Microwave interferometer for shock wave, detonation, and material motion measurements

Abstract

A microwave interferometer system which provides a continuous measurement of the position of interfaces, such as shock fronts, detonation fronts, or material surfaces, has been developed. The use of low-mass microcoaxial cable, some of which is commercially available, and stripline for conducting the microwave signal into the interior of an experiment makes the perturbation of the measuring system on the experiment small. Other microwave systems have measured the time of peak amplitude of a reflected signal, but the present system with proper matching produces a continuous record of position as a function of time. It is believed that this technique can be used to replace most uses of electrical contact or fiber-optic pins in high-explosive experiments. The amount of data far exceeds that which it is possible to obtain from pins or optical fibers. The matching of the microcoax to the microwave generator has an important effect on the accuracy of the technique. The effect of matching on accuracy will be described in detail, and matching methods will be discussed. A position accuracy of 0.2 mm has been attained. Several methods for extracting position information as a function of time are possible. These techniques are discussed, and their application to the analysis of an experiment is demonstrated. The data from the interferometer system is compared to a streak camera record in a phased, shock-breakout experiment. Agreement is quite good, and the superiority of this method over the pin method is demonstrated by comparing details of the streak and interferometer records.

Cookoff

“…This idealization provided an envelope of the true pressure behavior and provided a consistent method to compare shots under different conditions. To obtain the position data, we used a phase-unwrapping method to convert the quadrature interferometer signals into a continuous record of position vs. time [47].…”

Techniques for retrieval of the boundary displacement data in gas-dynamic experiments using millimeter-waveband radio interferometers

“…Block diagrams of the radio interferometers can be different. An alternate layout of the measurement setup, namely a 4-cm waveband interferometer [11], is presented in Fig. 1, where QH is a quadrature hybrid junction and DBM is a double balance mixer.…”

Using time-frequency analysis to determine time-resolved detonation velocity with microwave interferometry

“…Despite ongoing research since the early 1950's 3 and subsequent improvements made to microwave interferometers, [5][6][7] the explosives' community has been slow to apply time-frequency analysis for velocity measurements. Originally, a simple peak picking or zero crossing method was used to determine the average velocity at finite points in time.…”

“…Current phase unwrapping techniques attempt to account for all non-idealities in low quality signals at the expense of possibly introducing error through heavy filtering, normalization, and smoothing of the data. 6 Velocity is then numerically derived from position-time data, resulting in additional numerical errors and requiring additional filtering. The effort required to perform phase unwrapping is highly dependent on the quality of the MI signal, and it is possible that subtle transient features in velocity may be obscured after the application of heavy filtering.…”