E. T. Hinson

Full-wave modeling of the O–X mode conversion in the Pegasus toroidal experiment

The ordinary-extraordinary (O-X) mode conversion is modeled with the aid of a 2D full-wave code in the Pegasus Toroidal Experiment as a function of the launch angles. It is shown how the shape of the plasma density profile in front of the antenna can significantly influence the mode conversion efficiency and, thus, the generation of electron Bernstein waves (EBW). It is therefore desirable to control the density profile in front of the antenna for successful operation of an EBW heating and current drive system. On the other hand, the conversion efficiency is shown to be resilient to vertical displacements of the plasma as large as \pm 10 cm.

Characterization of peeling modes in a low aspect ratio tokamak

Peeling modes are observed at the plasma edge in the Pegasus Toroidal Experiment under conditions of high edge current density (Jedge ~ 0.1 MA m−2) and low magnetic field (B ~ 0.1 T) present at near-unity aspect ratio. Their macroscopic properties are measured using external Mirnov coil arrays, Langmuir probes and high-speed visible imaging. The modest edge parameters and short pulse lengths of Pegasus discharges permit direct measurement of the internal magnetic field structure with an insertable array of Hall-effect sensors, providing the current profile and its temporal evolution. Peeling modes generate coherent, edge-localized electromagnetic activity with low toroidal mode numbers n ≤ 3 and high poloidal mode numbers, in agreement with theoretical expectations of a low-n external kink structure. Coherent MHD fluctuation amplitudes are found to be strongly dependent on the experimentally measured Jedge/B peeling instability drive, consistent with theory. Peeling modes nonlinearly generate ELM-like, field-aligned filamentary structures that detach from the edge and propagate radially outward. The KFIT equilibrium code is extended with an Akima spline profile parameterization and an improved model for induced toroidal wall current estimation to obtain a reconstruction during peeling activity with its current profile constrained by internal Hall measurements. It is used to test the analytic peeling stability criterion and numerically evaluate ideal MHD stability. Both approaches predict instability, in agreement with experiment, with the latter identifying an unstable external kink.

Impedance of an intense plasma-cathode electron source for tokamak startup

An impedance model is formulated and tested for the ∼1 kV, 1 kA/cm2, arc-plasma cathode electron source used for local helicity injection tokamak startup. A double layer sheath is established between the high-density arc plasma ( narc≈1021 m−3) within the electron source, and the less dense external tokamak edge plasma ( nedge≈1018 m−3) into which current is injected at the applied injector voltage, Vinj. Experiments on the Pegasus spherical tokamak show that the injected current, Iinj, increases with Vinj according to the standard double layer scaling Iinj∼Vinj3/2 at low current and transitions to Iinj∼Vinj1/2 at high currents. In this high current regime, sheath expansion and/or space charge neutralization impose limits on the beam density nb∼Iinj/Vinj1/2. For low tokamak edge density nedge and high Iinj, the inferred beam density nb is consistent with the requirement nb≤nedge imposed by space-charge neutralization of the beam in the tokamak edge plasma. At sufficient edge density, nb∼narc is observed, ...

Full‐wave modeling of the O‐X mode conversion in the Pegasus Toroidal Experiment

The potential of an EBW heating scheme via the O—X—B mode conversion scenarios has been investigated for the PEGASUS toroidal experiment. With the 2D full‐wave code IPF‐FDMC the O—X conversion has been modeled as a function of the poloidal and toroidal injection angles for a microwave frequency of 2.45 GHz. Based on preliminary Langmuir probe measurements in the mode conversion layer, different density profiles have been also included in the simulations. A maximum mode conversion efficiency of approximately 80 % has been found, making EBW heating an attractive heating scheme for PEGASUS.