Pergamon A possible way to trigger lightning using a laser D. Wang,* T. Ushio,* Z.-I. Kawasaki,* K. Matsuura,* Y. Shimada,? S. Uchida ,t C. Yamanaka ,t Y. Izawa,$ Y. Sonoi# and N. Simokuras * Electrical Engineering Department, Osaka University, Yamadaoka 2-1, Suita 565, Japan t Institute for Laser Technology, 3-l l-20 Nakoji, Amagasaki 611, Japan 1 Institute for Laser Engineering, 2-1 Yamadaoka, Suita 565, Japan $ Kansai Electric Power Co., 3-l l-20 Nakoji. Amagasaki 611. Japan (Received in final form 19 Ma\, 1994 ; accepted 21 June 1994) Abstract--To study the possibility of triggering lightning with a laser plasma, laboratory laser-induced discharge experiments have been carried out and the following results were obtained. Both long straight and zigzag laser induced discharges bet&-en rod-rod electrodes were realized and the effective guiding effect of laser channels for electrical discharges was confirmed. With planeeplane electrode configuration. the investigation concerning the necessary condition for initiating and guiding a streamer reveals that the necessary electric field for initiating a streamer with a laser plasma depends closely on the absorbed energy used to produce laser plasma channels, the minimum electric field is about 200 kV/m, and that the necessary electric field for guiding a streamer by a laser plasma is lower than that for initiating a streamer and the minimum electric field is about 170 kV/m. We propose to use the local electric field near the top of a grounded tower as the necessary field for a laser-produced channel to trigger an upward leader and have proved that this is feasible. 1. INTRODUCTION To understand the characteristics of the lightning causing electromagnetic noise, one of the severest problems in the terrestrial electromagnetic environ- ment, simultaneous measurement of lightning and its remote electromagnetic fields is, without doubt, the most needed. However, the unpredictable nature of the striking point and time of lightning makes the measurement difficult. Control of the lightning dis- charges is the key to overcoming the difficulty. As a means of controlling a lightning discharge to a chosen point, the rocket triggering lightning technique has brought new knowledge about lightning physics and other related subjects (Uman, 1987 ; Willett et al., I988 ; 1989). However, the rocket triggering technique has its limitations since its response (rocket speed: I O2m/s) is not fast enough to control the propagation of a lightning leader (leader speed : 10’ m/s) and, in cases of failure of lightning triggering, the rocket and attached wires might destroy some ground facilities. In contrast, triggering a lightning with a laser, first suggested by Ball (1974), has the advantage of fast response and no harm to ground facilities. Since the effect of optical radiation on electrical discharges in gases was found in the 1960s (Keldish, 1965 ; Vladimirov et al., 1968), several research groups have carried out experiments of laser induced dis- charges for various purposes (Koopman (‘I ui., I97 I, 1973 ; Greig rt ul., 1978 ; Zhao ct ul.. 1991 ; Fujiwara et al., 1992 ; Kawasaki et al., 1992 ; Aihara c’t al., I992 ; Shindo ef al.. 1993; Honda et al., 1993; Nakamura et al., 1993). They investigated not only the relation between breakdown voltages and the distances of two electrodes, but also the effect of the polarity of the applied voltages as well as the characteristics of the leader propagation in the discharges. However, since a discharge is a non-linear phenomenon, we cannot use the relation obtained in laboratory to evaluate the necessary condition for realizing laser triggered lightning. It is apparent that a laser plasma has two effects rn inducing a final discharge; one is to initiate an arc streamer and the other is to guide the streamer. Here. the effects of laser plasma in both laser triggered dis- charge and laser triggered lightning are similar, and thus it is possible to use experimental data to estimate the necessary condition to trigger lightning with a laser. We investigated the guiding effect of a laser plasma by inducing both a long straight discharge and a zig- zag discharge between rod-rod electrodes. Then. 459