Multi frame synchrotron radiography of pulsed power driven underwater single wire explosions

Abstract

We present the first use of synchrotron-based phase contrast radiography to study pulsed-power driven high energy density physics experiments. Underwater electrical wire explosions have become of interest to the wider physics community due to their ability to study material properties at extreme conditions and efficiently couple stored electrical energy into intense shock waves in water. The latter can be shaped to provide convergent implosions, resulting in very high pressures (1-10 Mbar) produced on relatively small pulsed power facilities (100s of kA-MA). Multiple experiments have explored single-wire explosions in water, hoping to understand the underlying physics and better optimize this energy transfer process; however, diagnostics can be limited. Optical imaging diagnostics are usually obscured by the shock wave itself; and until now, diode-based X-ray radiography has been of relatively low resolution and rather a broad x-ray energy spectrum. Utilising phase contrast imaging capabilities of the ID19 beamline at the European Synchrotron Radiation Facility, we were able to image both the exploding wire and the shock wave. Probing radiation of 20-50 keV radiographed 200 μm tungsten and copper wires, in ∼2-cm diameter water cylinders with resolutions of 8 μm and 32 μm. The wires were exploded by a ∼30-kA, 500-ns compact pulser, and 128 radiographs, each with a 100-ps X-ray pulse exposure, spaced at 704 ns apart were taken in each experiment. Abel inversion was used to obtain the density profile of the wires, and the results are compared to two dimensional hydrodynamic and one dimensional magnetohydrodynamic simulations.

Numerical investigation of shock wave characteristics at microsecond underwater electrical explosion of Cu wires

“…For our experimental system, the skin depth is 0.29 mm, larger than the radii of the concerned wires; hence uniform current distribution was assumed. This is in consistency with previous experimental and numerical results that the radial distribution of the wire parameters is basically uniform during the microsecond UEWE [3,21,24]. Therefore, in order to reduce the complexity of modeling and amount of computation, the expansion process of the wire is described by a 0D hydrodynamic sub-model.…”

“…However, these models are usually time-consuming. The simulation results of these models and the experimental results using x-ray backlighting [24] have proved that the radial distribution of the wire parameters, e.g. density and temperature is basically uniform during the microsecond UEWE.…”

“…Eele (P pot+k − P ele η) (24) where E pot+k = E pot + E k and P pot+k = dE pot+k /dt. Due to the rapid increase of the electrical energy deposition power after the voltage peak, P ele η > P pot+k is satisfied between 1.9 µs to 5.4 µs as displayed in figure 13(a), leading to the drop of the efficiency in that period.…”

Electrical wire explosion as a source of underwater shock waves

“…Moreover, based on the 1D MHD results in the aforementioned literature, notable non-uniformities in the radial distribution of thermodynamic parameters such as density and temperature mainly appear during vaporization and earlier stages, and the EP is relatively uniform for most other times [41,95]. Recent high-resolution x-ray radiography also supported the 1D calculation results [101]. Nevertheless, one should be aware that the 0D simplification of the EP is valid provided for the uniform heating along the wire radius, and additional caution should be taken for very high current densities as significant inhomogeneity will occur during melting [78,102].…”

Use of synchrotron-based radiography to diagnose pulsed power driven wire explosion experiments

“…19 Hence, x-rays of this energy should lead to good contrast between the wires and water in the resulting radiograph. Using hydrodynamic simulations as described by Yanuka et al, 20 the water density behind the shock is ∼10% greater than in front, resulting in a difference in transmission across the shock of ∼3%. This is an upper bound on the radiograph contrast, which is significantly reduced by the <10 mm shock diameter from the majority of the experiment, and the decrease in x-ray path through the denser water closer to the shock, as the explosion is cylindrical.…”