Density Control
Density control regulates how many particles the plasma holds, using gas puffing and pellet injection against pumping and the density limit.
Why density matters
Plasma density enters fusion power quadratically, so more density means more output, but there is a limit. Above the density limit, the edge cools, radiation rises, and the plasma is prone to disruption. Density control keeps the plasma near the productive value while staying below this boundary.
Fueling actuators
- Gas puffing, which adds particles at the edge and is fast but shallow
- Pellet injection, which delivers frozen fuel deep into the plasma
- Supersonic or high-field-side injection for better penetration
- Recycling from the wall, an uncontrolled but real particle source
The estimation side
Line-integrated density is measured by interferometry in real time, giving the controller a fast signal. Reconstructing the density profile from several chords lets profile control act on where the particles are, not just how many. Wall recycling makes the particle balance history-dependent, which the controller must accommodate.
Interaction with other loops
Density couples to almost everything: to divertor detachment, which depends on upstream density; to impurity control, since fueling dilutes impurities; and to the density limit relative to current, which tightens during ramp-down. A density controller therefore cannot be tuned in isolation from the divertor and current loops.
In the Kronos program
The Hyperion breeder uses deuterium-tritium fuel, so density control also manages the fuel mix and interacts with tritium handling. Pellet injection provides the deep fueling needed to sustain the core density at the design operating point, while gas puffing trims the edge and supports detachment. These loops are coordinated in simulation ahead of operation.