Observation of Trivelpiece-Gould modes in a plasma-filled backward wave oscillator.

Abstract

Trivelpiece-Gould modes (TG modes) have been observed with frequencies up to the background plasma frequency in a plasma-filled X-band backward wave oscillator. The TG mode emission was broadband with observed frequencies ranging from 2.6 GHz, the lower limit of our frequency diagnostic, to ${\mathit{f}}_{\mathit{p}}$, the plasma frequency. Mode competition between the TG modes and the X-band ${\mathrm{TM}}_{01}$ mode prevailed when the background plasma density was below 6\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (${\mathit{f}}_{\mathit{p}}$\ensuremath{\sim}7 GHz). At a critical background plasma density of ${\mathit{n}}_{\mathrm{cr}}$\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (${\mathit{f}}_{\mathit{p}}$\ensuremath{\sim}8 GHz) a sudden power enhancement appeared simultaneously in both X band and the TG modes. Power enhancements in S and J bands were up to 8 and 3 dB, respectively, with the X band enhanced up to a factor of 6.