Plasma Density Feedback Control
Density control holds the plasma's particle content at a target by commanding gas and pellet fueling against real-time interferometer measurements.
Measuring density
The controlled quantity is usually the line-averaged electron density along a chord, measured by interferometry: a probing beam's phase shift is proportional to the integrated density it passes through. Multiple chords give a profile. This measurement is fast and reliable, making density one of the better-controlled plasma quantities.
Actuators
Fueling actuators are gas valves for fast, shallow response and pellets for deep, efficient core fueling. Pumping and wall retention remove or store particles on slower timescales. The controller blends these: gas for quick trims around the target and pellets for the bulk core inventory, respecting each actuator's speed and depth of deposition.
Control design
A density loop must handle a plasma that both fuels and empties itself: recycling from the wall returns particles, and the wall can absorb or release fuel depending on its condition. Because these effects vary, the loop often combines feedforward from the planned scenario with feedback trim, and it is tuned to avoid oscillation when edge-deposited fuel slowly reaches the core.
- Line-averaged density measured by interferometry
- Gas for fast trim, pellets for deep fueling
- Wall recycling adds a variable particle source
- Feedforward plus feedback for a stable target
Why it matters
Fusion power scales roughly with the square of density at fixed temperature, so density directly sets the reaction rate, while too high a density approaches a disruptive limit and can cool the plasma. Density control therefore lives between two edges. In the Kronos breeder design model, holding the D-T density at its set point is part of sustaining the simulated operating point; the machine is a design and simulation case, and no net-gain hardware claim precedes first tritium.
Density control is where fueling hardware meets the plasma physics of the reaction rate.