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Diagnosing an Experiment
Livermore scientists use a variety of complementary diagnostic tools during experiments with shaped charges. X-radiography produces shadowgraphs that provide experienced researchers with information about the jet's velocity, density, and mass distribution (Figure 3). The rotating-mirror framing camera, a kind of motion picture camera, can shoot millions of frames in a second. A typical shaped-charge jet-formation experiment lasts less than 30 microseconds, and the framing camera is usually set to record an image about once every microsecond. The exposure time for the framing camera may be anywhere from 100 to 200 nanoseconds, or billionths of a second.
The newest tool is the image-converter (IC) camera, which was developed at Livermore in the mid-1980s. A pulsed ruby laser is synchronized with the IC camera frames to provide illumination of the shaped charge. The electronic image tube that acts as the shutter for each image frame converts the photons of laser light reflected by the shaped charge to photoelectrons. These photoelectrons are accelerated by a high-voltage pulse onto a phosphor, where they are reconverted to photons that are then transmitted to the film. With exposure times of just 15 to 20 nanoseconds (up to ten times shorter than those of the framing camera) and a band-pass filter mounted on the camera to exclude extraneous light, the IC camera has supplied the first truly high-resolution images of the formation and early flight of a shaped-charge jet. The image in Figure 2(a) was taken with an IC camera and shows fine-scale features, including instabilities near the tip, the breakup of the material in the head, and even small ripples in the stem. Without the pulsed ruby laser illumination and the band-pass filter of the IC camera, this photograph would show only the hot gases encasing the jet as an extremely bright, luminous sheath.
The IC camera can record single frames at eight different times, stereo pairs of frames at four different times for three-dimensional photography, or combinations of each. The various frames may be focused on different portions of the jet, or they may be set to produce sequential photographs of the same portion of the jet.
In the high-resolution photographs, individual features on the jet surfaces as small as about 100 micrometers can easily be detected and followed as they evolve over time. When this information is combined with data from framing-camera images and x-ray shadowgraphs, Livermore researchers have at their disposal a detailed, verifiable record of the evolution of the jet. |