Localized current injection near the outboard midplane is used to form 0.1 MA plasma discharges with no induction supplied from a central solenoid in the ultra-low aspect ratio Pegasus Toroidal Experiment. The discharges are initiated by driving open-field-line currents that perturb the vacuum magnetic field such that the magnetic topology transitions to a tokamak-like configuration. The plasma is subsequently driven via helicity injection from the edge current sources and poloidal field induction. Intermittent n = 1 MHD activity is observed during periods of strong edge current drive and each event leads to a rapid inward expansion of the plasma volume and a drop in the plasma inductance. The plasmas are sufficiently turbulent such that the equilibrium approaches the lowest energy state described by Taylor relaxation theory. In agreement with that theory, the maximum I p scales with (I TF I inj/w)1/2, where I TF is the toroidal field rod current, I inj is the injected edge current and w is the radial width of the average poloidal magnetic flux in the driven open flux region.

Startup of a 0.1 MA tokamak plasma is demonstrated on the ultralow aspect ratio Pegasus Toroidal Experiment using three localized, high-current density sources mounted near the outboard midplane. The injected open field current relaxes via helicity-conserving magnetic turbulence into a tokamaklike magnetic topology where the maximum sustained plasma current is determined by helicity balance and the requirements for magnetic relaxation.

Peeling modes, an instability mechanism underlying deleterious edge localized mode (ELM) activity in fusion-grade plasmas, are observed at the edge of limited plasmas in a low aspect ratio tokamak under conditions of high edge current density (J(edge) ∼ 0.1  MA/m2) and low magnetic field (B ∼ 0.1  T). They generate edge-localized, electromagnetic activity with low toroidal mode numbers n≤3 and amplitudes that scale strongly with measured J(edge)/B instability drive, consistent with theory. ELM-like field-aligned, current-carrying filaments form from an initial current-hole J(edge) perturbation that detach and propagate outward.

A 0D circuit model for predicting in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface flux of finite- plasmas developed by Hirshman and Neilson (1986 Phys. Fluids 29 790) are modified and expanded to treat highly shaped, ultralow- tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the Pegasus spherical tokamak, and are found to agree within 15% of experimental . High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the first half of the current ramp, or 75 kA. The second half of the current ramp follows a limit imposed by power-balance as plasmas expand from high-A to ultralow-A. This shape evolution generates a significant drop in external plasma inductance, effectively using the plasma’s initially high inductance to drive the current ramp and provide  >70% of the current drive V-s. Projections using this model indicate the relative influences of higher helicity input rate and injector current on the attainable total plasma current.

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