Experimental observation and study of core transport barrier in DIII-D discharges with dominant electron cyclotron heating

Fusion power offers the prospect of abundant, carbon-neutral and sustainable energy. A fusion reactor’s main challenge is creating a controlled, confined and burning plasma at its core. Magnetic confinement fusion (MCF) is a promising route, and the tokamak approach is the the most widely adopted design among the MCF configurations. To achieve high fusion gain in tokamaks, the high-confinement mode (H-mode) has been explored and utilized. Compared to low-confinement mode (L-mode), H-mode operation produces better confinement and thus better performance, making it more attractive as the operational scenario for future reactors. One distinguishing feature of H-mode plasma is the presence of a transport barrier, a localized region of reduced transport, in the pedestal region at the edge of the plasma. Due to the strong dependence of fusion performance on pedestal parameters, transport in the pedestal region has been extensively studied. However, many questions regarding transport mechanisms in the plasma core remain unaddressed. This dissertation investigates the changes in core transport in tokamak plasmas discharges that transition to H-mode solely with electron cyclotron heating (ECH). This type of discharges often produce strong off-axis electron temperature peaking near the ECH heating location. In this work, we experimentally study the electron heat transport properties and identify evidence for the presence of internal transport barriers (ITBs) in the core region. These results are compared with numerical solutions of the time-dependent electron thermal equation, which matches the experimental observations. We proceed to study the formation of these ITBs and linked their formation to the modification of safety factor q by ECH during L-H transition. Other potential causes of ITB formation and future work are also discusses.Physic