Computer Simulation of Transport Driven Current in Tokamaks

Plasma transport phenomena can drive large currents parallel to an externally applied magnetic field. The Bootstrap Current Theory accounts for the effect of Banana Diffusion on toroidal current, but the effect is not confined to that transport regime, or even to toroidal geometry. Our electromagnetic particle simulations have demonstrated that Maxwellian plasmas in static toroidal and vertical fields spontaneously develop significant toroidal current, even in the absence of the "seed current" which the Bootstrap Theory requires. Other simulations, in both cylindrical and toroidal geometries, and without any externally imposed electric field, show that if the plasma column is centrally fueled, then an initial toroidal current grows steadily, apparently due to a dynamo effect. The straight cylinder does not exhibit kink instabilities because k_ {z} = 0 in this 2 + 1/2 dimensional model. When the plasma is fueled at the edge rather than the center, the effect is diminished. Fueling at an intermediate radius should produce a level of current drive in between these two limits, because the key to the current drive seems to be the amount of total poloidal flux which the plasma crosses in the process of escaping. In a reactor, injected (cold) fuel ions must reach the center, and be heated up in order to burn; therefore, central fueling is needed anyway, and the resulting influx of cold plasma and outflux of hot plasma drives the toroidal current. Our simulations indicate that central fueling, coupled with the central heating due to fusion reactions may provide all of the required toroidal current. The Neoclassical Theory predicts that the Bootstrap Current approaches zero as the aspect ratio approaches infinity; however, in straight cylindrical plasma simulations, axial current increases over time at nearly the same rate as in the toroidal case. These results indicate that a centrally fueled and heated tokamak may sustain its own toroidal current, even in the absence of the "seed current" which the Bootstrap Theory requires. Such "100% bootstrapped" tokamak plasmas have been realized in experiments. The Bootstrap and dynamo mechanisms do not drive toroidal current where the poloidal magnetic field is zero, and so the Bootstrap Theory asserts that a "seed current" must be maintained by some other means at the magnetic axis. The simulations and analytical theory presented here indicate that in tokamak conditions, there is an additional spontaneous current drive due to the preferential loss of particles whose current opposes the net plasma current. In a centrally fueled tokamak, this mechanism drives current even at the magnetic axis. If this effect functions in experiments as it does in our simulations, then Transport Driven Current would eliminate the need for any external current drive in tokamaks, except simple ohmic heating for initial generation of the plasma.