Picosecond high-voltage switching for multi-stage DC electron acceleration

For the acceleration of ultra high-brightness electron bunches, accelerating gradients of 1-2 GV/m have been achieved using a pulsed DC accelerator. To accelerate to higher energies, it would be desirable to extend this concept in a multi-stage configuration. To do this, and still maintain a high average accelerating gradient, it is necessary to switch the high-voltage pulses in less than 10 ps. A way to achieve this is to create a line focus between two electrodes using a high power femtosecond laser. At laser intensities above approximately 10¹⁸ W/m² tunneling ionization causes near-instantaneous ionization of a complete plasma channel between the electrodes. Because of the instantaneous ionization and high degree of ionization in the plasma channel, picosecond switching precision can be achieved in a properly designed matched geometry. This concept was tested using a prototype switch consisting of two coaxial transmission lines, interrupted by a 1 mm gap. One of the transmission lines was charged to 4.5 kV. A femtosecond Ti:Sapphire laser was used to close the switch. Using cylindrical optics, a line focus was created between the electrodes to form the conducting channel. At high power (≈0.2 TW) the jitter between the laser and the front of the transmitted high-voltage pulse was less than 15 ps.