Technical report | Development of Photon Doppler Velocimeter for Explosives Research

Abstract

 A Photon Doppler Velocimeter (PDV) was built for explosives research. The PDV is comprised of a 1550 nm continuous-wave single-mode fibre laser, an optical circulator, a photodetector and a GHz range capable oscilloscope. As a test, it was used to record the velocity history of the back surface of 5 mm thick steel plates driven by the detonation products of PE4 charges of mass 15 - 60 g. A bare fibre tip, located 1 - 4 mm away from the target was used as the optical probe. Time resolution of 7 ns was achieved. The free surface velocity effects were interpreted in terms of the elastic-plastic model of steel, Chapman-Jouguet pressure was evaluated, and Gurney equations were applied to describe the motion of the steel plate. The measurement limitations of the PDV system are determined, applications for explosives research are discussed, and the upgrade pathway is recommended.

Executive Summary

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 7 ns.

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.