A plasma wave tube and associated operating method are described in which a pair of cold-cathode electron beam generators discharge counterpropagating electron beams into an ionizable gas, preferably hydrogen or a noble gas, within a waveguide housing. A voltage within the approximate range of 4-20 kV relative to the waveguide housing is applied to the cathodes to produce electron beams with current densities of at least about 1 amp/cm.sup.2. The beams form a plasma within the gas and couple with the plasma to produce electron plasma waves, which are non-linearly coupled to radiate electromagnetic energy in the microwave to mm-wave region. A magnetic field is established within the waveguide between the cathodes to confine the plasma, and to control the beam discharge impedance. The gas pressure is held within the approximate range of 1-100 mTorr, preferably about 10-30 mTorr, to damp plasma instabilities and sustain the beam voltages, while the magnetic field is within the approximate range of 100-500 Gauss. A very rapid frequency slewing or chirping is achieved with a relatively high magnetic field that reduces the discharge impedance to the lower end of the permissible range. Frequency-stabilized operation is achieved with a lower magnetic field that increases the discharge impedance so that the beam current changes very slowly with time.