The characteristic duration over which a fusion plasma retains its thermal energy—one of the three factors in the triple product that determine whether a device can achieve net energy gain.
The energy confinement time, denoted τE, is defined as the ratio of the total stored thermal energy in the plasma (W) to the total power loss (Ploss):[1]
τE represents how quickly a plasma would cool if all heating were suddenly removed. A longer confinement time means less external heating is needed to maintain fusion-relevant temperatures.[4]
The most widely used scaling is IPB98(y,2) for H-mode tokamak plasmas:[1]
This scaling was derived from a multi-machine database and forms the primary basis for predicting ITER’s confinement performance.[3]
The discovery of H-mode at the ASDEX tokamak in 1982 by Friedrich Wagner approximately doubled τE compared with L-mode. This is a verified result reproduced on virtually every divertor tokamak worldwide. ITER’s baseline design assumes standard H-mode confinement (H98(y,2) = 1.0).[2]
τE ranges from tens of milliseconds in smaller devices to 1–3 seconds in large machines such as JET. ITER is projected to achieve τE ≈ 3.7 seconds at its nominal 15 MA operating point. Wendelstein 7-X has demonstrated values consistent with design predictions.[3]