Observations of strike line resiliency for non-resonant divertor configurations in a current-carrying stellarator

Abstract

Within the study of fusion energy, the stellarator concept’s lack of current drive and inherently steady state operation make it a promising candidate for fusion reactor designs despite the complexity of the core optimization and exhaust solution. Recent computational tools have allowed more advanced optimizations of the plasma core confinement and as such, alternative divertor concepts that can accommodate the needs of these optimizations have become an active area of interest within the field of stellarator research. In particular, the non-resonant divertor concept has garnered attention due to its utilization of a stellarator’s inherent 3D shaping to create resilient magnetic structures in an otherwise evolving chaotic magnetic field. Because the concept exploits the general features of a stellarator’s confining geometry, the non-resonant divertor yields itself to core optimization schemes that are unsuitable or incompatible with more mature divertor concepts, such as island or helical divertors. However, the non-resonant divertor has, thus far, only been studied computationally and an experimental validation of its resiliency properties is necessary before pursuing other relevant research goals. The Compact Toroidal Hybrid (CTH) can access a non-resonant divertor configuration by driving ohmic plasma current in its high vacuum rotational transform discharges. Strike lines from these discharges form a helical band around the outboard side of the vacuum vessel and are present across a range of internal plasma currents and surface shaping. To study the strike line properties, two Langmuir probe arrays were installed in CTH with one positioned near the center of the helical band and another closer to the end, near a pre-existing limiter. These probes measure the incoming ion flux across a range of magnetic equilibria to investigate the behavior of the strike line profiles and quantify the resiliency of the strike line locations. Non-resonant divertor strike lines in CTH are observed to shift by 2 - 4 cm over a ∼20 kA range or a change of ∼0.3 in the edge rotational transform. Based upon previously reported measurements, this provides an order of magnitude reduction in the movement of the strike line positioning when compared against the expected shifts of a similarly sized island divertor. Initial modeling of CTH non-resonant configurations also provides agreement in the width and positioning of the strike lines. Though most strike lines directly correspond to regions of high curvature along the confining geometry, a strike line at the limiter location is found to correspond to secondary field line structures that are more heavily influenced by changes in the magnetic equilibria. This dissertation characterizes the evolution of a CTH discharge from a limited geometry to a non-resonant divertor configuration and discusses the resilient strike line features of the edge chaotic magnetic field through the measurement of the strike line locations across a range of magnetic equilibria. Within the study of fusion energy, the stellarator concept’s lack of current drive and inherently steady state operation make it a promising candidate for fusion reactor designs despite the complexity of the core optimization and exhaust solution. Recent computational tools have allowed more advanced optimizations of the plasma core confinement and as such, alternative divertor concepts that can accommodate the needs of these optimizations have become an active area of interest within the field of stellarator research.