Picosecond high voltage switching of a pressurized spark gap

Laser wakefield acceleration promises the production of high energy electrons from table-top accelerators. External injection of (low energy) electrons into a laser wakefield puts extreme demands on the shortness and timing, i.e. a fraction of a plasma period, typically less than 100 fs. In order to meet these requirements, we have revisited the concept of pulsed DC acceleration. Simulations have shown that this concept can be successful if high voltage pulses (of the order MV) can be switched with picosecond precision. As a first step towards this goal, a 10 kV laser triggered pressurized spark gap was designed and built. One of the limitations on risetime and jitter in high voltage laser triggered spark gaps is the initial breakdown process. Since this is a stochastic process it will cause jitter, and the growth rate of the plasma will determine the fastest possible risetime of the pulse. A way to overcome this limitation is to create a line focus between the electrodes, using a high power femtosecond laser. At laser intensities above approximately 10/sup 18/ W/m/sup 2/ near-threshold or tunneling ionization causes near-instantaneous ionization of a complete plasma channel between the electrodes, much like a photoconductive semiconductor switch. Because of the instantaneous ionization and the high degree of ionization in the plasma channel, jitter and risetime are reduced considerably. We will present the first results from switching of a 10 kV spark gap with 3 mm inter-electrode distance, using a femtosecond Ti:sapphire laser. A line focus of the laser is created, using cylindrical optics. Folded-wave interferometry will be described to study the development of the plasma channel on femtosecond timescales.