Study of neutral transport and plasma fueling in the HIT-SI3 and HIT-SIU experiments

Abstract

Compared with more conventional toroidal magnetic confinement configurations for fusion plasmas, the spheromak has several favorable properties, among which is the potential to be Ohmicly heated all the way up to ignition temperatures. This would enable more economical fusion power plants. To this end, the HIT-SI family of experiments aim to demonstrate efficient formation and sustainment of spheromaks using Steady Inductive Helicity Injection (SIHI). The previous HIT-SI and HIT-SI3 experiments were limited in performance by high neutral density in the confinement region, and the need to continuously supply neutral gas fueling to the system. In this work the effectiveness of different methods to control neutral and plasma density in this class of devices was studied.Firstly, the performance of a set of existing passive pumping volumes was considered. To gain insight into the performance of these volumes, and the conditions of the edge plasma region, the plasma inside one of these volumes was characterized using a triple Langmuir probe. A 1D plasma-neutral transport model was then created to assess what combinations of edge plasma parameters most closely corresponded to the probe data. Although an exact match could not be achieved, this study suggests high neutral density and strong plasma-neutral coupling in the edge plasma region, which is expected to degrade HIT-SI3 spheromak performance. The subsequent HIT-SIU experiment was designed with several improvements to reduce the buildup of neutral gas and allow greater control of plasma density. One of these is a set of electrodeless plasma sources intended to ionize the neutral gas used to fuel the helicity injectors. As part of this work, these plasma sources were constructed, installed and characterized. It was determined that they operate as inductively-coupled plasma (ICP) sources and can reliably supply plasma to the helicity injectors over a wide range of density. During preliminary operations on HIT-SIU, these plasma sources were able to improve the speed and reliability of breakdown, as well as the consistency of plasma density. However, energetic plasma-material interactions were also observed, which necessitated removing the sources. A mitigation strategy was developed that will allow the plasma sources to be reinstalled, but in the meantime, HIT-SIU has demonstrated improved density control and performance when compared to its predecessors and is expected to perform even better once the new plasma sources are reinstalled.