RWM Critical Rotation Frequency and Beta Dependence in NSTX*

The resistive wall mode (RWM) can be stabilized by maintaining the plasma toroidal rotation frequency ($\omega _{\phi })$ above a critical rotation frequency ($\Omega _{crit})$. Recent experiments on NSTX seek to determine $\Omega _{crit}$ and rotation profile effects through actively braking plasma rotation by the application of external magnetic fields. Results from these experiments indicate that maintaining $\omega _{\phi }$ at the q = 2 surface above $\omega _{A}$/4q$^{2}$ is a necessary condition for RWM stability where $\omega _{A}$ is the local Alfven frequency. This result is in agreement with a theoretical model derived from a drift-kinetic energy principle. Similarity experiments with DIII-D are being performed to examine the aspect ratio dependence of the $\Omega _{crit}$ scaling. When $\omega _{\phi }$ at the q = 2 surface drops below $\Omega _{crit}$, the growth of internal kink/ballooning modes can prevent the RWM from terminating the discharge. A small beta collapse which drops $\Omega _{crit}$, accompanies this mode growth allowing a recovery of RWM rotational stabilization while maintaining $\beta _{N} \quad > \quad \beta _{N}^{no-wall}$.