Plasma Density Control
Regulating the plasma density with gas puffing and pellet injection while respecting confinement, fueling efficiency, and stability limits.
Why density matters
Fusion power scales with the square of density, so density is a primary performance lever - but it is bounded above by a stability limit and shaped below by fueling and pumping. Density control regulates fueling actuators to hold a target density profile through the discharge, balancing power output against the risk of density-limit disruptions.
The actuators
- Gas puffing: fast, easy to modulate, but fuels mostly the edge with modest efficiency
- Pellet injection: frozen fuel pellets deposit particles deeper in the plasma, more efficient and better for the core
- Pumping and wall conditioning: set the baseline particle balance
- Supersonic or high-field-side injection: improve fueling penetration
The control challenge
Density responds to fueling through slow, nonlinear particle transport with significant delay, and the wall acts as a large, drifting reservoir that absorbs and releases fuel. This makes density a hard loop: the plant is slow, uncertain, and time-varying. Controllers combine feedforward fueling waveforms with feedback on measured density, often using interferometry for the fast measurement.
The upper limit
Density cannot be pushed indefinitely. The Greenwald limit sets an empirical ceiling proportional to plasma current over cross-sectional area; approaching it degrades confinement and raises disruption risk. Density control must hold performance high while keeping a margin below this limit - a recurring theme in scenario design.
Profile, not just average
What matters for fusion is the density profile, especially in the core, not just the line-averaged value diagnostics read most easily. Pellet injection is favored where a peaked core profile is wanted, since edge gas puffing raises the average without efficiently fueling the center.