Reduced Model Prediction of Electron Temperature Profiles in Microtearing-dominated National Spherical Torus eXperiment Plasmas
Authors: S.M. Kaye , W. Guttenfelder, R.E. Bell , S.P. Gerhardt, B.P. LeBlanc, and R. Maingi
Abstract: A representative H-mode discharge from the National Spherical Torus eXperiment is studied in detail to utilize it 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 βe, ν*e, the MHD α parameter, and the gradient scale lengths of Te, Ti, and ne were examined as a prelude to performing linear gyrokinetic calculations to determine the fastest growing micro instability 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 βe and ν*e were relatively low, ballooning parity modes were dominant. As time progressed and both βe and ν*e increased, microtearing became the dominant low-κθ mode, especially in the outer half of the plasma. There are instances in time and radius, however, where other modes, at higher-κθ, may, in addition to microtearing, be important for driving electron transport. Given these results, 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 Te for times and locations where microtearing was determined to be important, but not as well when microtearing was predicted to be stable or subdominant.
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Published in: Physics of Plasmas (1994-present) 21, 082510 (2014); © 2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4893135]
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