An experimental method to measure the electric fields existing in a plasma wake produced by a ∼0.24 GW, ∼0.5 ns, 9.5 GHz microwave pulse traversing a plasma-filled waveguide is presented. The intensity of the second harmonic of a 30 fs 800 nm laser generated inside a gas-filled dielectric tube placed inside the waveguide is used to characterize the wakefield parameters. Three distinct decaying oscillations of the plasma wakefield, with peak amplitude of ∼20 kV/cm, were observed. The experimental results were confirmed by 3D large-scale plasma particle-in-cell simulations.

It was observed experimentally that after crossing a waveguide filled with a neutral gas a short powerful microwave pulse leaves a periodic glow of plasma along the waveguide, persisting for several tens of nanoseconds. A theoretical model is presented which in combination with numerical simulations proposes a possible explanation for this phenomenon.

This study investigates the ionization pressure threshold of a gas (air, helium, argon, and SF6 across a wide pressure range) filled dielectric tube when a ∼300 MW, ∼0.7 ns, 9.6 GHz high-power microwave (HPM) pulse propagates through it. The thresholds are determined as the pressure for which the energy of the transmitted HPM pulse decreases to ∼30%, which is close to the same HPM pulse's transmission coefficient when a metal rod fills the tube. These thresholds are found to be 0.4 × 105 Pa,105 Pa, 1.8 × 105 Pa, and 0.2 × 105 Pa, for air, argon, helium, and SF6, respectively. The measured intensity of the plasma light emission starts to decrease at a pressure which coincides with the pressure threshold determined by HPM pulse propagation. Additionally, at gas pressures 105 Pa transforms into streamer like plasma. Simplified numerical simulations of a microwave discharge in air at 1 × 105 Pa and 4 × 105 Pa are consistent with the experimental plasma light observations.

The electron velocity distribution function in the plasma, formed by gas ionization by a microwave pulse of sub-nanosecond timescale width and hundreds of megawatts power, is studied by a theoretical model and by 3D numerical simulations which confirm quite well the model. It is shown that the distribution function is defined by the field amplitude variation during the entire pulse. After the pulse's passage through the gas, the remaining plasma distribution function follows a decreasing power-law function. Experiments performed in a waveguide filled with helium gas confirm that energetic (from several keV to several tens of keV) electrons remain in plasma long after the pulse has crossed the experimental volume. These electrons continue the gas ionization over extended times up to tens of nanoseconds.

This study investigates the plasma formed by a ∼300 MW, ∼0.8 ns, ∼9.6 GHz high-power microwave (HPM) pulse propagating inside a waveguide filled by helium or hydrogen gas at different pressures. It was found that HPM traverses He and H2 gas at pressures <1 and 7 ns). Using visible spectroscopy, the time-resolved density of the helium and hydrogen plasmas was determined. The plasma density continues to increase after the HPM pulse has left the system. Additionally, it was found that the polarization of the emitted light coincides with the direction of the HPM propagation, which is explained by the existence of an axial electric field in the plasma. This field accelerates free electrons, which transfer momentum to bound atomic electrons. Radial emission of electrons with energies up to 100 keV was also registered. This validates the results with the results of numerical simulations, which show that a large proportion of the electron energy spectrum at the tail of the HPM pulse is with energies >10 keV.

Time- and space-resolved radiation emitted by the plasma produced by a 0.8 ns duration at full width half maximum, ~600 MW maximum power microwave (~9.6 GHz) pulse traversing a hydrogen-, helium-, or air-filled circular waveguide, is studied. Gas ionization by microwaves is an old subject but the regime investigated in the present experimental research, of very high-power microwaves and very short pulses using modern diagnostic tools, is new and follows a series of new studies performed so far only in our laboratory, revealing non-linear phenomena never observed before. In the present research, plasma radiation is observed along a slit made in a circular waveguide wall by either an intensified fast frame camera or a streak camera. Using calibrated input and output couplers, the transmission and reflection coefficients of the high-power microwaves were determined over a broad range of gas pressures, 0.1 kPa < P < 90 kPa. It was found that the intensity of the plasma light emission increases significantly after the high-power microwave pulse has left the waveguide. Depending on pressure, the radiation is either uniform along the slit, while the front of the emitted light follows the microwave pulse at a velocity close to its group velocity, or it remains in the vicinity of the input window, indicating that the plasma density is above critical density. It was also found that the radial distribution of radiation depends on pressure. At pressures 80 kPa, light emission is most intense on the axis where the electron oscillatory energy is ~100 eV and the ionization rate is maximal. We also studied the UV radiation from the plasma, the duration of which was found to be longer than the duration of visible light emission. This indicates the existence of energetic electrons for tens of ns after the high-power microwave pulse has left the observation region. Considering that the emitted light intensity depends on the plasma density and temperature, the observed data may be used for a comparison with the results of collisional radiative models if the electron time and spatial energy distribution is known.