Session HW13: Discovery Plasma Science

Creating Astrophysically Relevant Systems in the Laboratory in the High-Energy-Density Regime

High-energy-density physics systems are defined as systems with pressures greater than 1 million atmospheres, which often results in an ionized material at high temperature. These extreme conditions allow access to processes that occur in astrophysical regimes. High-energy-density experiments can provide insight into astrophysical processes, which are often observed from great distances under uncontrolled and unknown conditions.  In order for an experiment to be well-scaled to an astrophysical process, several specific conditions must be considered, including key governing equations, specific spatial scaling, and similar global dynamics. In many cases, these conditions can be met using high-energy-density experimental facilities, such as, high-energy laser or pulsed power devices. I will discuss general scaling rules and several astrophysically-relevant high-energy-density physics experiments, specifically an experiment conducted at the National Ignition Facility relevant to core-collapse supernova SN1993J, a red supergiant, where a radiative shock is near a hydrodynamically unstable interface. We found that significant energy fluxes from radiation and thermal heat conduction affect the hydrodynamics structure at the interface. In the experiments, a blast wave structure similar to those in supernovae is created in a plastic layer.  The blast wave crosses a three-dimensional interface that produces unstable growth dominated by the Rayleigh-Taylor instability. We have detected the evolution of the interface structure under these conditions and will show the resulting experimental and simulation data.   

Controlling plasma chemistry of atmospheric-pressure plasmas in helium with oxygen and water admixtures

Many applications of atmospheric-pressure plasmas (APPs) rely on a controlled delivery of known amounts of reactive oxygen species (ROS). Careful control of the plasma chemistry of these devices is therefore desired, but often challenging because of the complex, non-linear relations between plasma input parameters and ROS outputs. Nevertheless, combining (advanced) diagnostics and numerical modelling can enhance our understanding of the plasma chemistry, allowing better control and performance in plasma applications. In this work, the focus is on Helium-based APPs with molecular admixtures of O₂ and/or H₂O. First, case studies on using APPs for changing the wettability of plastic and the removal of photoresist are presented. Atomic oxygen was deduced to be the most relevant ROS for both these processes. Optimisation of O density in the plasma device was subsequently undertaken, showing a 100-fold improvement for the photoresist removal process.

A more complex chemistry that is of interest for applications is He + H₂O, producing ROS such as OH and H₂O₂. We investigated the kinetics of OH and O in an RF atmospheric-pressure plasma by UV absorption spectroscopy (OH) and VUV Fourier-Transform Absorption Spectroscopy (O). Alongside these measurements, a 0D plasma-chemical kinetics model was used to compare the experimental results and understand the reaction pathways. These investigations showed a strong coupling between the O and OH densities with specific production and destruction pathways changing with increasing water content. In addition, at low water content, small amounts of oxygen impurities significantly change the chemistry with a sharp increase in O density, showing that adding admixtures to the feed gas rather than relying on the ambient atmosphere, enhances control of the plasma chemistry. Finally, to further improve control of the mix of ROS in an APP, we investigated plasmas with both O₂ and H₂O admixtures. An independent control of OH, through the H₂O content, and O, through the O₂ content was observed.