The aim of this project has been the development of a Helium-I collisional-radiative model, so that the measured intensities of various helium spectral lines could be used to diagnose electron temperature and density in the HIT-SI3 device. The HIT-SI (Helicity Injected Torus - Steady Inductive) plasma research lab tests a novel confinement strategy in which the injection of current along helical magnetic fields forms a magnetic structure which confines the bulk plasma. Currently, the lab’s methods of measuring electron temperature (via Thompson scattering) and electron density (via a far-infra-red laser) are difficult, time consuming, and unreliable. Collisional-radiative modeling can fill both roles in a simple and inexpensive manner. In brief, it is an attempt to accurately model the expected emission profiles for various helium spectral lines of interest over a range of electron temperature and density values and then to extract those values which correspond to the lines’ measured intensities. Our model has been encoded following published methods, and takes into account electron impact excitation/de-excitation and ionization, radiative recombination, three body recombination, and spontaneous de-excitation. The first stage was the successful replication of both equilibrium and non-equilibrium published solutions. We extended these models to the HIT-SI3 operating regime by adding in ion recycling terms. The necessary optical systems for data collection were built, and then calibrated using a stabilized light source. Results so far show that equilibrium models should be sufficient, but have so far not produced useful results. However, when the density range is fixed to within known values, we find a temperature range of between 10-25 eV.

Explosive boiling is characterized by an unstable, non-uniform liquid/vapor interface consisting of multiple gaseous lobes. Explosive boiling at microscales is a process of extreme interest in the studies of micromechanical systems and actuation due to the nearly instantaneous pace and large amounts of kinetic energy associated with the process. These characteristics that make the explosive boiling phenomenon highly attractive for certain microscale applications also hinders the study and understanding of the dynamics of the unstable liquid/vapor interface, and thus the prediction of the overall behavior of the process. The objective of my research is to overcome the challenges posed by the small temporal and spatial scales by designing and building an experimental apparatus to observe the explosive boiling phenomenon. This apparatus consists of a test chamber in which the explosive boiling of a droplet of test fluid will be controlled by the host fluid temperature gradient, and a high speed camera used to capture the process in a high level of detail. The experimental results will be compared with theoretical results acquired from direct numerical simulation to elucidate the dynamics of the liquid/vapor interface. The experimental apparatus design and preliminary imaging results are presented.

As magnetohydrodynamic (MHD) systems exhibit incredible nonlinearity, computational models have become increasingly common for parsing difficult engineering problems in plasma experiments. The dynamic mode decomposition (DMD) is a principled, data-driven, equation-free approach to extracting coherent temporal-spatial structures from raw data, with particular success in hydrodynamics and fluid-flow problems. We consider the DMD applied to MHD systems in the particular example of spheromak plasmas simulated in the Helicity Injected Torus experiment. We find that the DMD successfully resolves magnetic field structures in simulated magnetic field measurements that closely match expected physical properties. We further find that, under certain constraints, the DMD provides a powerful tool for further validation of simulation to experiment. Given the nonlinearity of magnetohydrodynamic systems, existing mechanisms for feedback controlling plasmas as instabilities occur are limited. Because the dynamic mode decomposition effectively serves as an approximate, linear reduced order model built from experimental and simulated magnetic field measurements on the fly, it offers considerable promise as a predictive feedback controller. This is significant as the detection of and control over plasma instabilities remains a major barrier to the use of spheromaks for fusion energy.