The paper introduces a new model for assessing the exposure of free-standing structures and horizontal conductors above flat ground to direct lightning strokes. The starting point of this work is a recently developed criterion for positive leader inception, modified to account for positive leaders initiated under the influence of a negative descending lightning stroke. Subsequent propagation of the posltive leader is analysed to define the point of encounter of the two leaders which determines the attractive radius of a structure or the attractive lateral distance of a conductor. These parameters are investigated for a wide range of heights and return-stroke currents. A method for analysing shielding failure and determining the critical shielding angle is also described. The predictions of the model are compared with field observations and previously developed models.

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The triggering and guiding of leader discharges using a plasma channel created by a sub-joule ultrashort laser pulse have been studied in a megavolt large-scale electrode configuration ͑3-7 m rod-plane air gap͒. By focusing the laser close to the positive rod electrode it has been possible, with a 400 mJ pulse, to trigger and guide leaders over distances of 3 m, to lower the leader inception voltage by 50%, and to increase the leader velocity by a factor of 10. The dynamics of the breakdown discharges with and without the laser pulse have been analyzed by means of a streak camera and of electric field and current probes. Numerical simulations have successfully reproduced many of the experimental results obtained with and without the presence of the laser plasma channel.