We have developed a unique, high-energy microwave source that is capable of generating very long, high-power pulses in a simple, compact, and lightweight package. Called PASOTRON for Plasma-Assisted Slow-wave Oscillator, it integrates a novel combination of E-gun, plasma, and slow-wave-structure techniques to create a source capable of generating 100-μs pulsewidths while requiring no magnetic focusing fields. Long E-beam pulses are obtained at high current density using Hughes Hollow-Cathode-Plasma (HCP) E-gun which was previously developed for driving gas-discharge lasers. The gun overcomes the limitations for conventional high-power-microwave tubes which employ either thermionic cathodes that have limited current density (≤5 A/cm 2 ) or field-emission cathodes that offer high current density, but suffer from short pulsewidth capability (≪1 μs) because of plasma closure of the accelerating gap. The HCP E-gun provides both high-current-density (40 A/cm 2 ) and long-pulse operation without gap closure. It also requires no cathode-heater power. The gun employs a low-pressure glow-discharge inside a hollow cathode (HC) to obtain a uniform and stable plasma surface from which a high-current-density beam can be extracted. The plasma density is controlled by a low-voltage RC-discharge pulser to provide space-charge-limited emission of electrons. A de high-voltage electron-beam supply accelerates electrons across the gap while the hollow-cathode-pulser modulates the beam current to generate arbitrary pulse waveforms. The beam is focused electrostatically by shaping the gun electrodes; no magnetic fields are required. In practice, the accelerating gap consists of a multiaperture array in order to generate a large-area, high-current beam consisting initially of many individual beamlets.