Local Gyrokinetic Codes
Local (flux-tube) gyrokinetic codes simulate microturbulence in a small volume along one field line, delivering transport fluxes at modest cost.
The flux-tube idealization
Gyrokinetics reduces the six-dimensional kinetic problem to five by averaging over the fast gyromotion, evolving the distribution of gyrocenters. A local or flux-tube code exploits scale separation further: it simulates only a thin tube following one field line, assuming the equilibrium profiles and their gradients are constant across the small simulation domain.
This is justified when the turbulence correlation length is small compared with the machine size, which holds for ion-scale turbulence in conventional tokamaks. The payoff is a dramatic reduction in cost relative to a global simulation.
What it computes
The code evolves electrostatic and electromagnetic potential fluctuations and the perturbed distribution, driven by density and temperature gradients. Its principal output is the turbulent heat, particle, and momentum flux as a function of the driving gradients, which feeds directly into transport solvers.
Periodic boundaries and gradient drive
Because the domain is small, flux-tube codes use periodic boundary conditions perpendicular to the field and a twist-and-shift condition along it. The gradients that drive turbulence are held fixed as parameters, which makes the local approach a natural quasilinear or nonlinear flux calculator rather than a whole-device model.
Limits and design use
Locality breaks down when the ion orbit width is not small compared with the profile scale length, which is common in spherical tokamaks. For the Hyperion breeder, local runs give fast turbulence scans but are cross-checked against global simulations because the low aspect ratio stresses the flux-tube assumption. All such work is simulation preceding construction.
- Simulates one field-line tube
- Assumes constant gradients across the domain
- Outputs turbulent transport fluxes
- Cheap but limited at low aspect ratio