Abstract
Artificially triggered lightning has been studied at several laboratories by using lasers and rockets. The laser technique has certain advantages over the rocket technique in terms of its capacity for repetition and fast response to lightning. The primary limitation of the technique comes from the difficulty of producing a sufficiently long plasma in the air to guide lightning. However, the development of the MACH (Multi Active CHannel) focusing mirror1 has made the technique a promising one. The MACH mirror consists of three different concave minor surfaces that are arranged concentrically. To avoid laser light being absorbed by the plasma, the outer part of the laser beam focuses further from the mirror. Figure 1 is a picture of a guided discharge triggered by a 7.5 m long plasma with the three-focal-point MACH mirror. The length of the discharge itself is 8.5 m, and 1 m is due to an unguided discharge. A -12 MV impulse is applied to the upper electrode (pole). A 100-J TEA CO2 laser system is used and approximately 80% of the laser energy is estimated to be absorbed by the air. His effects of the plasma length on guiding were characterized by changing the electrode separation. Figure 2 is a plot of minimum, (threshold) impulse voltage (applied to the upper electrode) that is necessary to trigger the discharge as a function of the length of the discharge distance. Data of two types of the MACH mirror are plotted as a comparison. The threshold voltage is constant as long as the electrode separation is shorter than the plasma length and increases rapidly as the discharge distance between the tail of the plasma and the grounded electrode increases. This indicates that the discharge triggering depends primarily on the threshold of an initial discharge (leader) occurring between the upper electrode and the plasma. Once the leader reaches the plasma, the discharge progresses in the plasma freely.
© 1993 Optical Society of America
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