verified.EB is also used to adjust macroscopic parameters of plasmas,and then to modulate the amplitude and frequency of m-BAE.In summary,this thesis have investigated the characteristics of e-BAE and m-BAE,and verified the excitation mechanisms for e-BAE by the trapped fast electron precession and mBAE by the interaction between magnetic island and electrostatic GAM.We have also realized the modulation of m-BAE by ECRH,SMBI and EB.In addition,we also designed and built a new set of Alfvén antenna system and tested its performance,which provides engineering basis for the future investigation.The experimental and engineering results of this thesis may provide some implications on the excitation and control of BAE.