Physics and optimization of plasma startup in the RFP

Abstract

In the tokamak and reversed-field pinch (RFP), inductively driven toroidal plasma current provides the confining poloidal magnetic field and ohmic heating power, but the magnitude and/or duration of this current is limited by the available flux swing in the poloidal field transformer. A portion of this flux is consumed during startup as the current is initiated and ramped to its final target value, and considerable effort has been devoted to understanding startup and minimizing the amount of flux consumed. Flux consumption can be reduced during startup in the RFP by increasing the toroidal magnetic field, B ti , applied to initiate the discharge, but the underlying physics is not yet entirely understood. Toward increasing this understanding, we have for the first time in the RFP employed advanced, non-invasive diagnostics on the Madison Symmetric Torus to measure the evolution of current, magnetic field, and kinetic profiles during startup. Flux consumption during startup is dominantly inductive, but we find that the inductive flux consumption drops as B ti increases. The resistive consumption of flux, while relatively small, apparently increases with B ti due to a smaller electron temperature. However, the ion temperature increases with B ti , exceeding the electron temperature and thus reflecting non-collisional heating. Magnetic fluctuations also increase with B ti , corresponding primarily to low-n modes that emerge sequentially as the safety factor profile evolves from tokamak-like to that of the RFP.

Numerical characterization of plasma breakdown in reversed field pinches

“…Moreover, in RELAX there are efforts that have been done to investigate a low aspect ratio regime [36,37] due to its attractive advantages, such as enhanced bootstrap current and its simpler magnetic mode dynamics. Efforts have also been taken to optimize the startup on the RFP [38] and investigate the turbulence [39] in RFP plasma. These efforts move RFP investigations forward and make progress step by step and further make RFP an inspiring device for fusion energy study.…”

“…Since the breakdown is a fast process, the toroidal magnetic field is kept constant. The scanning range is 0.02 ∼ 0.07 T. Generally, for most RFP devices the B t in the startup is hundreds of Gauss [17,36,38]. From the experimental data available in KTX, the design value for phase I is about 0.35 T [20], which is designed for the aided reversal mode [43].…”

“…So, it is reasonable to apply this range. As for the error magnetic field, the scanning range is from zero to 30 G because in the breakdown phase it is quite small and from the B p distribution, it is the smallest in the magnetic axis, which is derived from the recent experimental work in [38].…”

An overview of diagnostic upgrade and experimental progress in the KTX

“…Over the last two years, a few methods have been tried to carry out wall conditioning, including wall baking, glow discharge cleaning and repeated low plasma current discharge cleaning, and it has been found that improvement in the maximum plasma current and realization of a longer RFP state are probably limited by the power supply, but not impurities from the wall. The maximum loop voltage at the present stage is about 150 V, which is not quite low compared with other RFP machines, however, the high loop voltage is not long enough to sustain plasma current rising to get through the RFP plasma start-up phase with high resistance [8]; thus what needs to be upgraded is the capacitance of the main ohmic capacitor bank. In addition, the results from the RFX and MST suggest that the QSH state tends to occur in large plasma current parameters [9][10][11][12][13].…”