Setup of laser system and beam transport optics. The output of a nanosecond frequency-doubled Nd:YAG laser is temporally compressed to a pulse length in a stimulated SBS cell filled with water. The beam is transported by relay imaging optics through vacuum tubes VT1 and VT2, expanded by a set of cylindrical lenses, and focused onto the leaks by a four-element lens FL. PBS1 denotes a thin-film polarizing beam splitter. Drawing not to scale.
Setup of the leak chamber and imaging system. A motorized lens (FL) and mirrors focused the SBS laser beam to a focal volume located from the surface of a stainless-steel chamber filled with gas. The leaked through twelve needles embedded in the chamber flange. The emission emitted from the resulting plasma was collected by a 2-m-long macrolens system consisting of an astronomical telescope and telephoto lens. The light was collimated into a 150-mm-diameter beam before traversing two interference filters. The resulting image of the leak was photographed by a thermoelectrically cooled CCD. Drawing not to scale.
Schematic of the lithographic photomask used to evaluate the image quality of the imaging system (a). Blowup of the photograph near the center (b) and edge (c) of the image circle. These chrome grids consist of -wide lines arranged with a pitch of 1 mm. The optics had a spatial resolution across most of the field, whereas coma and field curvature were negligible. Vignetting was seen only at the extreme edges of the image circle.
Emission spectra of the laser-induced breakdown plasma measured using pure gas and 1500 and 10 000 ppm by volume mixtures of in buffer gas at a pressure of 1 bar (a). Spectra measured for room air (b), showing the background line caused by water vapor (see text).
Photograph of the glow discharge of H (top) and N (bottom) lamps taken using the macrolens system and color CCD camera (a). Discharge tubes photographed using the interference filters (b). The filter angles were tuned to maximize the intensity of the signal relative to the N atomic emission.
CCD image of the top flange of the hydrogen chamber (a). Twelve needles arranged in a pattern resembling the letter “H” are embedded in the flange. Image of the emission emitted by the laser-breakdown plasma photographed using the filters (b). The focal volume of the laser was scanned over a area in a raster pattern. The positions of the twelve leaks are clearly seen.
CCD image of the emission from four leak points located at spacings 10 mm (a). -axis projection of the intensity of CCD signal (b).
Emission spectra of laser-induced breakdown plasma in pure gas, and 5000 and 20 000 ppm by volume mixtures of He in buffer gas at a pressure of 1.5 bar (a). Spectra measured for 500, 2000, and 10 000 ppm of He mixed into Ar buffer gas at (b). The He I line could not be clearly resolved at 500 ppm, due to several overlapping Ar I background lines.
Emission spectra of laser-induced breakdown plasma initiated in pure gas, and 3000 ppm by volume Ne mixed in at a pressure of 1 bar (a). Spectra measured for pure , and 3000 ppm of Xe mixed in buffer gas at 1 bar (b).
Wavelengths and level configurations of strong emission lines of atomic H, He I, N I, and Ar I, and singly charged ionic N II and Xe II in the wavelength regions , 542–550, and 583–593 nm.
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