Microwave Interferometry Page
Microwave Interferometry is a technique which uses constructive and destructive
interference patterns of reflected microwaves to measure both position and speed
of reflective fronts in the DDT event. Reflective fronts may initially include
a combination of the piston face, compaction waves and combustion fronts. Upon the onset of detonation, however, the interferometer signal is almost completely reflected, giving a very clean signal.
Microwave Interferometry diagnostics in the past often depend on an intrusive wave-guide
structure imbedded in the HE, which changes the phase of the reflected signal as it is crushed by the passing compaction, combustion, or detonation front. Some advantages of this type of interferometer is that it can use
relatively low frequency, low cost components, and can produce a clean signal without multiple reflections. The wave-guide structure used is often a fine strip of conductive material, however, and can easily be
damaged during assembly of the experiment especially when high densities of HE are used.
The microwave interferometer used in these experiments, shown above schematically, uses a mixture of lower frequency microwave components to obtain a high frequency (32GHz) microwave diagnostic. At this frequency, the microwave is able to move through free space, and may be guided through a material such as teflon, and down the DDT tube. The HE is nearly invisible to the high frequency microwaves, however, the microwaves are reflected partially by piston, compaction and combustions fronts present in the deflagration stage of the event, as can be seen in the following data from a DDT experiment:
A single, clean reflection would be represented by two sinusoidal signals, 90 degrees out of phase. Multiple incomplete reflections account for the distortion of the sinusoidal waves in the deflegration stage, which are widely spaced and nearly saw-toothed. The detonation stage (indicated by closely space sinusoidal signals), however, produces a clean reflection as seen in the expanded view below.