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Development of Photon Doppler Velocimeter for Explosives Research

Experimental methods of explosives research involve the determination of properties
of the detonation wave as it propagates inside the explosive charge, as well as
properties of the shockwaves propagating inside the inert materials adjacent to the
detonated charge. Photon Doppler Velocimeter (PDV) can be used to measure the
velocity of a shocked material’s free surface with nanosecond resolution. Photon
Doppler Velocimetry is a relatively novel technique which was first described by the
Lawrence Livermore National Laboratory (LLNL) authors in Review of Scientific
Instruments in 2006. The technique is based on mixing the light wave incident onto a
moving target, with the reflected light of changed frequency. The frequency of the
resulting beat wave is proportional to the velocity of the target. The technique is simple
and relatively inexpensive compared to competing methods such as VISAR and Fabry—
Perot interferometry.

This report describes the development of a PDV and an application of it to the
measurement of the velocity of steel shocked by detonating a PE4 explosive charge.
The PDV used a 100 mW 1550 nm infrared continuous wave single mode fibre laser, a
circulator, a photodetector, and an oscilloscope of a 2.5 GHZ bandwidth.

The obtained data allowed evaluation of the detonation pressure of PE4 and to
measure the terminal velocity of a steel plate accelerated by the explosive. The
maximum measurement velocity is 2—3 km/ s, and the demonstrated time resolution is
7ns.

The outcomes of the PDV trial show that the technique is working and can be applied
to a range of explosives research problems. The PDV could be used in the cylinder
expansion test where the acceleration of the cylinder wall under the pressure of the
expanding detonation gases is measured, and the performance of the explosive is
assessed. In another possible experiment, a flying plate test, the structure of the
detonation wave could be studied by recording a velocity history of an interface
between the explosive and a window.