Observation of laser-induced microscale knotted and unknotted vortex filaments on vaporizing tantalum surface

Abstract

surface superheating on a ns time scale is connected with the formation of a spinodal fluid which decomposes into a gaseous phase through microexplosions, generating vortex filament structures on the vaporizing surface. Vortex filaments associated with the Reynolds number Reϳ10 3 -10 4 are organized into regular, quasiregular, or chaotic structures. Homotopic operations transfer these structures into irreducible ones of a simple closed-loop type, showing that all of them are embedded in a three-dimensional torus either as a vortex ring ͑unknotted knot͒, as a cloverleaf ͑trefoil knotted knot͒, or as Hopf links ͑knotted knot͒. ͓S0163-1829͑96͒08432-9͔The paper deals with experimental evidence of generation and organization of vortex filaments in the explosive decomposition of a spinodal fluid ͑liquid Ta͒ into a gaseous phase on the time scale р10 ns. Spinodal fluid 1,2 is generated during surface superheating of metals in high-power, short-timescale laser-metal interactions when the surface layer behaves as a dielectric. 3 It thus, becomes transparent enabling a deep volume absorption of a laser beam. 3 The vapor pressure above the surface prevents boiling, and causes superheating of the liquid metal. 1,3 The calculations of Grosse 4 ͑based on the van der Waals theory͒ have shown that superheating may in principle occur for all metals, but that the spinodal curve for some of them is flat rather than cusplike. However, for refractory metals ͑Ti, Ta, Mo, etc.͒ the cusp is large, and superheating may reach even 10 3 -10 4 K. The system is pushed into a metastable region of a thermodynamic diagram, where thermal conductivity k→0, and specific heat Cp→ϱ. 1,2 Since fluctuations play a crucial role in the metastable phase, the system is not stable, and exists only for a short time, after which it decomposes into a gaseous phase through microexplosions of surface bubbles, characteristic of the onset of ''volume'' boiling. As usually assumed, this transition occurs at QӍ10 8 W/cm 2 .The transition from planar to ''volume'' boiling is not a well-elucidated problem from the aspect of the phase diagram, nor from the aspect of surface dynamics and the corresponding surface morphology. Ultrafast cooling after pulse termination causes the surface morphology to stay permanently frozen, thus enabling a posteriori analysis.Recent studies of these problems on metals, 5 semiconductors, 6 and superconductor ceramics 7,8 shed more light on superheating on a ns time scale. In spite of the difference in the absorption process and the superheating ͑sur-face or subsurface͒ in these materials, the surface morphology associated with fully developed volume boiling is the same. The most intriguing aspect relates to the onset of volume boiling ͑not fully developed͒ associated with the reach spectrum of dynamic phenomena, and generation of a strange morphology. 5 Our studies with a Q-switched Nd:YAG ͑yttrium aluminum garnet͒ laser, with 10 ns at half width at half-maximum, at a spot size ϳ3 -4 mm, QӍ10 7 W/cm 2 performed on small samples ϳ1ϫ1ϫ0,05...

Vortex Ring Reconnection in Laser–matter Interactions

“…The studies performed by our group in the regime of transition from planar-to-volume vaporization, at "10 ns, have shown the spontaneous formation of open-and closed-loop vortex "laments, showing various levels of the organization complexity (Lugomer 19961998a,b). Continuing the series of studies, this paper deals with generation of the microscale vortex rings and their interactions, especially with collision, reconnection and breaking, as a non linear and nonequilibrium process with strong dissipative character.…”

“…They travel in all directions and being re#ected from the sidewalls (of the small sample) they reverberate thorugh the sample, causing the interaction with surface hydrodynamic structures (vortex rings). The momentum transferred from the shock wave to the vortex ring causes its collision with other rings, the result of which is their interaction (Lugomer 1996(Lugomer , 1998Lugomer & Maksimovic 1997.…”

“…It thus becomes transparent, enabling a deep volume absorption of the laser beam. The vapour pressure above the surface prevents boiling, and causes superheating of the liquid metal (Lugomer 1996;Samokhin 1998, Samokhin & Uspensky 1977. The system is pushed into a metastable region of a thermodynamic diagram, where thermal conductivity kP0, and speci"c heat C N PJ (Lugomer 1996).…”

“…The vapour pressure above the surface prevents boiling, and causes superheating of the liquid metal (Lugomer 1996;Samokhin 1998, Samokhin & Uspensky 1977. The system is pushed into a metastable region of a thermodynamic diagram, where thermal conductivity kP0, and speci"c heat C N PJ (Lugomer 1996). The system is not stable, and exists only for a short time, after which it decomposes into a gaseous phase through microexplosions of surface bubbles, characteristic of the onset of the &&volume'' boiling.…”

Observations of Bubble Self-Organization in Laser-Matter Interactions

“…A SERIES OF STUDIES OF SELF-ORGANIZATION (SO) of various types of entities in laser}matter interaction (LMI), like droplets (Lugomer & MaksimovicH 1996), vortex "laments (Lugomer 1996(Lugomer , 1998Lugomer & MaksimovicH 1997), and bubbles (Lugomer & MaksimovicH 1999) has shown that various types of patterns of every entity can be generated. It has also been shown that these patterns change not only in the morphological sense, but also in the sense of topological complexity, depending (for "10 ns laser pulse) on laser power density Q N .…”