Formation of highly structured dense cores from exploding wires with 1--5 kAper wire
Conference
·
OSTI ID:20050987
The dynamics of the current-induced explosion of fine (7.5--40 {micro}m diameter) metal wires has been studied experimentally for times up to 12 {micro}s at 1--5 kA/wire. The main diagnostic technique is direct x-ray backlighting imaging of the exploding wires with 1 {micro}m scale spatial resolution, obtained by using very short-time (<0.5 ns) x-ray bursts from a collapsing X-pinch. By varying the parameters of the X-pinch and the starting time of the discharge it is possible to obtain the backlighting images at different moments of time after the discharge onset. In the high-current regime (> 30 kA per wire), exploding wires typically show fast evaporation and ionization of the wire surface forming an unstable coronal plasma around a dense core. However, in the low-current regime (<5 kA per wire), studies of wire explosions reveal very different behavior for most wire materials. It has been observed that a substantial portion of the wire material is not evaporated and ionized but remains in a condensed state for a very long time. The clear structure shown in the x-ray backlighting images of 7.5--13.5 {micro}m W wires as a function of time allows one to interpret different stages of the explosion as (1) efficient initial melting of the wire metal followed by (2) sudden volume boiling and (3) slow formation of a foam-like medium that consists of a liquid film sponge with vapor bubbles inside. If there is a vapor surrounding the wire, it is not detectable with the 2.5--4.8 keV x-ray backlighter (<10{sup 17}/cm{sup 2} areal density). During the foam formation, the wire remnant typically expands ten times in diameter, keeping the cylindrical form of the original wire. In the final stage (4) of the discharge (>5 {micro}s), when the current through the wire remnant is negligible, the slow cooling of the sponge leads to a tree-like structure with gradual condensation of the remaining liquid phase material into separate drops. No typical plasma phenomena such as pinching and collapse of the plasma column are observed in these low current experiments. This behavior is typical for metals with high melting and boiling temperatures, W, Mo, NiCr and Ti, which also have relatively high resistivity, but not for Au, which vaporizes rapidly and with little structure.
- Research Organization:
- Cornell Univ., Ithaca, NY (US)
- Sponsoring Organization:
- US Department of Energy
- OSTI ID:
- 20050987
- Country of Publication:
- United States
- Language:
- English
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