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The results presented herein include SparkJet characterization studies with a focus on understanding the actuator efficiency during stage 1 and the resulting flowfield. Experimental cavity pressure and arc power measurements are used to estimate efficiency whereas experimental microschlieren images are used to visualize the flowfield and quantify the velocity of distinct flow features. The experimental measurements used to estimate efficiency during stage 1 of a single SparkJet cycle were acquired using a single orifice SparkJet design in a quiescent environment. The experiments evaluated efficiency as a function of cavity volume (42.4-169.6 mm 3 ), orifice diameter (0.4-2.0 mm), and input energy (0.17-0.77 J per pulse). The results show that efficiency is a function of parasitic resistance in circuit components, high-temperature gas dynamics, and the ratio of input energy to internal energy of the cavity air, Q C ∕E. The circuit design used here was approximately 80% efficient and the pressure-based efficiency decreased from 40 to 20% as Q C ∕E increased from 5 to 60. Adjusting the pressure-based efficiency calculation to be a function of the measured arc energy, the conversion from arc energy to the energy available to raise the cavity pressure was 40-45% efficient. Using a SparkJet four-orifice array design (cavity volume of 363 mm 3 and each orifice 0.4 mm in diameter) in a quiescent environment, phase-averaged microschlieren images of a single SparkJet cycle reveal the rapid flow development when the first evidence of flow is seen 8 μs after the SparkJet initiation and a well-developed jet is visible by 100 μs. Also based on these calibrated images, the maximum jet velocity was estimated at 250 m∕s. Nomenclature C= capacitance, F C v = specific heat for a constant volume, J∕kg K E = internal energy, J I = current, A m = mass, kg n = frequency mode P = pressure, Pa Q = energy, J R = gas constant, J∕kg K T = temperature, K t = time, s U = jet velocity, m∕s V = voltage across capacitors, V γ = ratio of specific heats η = efficiency θ = molecular vibrational energy constant, K ρ = density, kg∕m 3 ω 0 = frequency, rad∕s Subscripts A = related to arc discharge C = related to capacitor P = related to pressure p = values corresponding to calorically perfect gas T = related to temperature 1 = conditions before arc 2 = conditions after arc

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