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.

Edge density profile modifications associated with edge localized modes (ELMs) and edge harmonic oscillations (EHOs) have been measured using fast profile reflectometry in DIII-D H-mode plasmas. Specifically, high time (down to 10 µs) and spatial (∼4 mm) resolution density profile measurements from 0 × 10 19 to 3.1 × 10 19 m −3 provide us with new tools to study the dynamics of pedestal perturbations. During ELMs, it has been observed that the SOL density profile expands outwards to the vessel wall while the pedestal density reduces. These measurements provide direct evidence for enhanced particle radial transport to the vessel wall at the onset of ELMs. A large SOL density profile radial expansion velocity is observed during ELMs. A comparison of the velocities for different pedestal densities is made. The density scale length, density at the wall, and density fluctuations during ELMs are determined as a function of pedestal density, as well as particle transport during ELMs. Measurements in quiescent H-mode plasmas indicate that the pedestal density profile is modulated at the fundamental frequency of the EHO. A comparison between reflectometer and beam emission spectroscopy measurements regarding the EHO density fluctuation profile is presented.

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The Pegasus Toroidal Experiment was developed to explore the physics limits of plasma operation as the aspect ratio (A) approaches unity. Initial experiments on the device found that access to high normalized current and toroidal beta was limited by the presence of large-scale tearing modes. Major upgrades have been conducted of the facility to provide the control tools necessary to mitigate these resistive modes. The upgrades include new programmable power supplies, new poloidal field coils and increased, time-variable toroidal field. First ohmic operations with the upgraded system demonstrated position and current ramp-rate control, as well as improvement in ohmic flux consumption from 2.9 MA Wb −1 to 4.2 MA Wb −1 . The upgraded experiment will be used to address three areas of physics interest. First, the kink and ballooning stability boundaries at low A and high normalized current will be investigated. Second, clean, high-current plasma sources will be studied as a helicity injection tool. Experiments with two such sources have produced toroidal currents three times greater than predicted by geometric field line following. Finally, the use of electron Bernstein waves to heat and drive current locally will be studied at the 1 MW level; initial modelling indicates that these experiments are feasible at a frequency of 2.45 GHz.

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