Measurements of Sparks inside HE Fireballs

We studied sparks inside HE fireballs, caused by triboelectric charge buildup on carbon particles in the detonation products gases. We studied spark emissions from fireballs created by the detonation of 25-g hemispherical charges. The charge was mounted on a 4-inch diameter steel rod to absorb the high detonation pressures and initiated by a RP-80 ignitor. The rod was flush mounted on a UV transparent PMMA plate. As the hemispherical fireball expanded, it could be photographed both from the side and from underneath. Fireball gases turbulently mix with air, forming a turbulent combustion layer near the outer edge of the fireball. This was visualized using framing-camera photography and 3-color pyrometry. The sparks were visualized using a UV photography system consisting of: (i) a UV lens, (ii) a high-speed image intensifier, and (iii) a booster and (iv) a relay lenses that feeds into a conventional high-speed camera. The sparks emit electromagnetic radiation. This was recorded by 4 different RF antennas covering different frequency ranges: 

            (i) a biconical (SAS-545) with bicon-spikes, with a frequency range of 30 MHz-1 GHz

            (ii) a discone (EM6105) with a frequency range of 10 kHz-2 GHz

            (iii) a high-frequency biconical (SAS-547) with a frequency range of 1 GHz-18 GHz

            (iv) a passive loop (ETS-6511) with a frequency range of 20 Hz-5 MHz

The recorded RF signals were compared with the high-speed photography to correlate the physical phenomena. Also measured were the visualized spark lengths; these were correlated to RF frequencies using the monopole theory relation:  where c is the speed of light and L the spark length. These were compared with spark lengths as predicted by the electrostatic version of our high-order Godunov code which simulates turbulent mixing in the HE fireball. LLNL-AABS-832909