Divertor Detachment Control
Detachment cools and dilutes the plasma before it reaches the divertor target, spreading heat over gas and radiation instead of concentrating it on a surface.
The heat-exhaust problem
The scrape-off layer channels exhaust power along field lines onto narrow divertor targets, where the unmitigated heat flux can exceed what any material tolerates. Detachment is the operating regime where the plasma pressure and heat flux drop sharply before the target because energy is radiated away and momentum is lost to neutral gas, cushioning the surface.
How it is driven
Detachment is promoted by raising neutral density near the target through fueling, and by seeding an impurity that radiates strongly. As the divertor cools, the ionization front moves off the target. Push too far and the front detaches all the way up toward the core, degrading confinement, so control aims for stable partial detachment rather than the extreme.
Sensing and feedback
Because the detachment front can move suddenly, control needs a fast indicator of its position: target current or Langmuir-probe signals, divertor spectroscopy, thermocouple or infrared temperature of the target, or a measure of the radiated fraction. The controller adjusts impurity seeding and gas puffing to hold the front at a target location, keeping the target protected without over-cooling.
- Detachment spreads exhaust over radiation and gas
- Driven by neutral density and impurity seeding
- Sensed by target current, spectroscopy, and IR
- Control holds the front stably, avoiding over-cooling
Stability of the loop
The detachment front's response can be nonlinear and hysteretic, so the loop is designed with care to avoid oscillation between attached and detached states. Feedforward from the planned scenario plus feedback trim on the seeding rate is a common structure. In the Kronos breeder design study, exhaust handling is part of the engineering basis for a high-power spherical tokamak; the machine is simulated, and detachment control is described here as a general subsystem behaviour.
Detachment control is where fueling, impurity seeding, and thermal monitoring converge to protect the divertor.