Chaotic advection in quiet discharges in tokamaks

A guiding-centre model for turbulent electron diffusion in quiet discharges in tokamaks is discussed. The dependence of the corresponding diffusion coefficient on the amplitude and steepness of the model has been studied. A half-way time-reversing procedure to assess the possible deviation often referred to as numerical diffusion is proposed. It is demonstrated that an implicit version of the Runge-Kutta integrator that leads to the same numerical diffusion as its explicit counterpart is 10-20 times faster. (UK)