The design, construction, and operation of an underexpanded jet facility developed as a test ow eld for laserbased supersonic combustion diagnostics are described. The continuous-running device accelerates hot gases from a 160-kPa, 2000-K, stoichiometric H2 /Air combustion chamber through a 3.15-mm i.d. alumina tube into a 11-kPa chamber. The resulting underexpanded jet of ame species provides a range of ow eld parameters (M = 1–4, T = 1700–500 K, P = 72.8–2.7 kPa, and 1⁄2 = 0.12–0.004 kg/m3 ) typical of practical supersonic combustion experiments. The modeling of the jet ow eld using an axisymmetric Navier–Stokes code is also described.