Optically Recording Velocity Interferometer System (ORVIS) For Subnanosecond Particle Velocity Measurements In Shock Waves

Abstract An optically recording velocity interferometer system (ORVIS) has been developed to mea­ sure particle velocity with subnanosecond resolution for shock waves in condensed matter. The fringe pattern of a wide-angle Michelson interferometer was focused as a set of dots on the slit of a high speed streak camera, resulting in a continuous record of the fringe posi­ tion as a function of time. This technique was employed to measure the particle velocity of a witness foil in a series of experiments to study the nature of detonation-driven shock waves. The present experiments demonstrated a time resolution of about 300 ps, and we be­ lieve that 20 ps resolution may be achievable with this technique. The improvement in time resolution of up to two orders of magnitude over current diagnostic techniques will be an aid in the study of several aspects of shock wave phenomena such as shock front thickness, detonation wave theory, fast relaxation at an impact plane, and fine structure in shock fronts associated with chemical reaction.IntroductionThe study of shock wave phenomena in condensed matter involves the measurement of velo­ city changes on the order of kilometers per second on a time scale of nanoseconds. In the past 20 years a number of techniques have been developed to make continuous, real time measurements of stress or particle velocity in shock wave experiments, but time resolution has generally been limited to a few nanoseconds.^ Because of this limitation, a number of important questions remain unanswered; for example, the risetime of the shock front is only known to an upper limit in a broad class of condensed materials and the initial relaxation rate caused by the reaction behind a detonation front in explosives can only be approxi­ mated.Laser interferometry has become a widely accepted method of measuring particle velocity at free surfaces and window interfaces because it is accurate and has the best time resolu­ tion previously possible.-'- Several interferometer systems have been used to observe shock wave phenomena including a Michelson displacement interferometer,2 a Sandia velocity inter­ ferometer with unequal but stationary legs,3/4 a viSAR (velocity interferometer system for any reflector) which is an adaptation of a wide-angle Michelson interferometer,1 and a Fabry-Perot velocity interferometer.5 The VISAR is the most commonly used of the interfero­ meters because spectral surfaces are not necessary, making it insensative to misalignment of the moving surface. The VISAR in a typical configuration consists of a wide-angle Michelson interferometer with a photomultiplier detection system and oscilloscopes for recording the resultant sine wave signal. This leads to a rather complicated but accurate data analysis. The typical time resolution is limited to several nanoseconds, however, by the photomulti­ plier tubes and oscilloscopes.In this paper we will describe an optically recording velocity interferometer system which we called ORVIS. This system uses an electronic streak camera to record interference fringe motion instead of photomultiplier tubes and oscilloscopes as in the VISAR. Streak camera recording in velocity interferometry was initially proposed by Parker and Chou.6 The most significant advantage to this approach over the VISAR is that the time resolution is primarily limited only by the streak camera capabilities. The experiments presented here were performed with a maximum time resolution of about 300 ps, but we believe that 20 ps resolution can be achieved with currently available cameras. Another advantage to this system is the simplified data reduction in comparison with photomultiplier detection. The recorded image of a fringe pattern changing position with time gives a direct indication of the velocity history of the monitored surface.Experimental TechniqueThe interferometer configuration consists of two legs with different temporal lengths as illustrated in Fig. 1. As in previous interferometer systems designed for use with diffuse reflectors, such as VISAR or WAMI,1 a fused silica etalon is used in one leg of the inter­ ferometer to provide a time delay. Laser light produced by an argon-ion laser passes through a 6mm hole in mirror Ml and collimating lens LI to the diffuse surface whose motion is to be monitored. The diffuse reflection is collected and collimated by lens LI, and is separated