We present a drilling method that is based on the phenomenon of local hot spot generation by near-field microwave radiation. The microwave drill is implemented by a coaxial near-field radiator fed by a conventional microwave source. The near-field radiator induces the microwave energy into a small volume in the drilled material under its surface, and a hot spot evolves in a rapid thermal-runaway process. The center electrode of the coaxial radiator itself is then inserted into the softened material to form the hole. The method is applicable for drilling a variety of nonconductive materials. It does not require fast rotating parts, and its operation makes no dust or noise.

The microwave-drill principle [Jerby et al., Science 298, 587 (2002)] is based on a localized hot-spot effect induced by a near-field coaxial applicator. The microwave drill melts the nonmetallic material locally and penetrates mechanically into it to shape the hole. This paper presents a theoretical analysis of the thermal-runaway effect induced in front of the microwave drill. The model couples the Maxwell’s and heat equations including the material’s temperature-dependent properties. A finite-difference time-domain algorithm is applied in a two-time-scale numerical model. The simulation is demonstrated for mullite, and benchmarked in simplified cases. The results show a temperature rise of $\sim {10}^3\mathrm{K}∕\mathrm{s}$ up to $1300\mathrm{K}$ within a hot spot confined to a $\sim 4\text{-}\mathrm{mm}$ width ($\sim 0.1$ wavelength). The input-port response to this near-field effect is modeled by equivalent time-varying lumped-circuit elements. Besides the physical insight, this theoretical study provides computational tools for design and analysis of microwave drills and for their real-time monitoring and adaptive impedance matching.

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)].

This paper presents a theoretical and experimental study of a locally induced microwave-heating effect implemented by a low-power transistor-based microwave drill. A coupled thermal-electromagnetic model shows that the thermal-runaway instability can be excited also by relatively low microwave power, in the range ~ 10-100 W, hence by solid-state sources rather than magnetrons. Local melting then occurs in a millimeter scale within seconds in various materials, such as glass, ceramics, basalts, and plastics. The experimental device employs an LDMOS transistor in an oscillator scheme, feeding a miniature microwave-drill applicator. The experimental results verify the rapid heating effect, similarly to the theoretical model. These findings may lead to various material-processing applications of local microwave heating implemented by solid-state devices, including local melting (for surface treatments, chemical reactions, joining, etc.), delicate drilling (e.g., of bones in orthopedic operations), local evaporation, ignition, and plasma ejection (e.g., in microwave-induced breakdown spectroscopy (MIBS) for material identification).