It is widely believed that low-frequency (lower than ion cyclotron frequency) electromagnetic perturbations are more important than high-frequency ones in transporting plasma in tokamaks, based on some non-conclusive observations and analytical theories. (This assumption can be verified numerically when we are able to do a full simulation including both low-frequency and high-frequency perturbations. This kind of verification is not possible at present due to the difficulties in doing a full simulation.)
If only the low-frequency perturbations are present, the Vlasov equation can be simplified. Specifically, some kind of symmetry of the perturbed particle distribution function in the phase space can be established if we choose suitable coordinates (independent variables) and split the distribution function in a proper way. The symmetry is along the so-called gyro-angle α in the guiding-center coordinates (X,v⊥,v∥,α). In obtaining the equation for the gyro-angle independent part of the distribution function, we need to average the coefficients of the equation over the gyro-angle α and thus this model is called “gyrokinetic”.
In deriving the gyrokinetic equation, the perturbed electromagnetic field is assumed to be known and of low-frequency. To do a kinetic simulation, we need to solve the field equation to obtain the perturbed electromagnetic field. It is still possible that high frequency modes (e.g., compressional Alfven waves and ΩH modes) appear in a gyrokinetic simulation. If the amplitude of high frequency modes is large, then the simulation is not meaningful since the gyrokinetic model is invalid in this case.