NPA/Int. 67 - 7 19.3.1967 Laser Induced Breakdown of Gases and the Interaction of Radiation with Gases by C. Grey Morgan Introduction 1 Since the first announcement by Terhume only four years ago that electrical breakdown of gases can be caused by focusing the out• put of a giant pulse laser, there has been a tremendous growth of interest in the phenomenon. Very many papers dealing with various aspects df ~he bre4k~ down have been published and already a healthy controversy appears to be in existence concerning the mechanism by which ~ases can become almost perfect conductors in times of a few nanoseconds under the in­ fluence of relatively long wavelength light alone. The observed phenomena are quite fascinating and may have far reaching consequences in our understanding of the mechanism of the interaction of radiation with matter. The properties of the gaseous plasmas created by laser induced breakdown have also been studied extensively during the last few years~'J Laser produced plasmas offer several distinct advantages over those produced by conventional means. They can be created at very high den­ sities in gases or from pellets of materials of interest, for example, lithium hydride, under clinically clean,conditions in vacuo. These plasmas have a unique set of properties including isotropy and initial electronic and ionic equilibrium with zero rnomentun and current in the plasma. These properties are of considerable practical importance in plasma physics and controlled thermonuclear research studies. Q-switched and non-Q-switched laser beams have also been used to initiate electrical discharges between electrodes in spark gap . 4,5,6. switches. This is an application of laser induced discharges which may have several distinct advantages over more conventional triggered spark gap switches and impulse generators, especially at - 2 - high voltage. They offer good synchronisation prospects and are ?, 8 currently being applied, together with a form of laser radar in a search for clumps or microparticles 9 which are sometimes considered to be the cause of breakdown in vacuum 1nsu. l at1on.. 10 To gain some idea of the magnitude of the parameters involved we may take a typical experimental result, say the case of laser in­ duced breakdown in neon at 2000 torr. It is found experimentally 11 5 2 to require a ruby laser beam power density of about 10 Megawatts/cm • Using Poynting 1 s theorems it can readily be shown that the associated electric vector field is a little less than lo7v/cm and the radiation pressure is 6 x 10 6 Newtons I m2 , i.e. about 10 12 times. grea t er th an th e solar radiation pressure on the earths surface. It is not surprising then that something violent happens. Apparatus and Experimental Observations The type of apparatus commonly used to study laser induced breakdown in gases is shown in figure 1. It consists essentially of a ruby or neodymium rod, an optical pumping system and a Q-switch, in this caso a Kerr cell shutter. This serves to suppress lasing action in the rod so that a very large population inversion, far above the threshold level, is achieved in the laser element. The shutter is then opened and coherent radiation, reflected between the mirrors, builds up rapidly and all the excess excitation is discharged in an extremely short time. The intensity of this giant pulse of radiation exceeds by several orders of magnitude that obtainable from an ordinary non-Q-switched laser flash. The giant pulse is focused by a lens to a point in the gas con- tained in a vessel. The vessel is provided with electrodes for charge collection and windows for visual observation and to enable measure- ments of the optical characteristics of the breakdown plasma to be made. Breakdown in atmospheric air, for example, is observed as a bright blue flash at the focus of the lens and is accompanied by a distinctive cracking noise. Figure 2 shows an example of laser in­ duced breakdown in air at atmospheric pressure in which four closely spaced plasmas were photographed. In some of our work at CERN wo have observed two very widely spaced breakdowns from tho same laser pulse. - J - In common with nll types of pulsed electrical discharges in gases, laser induced breakdown can. be divided into three distinct stages initiation, growth and extinction. The initiatory stage is tha time which elapses between the arrival of the laser radiation pulse in the focal region of the lens and the initiation, by the release of electrons and ions, of the growth of free electron and ion concentration in the gas. The formative growth stage is the subsequent period of amplif­ ication in the number of charged particles until the state of break- down is reached. Breakdown is arbitrarily defined as the attainment of a. certain electron concentration. The combined duration of the initiatory and formative growth times is exceedingly small and may be only a few nanoseconds. The final or extinction phase lasts for a time which may be two, or three orders of magnitude longer than the duration of the la~~~ flash, - 50 microseconds compared to about 30 nanoseconds. During this phase the plasm~ gradually dies away as a result of several pro~ cesses radiation, diffusion, recombination, attachment and s6 on~ 1 Initiation and Growth Mechanisms Several physical processes have been proposed by various workers as the mechanism.s of initiation and orowth. These include "cascade 11 or 11 avnlanche 11 growth by the quantum process of inverse . t .b. t • L • l • 1" • 11 • . 1 ,,_ 13 B remss rahl ung a sorp·ion w111c1 is pacysica y equiva~en~ t o exci• t - ation and ionisation caused by inelastic collisions between gas atoms and the free electrons which draw en(ffgy from the electromagnetic fi~ld Of the l~ser beam. Thfs is essentially an extrapolation to op- 1 '' tical frequencies (""10 ° Hertz) of well known microwave discharge l~, 15, 16, 17. th eory. "ff t . -" b t 18, 19. 20, .21, A d ~ eren view is proposeu y o her workers . ' 22, 23, who suggest that the observed growth of ionisation is the result of multiphoton absorption, that is the practically simultaneous ab­ sorption by ~n atom of several quanta, each having energy hV much less than the atomic ·excitatio!1 and ionisation energy. In th<:: case of heliti~ for example, no fewer than fourteen quanta from a rµby laser would' have to be absorbed simultaneously to release an elE;)ctron. The. process has been treated in terms of semi-classical field emission - 4 - theory in which the electron is regarded as tunneling out of the atom in a tim0; which is small compared with the period of oscillation . 24, 25. of the laser electromagnetic radiation. An alternative proposal 26 is. that the initiatory electrons are released as a result of Thermal ionization of the gas following heating of the gas by non-linear dielectric absorption. Most workers appear to prefer the cascade or avalanche growth of ionization which involves the absorption to radiation by electrons in free-free transitions and either collisional ionization or collie- ional excitation followed by photo-ionization. The cascade processes require for their initiation, the presence of at least one free electron in the focal volume early in the duration of the laser flash. To fix ideas, consider breakdown -9 in air at atmospheric pressure:- the focal volume may be about 10 -8 to 10 cc, so that the presence of an electron requires an equilibrium concentration in the atmosphere of about 109/cc. This is comparable to that obtained in a glow discharge and is certainly about a million times greater than the equilibrium density of ionization produced in the atmosphere by natural causes such as the passage of cosmic rays or the presence of local radioactivity and so on. In the absence of an initic:::l electron, no breakdown can occur by the cascade process alone however large the intensity of the laser radiation. Consequently, if natural phenomena are relied upon to pro­ vide the initial electron,breakdown will be erratic and will occur only when an electron is fortuitously liberated at the focus during the laser pulse. The breakdown time lag will then be statistically distributed as in conventional discharges. 27 However, no significant randomness is observed in the onset of laser induced breakdown. Ob- . 28 29 served fluctuations ' 'are far too small, indeed, by.several orders of magnitude to be attributable to a random appearance of initiatory electrons and in fact there does not appear to be a significant init­ iatory time lag. In view of these facts it follows that laser induced gas break­ down is unique insofar as it apparently requires no (~xternal source of ionization to initiate the breakdown process. This implies that the laser flash itself supplies the initiatory electrons in times of the order of a nanosecond. - 5 - This conclusion at once raises tho important question of h~w the laser radiation interacts with the atoms to release electrons. We will defer seeking an answer to this question for the time being and turn to examine some of the characteristics of the so-called cascade or avalanche processes of growth. Cascade Processes extrapolation to optical frequencies It is not· unnatural to rittempt to explain laser induced br,;ak~ down in. terms of an extrapolnt1on. of class1ca. 1 14' 30' -31 m1.crowave. br.eakdown, theory. Indeed, many of the characteristics of laser induced breakdown bear a strong r<;semblance to pulsed microwave clischarg.es. In treating the problem 1 we can tak;.i advantage~ of the fact that although five parameters c:.re involved, namely the ionisation potehtial of the gas atoms, the electric field associated with the laser beam, the wavelength of the laser light, the mean: free path of the charged particles in the gas and the size pf the focal region.