Overview

My research at UT Austin is funded under the Department of Energy’s National Nuclear Security Adminsitration PSAAP 3 framework (PSAAP = Predictive Science Academic Alliance Program). As an experimentalist in the project, I work closely with researchers on the computational side for comparison, model development, and validation. We also have strong collaborations with the National Labs (Sandia, Lawrence Livermore, and Los Alamos) to leverage our expertise with their resources, cross-pollinate methods and open research questions, and train students and postdocs.

(clicking on the bold face print will download corresponding PDFs)

50 kW inductively coupled plasma (ICP) torch

The UT Austin 50 kW ICP Torch facility (Applied Plasma Technologies, Corp, model APT-50) has a maximum DC input power of 50 kW and the driving magnetic field oscillates at a frequency of 6 MHz, i.e. the radio-frequency regime. It can run on a wide variety of atomic and molecular gases, such as nitrogen, oxygen, argon, methane, hydrogen, and mixtures thereof. The overarching goal of the project I am involved in is to develop a fully predictive model of this ICP torch, which represents a complex multi-physics problem with circuitry, electromagnetic fields, fluid mechanics, plasma chemistry, and radiation all being strongly coupled to produce the plasma properties we observe.

An interesting phenomenon we observed early on is that imperfect rectification leads to periodic oscilliations in the plasma properties at a frequency corresponding to an integer multiple of the domestic grid AC frequency, i.e. 180 Hz = 3 x 60 Hz. A first characterization of this effect was presented at the 2022 AIAA SciTech Forum with a follow-up talk at the 202 APS Division of Fluid Dynamics meeting.

We also utilized a nanosecond multiplex Coherent Anti-Stokes Raman Scattering (CARS) system to perform measurements of the rotational-vibrational equilibrium temperature in the plume of the ICP with air as the feed gas. This marks the first time CARS has been used for these kind of conditions and for such high equilibrium temperatures. We published our findings in Applied Optics.

The plasma in the plume has a bulk enthalpy on the order of tens of MJ/kg. Thus, it can be used to study aerothermochemstry effects occuring at the outer surface of vehicles moving at hypersonic speeds. Specifically, it is often assumed that the subsonic high-enthalpy plasma stream is representative of the hot region downstream of the bow shock created ahead of re-entry capsules and hypersonic leading edges. By increasing the spectral bandwidth of the Stokes laser in our CARS system and spatially registering the measurement volume position, we are able to measure spatially resolved profiles of temperature and probe the CARs spectra of carbon monoxide and molecular nitrogen simultenously. I presented some first results at the 2023 Gordon Research conference for Laser Diagnostics in Energy and Reacting Flows.

In the video below, a graphite sample exposed to the high-enthalpy air plasma from the ICP torch can be seen. The purple glow in the approach flow is due to production of the strongly radiating CN radical.

Capcatively coupled plasma (CCP) discharge

The CCP discharge, or glow discharge, is a low pressure plasma generator that provides a relatively uniform plasma in a cylindrical geometry. Within the project, the point of having the glow discharge is to study plasma chemistry, plasma models, and radiation effects without the more complex fluid mechanics and circuitry of the plasma torch. Because it is a smaller system, it is also easier to set up intrusive and non-intrusive diagnostics around it, or move the glow discharge to the diagnostics.

In the video below, the pressure inside the CCP plasma vessel is decreasing with time and the corresponding expansion of the plasma sheath can be observed. In the background is a Langmuir probe head.

Bayesian uncertainty quantification

Three recurring questions when comparing experimental and computational results are:

  • How close is close enough?

  • How uncertain are either results?

  • Can we improve the model based on experimental results when the targeted quantities are not measured directly?

To make the evaluation of experimental data more systematic, inlcude all known uncertainties, and set the stage for the inclusion of higher fidelity models in the interpretation of measurements, I am working on the Bayesian inversion of the measurement models we currently use. This is of particular interest for measurements where the inference of target parameters, such as temperature or electron density, depend on a large number of assumptions and uncertain parameters in the inversion of the measurement model, e.g. in optical emission spectroscopy. I recntly presented some prelimary results on this research thrust at the 2023 APS Division of Plasma Physics meeting.