Peaked density profiles in circular-limiterHmodes on the TFTR tokamak

Circular-limiter H modes are obtained on the TFTR tokamak during high-power neutral-beam heating. The transition is usually from the supershot to the H mode rather than the usual L to 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 ${\mathrm{\tau }}_{\mathit{E}}$ is enhanced over L-mode scaling by up to ≊2.5 times. The onset of edge-localized MHD modes shortly after the H-mode transition appears to limit ${\mathrm{\tau }}_{\mathit{E}}$. Limiter H modes of up to 1.5 s duration have been realized.