Coaxial Helicity Injection Non-inductive Startup on NSTX

Plasmas initiated by non-inductive transient coaxial helicity injection (CHI) have been successfully coupled to Ohmic ramp-up to achieve high-quality discharges in NSTX, reaching 800 kA and exhibiting H-mode. Conditioning of the divertor regions is found to improve both transient-CHI startup currents and coupling. Flux savings of transient-CHI startup plasmas over Ohmic ramp-up-only discharges has been demonstrated. Elimination of the central solenoid is a consideration for the design of toroidal confinement devices, which then require alternative methods for initiating the plasma current. The transient coaxial helicity injection (Transient CHI) [1, 2, 3] method of non-inductive startup, successfully developed on the Helicity Injected Torus, HIT–II, experiment [4], is implemented on the National Spherical Torus Experiment (NSTX) [5]. In this method, a plasma current is rapidly produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields, shown schematically in Fig. 1. An initial very small gas puff, introduced near the injector, is ionized by the applied voltage. The initial poloidal field configuration is chosen such that the plasma rapidly expands into the chamber. When the injector current decreases as the capacitor’s charge is depleted, magnetic reconnection occurs near the injector electrodes, with the toroidal plasma current forming closed flux surfaces. With careful conditioning of the divertor surfaces and minimizing the amount of injected gas, high-quality CHI-produced closed flux currents can be coupled to subsequent Ohmic current drive, and demonstrate flux savings. [6] (Details of HIT–II transient CHI results have been reported previously [1, 6, 7], while this paper reports recent NSTX results.)