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 ${\beta }_p$ values comparable to the aspect ratio $A({\beta }_p<~0.65A)$ and by confinement times close to those of Ohmic discharges. The high-${\beta }_p$ regime develops at an injection power ≥1.9 MW, a mean density ${\overline{n}}_e>~3\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$, and a $q\left(a\right)\mathrm{}$ value ≥2.6. Beyond these limits or in discharges with material limiter, low ${\beta }_p$ values and reduced particle and energy confinement times are obtained compared to the Ohmic heating phase.