Gas Puffing Control
Gas puffing feeds fuel to the plasma edge through fast valves, giving quick but shallow density control that anchors the fueling system.
How it works
A piezoelectric or solenoid valve releases a controlled burst of fuel gas near the plasma edge. The gas ionizes and adds particles, mostly to the outer plasma. Because the response is fast, of order milliseconds, gas puffing is the quickest fueling actuator, but the fuel it adds reaches only the edge, so it raises core density indirectly and slowly.
Control loop
A density controller compares a measured line-averaged density from an interferometer against a target and commands valve opening to close the gap. The valve's flow is roughly proportional to its drive within a calibrated range. Because edge fueling and core density are linked by transport, the loop is tuned to avoid overshoot when the added particles work their way inward.
Edge and divertor roles
Gas puffing does more than set density. Puffing near the divertor raises neutral pressure there and helps drive detachment, cushioning the target heat flux. Puffing can also modulate edge conditions that influence instabilities. These uses make the gas system a shared actuator between density control and divertor control, requiring coordination.
- Fast valves add fuel at the edge in milliseconds
- Density feedback sets valve opening from interferometry
- Edge fueling reaches the core only slowly
- Also used to drive divertor detachment
Limits
Because the fuel is deposited at the edge, heavy gas puffing to raise core density can cool the edge and degrade confinement before the core responds, so deep fueling is left to pellets. The two methods are combined: pellets for core inventory, gas for fast trim and edge conditioning. In the Kronos breeder design model, fueling control balances gas and pellets to hold the D-T density of the simulated plasma; the machine is not built.
Gas puffing is the responsive, shallow half of a two-actuator fueling strategy.