Doppler Velocity Interferometer Research Articles

Forward modeling of Doppler velocity interferometer system for improved shockwave measurements.

When near-instantaneous shocks are recorded by using a Doppler velocity interferometer velocity interferometer system for any reflector (VISAR), they typically exceed the detector's ability to react and "skipped fringes" result, where its visibility briefly reduces. Traditionally, replacing skipped fringes required guesswork in analysis, which increased arrival time errors. Second, the use of long but velocity-sensitive interferometer delays with fast detectors which can resolve the delay has traditionally been avoided because of the fear of confusing the arrival time signal. However, shorter delays produce smaller fringe phase shifts per velocity and, thus, can decrease velocity precision. We realize that while some loss of fringe information occurs at shock events, this is often just a partial loss and the residual fringe information can still hold valuable information. We describe a forward model (FM) of the interferometer action and detector blurring that assists with VISAR fringe analysis at skipping events: (1) more precise shock arrival times, (2) arrival time precision not limited by long delays, and (3) improved ghost subtraction, which improves accuracy over a broad time region. We demonstrate the utility of a FM on National Ignition Facility and Laboratory for Laser Energetics Omega shots and discuss data process methods to increase the high time resolution of VISAR systems despite their limitations and complexities.

Ghost fringe removal techniques using Lissajous data presentation.

A VISAR (Velocity Interferometer System for Any Reflector) is a Doppler velocity interferometer which is an important optical diagnostic in shockwave experiments at the national laboratories, used to measure equation of state (EOS) of materials under extreme conditions. Unwanted reflection of laser light from target windows can produce an additional component to the VISAR fringe record that can distort and obscure the true velocity signal. Accurately removing this so-called ghost artifact component is essential for achieving high accuracy EOS measurements, especially when the true light signal is only weakly reflected from the shock front. Independent of the choice of algorithm for processing the raw data into a complex fringe signal, we have found it beneficial to plot this signal as a Lissajous and seek the proper center of this path, even under time varying intensity which can shift the perceived center. The ghost contribution is then solved by a simple translation in the complex plane that recenters the Lissajous path. For continuous velocity histories, we find that plotting the fringe magnitude vs nonfringing intensity and optimizing linearity is an invaluable tool for determining accurate ghost offsets. For discontinuous velocity histories, we have developed graphically inspired methods which relate the results of two VISARs having different velocity per fringe proportionalities or assumptions of constant fringe magnitude to find the ghost offset. The technique can also remove window reflection artifacts in generic interferometers, such as in the metrology of surfaces.

Study of the laser-driven spallation process by the velocity interferometer system for any reflector interferometry technique. I. Laser-shock characterization

This article presents the technique for measuring an amplitude and a profile of a laser-generated stress pulse using a Doppler velocity interferometer velocity interferometer system for any reflector (VISAR). The compressive stress, produced by a laser pulse, moves target’s rear free surface. The velocity of this displacement recorded by VISAR can then be related to the laser-driven load applied at the front face of the sample. In the first part of our study, the effect of the laser power density on the shape of the pressure pulse was experimentally and numerically studied for aluminum samples. Subsequently, this method was applied to other materials (copper, molybdenum, and tantalum) in order to find an influence of the material kind on the amplitude and the shape of the loading profile.

Study of the laser-driven spallation process by the VISAR interferometry technique. II. Experiment and simulation of the spallation process

This article presents the experimental and numerical study of laser-driven spallation using a Doppler velocity interferometer VISAR (velocity interferometer system for any reflector). Experiments were carried out on two ductile metals: aluminum and copper. A kinetic damage model was developed in order to predict the damage effects on the mechanical properties of the material and on the waves propagation through the sample. Laser-driven loadings were simulated by a temporal pressure profile, applied at the front surface of the sample. The amplitude and shape of this profile is discussed in Part I [J. Appl. Phys. 83, 1224 (1998)]. The data input to the model was determined for both materials thanks to the comparison of experimental and numerical free-surface velocity profiles. Good agreement was obtained between experimental and predicted results with the model for the spallation threshold and for the damaged zone location.

Deformation Modes in Fiber glass, under Weak Impact

Spall strength and shock front structure of commercial fiberglass were studied at pressures between 0.3-0.8 GPa, using Doppler velocity interferometer technique (VISAR). Substantial changes in the spall strength and shock front rise time were observed. These changes allow us to propose that fiberglass performs three modes of the deformation at the different levels of compression curve similar to porous materials and some dispersed composites.

Fast deformation velocity measurement using laser Doppler velocity interferometer

A mild steel projectile (65.6 g) with an elevated impact area (1.0 cm in diameter) is driven to 25.6 m/s by shock wave toward the fixed target plate with thickness 0.5–0.8 cm and the bulging deformation at the polished free surface is measured by using a laser Doppler velocity interferometer. Initially thick target (1.2–2.4 cm) is used for the determination of the velocity of the elastic wave in the solid. In the thinner targets, the free surface gets bulging plastic deformation. Its maximum bulging velocity, which is thickness dependent, is determined for the thickness 0.5–0.8 cm. It is found the maximum bulging velocity decreases approximately inversely with respect to the thickness of the target. The time-dependent and target thickness behaviors of bulging plastic deformation of the mild steel target are obtained for the constant impact velocity, and they are discussed on the basis of data obtained by using the laser Doppler velocity interferometer.