A laser diagnostic for lineshape-based gas temperature and pressure measurements targeting a single atomic potassium absorption transition

A single-line laser absorption diagnostic technique has been developed for rapid measurements of temperature and pressure in high-enthalpy gas environments. This technique targets atomic potassium via the absorption lineshape of the D2 transition near 766.7 nm. Potassium is a convenient spectroscopic target because it has strong resonance transitions, is naturally occurring in many experimental facilities, and is active in the near-infrared where robust and cost-effective optical equipment is easily available. The approach was tested and validated at a measurement rate of 50 kHz in a shock tube at conditions ranging from 800–3600 K and 0.04–1 atm. Collisional broadening coefficients for potassium vapor in argon and in nitrogen were extended to this temperature range. The approach is most sensitive to pressure at low temperatures and high pressures, where the lineshape is most Lorentzian in form; conversely, it is most sensitive to temperature at high temperatures and low pressures where the lineshape is most Gaussian in form. At intermediate temperatures and pressures, it is sensitive to both quantities. Measurement uncertainties were as low as 6% in temperature and pressure. The technique can also be extended to other species for developing similar single-line diagnostics sensitive to different regions in temperature and pressure space.