gas puffing is fueling or edge-cooling a 📝plasma by injecting neutral gas at the boundary.
It is the simplest tool in the fueling kit: fast valves in the vessel wall release measured puffs of gas, which cross the vacuum gap and meet the plasma edge. There the atoms are ionized almost immediately, which is both the method's convenience and its limit — the fuel joins the plasma at the boundary and must diffuse inward, so gas puffing feeds the edge well and the core poorly. Its near-neighbor, 📝pellet injection, exists precisely to beat that limit, delivering frozen fuel ballistically past the edge; the standard division of labor is puffed gas for the boundary and density control, pellets for the core. Gas injection also provides the very first fuel of a discharge: the neutral fill that a 📝Tokamak ionizes at startup.
The second job named in the definition is thermal, not nutritional. Puffing gas — fuel species or seeded impurities — into the exhaust region cools the plasma streaming toward the 📝divertor, radiating power away before it concentrates on material surfaces and helping push the exhaust into detachment. That makes the humble gas valve one of the working instruments of heat-flux management in machines whose divertors face some of the highest power densities in engineering.