A ring-amplified shock tube for variable-gain, multi-wavelength absorption spectroscopy

A ring-amplified shock tube (RAST) is presented as a new facility design for high-sensitivity laser absorption spectroscopy (LAS) in shock-heated experiments. By integrating a confocal, segmented circular multi-pass cell (SC-MPC) into the test section, the RAST decouples optical pathlength from tube diameter, enabling multi-pass absorption spectroscopy (MPAS) with selectable pathlengths ranging from 0.15 m to 11 m using single-entry alignment. Pathlengths well in excess of 11 m are possible by combining multiple modes through re-entrant beam alignment. A high-temperature air chemistry experiment targeting O2(X3Σ) at 221.6 nm, O(3s5So2) at 777 nm, N2(A3Σ) at 868 nm, and N(3s4P5/2) at 868 nm demonstrates multi-wavelength, variable-gain measurements of absorption transitions previously undetectable at the target conditions. Results confirm robust beam alignment, low cavity noise, and minimal emission interference. The RAST provides substantial improvements in sensitivity and diagnostic flexibility for kinetics and spectroscopy applications.