Gyrokinetic Turbulence Codes
Gyrokinetic codes simulate the microturbulence that dominates cross-field transport by solving a reduced kinetic equation on the gyro-averaged distribution.
The gyrokinetic reduction
Full kinetics tracks each particle in six-dimensional phase space, which is intractable for turbulence over transport timescales. Gyrokinetics averages over the fast gyration of particles around field lines, removing one velocity dimension and the fast cyclotron timescale while keeping the physics of drift-wave turbulence. The result is a five-dimensional equation for the gyro-averaged distribution function.
What the turbulence does
Small-scale instabilities driven by density and temperature gradients (ion-temperature-gradient modes, trapped-electron modes, electron-temperature-gradient modes) grow into turbulence that transports heat and particles across flux surfaces far faster than collisions alone would. This turbulent transport sets the confinement quality of the plasma.
Numerical approaches
- Eulerian (continuum): the distribution is evolved on a fixed phase-space grid
- Lagrangian (particle-in-cell): marker particles sample the distribution and deposit onto a spatial grid
- Semi-Lagrangian: combines a grid with characteristic tracing
Outputs
A gyrokinetic run yields turbulent heat and particle fluxes, growth rates and frequencies of the dominant modes, and the structure of the turbulence. These fluxes can be fed into transport solvers, and the growth rates underpin the reduced quasilinear models used for fast prediction.
Cost and scope
Gyrokinetic simulations are among the most computationally demanding in fusion, especially global, multi-scale, electromagnetic runs. Analysts choose the smallest configuration that captures the physics of interest: a local flux-tube for a transport coefficient, a global domain for large-scale structure.
For the Hyperion breeder, gyrokinetic estimates inform whether the assumed confinement is consistent with the expected turbulent transport at the design gradients.