A detailed investigation on flame structures and stabilization mechanisms of confined high momentum jet flames by one-dimensional (1D)-laser Raman measurements is presented. The flames were operated with natural gas (NG) at gas turbine relevant conditions in an optically accessible high-pressure test rig. The generic burner represents a full scale single nozzle of a high temperature FLOX® gas turbine combustor including a pilot stage. 1D-laser Raman measurements were performed on both an unpiloted and a piloted flame and evaluated on a single shot basis revealing the thermochemical states from unburned inflow conditions to burned hot gas in terms of average and statistical values of the major species concentrations, the mixture fraction and the temperature. The results show a distinct difference in the flame stabilization mechanism between the unpiloted and the piloted case. The former is apparently driven by strong mixing of fresh unburned gas and recirculated hot burned gas that eventually causes autoignition. The piloted flame is stabilized by the pilot stage followed by turbulent flame propagation. The findings help to understand the underlying combustion mechanisms and to further develop gas turbine burners following the FLOX concept.