Density gradient bifurcation in tokamaks

Certain drift waves in tokamaks have the property that they become unstable when the temperature gradient exceeds a critical value, which increases with increasing density gradient. If instabilities of this type dominate the transport of heat and particles then it is shown that the particle flux can be a multivalued function of the density gradient. A bifurcation to a reduced transport state can then be driven through the density gradient. The conditions for a bifurcation to exist are derived. In order for this bifurcation to be observable, trapped particle instabilities must be suppressed by some other mechanism. (author)