Abstract
For a long time, the nitromethane (NM) ignition has been observed with different means such as high-speed cameras, VISAR or optical pyrometry diagnostics. By 2000, David Goosmann (LLNL) studied solid high-explosive detonation and shock loaded metal plates by measuring velocity (Fabry-Pérot interferometry) in embedded optical fibers. For six years Photonic Doppler Velocimetry (PDV) has become a major tool to better understand the phenomena occurring in shock physics experiments. In 2006, we began to use in turn this technique and studied shock-to-detonation transition in NM. Different kinds of bare optical fibers were set in the liquid; they provided two types of velocity information; those coming from phenomena located in front of the fibers (interface velocity, shock waves, overdriven detonation wave) and those due to phenomena environing the fibers (shock or detonation waves). We achieved several shots; devices were composed of a high explosive plane wave generator ended by a metal barrier followed by a cylindrical vessel containing NM. We present results.
Highlights
By 2004, Oliver Strand from LLNL [1, 2] designed a new Photonic Doppler Velocimetry (PDV) chain built with telecom components around a displacement interferometer, working in the infra-red spectrum (1.55 μm) and applied to shock physics experiments
Studying shock physics phenomena within transparent matter has proven to be possible by using simple bare optical fibers; graded-index and multimode step index fibers as probes but connected to classical singlemode fibers, are more sensitive and deliver better signals than the singlemode fiber as probe
These fibers act first, as a no-contact probe as an intrusive probe, at least with detonation wave-like interfaces. Velocities of both shocks and overdriven detonation located ahead of and within a fiber submerged in nitromethane have been measured
Summary
By 2004, Oliver Strand from LLNL [1, 2] designed a new PDV chain built with telecom components around a displacement interferometer, working in the infra-red spectrum (1.55 μm) and applied to shock physics experiments. In 2006 we built a similar PDV chain (we call it “Heterodyne Velocimetry” or HV) [3]. We successfully carried out a few gun and high-explosive shots with it. We modified the design to improve the equipment velocity range (up to 10 km/s) by adding a frequency shifted second laser. Shock-detonation transition study in transparent high-explosive such as nitromethane (NM) became possible. Because the high explosive medium was liquid, we set bare optical fibers acting as PDV probes, directly in this medium.
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