| Computer simulations of two different drift wave models, one for collisionless trapped electron modes (CTEM) and one for ion temperature gradient driven modes (ITG), have revealed evidence of nonlinear instability. The mechanism for this instability is related to nonlinear excitation of a damped branch in the drift wave dispersion relation which is nonorthogonal to the growing branch. Because of the manner in which it is excited, the damped branch shows strong phase correlation with the growing branch. As a result, terms in the energy evolution equation related to the phase between branches play an important role in these systems.; We present simulation evidence of this instability as well as a general theory for this type of effect. Solving for the relative phase between dispersion branches using standard statistical closures used in turbulence theory provides us with a method of quantifying and predicting this effect. In addition, we explore its implications for various approximations used in plasma turbulence theory. |
