Photoconductive Switching

A more general description of the project appeared in Cursor, the Eindhoven University of Technology newspaper (in Dutch only):

Photoconductive switching of an atmospheric, air filled, high voltage spark gap

High voltage pulses are widely used nowadays in many applications ranging from radar to EUV sources and from nuclear fusion experiments to waste-water treatment. Next to better synchronization purposes, ultra-short high voltage pulses with picosecond rise time and time stability (jitter) lead to even more applications like compact pulsed DC electron acceleration, the creation of high intensity THz-radiation that can be used as a harmless alternative to X-rays and extension of the research on electroporation of biomedical materials.

When switching high voltages there are three important parameters:
Amplitude of the switched pulse
Rise time of the switched pulse
Shot-to-shot time stability (time-jitter) of the switched pulse

The tendency of high voltage switching is to be able to switch even higher voltages and currents in even shorter times with, favorably, no time-jitter.

Currently, two types of high voltage switches that are mainly used for fast high voltage switching are the semiconductor-switch and the laser triggered spark gap.

In a semiconductor-switch the two conductors are separated by a piece of semiconductor. When illuminating the complete semiconductor by a short-pulse laser, electron-hole pairs make the semiconductor to be conducting over its total length and the switch is closed. The benefits and drawbacks of a fast semiconductor switch are given in figure 1.

Figure 1: benefits and drawbacks of semiconductor switches

In a laser-triggered spark gap either a gas or a liquid separates the two conductors. A laser is focused in the gap between the two electrodes or on one of them, creating some free electrons. These electrons are accelerated by the applied electric field in the gap, creating more free electrons and finally, via avalanche and/or streamer formation, a breakdown. This conducting channel closes the switch. Notice that this breakdown process is a stochastic one. The benefits and drawbacks of a gas-filled laser triggered spark gap switch is given in figure 2.

figure 2: benefits and drawbacks of gas-filled laser triggered spark gaps

We now developed a switch that combines the benefits of both the semiconductor switch and the laser triggered spark gap switch (figure 3). By using an atmospheric air-filled spark gap in combination with a femtosecond, high power laser, we are able to ionize the complete gap at once. No stochastic processes dominate the breakdown anymore, which greatly enhances the shot-to-shot time stability.

figure 3: the photoconductive switch, a combination of benefits

Our spark gap setup is depicted in figure 4. Ports are present for the switching-laser and for diagnostic-purposes. Cylindrical lenses are used to create a laser-focus that is able to ionize the complete gap at once.

figure 4: our spark gap setup

The combination of the high power laser and the cylindrical focus revealed a new and interesting transition from the conventional laser triggered regime: The full voltage can be switched within the first couple of picoseconds and time-jitter has almost disappeared because it is no longer determined by the stochastic plasma processes.

More details on the experimental results can be found in the following publication:

G.J.H. Brussaard and J. Hendriks, Photoconductive switching of a high-voltage spark gap, Applied Physics Letters, 86, 081503 (2005)

J. Hendriks, B.H.P. Broks, J.J.A.M. van der Mullen, G.J.H. Brussaard, Experimental investigation of an atmospheric photoconductively switched high-voltage spark gap, J. Appl. Phys., 98, 043309, (2005)

We also performed simulations to investigate the electrodynamic details of this ultrafast switching process. These can be found in:

J. Hendriks, S.B. van der Geer and G.J.H. Brussaard, Electrodynamic simulations of a photoconductively switched high voltage spark gap, Journal of Physics D: Applied Physics, 38 2798-2803 (2005)

J. Hendriks, S.B. van der Geer and G.J.H. Brussaard, Spark gap optimization by electrodynamic simulations , Journal of Physics D: Applied Physics, 39 274-280 (2006)