Plasma startup in the National Spherical Torus Experiment using transient coaxial helicity injection

A method of plasma current generation known as coaxial helicity injection (CHI) has been successfully applied in the National Spherical Torus Experiment [M. Ono, S. M. Kaye, Y.-K. M. Peng, Nucl. Fusion 40, 3Y 557 (2000)] to form closed, nested magnetic surfaces carrying a plasma current up to 160 kA. In some discharges the generated current persists for surprisingly long, ∼400 ms. While the CHI method has previously been studied in smaller experiments, such as the Helicity Injected Tokamak (HIT-II) [R. Raman, T. R. Jarboe, B. A. Nelson, Phys Rev. Lett. 90, 075005 (2003)] at the University of Washington, the significance of these results are (a) demonstration of the process in a vessel volume thirty times larger than HIT-II on a size scale more comparable to a reactor, (b) a remarkable multiplication factor of 60 between the injected current and the achieved toroidal current, compared to six in previous experiments, and (c) for the first time, fast time scale visible imaging of the entire process that shows discharge formation, disconnection from the injector, and luminous structures consistent with the reconnection of magnetic field lines and closed flux surfaces. These significant results indicate favorable scaling with machine size.