Pulsed Laser Stereophotography Of Miniature Exploding Foils

Abstract

AbstractA pulsed -laser stereophotography system is used to evaluate the physical changes takingplace in close (1 pm) proximity to the actinic plasma of miniature exploding foils. The system records stereopairs of photographs at 3 nsec exposures and 20X magnification. The image size is 50 mm square with resolution 40 lines /mm. The purpose of the laser is tosupply a short (3 nsec) pulse of monochromatic light. This laser light reflected off thetest specimen is then collected by the stereocamera. The 3 -nsec pulse minimizes motionblur and the monochromaticity permits filtering out all wavelengths except that of the laser(694.3 nm) wavelength. This permits the self -light of the exploding foils to be, for allpractical purposes, eliminated from the stereophotographs. The metal foils provide satis-factory reflection and the plasma acts as an optical absorber and provides for the necessarycontrast. The camera includes 1.0 -nm narrow band pass filters to prevent recording theexploding foil self -light. The stereopairs are read on a stereo- comparagraph normally usedfor aerial photographs that has been calibrated for reading these stereophotographs. Sincethese exploding foils have typical function times of 500 nsec, the stereophotographs aresynchronized in time with the event under test. Stereophotographs can be recorded at anypreselected time to within ± 20 nsec. This technique provides a means to closely evaluateevents near optically intense plasma.IntroductionFor the past few years, Mound Laboratory has been engaged in the dynamic evaluation ofminiature (1 mm2) exploding foils. These foils are electrically exploded with an electricalcapacitor discharge unit (CDU) that causes the foil to be totally vaporized in less than0.5 x 10_6 sec (500 nsec) from the start of the current flow. The actual change of statefrom solid to vapor normally occurs in 0.1 x 10-6 sec. These foils are extremely actinic.Since we are interested in the method and pattern by which these foils explode, it becomesnecessary to photograph them over a wide dynamic optical output range. We must be able tosimultaneously resolve the metal foil surface and this intense plasma. Conventional self -light streak and electronic framing camera photography were used with limited success.Therefore, we felt the need to use pulsed laser photography with the added benefit of astereocamera. This system provides a framing camera photograph of short exposure (3 nsec)with no self -light from the foil. The laser illuminates the exploding foil and the laserlight reflected off the foil is collected by the stereocamera to record the stereophoto-