This Letter reports the first measurements of the fast confined $\alpha$-particle energy distribution in a fusion plasma. The pellet charge exchange technique shows the fusion generated $\alpha$'s in the core of the Tokamak Fusion Test Reactor plasma slow down classically, and appear to be well confined. Preliminary indications are that stochastic ripple effects are responsible for steepening the energy distribution outside the plasma core $(r/a\gtrsim 0.35)$. Sawteeth mixing of fast $\alpha$'s is suggested in data during the post-beam-heating plasma decay.

• Received 29 March 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.846