Resonance radiation plasma (photoresonance plasma)
`
`I. M. Beterov, A. V. Eletskii, and B. M. Smirnov
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`Usp. Fiz. Nauk 155, 265-298 (June 1988)
`
`A plasma formed by the action on a gas of monochromatic radiation whose frequency
`corresponds to the energy of a resonance transition in the atom is studied. The elementary
`methods of creating and studying a plasma of this type are analyzed. The kinetics offormation of a
`photoresonance plasma is studied, including collision processes with participation of excited
`atoms leading to formation of molecular ions and highly excited atoms, processes of stepwise
`ionization and triple recombination, and radiative processes. A photoresonance plasma is
`characterized by a high electron density with a relatively low electron temperature; for this reason
`the condition ofideality is more easily violated in a plasma of this type. Some ways of utilizing a
`photoresonance plasma are presented.
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`TABLE OF CONTENTS
`
`1. Introduction ......................................................................................................535
`2. Types of photoresonance plasmas and methods of preparing them .............. ..536
`2.1. Photoresonance non-laser plasmas. 2.2. Photoresonance laser plasmas.
`2.3. Quasiresonance plasmas. 2.4. Beam and jet photoresonance plasmas.
`. Elementary processes in a photoresonance plasma ........................................ ..54l
`3.1. Photoprocesses. 3.2. Collision of electrons with excited atoms. 3. 3. Ioniza-
`tion with participation of excited atoms.
`. Properties of photoresonance plasmas ............................................................ .. 545
`4.1. Establishment of equilibrium in photoresonance plasmas. 4.2. Nonideal
`photoresonance plasmas.
`. Optogalvanic spectroscopy ............................................................................. ..55l
`5.1. The optogalvanic efi‘ect. 5.2. Laser isotope analysis.
`. Conclusion ....................................................................................................... ..552
`References ........................................................................................................ ..553
`
`1. INTRODUCTION
`
`One of the methods of creating a plasma involves the
`action of optical resonance radiation on a gas. This method
`was first realized by Mohler and Boeckner,‘ who observed
`the formation of ions upon irradiating cesium vapor with
`resonance radiation. Thus they established the possible oc-
`currence in the gas of the process of associative ionization, in
`which an electron and a molecular ion are formed by colli-
`sion ofexcited and unexcited atoms, so that the energy need-
`ed for ionizing the atom is released through formation of a
`molecular ion. Studies of photoresonance plasmas (PRPS)
`began with the study of Morgulis, Korchevoi, and
`Przhonskiiz in 1967. By illuminating cesium vapor with res-
`onance radiation to obtain a gas with a high concentration of
`excited atoms, they found as a result that a plasma is formed
`with a rather high concentration of charged particles. Since
`the ionization energy of the cesium atom (3.89 eV) exceeds
`by more than twofold the energy ofa resonance photon ( 1.39
`or 1.45 eV), this result indicated a complex, multistep char-
`acter of the kinetics of the ionization of cesium atoms under
`the conditions studied. The subsequent detailed studies of
`this kinetics3‘5 have permitted obtaining rich information on
`the mechanisms and rates of processes involving excited
`atoms.
`
`The formation of a photoresonance plasma is accompa-
`nied by various phenomena that occur in gases. Thus, the
`ionization of a gas under the action of resonance optical radi-
`ation is one of the fundamental mechanisms of formation of
`
`an ionization wave in the gas, which propagates upon apply-
`
`ing an external electric field.” This same mechanism plays
`the decisive role in the phenomenon of ionization of a gas
`ahead of the front of a strong shock wave in the gas.7 Irradia-
`tion of a gas with optical resonance radiation is used as one of
`the methods of preliminary ionization of the active medium
`of high-pressure molecular lasers.3 This enables one to cre-
`ate a plasma homogeneous throughout the volume, while
`avoiding the factors that favor the development of instabili-
`ties and spatial inhomogeneities of the active medium} The
`stated method of creating a high-density plasma homoge-
`neous throughout the volume has attracted the attention of
`investigators also in connection with the problem of heating
`thermonuclear targets with beams of light ions.“ In this case
`the ionization of the gas under the action of resonance radi-
`ation enables one to create for a short time an extended plas-
`ma channel, which serves for transport of the ion beam to the
`target, while hindering electrostatic repulsion of the ions.“
`The potentialities of study of photoresonance plasmas,
`as well as the set of their applications, have been expanded by
`the invention of frequency-tunable lasers. On the one hand,
`this has enabled considerable increase in the fluxes of reso-
`
`nance radiation transmitted through the gas, and on the oth-
`er hand, study of the processes that occur upon optical exci-
`tation of various states of the atom. The photoresonance
`plasma formed by using tunable lasers is used as a nonlinear
`element in frequency transformation of laser radiation, "’ as
`a source of ions of a given type,‘ "'3 etc.
`The set of phenomena that occur in a photoresonance
`plasma is closely bordered by the optogalvanic effect, which
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`535
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`Sov. Phys. Usp. 31 (6), June 1988
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`0038-5670/88/060535-20$01.80
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`© 1989 American Institute of Physics
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`535
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