R.G. O'Neill

Non-inductive solenoid-free plasma start-up using coaxial helicity injection

Experimental results on the transfer of a coaxial-helicity-injection (CHI) produced discharge to inductive operation are reported. By self-consistently increasing both the injector flux and the externally produced toroidal flux, the useful CHI produced closed flux current has been increased to 100 kA, which is retained during the inductive ramp. CHI started plasmas outperform inductive-only discharges and consume less volt-seconds. These significant results were obtained on the helicity injected torus-II (Jarboe T.R. 1989 Fusion Technol. 15 7) spherical torus experiment (major/minor radius of 0.3/0.2 m).

Demonstration of steady inductive helicity injection

Initial results demonstrating the concept of constant inductive helicity injection are presented. Constant helicity injection is achieved using two oscillating inductive helicity injectors, with the goal of producing a bow tie spheromak. Each injector is a 180° segment of a reverse field pinch and they are driven 90° out of phase. Approximately 5 MW of power is injected during the 6 ms pulse, and the input power has been maintained at a fairly constant value by directly fuelling the injectors with neutral gas. Motivation for the experiment is given, including beta-limit calculations for the bow tie spheromak. Fuelling the injectors with neutral gas during the discharge is shown to produce injector parameters that are more constant in time. A series of discharges with increasing power input shows a promising increase in toroidal current. Unique construction techniques of the experiment are also described.

Recent results from the HIT-SI experiment

New understanding and improved parameters have been achieved on the Helicity Injected Torus with Steady Inductive helicity injection current drive (HIT-SI) experiment. The experiment has a bowtie-shaped spheromak confinement region with two helicity injectors. The inductive injectors are 180° segments of a small, oval cross section toroidal pinch. Spheromaks with currents up to 38 kA and current amplification of 2 have been achieved with only 6 MW of injector power. The Taylor-state model is shown to agree with HIT-SI surface and internal magnetic profile measurements. Helicity balance predicts the peak magnitude of toroidal spheromak current and the threshold for spheromak formation. The model also accurately predicts the division of the applied loop voltage between the injector and spheromak regions. Single injector operation shows that the two injectors have opposing, preferred spheromak current directions. An electron locking relaxation model is consistent with the preferred direction, with ion Doppler data and with bolometric data. Results from higher frequency operation are given. The impact of the new understanding on the future direction of the HIT programme is discussed.

Observation of persistent edge current driven by coaxial helicity Injection

Coaxial Helicity Injection (CHI) has been used on the National Spherical Torus Experiment [Ono et al., Nucl. Fusion 40, 557 (2000)], the Helicity Injected Torus (HIT) [Nelson et al., Phys. Rev. Lett. 72, 3666 (1994)] and HIT-II [Jarboe et al., Phys. Plasmas 5, 1807 (1998)] to initiate plasma and to drive up to 400 kA of toroidal current. On HIT-II, CHI initiated discharges have been successfully coupled to Ohmic sustainment [Raman et al., Phys. Plasmas 11, 2565 (2004)]. This paper presents the first results on the use of CHI to demonstrate the persistence of edge current drive in a pre-established single null diverted inductive discharge. Edge current drive has the potential to improve plasma stability limits [Menard et al., Nucl. Fusion 37, 595 (1997)]. While most current drive methods drive current in the interior of the plasma, CHI is well suited for driving current in the edge plasma.