Measurements of railgun generated supersonic plasma jet propagation and two jet oblique merging.

摘要

Imploding spherical plasma liners have been proposed as a possible method for creating high-energy-density (HED) laboratory plasmas and as a standoff driver for magneto-inertial fusion (MIF). The Plasma Liner Experiment (PLX) planned a three-phase experimental program to study the feasibility of using railgun-driven supersonic jets to form imploding spherical plasma liners. The three phases are to investigate single-jet evolution during propagation, to merge 2-5 jets to assess the suitability of merging for liner formation, and to merge 30 jets in spherical symmetry to form a complete liner. We present here details of single-jet propagation and two-jet oblique merging experiments completed on PLX.;A key component of this dissertation was the design, implementation, and operation of a novel 8 chord, fiber-coupled interferometer based on a long coherence length (> 100 m) 561 nm diode-pumped solid state laser. This interferometer was a critical diagnostic in both single-jet propagation and two-jet merging studies. The long laser coherence length and fiber-optic design allowed signal and reference path lengths in the interferometer to be mismatched by many meters without signal degradation, greatly simplified interferometer optical layout, and added flexibility in interferometer positioning for both propagation and merging experiments. The interferometer sensitivity to ions, neutral atoms, and electrons required development of a phase shift analysis that incorporated the presence of neutrals, impurities, and multiply ionized species. Interferometry, coupled with spectroscopic ionization fraction estimates, was used to assess time resolved density profile measurements. Survey spectroscopy inferred both Te and ionization fraction f via non-local-thermodynamic-equilibrium (non-LTE) atomic/equation-of-state (EOS) modeling. A fast CCD camera and photo-diode array allowed for assessment of plasma emission for velocity and jet profile measurements.;Initial jet parameters were ne ∼ 10E 16 per cc, Te ≈ 1.4 eV, velocity v ≈ 30 km/s, sonic Mach number M ≈ 14, diameter ≈ 5 cm, and length ≈ 20 cm. Interferometry in conjunction with CCD line-out data showed that the average jet density decreases by a factor of ten after propagating 40 cm, which is at the very low end of the 8--160 times drop predicted by ideal hydrodynamic theory. In oblique merge experiments, interferometry identified a density increase consistent with shock formation as opposed to simple plasma interpenetration, and the consistent formation of a density structure (with scale length ∼ lambdaii') near the merge plane. Imaging showed formation of a multi-peaked emission structure transverse to the jet-merging plane with widths similar to the density structure. Since the merging regime was semi-collisional and the counter-streaming ion collisionality was comparable to the merged-structure size, we interpreted the observations using both hydrodynamic oblique shock and multi-fluid plasma theory and simulations. We find that our observations were consistent with oblique shock theory and a collisional, one-dimensional, multi-fluid plasma simulation.