Publications

A colinear Second-Harmonic Orthogonal Polarized (SHOP) interferometer diagnostic capable of making electron areal density measurements of plasmas formed in Magnetically Insulated Transmission Lines (MITLs) has been developed.

Experimental measurements of low density plasmas forming in Magnetically Insulated Transmission Line (MITL) regions are desired to improve our understanding of current loss and power flow. Therefore, a new optical interferometer diagnostic was commissioned via this LDRD project. To measure the expected 10¹³ - 10¹⁷ cm^-3 electron densities inside the 0.5 - 6 mm Anode-Cathode (A-K) gaps, a colinear SHOP interferometer diagnostic was constructed. The diagnostic was initially fielded on the University of New Mexico (UNM) Helicon-Cathode (HelCat) plasma device which provided a highly repeatable and well understood plasma source for which the colinear SHOP interferometer’s functionality could be verified and measured. Utilizing the highly repeatable plasma source and shot averaging, the interferometer was able to achieve an areal density sensitivity of 1×10¹⁴ cm^-2. This work at UNM lead to a Review of Scientific Instruments (RSI) publication [20], DOI:10.1063/5.0101687. After the diagnostic’s capability was proven at UNM, the colinear SHOP interferometer was commissioned for use on the Sandia National Laboratories (SNL) Mykonos accelerator. Here, it provided the first temporal areal density measurements of plasma formation in a parallel plate MITL. The diagnostic was able to achieve a single shot (no multi-shot averaging like at UNM) areal density sensitivity of 1×10¹⁵ cm^-2 along a ~ 2mm probing path length, which provided adequate capability to conduct fundamental physics research of MITL plasma formation. CHICAGO and ALEGRA simulations support the diagnostics experimental findings. More experimental and computational work will continue, likely leading to another publication(s). The smaller scale Mykonos accelerator work has also provided justification that the colinear SHOP interferometer is a capable diagnostic for measuring plasma areal densities in the inner MITL and convolute regions of larger TW-class accelerators like SNL’s Z machine.

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Absolute calibration of streaked visible spectroscopy systems has been performed at Z-machine at Sandia National Labs in order to determine temperatures of electrode surfaces during the current pulse. The ability to calibrate the full system, including all fiber optic runs and probes is crucial to understanding errors in the calibration process. The calibration procedure involves imaging a blackbody light source, with a known spectral radiance which is coupled to an integrating sphere. This source is streaked slowly over a few ns using Sydor streak cameras. The slow sweep is converted to a 100-500ns sweep by imaging a bright light source on both sweep rates, and obtaining wavelength and time dependent correction curves. Any broadband light source or several laser lines of differing wavelengths can be used for this correction. This technique has yielded temperature estimates of several eV in the Z convolute.

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Sandia National Laboratories is pursuing a variation of Magneto-Inertial Fusion called Magnetized Liner Inertial Fusion, or MagLIF. The MagLIF approach requires magnetization of the deuterium fuel, which is accomplished by an initial external B-Field and laser-driven pre-heat. Although magnetization is crucial to the concept, it is challenging to couple sufficient energy to the fuel, since laser-plasma instabilities exist, and a compromise between laser spot size, laser entrance window thickness, and fuel density must be found. Ultimately, nonlinear processes in laser plasma interaction, or laser-plasma instabilities (LPI), complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation and beam-spray. We determine and discuss key LPI processes and mitigation methods. Results with and without improvement measures are presented.

A hybrid fs/ps pure-rotational CARS scheme is demonstrated in the product gases of premixed hydrogren/air and ethylene/air flat flames. Near-transform-limited, broadband femtosecond pump and Stokes pulses impulsively prepare a rotational Raman coherence, which is later probed by a high-energy, frequency-narrow picosecond pulse, generated by sum-frequency mixing of linearly chirped broadband pulses with conjugate temporal phase. Spectral fitting is demonstrated for both shot-averaged and single-laser-shot spectra. Measurement accuracy is quantified by comparison to adiabatic-equilibrium calculations for the hydrogen/air flames, and by comparison to nanosecond CARS measurements for the ethylene/air flames. Temperature-measurement precision is 1-3% and O2/N2 precision is 2-10% based on histograms constructed from 1000 single-shot measurements acquired at a data rate of 1 kHz. These results indicate that hybrid fs/ps rotational CARS is a quantitative tool for kHz-rate combustion temperature/species data.

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