J Jimi Hendriks

Photoconductive switching of a high-voltage spark gap

We have demonstrated photoconductive switching of a gas-filled spark gap. A femtosecond Ti:sapphire laser was focused in a 1 mm spark gap biased at 4.5 kV. There is a clear transition between triggered operation, when only part of the path between the electrodes is ionized, and photoconductive switching, when the entire length of the gap is ionized directly by the laser. The measured standard deviation of the time fluctuations between the rising edge of the transmitted electrical pulse and the laser was less than 15 ps.

Experimental investigation of an atmospheric photoconductively switched high-voltage spark gap

We report on the experimental investigation of the photoconductively switched gas-filled spark gap. When the laser intensity of a femtosecond laser is high enough (around 1018Wm−2), a plasma can be created that spans the complete distance between the electrodes. The gas-filled spark gap is then closed on a femtosecond time scale, similar to photoconductive switching of a semiconductor switch. Stochastic breakdown processes, such as avalanche and streamer formation that cause the breakdown in laser triggered spark gaps, are passed over, which results in faster rise time and less jitter. Measurements of the switched pulses as a function of laser energy were performed in a 1-mm gap at an applied voltage of 4.5 kV. A clear transition from triggering to switching was measured with increased laser energy. Measurements of the output pulses with the gap filled with nitrogen at 1 atm showed results very similar to measurements in air in the same gap. In the switching regime, the amplitude of the switched pulse did...

Electrodynamic simulations of a photoconductively switched high voltage spark gap

We present a full three-dimensional electrodynamic model to simulate a photoconductively switched high voltage spark gap. This model describes the electromagnetic field-propagation in a coaxial spark gap set-up, which determines the rise time of the switched pulse and reveals the influence of discontinuities, such as view ports, on the pulse shape and the rise time. Existing inductive lumped element and transmission line models, used to model laser-triggered spark gaps, are compared with our electrodynamic model. The rise time of the switched pulses in the different models does not differ significantly. In the electrodynamic simulation, a curvature of the electric field wave front is visible, resulting from the presence of non-TEM modes near the gap. Furthermore, oscillations on the output signal are revealed. These oscillations are caused by internal reflections on the inner and outer conductors. Our electrodynamic model is able to visualize the rise time evolution by monitoring the electric field-propagation in the gap region. The presence of view ports in the set-up increases the rise time at the output significantly and induces, owing to internal reflections, extra oscillations in the signal.

Theoretical investigation of a photoconductively switched high-voltage spark gap

In this contribution, a photoconductively switched high-voltage spark gap with an emphasis on the switching behavior is modeled. It is known experimentally that not all of the voltage that is present at the input of the spark gap is switched, but rather a fraction of it drops across the spark gap. This voltage drop depends on the voltage that is present at the input of the spark gap with higher voltages resulting in a smaller drop. We have investigated two possible causes of this: the cathode fall and the resistance of the plasma arc. Using an analytical model of the cathode fall, we have established that the cathode fall can be excluded as the cause of the observed voltage drop. A one-dimensional, time-dependent non-local thermal equilibrium fluid model of the arc plasma has been made. Using this model, the plasma properties have been analyzed for various values of the switched current with emphasis on the conductivity. A good qualitative match between the observed and the simulated dissipation in the ga...

Photoconductive Operation of a Laser Triggered Spark Gap

The operation of a photoconductive spark gap switch is reviewed. It is shown that it is possible to produce pulses far below the self-breakdown voltage of the gap. The resistance of the switch depends on the switched current through the switch. Electro-dynamic simulations show that the rise time of the pulse depends on the layout of the switch and is approximately 35 ps for the setup used here.

Spark gap optimization by electrodynamic simulations

When switching times are no longer dominated by the plasma formation time, such as for photoconductive switching of high-voltage spark gaps, electrodynamic details of the switching process determine the rise time and pulse shape of the switched pulse. We show that the commonly used zero-dimensional lumped element and one-dimensional transmission line theory are no longer sufficient for optimizing such fast-switching devices, because important electromagnetic-field propagation in three dimensions is neglected. In order to improve the output of the photoconductively switched spark gap, we developed an optimization procedure for spark gap geometries based on full three-dimensional electrodynamic simulations. By monitoring the electromagnetic-field propagation in time, it will be shown that the initial electromagnetic-field disturbance in the gap reflects at the outer conductor and interferes with the initial field. The reflection and interference are essential for the shape of the output signal. We propose the following optimization procedure to improve the output of the photoconductively switched coaxial spark gap. Initially, the reflection and interference can be influenced by reshaping the inner conductor. The outer conductor can be used to fine-tune the system to get an output pulse with a sharp rising edge and no significant oscillations. We also present the optimal spark gap geometry that gives the best output signal at photoconductive switching.

Photoconductive Switching of a High Voltage Spark Gap

Summary form only given. Laser wakefield acceleration promises the production of high energy electrons from table-top accelerators. External injection of a relativistic electron bunch into a laser wakefield requires bunches of the order of the plasma wavelength, typically 100 fs. Acceleration fields necessary to create these bunches have to be of the order GV/m. RF technology has reached its limit of acceleration with fields of the order 100 MV/m, but pulsed DC acceleration can go up to GV/m gradients. Compact pulsed DC acceleration in the GV/m region is possible if high voltage pulses of the order MV can be switched on ps timescales with ps timing precision. Presently rise time and jitter of high voltage pulses in laser-triggered spark gaps are limited to the (sub)-nanosecond regime by the initial, stochastic breakdown processes in the gap. Picosecond switching precision can only be achieved if these stochastic breakdown processes, like avalanche- and streamer formation, are omitted. 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, jitter is reduced significantly and ps switching precision can be achieved. We have demonstrated photoconductive switching of an atmospheric high voltage spark gap. A 200 femtosecond, 1-35 mJ Ti:sapphire laser pulse is cylindrically focused into a 1 mm, air filled, spark gap biased at 4.5 kV. A clear transition is measured between triggering, when the gap is only partially ionized, and photoconductive switching, when the entire gap is almost instantaneously ionized by the laser. The measured rise time of the photoconductively switched high voltage pulse is smaller than 100 ps and the time jitter is less than 15 ps. We also measured at a smaller gap distance and with a flow of nitrogen in the gap. From measured Vapplied-Vout curves and preliminary simulation results, a qualitative description of the plasma behavior is deduced.

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.

Photoconductive Switching of an Air-Filled High-Voltage Spark Gap: Pushing the Limits of Spark Gap Switching

In this contribution we present the recent results on photoconductive spark gap switching. This new way of switching combines the benefits of both laser-triggered spark gap switches and photoconductive semiconductor switches. High voltages can now be switched with rise times of the order ps and almost no time jitter. We will also show that for this new way of switching, conventional theory is no longer sufficient to describe the switching behavior. A new approach of theoretical spark gap optimization is required to push the limits further