Time Evolution of the Pinch Region of a Deflagration Plasma Accelerator

TL;DR

This work addresses time-resolved evolution of the pinch region in a coaxial plasma accelerator (CHENG) by employing time-resolved optical emission spectroscopy on the Hβ line, with Abel inversion used to recover radial density profiles. Distinguishing deflagration and detonation modes, it finds deflagration to produce very high core densities up to ~$4 \times 10^{23}\ \mathrm{m^{-3}}$ while detonation yields a broader, lower-density pinch (~$3 \times 10^{21}\ \mathrm{m^{-3}}$ peak) and shorter lifetimes. A Voigt-profile analysis of Hβ, coupled with energy-balance arguments, provides an upper bound on core pinch temperatures of ~550 eV, though instrument resolution and data near the core limit precise temperature determinations. The results corroborate prior Schlieren measurements and time-integrated OES, highlighting the potential and challenges of time-resolved pinch diagnostics and pointing toward higher-resolution, intensified setups for direct temperature measurements.

Abstract

A spectroscopic study of a plasma deflagration accelerator was carried out to investigate the temporal evolution of plasma density within the pinch region. A half-meter imaging monochromator, paired with a fast (10 MHz) camera operating at 1 MHz, was used to collect broadened chord-integrated spectral lines from the pinch region of a plasma deflagration device. Specifically, images of the $H_β$ and the $H_α$ lines were taken - with the $H_α$ used to find the background continuum. Voigt fits of the Abel inverted H$_β$ emission lines allowed for determination of the radial profile of the number density in the pinch at intervals of 1 $μ$s. This provided insight into the formation, growth, and decay of the pinch in both the deflagration and detonation modes of the accelerator. It was found that the maximum density for the deflagration increased from $\sim 10^{20}$ m$^{-3}$ 1.4 cm away from the core of the pinch to $\sim 10^{23}$ m$^{-3}$ at the core of the deflagration pinch. In contrast, the detonation pinch featured a broader zone of relatively constant density on the order of $\sim 10^{20}$ m$^{-3}$. Furthermore, an energy balance of the plasma in the deflagration pinch and downstream, as informed by prior work, was done to get an estimate of temperatures in the core of the pinch where measurements are not currently possible revealing potential temperatures on the 550 eV in the core of the pinch suggesting departures from some of the assumptions in the analysis.

Figures (11)

• Figure 1: Long exposure (5 s) image of the CHENG device during operation prominently showing the deflagration pinch region - the area of interest for this experiment.
• Figure 2: The experimental setup included two f/4 lenses for focusing light on the entrance slit of the spectrometer half and a Shimadzu high speed camera gate for 1 $\mu$s with an exposure time of 500 ns
• Figure 3: A mercury lamp was used for spatial and spectral calibration. A spatial calibration factor of 0.076 mm/pixel was found with a spectral calibration factor of 1.025 $\text{\AA}$/pixel
• Figure 4: Example frame featuring the broad $H_\beta$ line and some other spectral lines in the pinch region during a deflagration. The picture image spans 190 $\text{\AA}$ of the spectrum (area demarcated in green), and the center of the pinch is about 30 pixels from the top of the image
• Figure 5: Abel inverted image of an acquired emission image after dark-field and flat-field corrections. The image has been cropped to show the field of view and re-leveled for visibility