Divertor Detachment Control
Regulating the plasma edge so the heat reaching the divertor is radiated away before it can damage the target.
The exhaust problem
The power leaving the plasma core is funneled along field lines to the divertor targets, where it can exceed what any material can survive as a concentrated load. Detachment is the operating state in which most of that power is radiated and dissipated in the divertor volume before reaching the target, spreading and softening the load. Controlling detachment is controlling power exhaust.
What detachment is
As edge density and radiation rise, the plasma near the target cools and the pressure along the field line drops, so the plasma effectively detaches from the surface - particle and heat fluxes to the target fall sharply. Detachment protects the target, but too much detachment cools the edge enough to degrade core confinement or trigger instability. The useful state is a controlled, partial detachment.
The actuators
- Impurity seeding (for example nitrogen, neon, argon) to enhance edge radiation
- Divertor and edge fueling to raise density near the target
- Pumping to set the neutral balance
- Heating, which sets the power that must be exhausted
The control challenge
Detachment is a strongly nonlinear, threshold-like phenomenon: near the detachment front the plasma is sensitive, and the front can move rapidly between attached and over-detached states. Control must hold the front in a narrow window using indirect measurements (target heat flux, edge radiation, spectroscopic signals), which makes it a hard regulation problem with tight margins.
Coupling to core control
Detachment control cannot be done in isolation: raising edge density and radiation affects the Greenwald fraction and the core, so it is coordinated with density and radiated-power control. Managing the edge and the core as one coupled system is essential to a reactor-relevant scenario.