Ultrafast high-power microwave window breakdown: Nonlinear and postpulse effects

The time- and space-dependent optical emissions of nanosecond high-power microwave discharges near a dielectric-air interface have been observed by nanosecond-response four-framing intensified-charged-coupled device cameras. The experimental observations indicate that plasma developed more intensely at the dielectric-air interface than at the free-space region with a higher electric-field amplitude. A thin layer of intense light emission above the dielectric was observed after the microwave pulse. The mechanisms of the breakdown phenomena are analyzed by a three-dimensional electromagnetic-field modeling and a two-dimensional electromagnetic particle-in-cell simulation, revealing the formation of a space-charge microwave sheath near the dielectric surface, accelerated by the normal components of the microwave field, significantly enhancing the local-field amplitude and hence ionization near the dielectric surface. The nonlinear positive feedback of ionization, higher electron mobility, and ultraviolet-driven photoemission due to the elevated electron temperature are crucial for achieving the ultrafast discharge. Following the high-power microwave pulse, the sheath sustains a glow discharge until the sheath collapses.

Figure 1

Illustration of the schematic experimental setup for detecting the plasma discharge at the dielectric-air interface by using a four-frame ICCD and directional couplers (not to scale, with the discharge region marked by blue dotted ring).

Figure 2

Typical waveforms of the incident inline pulse (black circles) and the far-field radiation pulse (yellow diamonds) and two trigger waveforms of the four-frame ICCD (red triangles and blue squares).

Figure 3

Development and evolution of the HPM window surface breakdown in air at 760 Torr for a 9.9-GHz, 1-GW, 15-ns pulse. The pictures are taken relative to the observation area of the ICCD placed in Fig. 1, with the dielectric-air interface represented by the black dotted line and the round window showing the boundary dimensions of the image intensifier after enlarging by lenses of the ICCD. The times are (a) 5-ns, (b) 10-ns, (c) 20-ns, and (d) 25-ns images of visible light in pseudocolor.

Figure 4

(a) Time-averaged amplitude and (b) transient vector profile at phase 0° of the local E field near the dielectric-air interface without plasma in the Y-Z plane view (the dielectric window located to the right of the black line, with a peak E field of 120 V/m at an incident power of 0.5 W).

Figure 5

(a) Temporal and spatial evolution of plasma profiles in two-dimensional PIC simulations: initial uniform density (cyan squares), transient electron density $n_e$ under the initial preset accumulated negative charge (time 5 ns) (black diamonds), $n_e$ (purple up triangles) and ion density $n_i$ (blue down triangles) near saturation (time 8 ns), and transient thermal electron energy (red squares). (b) Temporal evolution of the total $E_z$-field profile (defining the negative field pointing toward the dielectric): initial field (red circles), developing field (purple up triangles), near saturation (blue down triangles), collapsing sheath after the HPM pulse (black diamonds), and $E_z$ for microwave incident at a 5° angle (cyan squares).