Evidence for Nanoparticles in Microwave-Generated Fireballs Observed by Synchrotron X-Ray Scattering

The small-angle x-ray scattering method has been applied to study fireballs ejected into the air from molten hot spots in borosilicate glass by localized microwaves [V. Dikhtyar and E. Jerby, Phys. Rev. Lett. 96 045002 (2006)]. The fireball’s particle size distribution, density, and decay rate in atmospheric pressure were measured. The results show that the fireballs contain particles with a mean size of $\sim 50\mathrm{nm}$ with average number densities on the order of $\sim {10}^9$. Hence, fireballs can be considered as a dusty plasma which consists of an ensemble of charged nanoparticles in the plasma volume. This finding is likened to the ball-lightning phenomenon explained by the formation of an oxidizing particle network liberated by lightning striking the ground [J. Abrahamson and J. Dinniss, Nature (London) 403, 519 (2000)].

Figure 1

The experimental setup including the microwave cavity in which the fireball is created [8] adapted for the synchrotron scattering experiment. The distance between the microwave cavity and the SAXS detector was 5 m with an evacuated flight tube after the cavity.

Figure 2

An optical image of a stable fireball generated in the setup shown in Fig. 1. The nipple at the bottom of the fireball demonstrates its origin from a molten drop of liquid detached from a hot spot in the solid substrate.

Figure 3

Normalized SAXS intensity, $I(q)$, as a function of momentum transfer, $q$, for a typical fireball in our experiment. The continuous line corresponds to a fit to Eq. (1) with $R_g=690\pm 13\AA$, $G=1.05\pm 0.025$, $B=(2.08\pm 0.43)\times {10}^{-9}$, and $d=3.4\pm 0.04$. The dotted line represents a fit with $d=4$ fixed.

Figure 4

Decay of the normalized SAXS intensity $I(q)$ after the microwave energy has been turned off. The signal level reaches the background scattering levels after about 4 s. The continuous line for the upper curve corresponds to a fit to Eq. (1) with $R_g=622\AA$, $G=1.53$, $B=5.16\times {10}^{-10}$, and $d=3.8$.