Peaked density profiles in circular-limiter H modes on the TFTR tokamak.

Circular-limiter {ital H} modes are obtained on the TFTR tokamak during high-power neutral-beam heating. The transition is usually from the supershot to the {ital H} mode rather than the usual {ital L} to {ital H} transition, and thus is obtained in a low-recycling environment with core fueling mainly from the heating beams. As a result, the density and pressure profiles are highly peaked at the center. The global confinement time {tau}{sub {ital E}} is enhanced over {ital L}-mode scaling by up to {approx}2.5 times. The onset of edge-localized MHD modes shortly after the {ital H}-mode transition appears to limit {tau}{sub {ital E}}. Limiter {ital H} modes of up to 1.5 s duration have been realized.