A reduction in the rise time of a 2.85 GHz high power microwave (HPM) pulse is achieved by implementing an overvoltaged spark gap inside a waveguide structure. The spark gap is oriented such that when triggered, the major electric field component of the dominant TE10 mode is shorted. The transition from a transmissive to a highly reflective microwave structure in a relatively short period of time (tens of nanoseconds) creates a means to switch multimegawatt power levels on a much faster timescale than mechanical switches. An experimental arrangement composed of the waveguide spark gap and a high power circulator is used to reduce the effective rise time of a HPM pulse from a U.S. Air Force AW/PFS-6 radar set from 600 ns down to 50 ns. The resulting HPM pulse exhibits a much more desirable excitation profile when investigating microwave induced dielectric window flashover. Since most theoretical discussions on microwave breakdown assume an ideal step excitation, achieving a "squarelike" pulse is needed if substantial comparison between experiment and theory is sought. An overview of the experimental setup is given along with relevant performance data and comparison with computer modeling of the structure.