Fireball Ejection from a Molten Hot Spot to Air by Localized Microwaves

A phenomenon of fireball ejection from hot spots in solid materials (silicon, germanium, glass, ceramics, basalt, etc.) to the atmosphere is presented. The hot spot is created in the substrate material by the microwave-drill mechanism [Jerby et al., Science 298, 587 (2002)]. The vaporized drop evolved from the hot spot is blown up, and forms a stable fireball buoyant in the air. The experimental observations of fireball ejection from silicate hot spots are referred to the Abrahamson-Dinniss theory [Nature (London) 403, 519 (2000)] suggesting a mechanism for ball-lightning initiation in nature. The fireballs observed in our experiments tend to absorb the available microwave power entirely, similarly to the plasmon resonance effect in submicron wavelengths [Nie and Emory, Science 275, 1102 (1997)].

Figure 1

An illustration of the fireball ejection from a molten hot spot in a silicate substrate: (I) a hot spot is induced in a thermal-runaway process by the microwave-drill technique on the substrate’s surface. (II) The microwave drilling bit is pulled out slowly from the molten hot spot, dragging out a drop of the substrate material that blows up in the air. (III) The vaporized drop evolves to a stable fireball floating in the cavity’s atmosphere.

Figure 2

The experimental setup consists of a waveguide fed by a 0.6 kW magnetron. The microwave energy is concentrated by the microwave-drill bit to form a molten hot spot in the substrate material from which the fireball evolves. A mirror made of vanes and the slot along the waveguide enable a direct view into the cavity.

Figure 3

The fireball initiation process in the microwave cavity, observed through the vanes shown in Fig. 2. (a) A molten hot spot is created by the microwave drill in the substrate material in a thermal-runaway process. (b) A fire column is ejected from the molten hot spot. (c) The fireball is detached from the hot spot and confined like a glowing elastic balloon buoyant in the air. The fireball is floating freely then near the cavity ceiling.

Figure 4

Other effects observed in the fireball experimental setup: (a) a burned trace left by a traveling fireball on the Teflon plate shown in Fig. 2. The fireball flew along the cavity toward the microwave source ($\sim 17\mathrm{cm}$). The burned trace shows the fireball’s shape. (b) A secondary fireball excitation mode in which the original fire column remains. The secondary fireball moves to the adjacent peak of the standing wave in the cavity ($\sim 8\mathrm{cm}$ toward the magnetron) and resides there separately.