"Modeling UV and X-ray Spectra from the Swarthmore Spheromak Experiment"
Victoria Swisher

Summary:

Most of the ordinary matter in the universe is in the plasma state, so studying plasma is vital for our understanding of the universe. Research into plasma properties and behavior will also contribute to the development of fusion power. Plasma is defined as "a quasineutral gas of charged neutral particles which exhibits collective behavior." Quasineutral means that the gas is neutral enough that the electron density and the ion density are approximately equal to the common density of the gas, but not so neutral that all interesting electromagnetic forces disappear.

The goal of Victoria's project was to use PrismSPECT models to infer information about the impurity levels and temperature of the SSX plasma from data taken with the SSX soft x-ray detector (SXR). The soft x-ray detector can take data through four different filters simultaneously, and the ratio of the flux through these filters can tell us about plasma properties. Victoria includes a section of background about SSX--the information covered can be found in my summaries of Jerome and Slava's theses.

The SSX plasma is not in Local Thermodynamic Equilibrium (LTE) because its density is too low. Instead, we can model its state as coronal equilbrium, in which all upward atomic transitions are collisional and all downward transitions are radiative. Radiative transitions include transitions between bound states and free-bound transitions (recombination/photoionization). Collisional transitions include electron impact (excitation/deexcitiation), impact ionization/three-body recombination, and dielectronic recombination/autoionization. PrismSPECT is able to separate the contributions of free-free, bound-free, and bound-bound transitions to a single spectrum. Simulations using this capability showed that the SSX x-ray spectrum is dominated by bound-bound emission. Since collisional excitation is required before x-rays can be produced, line strengths should be dependent on the density of the plasma. However, increasing the density should have a proportional effect on all lines, so relative line intensities should not be effected.

Simulations were run assuming that the plasma contained 97% hydrogen (by number) and 1% each of carbon, oxygen, and nitrogen (to represent trace amounts of water, air, and dust left in the plasma chamber). Element models from Atomic Model Builder with between 450 and 1000 levels were used in the PrismSPECT calculations. PrismSPECT is capable of running either steady-state simulations, in which density, temperature, and ionization state fractions do not change with time, or time-dependent simulations in which these parameters can vary. Early results suggested that the time-independent simulations did not accurately describe the SSX plasma, because the CIII 229.687 nm line that had been experimentally observed at SSX using ion Doppler spectroscopy (IDS) appeared in the simulated spectra only at very low temperatures. Therefore, time-dependent simulations were run in which temperature and density were varied at 10 μs intervals in order to better approximate the conditions in SSX as the plasma is formed in the gun and then ejected into the central chamber. However, steady state simulations were also used in order to achieve a qualitative understanding of the temperature profile during an SSX shot.

The four filters for the SXR were chosen to have non-overlapping response curves and peak sensitivities at temperatures below 50 eV, since previous work suggested that SSX plasma temperatures were between 0 and 50 eV. An IDL code was used to sum the high-resolution spectra calculated by PrismSPECT over the range of responsivity for each filter. From steady-state simulations, it was determined that line intensity ratios have little dependence on the impurity fraction in the plasma, but a strong dependence on plasma temperature. Therefore, the first stage of analysis focused on comparing PrismSPECT spectra to SXR data in order to determine a best-fit value for the plasma temperature. The SXR data was very noisy, so it was difficult to achieve accurate results, and two of the four filters had such low signals for each shot that they could not be used in the final analysis. However, it was determined that the temperature in SSX spikes immediately after the magnetic reconnection event and then decreases over the remainder of the shot.

Applications to my research:

My project this summer is in many ways an extention of Victoria's. Building on the work she did, I hope to use PrismSPECT simulations to further constrain the properties of the SSX plasma. I will compare simulations to data from IDS and SXR, possibly using some of Victoria's code to bin the PrismSPECT spectra to match the low-resolution SXR spectra. Given the limitations of steady-state simulations for accurately describing the experimental conditions, further use of time-dependent simulations will be crucial for advancing our understanding of plasma dynamics and the reconnection process.