A. Robledo-Martinez

Dipolar field and plasma expansion at the onset of laser-induced breakdown in a uniform dc field

An experimental investigation into the initial phases of laser-induced breakdown in air, with and without an external field, is presented. The plasma is produced by focusing the light of a Nd : YAG laser between two parallel plates connected to a dc bias supply. The diagnostics employed included fast photography and electric field measurements with a D-dot field probe. It is found that the laser power threshold required to initiate breakdown increases when an external field transverse to the laser beam is applied; the effect is not observed when the field is parallel to the beam. Measurements performed with a field probe show that the plasma produces a dipolar electric field that is proportional to the strength of the bias. The dipole is caused by charge redistribution over the plasma surface rather than by charge creation at separate points. The estimated plasma-polarizability coefficient associated with the dipole is a function of the beam energy and focal lengths employed only.

Time-resolved diagnostic of an impulse discharge in variable pressure air

The effect of gas pressure on the characteristics of a short-gap lightning discharge in air was investigated. For the tests, 70 ns front lightning pulses were applied to a short (11 cm) point-plane gap under variable pressure. The diagnostics employed included electric current and field measurements, spectroscopy in the visible and fast-frame photography. We found that the pressure has a clear effect on the electric field at the plane. For low pressures, the high fields measured (~7 kV cm−1) are comparable to the Laplacian field, indicating that very little ionization takes place in the gap at this pressure; at higher pressures the space charge contributes substantially to the field magnitude. The effect of pressure on the current pulse was, in contrast, minimal; its peak amplitude and shape remained largely unaffected by pressure. Time-resolved spectroscopy allowed the determination of the instantaneous electron density and temperature to be made; the latter, for example, was found to reach 33 000 K at t ~ 1 µs for most of the pressures employed. Using the measured temperature and radius we made estimations of the arcs resistance. We found that the Spitzer resistivity model gives values of resistance that are compatible with the experimental data obtained.

Virtual anode effect in the propagation of positive streamers

Experiments made in a parallel-plate arrangement, where one of the plates is the ground plane and the other a positively charged dielectric sheet, show that at low air pressure the dielectric injects streamers onto the surrounding air. The similarities and differences between the streamers thus produced and those emitted from electrodes are discussed. The streamers here obtained were found to have diameters and speeds that are clearly dependent on pressure. Often the streamers were seen to decelerate markedly to the point of coming to a complete stop and on occasions even returning back to the dielectric they came from. The deceleration and the reflection of the streamers are attributed to a virtual anode created by the electrostatic image of the heads charge behind the ground plane. Field calculations performed corroborate this hypothesis. Through the use of an equation of motion of the positive head, it is then possible to obtain an estimate of the magnitude of its charge. The charge thus obtained was ...

Measurement of the temperature and the resistivity of an impulse arc in variable-pressure CO2

The properties, as a function of pressure, of an arc produced by fast impulses in a carbon dioxide (CO2) atmosphere were investigated. A time-resolved spectroscopy technique consisting of a spectograph coupled to a fast camera was implemented to analyse the light emitted by the arc. Using this technique the arcs spectrum could be obtained at different moments of the development of the discharge. The use of a multi-element Saha–Boltzmann method allowed both the temperature T and the electron density ne of the arc to be obtained. These were found to have peak values of T = 28 000 K and ne ~ 9 × 1017 cm−3 at the time of maximum current. The measured temperatures were incorporated into a variation of the Spitzer model to obtain the plasma resistivity. With it, the arc resistances were calculated and found to be consistent with the measured currents (~1.85 kA). Mass spectroscopy sampling of the spent gas shows that CO2 degraded very little after several dozen kiloampere shots.

Comparison between low‐pressure laboratory discharges and atmospheric sprites

The discharge of a charged dielectric in low pressure air has characteristics that resemble some of the features of mesospheric discharges. The dielectric discharges in steps when the pressure of the surrounding air is gradually reduced from nearly atmospheric to ~0.01 torr. The set-up employed here decouples the discharge from the power supply and, thanks to that, unique properties of the discharge manifest themselves. For example: in the pressure interval ~10-100 torr streamers are emitted from the surface of the dielectric but when the pressure decreases to 2-16 torr these are replaced by spherically symmetrical discharges that we call peonies. These have interesting properties, like: a) they do not produce electrical field, b) they remain static and c) their size increases with decreasing pressure. The peonies are a type of discharge that has not been reported before. They resemble sprite beads and are assumed to consist of large avalanches that do not lead to the formation of a streamer. At further lower pressures, in the interval 0.01-0.1 torr, diffuse volume discharges were observed that have some morphological similarities with sprite halos and the top of columnar sprites. The spectrographic measurements carried out show that the discharges have bands from the first and second positive system in N2 as well as lines of N2+. Quenching of the first negative system of N2 was observed at 3 torr. In this work it was also observed how a cosmic ray can go on to trigger a discharge inside the experimentation chamber.

Light focusing from large refractive indices in ionized air

The sheath that surrounds a laser-induced plasma in air was investigated with a light probe. The sheath is a 3-mm-thick halo of ionized gas created by ultraviolet light emitted from the plasma core. A beam of laser light tracked with a streak camera was employed to probe it. It was found that in the first hundreds of a nanosecond after plasma inception, the beam is deflected towards the plasma center. This result points to a high refractive index inside the sheath. The index values obtained (up to 2.0) are due to an increased susceptibility caused by a bulk electric polarization. Using previous results on the sheath’s electric polarizability values, the refractive index was calculated and was found to agree with the observed indices. The application of the electric polarization model to the guiding of laser beams and to plasma lenses is also discussed.

Negative Refractive Index in a Plasma Induced by Laser

Summary form only given. In a previous research the authors demonstrated that a layer of cold, weakly-ionized gas surrounds the plasma produced by a focused laser. This layer is like a halo or corona that contains a population of free electrons that make the layer behave like a conductor. It was also found that this layer has a thickness of 2-3 mm. The mechanism by which this outer layer is ionized is by UV light from the plasma; in addition, some of the free electrons produced by the focused laser beam may drift to the outer layer. An experiment was devised that consisted in shining a pulse of green light (fiducial) from an Nd:YAG laser through the outer layer of the plasma produced by focusing the IR light from the same source. The two components were separated by means of a frequency-doubler and a separator. The light emitted from the plasma and the fiducial was simultaneously registered by a streak camera with a resolution better than 100 ps. For times close to the plasma onset and energies well above threshold, it was found that when the fiducial passes through the outer layer it is deflected towards the plasma. Calculations of the refractive index using classic plasma formulae predict a deflection away from the plasma. For this reason the observed deflection in this case is attributed to a negative refractive index. A plasma with its diamagnetic properties and its conductor-like behavior complies with the requirements of a left-handed medium that has a negative index