A simple high-voltage high current spark gap with subnanosecond jitter triggered by femtosecond laser filamentation

We describe a simple, sturdy and reliable spark gap operating with air at atmospheric pressure and able to switch currents in excess of 10 kA with sub-nanosecond jitter. The spark gap is remotely triggered by a femtosecond laser filament. Many applications rely on the initiation of a well controlled discharge between two charged electrodes. For high current applications, a gas-filled trigger is necessary. The breakdown of the gas filling the inter-electrode gap D is obtained by the introduction of an important electric field perturbation between the electrodes. This perturbation is achieved either by a high voltage (HV) trigger pulse through a third electrode, or by illumination of the inter-electrode region with a laser pulse. An outstanding feature of laser triggering is the absence of electrical coupling between the discharge electrodes and the triggering circuit, leading to a safe mode of operation. Since the first laser-triggered switching experiments in the early 1960s, a wide variety of spark gap geometries and laser types have been investigated 1. It turned out that focusing a laser axially rather than transversally inside the gap along the electrodes gave a better temporal stability of the switched pulse. Using nanosecond laser pulses, several authors have reported the switching of hundreds of kV across an air filled gap of cm length with a nanosecond jitter time 2-7. The jitter could be reduced to hundred of picoseconds with a SF 6 filled cell 8. Using femtosecond laser pulses, other authors even achieved a jitter in the range of tens of picoseconds in air at atmospheric pressure but with significantly lower operating voltage (4.5 kV) in a mm wide gap 9. In this paper we present results on atmospheric air spark gap triggering with help of femtosecond laser filamentation. Triggering via filamentation has several advantages. Since filament generated plasma column can be created at a distance of several hundreds of meters from the laser, remote triggering can be achieved. Because filaments have a near constant longitudinal electron density over several meters, they connect the two electrodes quasi-instantaneously, circumventing the complex process required in the case of an initial localized plasma, and reducing thereby the jitter. Thus, this spark gap combines the advantages of large inter-electrode air gaps capable of switching high voltages and the low jitter of narrow spark gaps or SF 6 filled cells. Main disadvantage is the requirement of a high power femtosecond laser, a commercially available but …