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 cavity modes through re-entrant beam alignment. Design details of the constructed facility are provided and complemented with a generalized analytical framework for calculating the modes and beam pattern characteristics of SC-MPC optical cavities. A high-temperature air chemistry experiment targeting O₂(X³Σ) at 221.6 nm, O(3s ⁵S2o) at 777 nm, N₂(A³Σ) at 868 nm, and N(3s ⁴P_5/2) 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.