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Low impedance relativistic electron accelerators currently produce nominal 50 ns pulses that are capable of power levels near 1 Terawatt at impedances near 1 Ohm. The time-dependent diode impedance characteristic plays a major role In efficiently coupling the pulse line power to an electron beam. In an effort to establish the desired accelerator impedance match early in the pulse, experimentalists have investigated cold cathode vacuum breakdown and subsequent space charge limited emission during the ~ 100 kV machine prepulse. This machine prepulse is due to capacitive coupling across the accelerator switches, and consequently cannot be independently studied and optimized. In this paper, a technique for externally introducing a typically 100 kV, low power conditioning pulse prior to the main pulse of a low impedance relativistic electron diode is described, along with techniques for reducing the machine prepulse to less than 5 kV. For various cathode geometries, the breakdown field, closure velocity, and time-dependent impedance established by this external prepulse is measured and compared with an empirical model of space charge limited emission from a hydro-dynamically expanding plasma. 3 Experimental evidence is presented that the high current accelerator impedance is effectively controlled by the relative time delay between the start of the prepulse and the main pulse.