Sodium tracer measurements of an expanded dense aluminum plasma from e-beam isochoric heating

Abstract

Spatially and temporally resolved visible absorption spectroscopy is performed on sodium D-lines present as surface contaminants on an expanded dense aluminum plasma plume. An 80-ns FWHM, intense, relativistic electron beam deposits 5.4 J into a 100-μm-thick Al foil, which isochorically heats and subsequently hydrodynamically expands the material through the warm dense matter state and into a classical-like plasma state, with a coupling parameter of approximately 0.2 and a degeneracy parameter of approximately 270. The Na contamination, carried along with the expanding plume, shows saturated absorption features in the dense Al continuum for λ> 450 nm. X-ray photoelectron spectroscopy and laser-induced breakdown spectroscopy confirm Na is a surface contaminant with an atomic concentration of ∼0.1% when interrogating identical foil samples. A spectroscopic-quality radiation transport model is used to post-process 2D hydrodynamic simulations to interpret the plasma conditions based on the measured Na 3p-3s doublet line profiles. A sodium number density of 3×1015 cm−3 best matches the experimental spectra, which originate from a dense surface plasma with ne=3.0±0.8×1018 cm−3.

Diagnosing the DARHT Electron Beam-Target Interaction and Hydrodynamic Expansion [Dissertation]

“…This gradient can span several decades of density in 100s of µm. The majority of the present plume measurements, especially for spectroscopy, are made well after the material has disassembled [3,68,69]. A consequence of measuring a plasma plume with a severe density gradient is the opacity effects and attenuation As the plasma plume expands, it radiatively cools, slowing its expansion velocity and reducing its temperature and density.…”

Diagnosing the DARHT electron beam-target interaction and hydrodynamic expansion [Slides]

“…The Axis-I LIA produces an intense, relativistic electron bunch with an 80-ns Full-Width at Half-Maximum (FWHM) at an energy of 19.8 MeV and a current of 1.5 kA. In these beam-target interaction experiments, the beam is incident upon range-thin low-Z metal foils and collisionally deposits energy in a two-stage heating process [3,[67][68][69]. Prior to hydrodynamic disassembly of the foil, the heating is isochoric.…”

“…The second formulation from Equation 2.5 is used to infer roughly the electron temperature by measuring the expansion velocity v max . This can be obtained by performing a series of time-gated plasma plume expansion (Chapter 4, Section 4.2.1) or shadowgraph (Chapter 4, Section 4.2.4) measurements and then using the following relation [69,78]:…”

“…collecting, collimating, and dispersing the radiation produced by the plasma, do not work as the plasma is dominated by absorption effects and very low emissivity, relative to hot plasmas. Present measurements have recorded densities up to 10 19 cm −3 with 532-nm interferometry [83], and visible spectroscopy via Stark broadening has measured densities up to 10 18 cm −3 [3,68,69]. While these measurements do constrain and demonstrate the WDM state to be present, the WDM state has not been directly measured.…”

“…This allows us to quantify the energy density (J/m 3 ) deposited into the target foils. Figure 4.5 shows the diagnostic layout for measuring near-field OTR in the downstream barrel diagnostic region on DARHT Axis-I [68,69,83]. The light is captured by a PiMAX4 image intensified Charge-Coupled Device (CCD) camera [101].…”