Study of the Frequency Spectrum, Noise and Harmonics in the Output Radiation of the Pasotron

Abstract

Summary form only given. The plasma-assisted slow-wave oscillator (pasotron) was first developed at Hughes electron dynamics to meet the need for mobile microwave sources capable of generating high pulse energies. The gun plasma provides a source of electrons for generating high-current electron beams and ions for neutralizing space charge forces on the beam. By controlling the density and extent to which gas propagates into the system, the beam may be focused and transported downstream while the RF structure is maintained essentially at vacuum. One distinct advantage of the pasotron is that beam transport is accomplished purely by ion focusing and not by an externally applied magnetic field. Work on helix pasotrons has continued at the University of Maryland where 1 MW of output power in L-band at efficiencies greater than 50% has been achieved. Even though significant progress has been made, there is reasonable concern about the spectral purity and noise content in the radiated power. It is well known that ions from the residual gases in microwave tubes may cause low-frequency fluctuations in the beam current. This effect manifests itself as noise and spurious sidebands in the output spectrum. These issues motivated a study of the spectral characteristics of the pasotron which included measurements of the radiation line width, spurious sidebands, wideband noise and power at the second harmonic of the operating frequency. The results show that the line width measured at -20 dBc is less than 30 kHz, which is the resolution limit of the pulse, wideband noise is less than -105 dBc for frequencies greater than 30 MHz off carrier, and spurious sidebands and the second harmonic are -50 dBc. The measured spectral performance compares favorably with or better than many commercial microwave vacuum tubes especially at megawatt power levels. Furthermore, the results suggest that plasma in the pasotron is largely confined to the gun and beam transport regions and has minimal impact on the RF spectrum. Sideband spurs at a level of -50 dBc at +/-3 MHz off carrier are observed which are consistent with the ion oscillation frequency in the beam focusing region. Results from measurements of frequency and mode stability and the frequency-pulling effect will also be presented