DOE STTR Phase I/II: Low-Impurity, Electrode-less Pre-ionizer Plasma Gun for Innovative Confinement Concepts

EPS installed onto the HIT-SI experiment at the University of Washington.

Innovative Confinement Concepts (ICC), like the spheromak and the field reversed configuration (FRC), provide an opportunity to study magnetic confinement in geometries other than the tokamak and may provide an alternative path to fusion energy. Many ICCs require a pre-ionizer or plasma injector in order to improve shot-to-shot repeatability. A pre-ionizer must meet several critical criteria so that it does not interfere with the experiment including high ionization percentage, low impurity production, injection from a remote location and low electromagnetic interference. An example of this need is seen in the Helicity Injected Torus with Steady Inductive current drive (HIT-SI) at the University of Washington. HIT-SI is designed to study the process of magnetic relaxation that generates an axisymmetric spheromak equilibrium from a nonaxisymmetric helicity injection configuration. The current pre-ionizer for HIT-SI utilizes a RF antenna operating at 13.56 MHz and the pre-ionizer violates all the desirable characteristics listed above. To operate HIT-SI in the desired operational regime a new solution for the production highly-ionized, low impurity starter plasma is now required.

Cross sectional view of EPS discharge as viewed with a high-speed Phantom camera at HIT-SI.

Recent experimental work on the High Power Helicon eXperiment (HPHX) at the University of Washington and Eagle Harbor Technologies (EHT) has lead to development of an electrode-less inductive plasma source that produces highly ionized low-impurity plasma that may be an ideal solution for producing the necessary pre-ionizer plasma desired by the HIT-SI and other ICC experiments. This source can be configured to operate over a wide range of output plasma parameters. These include operation at high density (> 1020 m-3), low temperature (~ 2-5 eV) and as a highly collimated beam. The source is unique since it produces a fully ionized plasma and does not require a background gas to operate. It can be configured to inject plasma into a system without requiring or injecting attached magnetic flux. The pulsed high velocity and density nature of the plasma source allows for a very large ion flux.  The new plasma sources are currently in use at both the Advanced Propulsion Laboratory and Steady Inductive Helicity Injected Torus experiments at the University of Washington.

In the fusion science community, a high power inductive plasma gun that provides a directed, high energy plasma beam could be a replacement for several arc plasma sources used for plasma heating schemes, startup and refueling and current drive applications. The inductive coil geometry of the helicon source allow for electrode-less high density plasma production removing issues of electrode impurities. The unique nature of the downstream whistler wave has many similarities to the RMF current drive used in several confinement concepts for the generation of fusion energy and additional research into this area could provide increased capabilities in this area.

This work resulted in a paper: Reduction of plasma density in the Helicity Injected Torus with Steady Inductance experiment by using a helicon pre-ionization source published in Review of Scientific Instruments.