Reduced model prediction of electron temperature profiles in microtearing-dominated NSTX plasmas

A representative H-mode discharge from the National Spherical Torus Experiment (NSTX) is studied in detail as a basis for a time-evolving prediction of the electron temperature profile using an appropriate reduced transport model. The time evolution of characteristic plasma variables such as $\beta_{\mathrm{e}}$, $\nu_{\mathrm{e}}^{\mathrm{\ast }}$, the MHD $\alpha $ parameter and the gradient scale lengths of T$_{\mathrm{e}}$, T$_{\mathrm{i\thinspace }}$and n$_{\mathrm{e\thinspace }}$were examined prior to performing linear gyrokinetic calculations to determine the fastest growing microinstability at various times and locations throughout the discharge. The inferences from the parameter evolutions and the linear stability calculations were consistent. Early in the discharge, when $\beta _{\mathrm{e\thinspace }}$and $\nu_{\mathrm{e}}^{\mathrm{\ast \thinspace }}$were relatively low, ballooning parity modes were dominant. As both $\beta_{\mathrm{e\thinspace }}$and $\nu _{\mathrm{e}}^{\mathrm{\ast \thinspace }}$increased with time, microtearing became the dominant low-k$_{\mathrm{\theta \thinspace }}$mode, especially in the outer half of the plasma. There are instances in time and radius where other modes, at higher-k$_{\mathrm{\theta }}$, may be important for driving electron transport. The Rebut-Lallia-Watkins (RLW) electron thermal diffusivity model, which is based on microtearing-induced transport, was used to predict the time-evolving electron temperature across most of the profile. The results indicate that RLW does a good job of predicting T$_{\mathrm{e\thinspace }}$for times and locations where microtearing was determined to be important, but not as well when microtearing was predicted to be stable or subdominant.