A nanosecond pulsed plasma discharge located within a wall cavity is used to ignite jet flames (hydrogen and ethylene) in supersonic crossflows. The nonequilibrium plasma is produced by repetitive pulses of 15 kV peak voltage, 20 ns pulse width and 50 kHz repetition rate. Sonic fuel jets are injected into free stream air of Mach numbers Ma = 1.7 to Ma = 3.0. The flow pattern and shockwaves induced by the fuel jets and cavity are characterized by Schlieren imaging and planar laser induced fluorescence is used to image the distribution of OH radicals. Combustion is found to be enhanced by the plasma discharge, with the time evolution of the flame suggesting that enhancement is caused by the reduction in ignition delay time. Similar trends are observed with both hydrogen and ethylene fuel injection. The experimental results for hydrogen combustion are interpreted using a simple model in which the pulsed plasma serves as a repetitive source of reactive radicals. The reaction kinetics following radical production is evolved numerically, confirming a reduction in ignition delay comparable to that seen in the experiments. The model description allows us to predict how a broader range of plasma operation conditions will affect combustion. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.