Edge Kinetic Codes
Edge kinetic codes solve the plasma distribution function across the separatrix, capturing turbulence and non-Maxwellian effects that edge fluid closures cannot represent.
Why kinetic at the edge
The tokamak edge violates the assumptions behind fluid closures. Gradients are steep on the scale of the ion orbit width, the plasma is only marginally collisional, and the distribution function departs from Maxwellian. Edge kinetic codes abandon the fluid moments and evolve the full distribution function f(x,v,t), typically with a gyrokinetic or drift-kinetic formulation, across both closed and open field lines.
This lets them capture edge turbulence, the formation of the pedestal, blob transport in the SOL, and the sheath physics at material surfaces within one framework, at the price of far greater computational expense than fluid edge codes.
Total-f and the X-point
Edge kinetic codes usually solve for the total distribution rather than a small perturbation, because the edge has order-unity fluctuations and large mean flows. Handling the magnetic separatrix and X-point, where flux coordinates break down, requires unstructured or cylindrical meshes and special numerical care.
Coupling to neutrals and walls
Realistic edge kinetics still needs neutral recycling and wall interaction, so these codes embed or couple to neutral transport and sheath models. The combination is among the most demanding multiphysics problems in fusion, routinely consuming large HPC allocations.
Design relevance
Edge kinetic simulation informs pedestal and divertor predictions where fluid transport coefficients are uncertain. For the tightly shaped Hyperion breeder, kinetic edge studies help bound the cross-field transport that fluid divertor models must assume, strengthening the simulation case ahead of construction.
- Evolves the full distribution function at the edge
- Captures turbulence, pedestal, blobs, sheaths
- Handles the X-point without flux coordinates
- Very high computational cost