Measurements of fuel mole fraction were taken in a model scramjet cavity flameholder using nanosecond-gated laser-induced breakdown spectroscopy with direct spectrum matching. A fuel surrogate, carbon dioxide (CO_2), was used to conduct statistical measurements at locations which the laser breakdown initiated sustained combustion in the model scramjet. Verification and calibration testing was performed in a variable pressure combustion chamber to create a direct spectrum matching calibration matrix and confirm the suitability of CO_2 as a surrogate for ethylene (C_2H_4) fuel testing. CO_2 was shown to be a good surrogate for ethylene in the scramjet cavity, both in matching fuel/surrogate mole fraction and gas density for conditions with sufficiently high mole fraction. The use of CO_2 allowed for statistical measurements of fuel surrogate mole fraction at locations within the cavity that were ignited by the laser spark.
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