Microtearing instabilities and electron thermal transport in low and high collisionality NSTX discharges

Journal article, 2021

Microtearing mode (MTM) real frequency, growth rate, magnetic fluctuation amplitude, and resulting electron thermal transport are studied in
systematic NSTX scans of relevant plasma parameters. The dependency of the MTM real frequency and growth rate on plasma parameters,
suitable for low and high collision NSTX discharges, is obtained by using the reduced MTM transport model [T. Rafiq et al., Phys. Plasmas 23,
062507 (2016)]. The plasma parameter dependencies are compared and found to be consistent with the results obtained from MTM using the
gyrokinetic GYRO code. The scaling trend of collision frequency and plasma beta is found to be consistent with the global energy confinement
trend observed in the NSTX experiment. The strength of the magnetic fluctuation is found to be consistent with the gyrokinetic estimate. In earlier
studies, it was found that the version of the multi-mode (MM) anomalous transport model, which did not contain the effect of MTMs, provided
an appropriate description of the electron temperature profiles in standard tokamak discharges and not in spherical tokamaks. When the MM
model, which involves transport associated with MTMs, is incorporated in the TRANSP code and is used in the study of electron thermal transport
in NSTX discharges, it is observed that the agreement with the experimental electron temperature profile is substantially improved.