Wake Excitation in a Cylindrical Waveguide Driven by a Gigawatt Sub-Nanosecond K-Band Microwave Pulse

We present the first experimental observation of the predicted
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plasma wake excitation driven by a powerful gigawatts sub-nanosecond K-band microwave (26 GHz, > 1 GW, <0.5 ns) in a cylindrical waveguide preliminary filled by a plasma produced by a flashboard. The plasma time dependent parameters were studied using single Langmuir probe, microwave interferometry, microwave cut-off and visible spectroscopy. The sub-nanosecond microwave pulse was generated by a super-radiant backward wave oscillator driven by an annular electron beam (∼320 keV, ∼1.6 kA, ∼5 ns), produced by a magnetically insulated foilless diode powered by a ∼320 kV negative polarity high-voltage generator of ∼0.5 ns rise-time improved by a gas-filled spark-gap switch. The electron beam propagates through a slow-wave structure guided by an 8 T axial magnetic field. Frequency-modulation during the transmitted microwave pulse is observed, the result of the dynamics of the plasma density modulation during the wake excitation. 1D analytical modeling and 3D PIC (Particle-in-Cell) simulations confirm the connection between the transmitted microwave pulse frequency and the plasma density modulations.