The decay time of stored energy was measured when additional heating power decreased abruptly. The energy decay time is almost constant at 15 ms, irrespective of the gross energy confinement time ${\tau }_E^G$, which is changed by ion-cyclotron range of frequency heating up to 1.5 MW. The decay time coincides with a confinement time of the additionally heated power defined by ${\tau }_{\mathrm{ad}}=\frac{\Delta W_T}{\Delta P_{\mathrm{tot}}}$, which indicates that the incremental energy from the additional heating behaves independently of the gross plasma energy. It is also shown that ${\tau }_{\mathrm{ad}}$ is almost independent of density and plasma current as well as heating power.

• Received 19 February 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.2814