Single-Shot Terahertz Time-Domain Spectroscopy for Reactive Plasma Flows

In this work, we develop a chirped terahertz time-domain spectroscopy diagnostic to probe the non-equilibrium state of chemically reacting plasma flows. The diagnostic is capable of measuring species densities over characteristic scales that govern transport, reactivity, and instability formation. The diagnostic leverages chirped pulse technology to scan broad spectral regions spanning ∼ 100 GHz - 2 THz all in a single shot from a femtosecond laser. This enables the diagnostic to achieve temporal resolutions < 10⁻¹² s and spatial resolutions ∼ 1 mm during a single snapshot or view dynamics of plasma flows at the 2 MHz repetition rate of the laser. Together, these capabilities are leveraged to target rotational transitions and measure the concentrations of molecules in air plasmas quantitatively over space and time (CO₂, N₂, NO, and CO). The line strength and shapes are used to extract information about the thermodynamics including characteristic temperatures. The spectral phase is monitored simultaneously to measure electron densities spanning 10¹⁷ - 10¹⁹ m⁻³.