Kinetic-MHD and Hybrid Codes
Kinetic-MHD codes couple a fluid bulk plasma to a kinetic treatment of energetic particles, capturing wave-particle resonances fluid models cannot.
When fluids are not enough
Some instabilities are driven by a small population of fast particles, from neutral-beam injection, radio-frequency heating, or fusion-born alphas, whose distribution is far from Maxwellian and whose dynamics involve resonances with waves. Pure fluid MHD cannot represent these. Kinetic-MHD, or hybrid, codes solve the bulk plasma as an MHD fluid but treat the energetic species kinetically, coupling the two through the momentum equation.
The fast-particle pressure or current enters the fluid force balance, while the fluid fields push the kinetic markers. This captures the drive and damping of Alfven eigenmodes and fishbones that set fast-ion confinement.
The coupling scheme
Typically the energetic population is represented by particle-in-cell markers evolving in the MHD fields, contributing a kinetic closure term to the fluid pressure tensor. The pressure-coupling and current-coupling schemes are two standard formulations differing in which moment feeds back to the fluid.
Alfven eigenmodes
The central application is the stability of shear Alfven eigenmodes destabilized by fast-ion pressure gradients. If these modes grow, they can eject energetic particles before they thermalize, degrading heating efficiency and threatening the first wall, so predicting them is a safety and performance concern.
Design relevance
For the Hyperion breeder, the confinement of fusion-born and beam fast ions bears on heating and on wall loads. Kinetic-MHD simulation assesses whether Alfvenic activity would expel fast ions at the design point. In the Aegis and MetroVolt burner, energetic D and He-3 populations make such analysis central, all at simulation stage.
- Fluid bulk plus kinetic energetic species
- Coupled through the momentum equation
- Predicts Alfven eigenmode drive and damping
- Governs fast-ion confinement and wall loads