Switching of circular waveguide H11 mode by plasma of gas microwave discharge

The report presents the results of experimental study of switching the H₁₁ mode by gaseous discharge plasma formed in microwave switches made of S-band circular waveguides. The objective of the study was switching executed by a microwave discharge inside a quartz tube located in the polarization plane of the switched wave in a regular waveguide or switching arm of the waveguide H-tee. The main conditions of the effective switching of H₁₁ mode in the waveguide capable to transmit the higher E₀₁ mode as well were determined by model experiments at low power level by imitating a plasma channel by a section of thin wire inserted into the discharge area. The portions of total power in transmitted, reflected and dissipated waves during the switching process in nitrogen, argon and argon and SF₆ mixture was also estimated during the experiments run at high power level. The switching was realized inside the tube under insulating gas pressure of ~10-760 Tor and with transmission of microwave pulse with pulse power 2 MW and pulse width of 3mcs along the switch. It was found out that the switching process is more effective during first 10-20 ns. In particular it was determined that during this time the working mode power may be amplified by the plasma of microwave discharge. Therefore the assumption about the possible overestimation of the active microwave pulse compressor amplification with plasma gas discharge switches was formulated. The first data concerning switching of H₁₁ mode in the regular waveguide with the diameter close to the critical diameter for H₀₁ and E₁₁ modes, switching in the H-ee made of such waveguides and switching in the transmitting type cavity which is cut off for the upper resonant modes were obtained. The influence of mode coupling and weak reflections forming resonant cavities inside the waveguide on the switching process were noticed. Operation of working prototypes of S-- and active pulse microwave compressors demonstrated that the use of plasma microwave switches with the H₁₁ mode of circular waveguide allowed producing steady nanosecond microwave pulses with the pulse power of up to 1 GW, pulse repetition frequency of up to 100 Hz and when the power losses were about 1...2 dB.