<title>FOREX-A Fiber Optics Diagnostic System For Study Of Materials At High Temperatures And Pressures</title>

Abstract

We have successfully fielded a Fiber Optics Radiation EXperiment system (FOREX) designed for measuring material properties at high temperatures and pressures on an underground nuclear test.The system collects light from radiating materials and transmits it through several hundred meters of optical fibers to a recording station consisting of a streak camera with film readout. The use of fiber optics provides a faster time response than can presently be obtained with equalized coaxial cables over comparable distances. Fibers also have significant cost and physical size advantages over coax cables. The streak camera achieves a much higher information density than an equivalent oscilloscope system, and it also serves as the light detector.The result is a wide bandwidth high capacity system that can be fielded at a relatively low cost in manpower, space, and materials.For this experiment, the streak camera had a 120 ns time window with a 1.2 ns time resolution.Dynamic range for the system was about 1000. Beam current statistical limitations were approximately 8% for a 0.3 ns wide data point at one decade above the threshold recording intensity.Description of apparatus Figure 1 is a schematic diagram of one data channel of the system. The source was buried 494 m underground. A black body light source whose light output was to be measured was located at the bottom of an evacuated stainless steel tube 5.8 m in length with an inner diameter of 22 mm. The light beam was reflected out of the tube at 90° by an optical -grade flat Be mirror and focused by a radiation resistant supracil lens with a 100 mm focal length. A glass step filter prevented light with wavelengths shorter than 500 nm from entering an optical fiber positioned at the focal point of the lens system.The optical signal was transmitted uphole by a graded -index optical fiber line consisting of a 1.8 m downhole section and a 760 m section from downhole to the surface recording station.System dynamic range was enhanced by connecting the line to a fiber beam splitter coupler with one input and two output arms having output ratios of 15 :1.The coupler output fibers were attached to a linear fiber array that defined the entrance aperture for the streak camera. A lens imaged the array onto the streak camera with a magnification of 1. A band -pass filter between the array and lens selected a 5 nm wide wavelength band centered at 650 nm, limiting the effect of material dispersion in the fibers. The array and beam splitter fibers all had a core diameter of 50 pm.The downhole fibers had core diameters of either 50 or 62 um and were packaged in two cables having eight fibers each. Corning graded index fibers were used throughout, and sections were connected with Amphenol connectors. Be mirror Band -pass filter Fiber Optical coupler fiber cable Vacuum system UV glass filter Lens Radiation resistant lens Light source Streak camera Film MCP intensifierFigure 1.FOREX, a long-distance wide -band optical recording system, offers several advantages over systems using coaxial c...