Radiated Power Control
Managing how much of the plasma's power is radiated - useful for protecting the wall, dangerous if it runs away.
Radiation as a tool and a hazard
Impurities in the plasma radiate power. A controlled amount is useful: it spreads exhaust power over the wall and enables divertor detachment. Too much is dangerous: excessive core radiation cools the plasma, can drive a radiative collapse, and is a common step on the path to a density-limit disruption. Radiated-power control keeps radiation in the useful band.
The measurement
Bolometers measure the radiated power, resolving it into core and edge contributions across the plasma cross-section. This is a fast, robust, control-grade diagnostic. The radiated fraction - radiated power over input power - is a key control signal: rising toward one warns of impurity accumulation and impending collapse.
The actuators
- Impurity seeding to increase edge radiation deliberately
- Heating to offset radiative cooling
- Fueling and pumping to influence impurity transport
- Sawtooth and mode control to flush accumulating impurities from the core
Core versus edge
Where radiation happens matters. Edge radiation is beneficial - it dissipates exhaust power before it reaches the divertor. Core radiation is harmful - it removes power from where fusion needs it and can collapse the temperature. Control aims to promote edge radiation for exhaust while suppressing core radiation from accumulated high-Z impurities.
The runaway risk
Radiation and cooling can form a positive feedback: cooling raises certain impurity radiation, which cools further. Left unchecked this is a radiative collapse that ends in disruption. Radiated-power control watches the radiated fraction and impurity content as early-warning signals, feeding the disruption predictor and avoidance logic before the loop runs away.