Regime of Improved Confinement and High Beta in Neutral-Beam-Heated Divertor Discharges of the ASDEX Tokamak

A new operational regime has been observed in neutral-injection-heated ASDEX divertor discharges. This regime is characterized by high ${\ensuremath{\beta}}_{p}$ values comparable to the aspect ratio $A({\ensuremath{\beta}}_{p}l~0.65A)$ and by confinement times close to those of Ohmic discharges. The high-${\ensuremath{\beta}}_{p}$ regime develops at an injection power \ensuremath{\ge}1.9 MW, a mean density ${\overline{n}}_{e}g~3\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and a $q(a)$ value \ensuremath{\ge}2.6. Beyond these limits or in discharges with material limiter, low ${\ensuremath{\beta}}_{p}$ values and reduced particle and energy confinement times are obtained compared to the Ohmic heating phase.