H.P. Mercure

Triggering and guiding high-voltage large-scale leader discharges with sub-joule ultrashort laser pulses*

The triggering and guiding of leader discharges using a plasma channel created by a sub-joule ultrashort laser pulse have been studied in a megavolt large-scale electrode configuration (3–7 m rod-plane air gap). By focusing the laser close to the positive rod electrode it has been possible, with a 400 mJ pulse, to trigger and guide leaders over distances of 3 m, to lower the leader inception voltage by 50%, and to increase the leader velocity by a factor of 10. The dynamics of the breakdown discharges with and without the laser pulse have been analyzed by means of a streak camera and of electric field and current probes. Numerical simulations have successfully reproduced many of the experimental results obtained with and without the presence of the laser plasma channel.

Modeling the triggering of streamers in air by ultrashort laser pulses

The physical processes involved in the triggering of ionization waves (streamers) by ultrashort laser pulses, focused in air at 350 Torr and in a uniform electric field, are investigated by means of a one-dimensional (1-D) numerical model. The model describes the interaction of the laser pulse with air and takes into account many of the reactions in the laser-created plasma as well as the radial expansion of the plasma. Consequences of the model are that the threshold electric field for the appearance of streamers is an increasing function of the delay between the laser pulse and the electric field pulse and a decreasing function of the laser energy. Also, it appears that the electron temperature, the plasma density and radius, and the conduction of heat across the plasma boundaries play major roles in the capacity of the laser-created plasma to trigger streamers. The results of the model are compared with the available experimental data.

The influence of electron density on the formation of streamers in electrical discharges triggered with ultrashort laser pulses

In an ongoing program using ultrashort laser pulses to provoke discharges in air over considerable distances at electric fields below breakdown threshold, we have studied the conditions for the onset of streamers in such laser-produced plasmas, both experimentally and through numerical simulations. The results demonstrate the importance of the electron density and of its gradient on the generation of streamers. Also, a significant reduction of the breakdown voltage for a 30 cm plane-plane gap in air was observed with a laser pulse energy of 15 mJ. Finally, a direct comparison of laser-induced breakdown in air and in nitrogen shows the influence of electron attachment to oxygen on the discharge process.

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. I. Experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.

Insulator pollution performance at high altitude: major trends

A survey of the data available from the literature was conducted in order to assess major trends in insulator performance with altitude. The literature comprised tests on individual insulator strings both under natural and artificial conditions along with actual live-in performance. The major trends reported here suggest a pressure dependence (p/po)0.35 for DC and (p/po)0.5 for AC, of V50 for standard polluted insulators at altitudes lower than about 4 km (p ~ 0.6 atm). These pressure dependences impose a derating of the relative performance of leakage path length or flashover voltage with altitude, namely 6%/ km for AC, 5%/ km for positive DC and 4%/ km for negative DC applied voltages. Dielectric breakdown, in contrast, imposes a 10.5%/ km degradation of insulating air gaps with altitude.

Spectroscopic study of ultrashort pulse laser-breakdown plasmas in air

Time-resolved visible spectroscopy of plasmas produced by laser breakdown of air using femtosecond Nd laser pulses (50–300 mJ, 500 fs) reveals features not observed with nanosecond laser pulses. Emission is initially dominated by molecular lines, specifically the second positive system of N2 and the first negative system of N2+. This is followed by continuum emission with a growth time of ∼3 ns and a decay time of ∼30 ns. Atomic lines of N and O emerge from the decay of the continuum and last up to 1 μs; only faint ionization lines are observed. Several of the atomic lines are initially strongly broadened, narrowing over a period of 100 ns.

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. II. Modeling

We have used the Bondio-Gallimberti model of positive leader propagation to simulate laboratory experiments of laser triggering and guiding of upward leaders initiated from a ground rod. The model proves to be capable of reproducing all the important features of laser-guided leader propagation that have been observed experimentally. The leader guiding effect of the laser-created plasma channel is taken into account in the model by adjusting the value of the charge per unit length of the leader, which has been measured in the laboratory to be lower for a laser-guided leader than for an ordinary one. The charge per unit length of the leader is related in the model to the critical temperature at which the air in the transition region at the leader tip must be heated to be conductive enough to become a new leader portion. For an ordinary leader, this critical temperature is 1500 K, at which the electrons all detach from the negative ions in the leader corona, increasing the air conductivity. We give the interpretation that in the case of the laser-guided leaders, because of the relatively high density of negative ions per unit length in the laser-ionized channel, the right conditions of conductivity can be met in the transition region without the electrons being all detached from the ions, allowing a reduction of the critical temperature and of the charge per unit length.

Modeling of the air plasma near the tip of the positive leader

With the aim of elucidating the role of thermal effects in air in the leader propagation, the properties of the ionized air (charged species densities, electric field, and air temperature) in a limited region near the tip of the positive leader were calculated using a two-dimensional axisymmetric numerical model. Charged-particle densities and the electric field were treated consistently. Compared with the geometrical field, the actual electric field appears to be much more uniform and is strongly reduced at the tip due to the plasma shielding. As a consequence of the field damping, thermal effects are determined to play a negligible role outside the leader boundary.

Study of laser-induced breakdown in a 30-cm air gap under a uniform field

In an ongoing effort to understand how ultrashort pulse lasers can be used to trigger spark discharges in air over considerable distances at electric fields much below that of self-breakdown, we have studied the influence of the laser pulse energy on the development of an electrical discharge over a 30-cm air gap in a uniform field geometry. In addition to the purely electrostatic effects associated with the production of free electrons as the laser beam is focused in the gap, the results highlight the importance of thermal effects to trigger the discharge when the plasma is produced several tens of microseconds before the electric field is applied. One of the most important mechanisms leading to breakdown of the gap is the production of a space leader from the laser-created plasma. Results showing the propagation of such a space leader in a 30-cm gap are presented for the first time.

A laser-triggered spark gap for 750 kV, 20 ns risetime impulse generator

A description is given of a nanosecond-risetime, 750-kV spark gap impulse generator that is triggered by an ultraviolet laser pulse (KrF, lambda =249 nm). The SLITS (SF/sub 6/ laser-induced triggering system) is an add-on fast switching capability that makes use of the existing equipment (Marx, AC or DC) and can be triggered at voltages as low as 10% of the spontaneous breakdown voltage of the gap. The output voltage can be varied simply by adjusting the internal delay of the laser or the source voltage, eliminating the awkward adjustment of the sphere gaps required in a conventional pulse generator. Jitter is reduced too less than 10 ns at 80% of the spontaneous breakdown voltage, making the output waveform highly reproducible. >