C. DeMichelis

Laser induced gas breakdown: A bibliographical review

This paper provides a review of papers published on laser induced gas breakdown. The material is divided into four parts: 1) breakdown mechanism, 2) plasma expansion under the influence of the laser pulse, 3) plasma decay after the end of the laser pulse, and 4) bibliography of papers published on the subject up to the end of May, 1968.

Breakdown and self-focusing effects in gases produced by means of a single-mode ruby laser

Using a passively Q -switched ruby laser, operating in a single axial and transverse mode, sparks produced in a number of gases at pressures ranging from 760 to 9000 mmHg have been studied. Breakdown threshold measurements for both single and multimode laser radiation have been made and in addition, the characteristics of the sparks produced with single-mode radiation have been investigated. Photography at 90° of the scattered laser light showed the existence of scattering regions with transverse dimensions not exceeding the 5-μ resolution of the optical system. Furthermore, a large amount of laser light is scattered in the forward direction. The intensity, angular distribution, and spectral characteristics of this scattered radiation have been determined. Various possible mechanisms that could account for these phenomena are discussed, and it is concluded that self-focusing of the laser beam after the initiation of the breakdown process may be occurring.

PRELIMINARY EVIDENCE FOR SELF‐FOCUSING IN GAS BREAKDOWN PRODUCED BY PICOSECOND LASER PULSES

An investigation of the sparks produced by picosecond pulses from mode‐locked Nd:glass and ruby lasers indicates the existence of breakdown regions of diameter less than 5 μ. These results support the hypothesis that self‐focusing of the beam may be involved in the breakdown process.

WAVELENGTH DEPENDENCE OF LASER‐INDUCED GAS BREAKDOWN USING DYE LASERS

Two tunable dye lasers, operating in the region 7000 to 8500 A, have been used to study the wavelength dependence of laser‐induced gas breakdown. The results confirm the existence of a threshold maximum for argon and xenon, and indicate that the shape and location of the peak depend on the gas investigated.

SUBNANOSECOND SCHLIEREN PHOTOGRAPHY OF LASER‐INDUCED GAS BREAKDOWN

Second harmonic radiation from a mode‐locked neodymium:glass laser, providing a 400‐nsec‐long train of picosecond pulses, has been used as a light source for Schlieren photography of a laser‐induced spark. The beam from a ruby laser, Q‐switched by means of a Pockels cell, was focused in air to produce breakdown and synchronization of the two lasers was achieved by switching the Pockels cell with a spark gap illuminated by the mode‐locked pulse train.

Production of a spark by a train of mode - locked laser pulses

Abstract The production of a spark in atmospheric air by means of a mode-locked laser beam has been investigated and some observations on the breakdown threshold conditions and subsequent development of the spark are reported.

LASER‐PRODUCED SPARKS IN A 200‐kG MAGNETIC FIELD

Laser‐produced sparks in air, butane, and helium at atmospheric pressure have been studied in a 200‐kG magnetic field. The influence of the magnetic field on the breakdown threshold, on the visible radiation emitted by the plasma, and on the plasma expansion has been investigated.

A mode-locked laser as a light source for Schlieren photography

Second harmonic radiation from a mode-locked neodymium: glass laser has been used to carry out time-resolved Schlieren studies of sparks produced by focused ruby laser radiation. Synchronization of the two lasers was achieved by means of a spark gap triggered by the neodymium laser radiation, which switched a Pockels cell within the ruby laser cavity. The use of this technique permitted a detailed study of the plasma development during the initial stage immediately after breakdown and results were obtained in four gases (neon, argon, nitrogen, and air) at pressures between 60 and 1520 torr, using peak laser powers ranging from 35 to 300 MW and pulse risetimes between 14 and 24 ns. The results obtained indicate that, on the rising part of the laser pulse, the motion of the plasma along the laser beam axis can be explained in terms of a traveling ionization-breakdown wave.

Plasma production by laser beam irradiation of a single solid particle

Abstract A study has been made of the plasma produced by focusing a Q -spoiled ruby laser beam onto a single particle of lithium hydride suspended in a vacuum. Streak photographs have been taken and the electron temperature measured from the soft X-ray emission.

FREQUENCY BROADENING IN LASER-INDUCED SPARKS,

Abstract The spectrum of forward scattered radiation resulting from laser-induced air breakdown has been investigated using a single-mode ruby laser and a mode-locked neodymium: glass laser. The broadening observed provides additional evidence for self-focusing mechanism associated with the production of laser sparks.