The Saturation of Beta in W7-AS

Quasi-stationary, MHD-quiescent discharges with $\left\langle \beta \right\rangle $ up to 3.5{\%} were sustained in the W7-AS for more than 100 energy confinement times.. The achieved $\left\langle \beta \right\rangle $ is limited by confinement, not stability, and is above the linear ideal stability threshold. Neutral beam heating power scans are analyzed for vacuum iota values of 0.445 and 0.575. The scans saturate at $\left\langle \beta \right\rangle $ values of 3.1{\%} and 2.1{\%}, respectively. At low power, both scans show confinement incrementally varying as $\tau _{E}\propto P_{NB}^{-0.5} $. At high $P_{NB} $, the confinement incrementally varies as $\tau _{E}\sim P_{NB}^{-0.8} $. In both scans, only the central $T_e $ responds to increasing power. $T_e$ and $\nabla T_e$ do not change appreciably in the outer region of the plasma as $P_{NB} $ increases, indicating an increase in local thermal diffusivity. The edge$\nabla T_e $ is approximately a factor of two larger for lower iota. The role of the magnetic flux topology has been analyzed using the PIES 3D equilibrium code. It calculates that a stochastic field region forms at the edge with increasing $\left\langle \beta \right\rangle $, and that it reduces the minor radius by $\sim $30{\%} at the saturated $\left\langle \beta \right\rangle $ value. In the higher iota scan, this occurs at lower $\left\langle \beta \right\rangle $ and the stochastic region has a shorter connection length, plausibly explaining the reduced $\nabla T_e $, and lower saturated$\left\langle \beta \right\rangle $.