Time-frequency characteristics of microwaves generated by hypervelocity impact

When space debris impinges upon a spacecraft shield structure at an ultra-high speed, part of the debris’ kinetic energy is converted into electromagnetic energy, which radiates outward in the form of electromagnetic waves and can interfere with the spacecraft's normal communication. To investigate the radiation characteristics of the emitted electromagnetic waves, a series of experiments were conducted using a two-stage light gas gun, in which a spherical aluminum alloy (diameter:6.4 mm) projectile impinges on a 23-mm-thick target of the same material at velocities of 3.3–6.3 km/s. A real-time spectrum analyzer and super-heterodyne receiver were used to measure the microwaves in the frequency and time domains, respectively. Then, the time–frequency spectrum of the microwaves was obtained by Hilbert-Huang Transform. Further, two mechanisms of microwave generation—plasma expansion and material destruction—in an aluminum–aluminum impact are proposed, and the radiation powers of the two mechanisms are estimated based on the theoretical analysis and experimental results. Finally, the effect of the plasma cutoff frequency on the microwave propagation is explained, and the relationship between the radiation mechanism and the impact velocity is obtained. When the impact velocity is less than 5.2 km/s, material destruction is the main mechanism of microwave radiation, whereas at impact velocities > 5.2 km/s, plasma expansion dominates.