Keith Loebner

Evidence of Branching Phenomena in Current-Driven Ionization Waves.

This Letter reports the first fully consistent experimental observations of current-driven ionization waves conforming to the magnetohydrodynamic Rankine-Hugoniot model for hydromagnetic shocks. Detailed measurements of the thermodynamic and electrodynamic plasma state variables across the ionization region confirm the existence of two types of waves, corresponding to the upper and lower solution branches of the Hugoniot curve. These waves are generated by pulsed currents in a coaxial gas-fed plasma accelerator. The coupling between the state variables of this complex, transient, three-dimensional system shows a remarkable quantitative agreement of less than 8% deviation from the quasisteady, one-dimensional theoretical model.

High-Velocity Neutral Plasma Jet Formed by Dense Plasma Deflagration

High-velocity high-energy-density plasma jets are an invaluable tool for studying plasma-material interactions under extreme thermal, physical, chemical, and electrostatic stresses. Such a jet has been generated in an experimental facility utilizing a pulsed plasma deflagration accelerator. Time-averaged and fast-frame-rate intensified charge-coupled device images of the plasma jet are presented, showing a concentrated high-density plasma near the exit that is characterized with laser interferometry.

Radial magnetic compression in the expelled jet of a plasma deflagration accelerator

A spectroscopic study of a pulsed plasma deflagration accelerator is carried out that confirms the existence of a strong compression in the emerging jet at the exit plane of the device. An imaging spectrometer is used to collect broadened Hα emission from a transaxial slice of the emerging jet at high spatial resolution, and the radial plasma density profile is computed from Voigt fits of the Abel inverted emissivity profiles. The plasma temperature, determined via Doppler broadening of impurity line emission, is compared against the temperature predictions of a radial magnetohydrodynamic equilibrium model applied to the measured density profiles. Empirical scaling laws developed for the plasma density, combined with the measured and predicted temperatures, indicate that a radially equilibrated Z-pinch is formed within the expelled plasma jet at the exit plane during the deflagration process.

Damage Morphologies in Targets Exposed to a New Plasma Deflagration Accelerator for ELM Simulation

Transient events in fusion power plants such as DEMO and ITER are known to pose a severe threat to plasma facing components (PFCs) due to melting and erosion after repeated edge localized mode (ELM) loads. In situ experimental testing of potential PFC materials at fusion relevant conditions is difficult and expensive, and as a result plasma devices capable of replicating the desired transient conditions are of increasing interest. At Stanford University, an experimental facility designed to mimic the heat flux, particle fluence, and other key characteristics of ELMs and disruption events in a controlled setting has been developed. A pulsed plasma accelerator operating in the deflagration mode is used to generate high-velocity (40-100 km/s) directed plasma jets that are stagnated on target material samples. In this paper, we present probe data characterizing the plasma parameters of the accelerated plume using hydrogen as the working gas, as well as preliminary target studies of silicon and copper witness plates exposed to pulses at various total and peak shot energies. Results from the probe analysis indicate achieved energy fluxes and heat flux parameters that are ELM-like, and the observed linearly correlated damage morphologies on the witness plates indicate that initial surface roughness plays a significant role in the growth characteristics of surface damage patterns. These results lay the groundwork for future studies of ELM-like loading on reactor-relevant materials using the Stanford plasma accelerator facility.

A fast rise-rate, adjustable-mass-bit gas puff valve for energetic pulsed plasma experiments

A fast rise-rate, variable mass-bit gas puff valve based on the diamagnetic repulsion principle was designed, built, and experimentally characterized. The ability to hold the pressure rise-rate nearly constant while varying the total overall mass bit was achieved via a movable mechanical restrictor that is accessible while the valve is assembled and pressurized. The rise-rates and mass-bits were measured via piezoelectric pressure transducers for plenum pressures between 10 and 40 psig and restrictor positions of 0.02-1.33 cm from the bottom of the linear restrictor travel. The mass-bits were found to vary linearly with the restrictor position at a given plenum pressure, while rise-rates varied linearly with plenum pressure but exhibited low variation over the range of possible restrictor positions. The ability to change the operating regime of a pulsed coaxial plasma deflagration accelerator by means of altering the valve parameters is demonstrated.

Characterization of a plasma deflagration accelerator as an ELM replicator for PFC testing

Transient events in fusion power plants such as DEMO and ITER are known to pose a severe threat to plasma facing components (PFCs) due to melting and erosion after repeated edge localized mode (ELM) loads. Experimental testing in situ of potential PFC materials at fusion relevant conditions is difficult and expensive, and no facility currently in operation accurately replicates the salient plasma environment. At Stanford University, an experimental facility designed to mimic the heat flux, particle fluence, and other key characteristics of ELMs and disruption events in a controlled setting is under development. A pulsed plasma accelerator operating in the deflagration mode is used to generate high velocity (40-100 km/s) directed plasma jets that are stagnated on target material samples. In this work, we present probe data characterizing the plasma parameters of the accelerated plume using hydrogen as the working gas, as well as preliminary target studies of copper tokens exposed to pulses at various total and peak shot energies. Results from the probe analysis indicate achieved energy fluxes and heat flux parameters that are ELM-like, and the observed damage morphologies on the witness plates indicate that initial surface roughness plays a significant role in the growth and characteristics of surface melt patterns.