Electron thermal transport in the Madison symmetric torus

Due to diagnostic improvements and the development of the MSTFit equilibrium reconstruction code, it has become possible for the first time to accurately characterize the transport behavior of MST plasmas over the sawtooth cycle. Magnetic fluctuations in the MST reversed-field pinch, which rise sharply at the crash, play a significant role in the transport of heat and particles. The RFP configuration is a good test bed for studying magnetic fluctuation induced transport since the overlapping magnetic tearing mode islands create a large radial region in which the magnetic flux surfaces are destroyed and field lines wander stochastically. The measured electron thermal conductivity in this stochastic region agrees with Rechester-Rosenbluth like predictions from a fluctuating magnetic field.; Time evolving profiles are measured to understand electron heat and particle transport in the MST. In particular, electron temperature, electron density, and current density profiles have been measured during a “Standard” plasma sawtooth cycle. The MHD activity during the sawtooth cycle is examined as an a priori, time-evolving condition that is affecting the plasma equilibrium and consequently the transport of heat and particles. The cause of the MHD behavior is not central to this thesis. Rather, the effects of the fluctuations on transport are considered. The current density peaks up as the sawtooth crash is approached. This current peaking pushes the plasma farther away from the Taylor minimum-energy state and causes (possibly dynamo driven) tearing instabilities to grow. At the crash the current density profile broadens, resulting in a flatter λ-profile, and a plasma that is closer to a Taylor minimum-energy state. Another effect of the peaking current density is to broaden the q-profile. Lower q-shear results in a more stochastic magnetic field in the region where tearing mode islands overlap. Greater field stochasticity leads to enhanced transport of heat and particles by the electrons, and the electron temperature is observed to drop. After the sawtooth crash, the current density broadens, the q-profile peaks, the q-shear is increased in the region of overlapping islands, the field stochasticity is reduced, and the electron heat transport falls.