End-member compositions of plumes from volcanic eruptions range from nearly pure steam to heavily particle-laden gas flows. In all cases, if the plumes erupt from a high-pressure reservoir, they are initially supersonic jets that may have complex internal flow structures not easily documented in the field. In the laboratory, some properties of volcanic jets can be investigated with particle-laden flows, but other properties can only be investigated in optically transparent flows. We examine the relation of unsteady jet structure to reservoir conditions for optically transparent flows. We have developed an experimental shock tube facility capable of achieving pressure ratios up to ~150 with reservoirs of different shapes. Time-resolved schlieren visualization is combined with pitot pressure measurements to interrogate the structure of the underexpanded jet flow. We have done preliminary experiments at a pressure ration of 40 with air, with two reservoirs that are 12.6 and 20 cm in length. These initially produce well-defined supersonic jets that have properties (shape of the underexpanded jet; barrel shocks, Mach disk shocks) which we have bench-marked against other experiments and simulations. Estimated durations of the supersonic portions of the flow from pressure decay calculations are ~45 and ~75 ms, respectively. On these time-scales, the experimental jets collapse; the plume boundary and internal barrel shocks tighten and the Mach disk shock moves toward the vent, until subsonic conditions occur
