Successful Suppression of Pulse Shortening in an $X$ -Band Overmoded Relativistic Backward-Wave Oscillator With Pure $\mathrm{TM}_{01}$ Mode Output

Long-pulse operation of relativistic backward-wave oscillators (RBWOs) is seriously handicapped by pulseshortening phenomenon. Explosive emission plasma formed on the surface of slow-wave structures (SWSs) due to RF breakdown is considered to be the predominant cause of pulse shortening in RBWOs. Utilizing overmoded SWSs with large diameter is effective for decreasing the field strength on the surface of SWSs and the possibility of RF breakdown. However, multiple modes may be output simultaneously in overmoded RBWOs. In this paper, both the power capacity and output mode purity of an X-band overmoded RBWO are substantially improved through optimizing the profile of the SWSs and geometric parameters of the whole electrodynamic structures. As a result, we successfully suppress pulse shortening and achieve pure TM
₀₁
mode output simultaneously. The first ever reported experimental result is that the pure TM
₀₁
mode is radiated with a power of 2 GW, a pulse duration of 116 ns, a power efficiency of 28%, and a repetition rate of 30 Hz.