Doppler spectroscopic measurement of the axial and transverse expansion of a hypersonic plasma jet in PJMIF

The work reported in this paper addresses the feasibility, the technical issues, and practical challenges in the application of Doppler spectroscopy in diagnosing the motion of ions and their symmetry properties in a relatively unfamiliar context of a fast-moving, low-temperature, partially ionized, high-density, high-Mach number plasma jet being developed for plasma jet driven magneto-inertial fusion. The paper documents our experience in developing a multi-channel spectroscopic system on the STG1 coaxial plasma gun development platform as well as the computational methodologies and computer codes to deconvolve the axial and transverse Doppler spectroscopic data. For the first time, two-channel Doppler spectroscopy and line-ratio spectroscopy provide direct evidence to support the proposition that the plasma jet launched by the coaxial plasma gun is axially symmetric. Surprisingly, we found that there is a broad distribution of ion velocities both axially and transversely in the plasma jet. Interestingly, the transverse and axial ion velocity distributions are found to be Gaussian despite their broad distributions. For a representative shot launched by the STG1 plasma gun, the results show that the average ion axial velocity is 100±3 km/s and temperature is low as 1.25±0.02 eV with ion acoustic velocity estimated as 1.95 km/s, suggesting a bulk plasma jet Mach number of nearly 51.3. Yet the average ion expansion velocity obtained from the Doppler spectroscopic measurement is 6.5±4.1 km/s implying a much lower Mach number (15±9) for the overall plasma flow. The resolution of the discrepancy between these two values of Mach number begs for further research in the future.