THEORY OF SELF-ORGANIZED CRITICAL TRANSPORT IN TOKAMAK PLASMAS

A theoretical and computational study of the ion temperature gradient (ITG) and eta(i) instabilities in tokamak plasmas has been carried out . In a toroidal geometry the modes have a radially extended structure and their eigenfrequencies are constant over many rational surfaces th at are coupled through toroidicity. These nonlocal properties of the I TG modes impose a strong constraint on the drift mode fluctuations and the associated transport, showing self-organized criticality. As any significant deviation away from marginal stability causes rapid temper ature relaxation and intermittent bursts, the modes hover near margina lity and exhibit strong kinetic characteristics. As a result of this, the temperature relaxation is self-similar and nonlocal, leading to ra dially increasing heat diffusivity. The nonlocal transport leads to Bo hm-like diffusion scaling. Heat input regulates the deviation of the t emperature gradient away from marginality. We present a critical gradi ent transport model that describes such a self-organized relaxed state . Some of the important aspects in tokamak transport like Bohm diffusi on, near marginal stability, radially increasing fluctuation energy an d heat diffusivity, intermittency of the wave excitation, and resilien t tendency of the plasma profile can be described by this model, and t hese prominent features are found to belong to one physical category t hat originates from the radially extended nonlocal drift modes. The ob tained transport properties and scalings are globally consistent with experimental observations of low confinement mode (L-mode) discharges. The nonlocal modes can be disintegrated into smaller radial islands b y a poloidal shear flow, suggesting that the transport changes from Bo hm-like to near gyro-Bohm. (C) 1996 American Institute of Physics.