NONLINEAR DYNAMICS OF LINEARLY UNSTABLE n = 0 ELECTROSTATIC PERTURBATIONS IN CONVENTIONAL TOKAMAK PLASMAS

The nonlinear dynamics of axisymmetric radially localized oscillations of the electric potential in a tokamak with stationary toroidal plasma rotation is investigated. In the linear approximation, these oscillations split into two independent branches: geodesic acoustic modes (GAMs) and low-frequency zonal flows (ZFs). The stability of the latter is determined by the specifics of plasma equilibrium, and the frequency/growth rate – by the velocity of stationary rotation. It is shown that the nonlinear dynamics of the electric potential and the associated fluctuations of pressure, density, and longitudinal plasma velocity within the MHD model has integrals of motion. The evolution of the electric potential and hydrodynamic plasma characteristics is calculated for different velocities of stationary plasma rotation and for different initial values of the electric field perturbations. The regime, which initial stage corresponds to linearly unstable ZF, is studied in detail. It is shown that at the nonlinear stage, the fluctuations of electric potential reach amplitude-limited oscillations of both low frequency and GAM. The resulting oscillation spectrum exhibits GAM frequency splitting and intermittency.