Measurement of the Number Density in Pulsed (Gas/Plasma) Jets using a 2-Dimensional, Second-Harmonic, Dispersion Interferometer

A (two-color) second-harmonic, dispersion interferometer (SHDI) is under development that uses a common-mode, single-beam path to measure a sample's dispersive-phase shift. The phase shift exists between the primary 1064 nm beam, Δt = 0.6 ns, 200 μJ, and its 532 nm second harmonic. Post-sample, the primary-beam phase information is transferred to a second, second-harmonic beam, and the dispersive-phase change is eventually measured between the two SH beams. Achromatic telescopes, before and after the sample, provide a 5X beam magnification/demagnification, which allows for large-area illumination, A ≤ 10 cm². The use of these telescopes unfortunately introduces a sizeable ``noise" background present on all measurements, as a radially-symmetric, dispersive-phase shift (Δφ > 2π radians). This phase noise is quantified and subsequently removed from all sample measurements using various digital techniques. The instrument has now been tuned to measure dispersive-phase changes as low as 10^-2 radians, at spatial resolutions of order, Δ x ≈ 0.1 mm using samples that include: fused-silica objects (wedge, plano-concave and plano-convex lenses) and pulsed-jets of argon (gas and plasma). For the latter, the minimum measurable line-density product is, ∫n_gasdl ~10¹⁵ cm^-2 and number density, n_gas ~ 10^17-19cm^-3 and for plasma is, ∫n_plasma dl ~10¹⁵ cm^-2 and n_plasma ~ 10^14-16cm^-3. Design enhancements are currently underway to extend the SHDI performance to provide a higher noise-rejection ratio, S/N ≤ 10^-2.8, larger-area imaging, A ~10 -100 cm², and higher, frame-capture rates, fps ~ 0.1 - 1 kHz.