Small scale laser-triggered electrical discharges and their application to characterization of plasma channel induced by light filaments

Abstract

Since the first demonstration of laser-induced electrical discharge [1] control of laser-triggered lightning discharges for lightning protection has became a long pursued task holding a great potential for practical applications [2–4]. Instead of high-energy continuouswave lasers such as CO2, emitting in the far infrared, the ultrashort pulse lasers with much shorter wavelengths (from near-infrared to ultraviolet) have been proven to be more advantageous offering a possibility to obtain continuous long ionized channels from the light filaments [5, 6]. It is well known that when a femtosecond laser pulse with power above the critical power for self-focusing propagates in the atmosphere, it forms a long white-light filament, which ionizes the molecules of air [7]. This ionized trail (narrow plasma channel) has been considered to be useful for triggering and guiding the electrical discharges [8]. A number of laboratory experiments had proven the validity of this approach [9–13], and guided megavolt discharges with spark lengths up to 3.8 m [14] were reported to date. Physical processes underlying a laser-triggered discharge are basically understood [12, 15]. A simple explanation is that the light filament by means of multiphoton ionization produces a short-lived (in the range of nanoseconds [16]) conducting plasma channel [17, 18] that serves as a precursor for discharge. In long gaps free electrons are then accelerated by the external electric field and start a local avalanche process – the ionization wave, called streamer. The avalanche zone expands towards the direction of the opposite electrode developing a self-sustained conducting path (leader). Once a contact between the electrodes is established, an electrical discharge occurs. However, plasma filament cannot trigger an electrical discharge by itself because of its short lifetime, and the electrical discharge (or breakdown) appears after a certain delay (ranging from nanosecond to microsecond time scale) with respect to the optical pulse. As follows from the model, the buildup time for electrical discharge is governed by transient processes (electron drift velocity and avalanche ionization [6]) to provide the critical electron density for a conducting channel. Somewhat different scenario is proposed for discharges in small gaps, assuming a constant electron density produced by the laser pulse. The current circulating in the laser-induced plasma channel produces significant heating of a thin air column by the Joule effect, which in turn is followed by radial expansion of the heated volume, and air depression meets the conditions providing a path for a guided discharge [13]. The number of free electrons in air under atmospheric pressure in normal conditions is usually very low (estimated to be less than 10 cm [19]) as compared with the atmospheric molecular density of 3·10 cm, therefore, if external electric field is applied, build-up time for a spontaneous discharge is long and typically requires a strength of external electric