Formation kinetics and parameters of a photoresonant plasma
`
`A. V. E|etskii,Yu.N. Zaitsev, and S.V. Fomichev
`
`(Submitted 28 July 1987)
`Zh. Eksp. Teor. Fiz. 94, 98-106 (May 1988)
`
`The kinetics of a plasma produced by applying to a metal vapor pulsed resonant radiation whose
`frequency corresponds to the energy of a resonant transition in the atom is investigated. It is
`established that the principal ionization mechanism in such a plama, which ensures practically
`total ionization of the vapor at relatively low energy inputs, is stepwise ionization of the atoms by
`electron impact. Under conditions of above-equilibrium occupancy of the resonantly excited
`state, this process takes place at a constant electron temperature T, . The calculated values of T,
`agree with the measurement results. The possibility of producing a nonideal photoresonant
`plasma is analyzed; it is shown that the action of resonant radiation increases the nonideality
`parameter by 1.5 to 2 times compared with an equilibrium plasma. The parameters of the
`equilibrium plasma produced by relaxation of a photoresonant plasma are calculated. These
`parameters (Ne ~10‘°—10” cm”3, Te ~ 0.4-0.6 eV) are not reached if the plasma is produced by
`traditional methods. It is shown that an equilibrium plasma with arbitrary electron density can be
`produced by the photoresonance method.
`
`1. A photoresonant plasma (PRP) is produced when a
`gas is acted upon by monochromatic radiation whose fre-
`quency corresponds to a resonance transition of the gas
`atom.‘ Even the first experimentsz aimed at producing PRP
`and at investigating its properties have shown that this inter-
`esting physical object differs substantially from a plasma
`produced by traditional methods (gas discharge, charged-
`particle beam, etc. ). Interest in the investigation of PRP has
`increased substantially in recent years in view of the advent
`of high-power tunable visible-light lasers, which are used to
`produce the PRP. Experiments performed with the aid of
`such sources‘ have shown that even relatively low-power
`laser emission suflices to obtain PRP of Na (Ref. 3), Li
`(Ref. 4), Cs (Ref. 5), Ba (Ref. 6), Mg (Ref. 7) and other
`vapors with quite high ionization. In contrast to a gas-dis-
`charge plasma, electrons in PRP are not subject to the accel-
`erating action of an electric field, so that their temperature
`remains relatively low. By the same token, the resonant ac-
`tion of the radiation on the gas is a special method of produc-
`ing a plasma having extremal physical properties, viz., high
`degree of ionization at a low electron temperature. Produc-
`tion of a supercooled plasma with close to one particle in the
`Debye sphere is reported in Ref. 8. Plasma with such param-
`eters cannot be produced by other methods. The purpose of
`the present study was an investigation of the mechanisms
`and formation kinetics of a PRP and determination of the
`
`best attainable properties of this object.
`2. We shall consider the PRP produced when metal va-
`por of density N is acted upon by monochromatic radiation
`of intensity J. For the sake of argument we consider the con-
`ditions of the experiment of Ref. 8, where Na vapor at a
`density 10'°—4~ 10” cm“3 was exposed to 25-ns pulses of
`resonant radiation of intensity 6-107 W/cmz, as a result of
`which PRP was produced with parameters NC ~10” cm”
`and Te ~O.2—O.3 eV. The cited paper contains a most com-
`plete description of the PRP parameters.
`In the initial state of PRP production, the main source
`of the charged particles are the collisions between resonantly
`excited atoms, which either lead directly to formation of
`electrons and ions (associative ionization)°:
`
`A‘+A‘—>A2++e,
`
`or lead to formation of highly excited atoms°:
`
`A‘+A’—>A"+A,
`
`(1)
`
`(2)
`
`which undergo associative ionization
`A
`(3)
`A--+{ _}»A,++e
`when colliding with normal or resonantly excited atoms.
`At high substantially above-equilibrium density of the
`resonantly excited atoms, the processes (1)—(3) lead to a
`rapid growth of the electron density in the PRP. Thus, in the
`case of Na the rate constant of process ( 1 ) is z 3 - 10“ ‘ ‘ cm3/
`s (Ref. 9). Therefore the use of resonant radiation of saturat-
`
`ing intensity (J2 102-103 W/cmz) at an Na vapor density
`~10‘°—10” cm” ensures effective ionization at a level
`~10“ cm‘3/s. Thus, even in the first few tens of nanose-
`conds of the PRP lifetime the electron density in it exceeds
`~ 10'’ cm”. This creates a more effective channel for the
`formation of charged particles, due to the stepwise ioniza-
`tion of the atoms by electron impact. In fact, the rate of
`formation of charged particle via associative ionization (1)
`does not depend on Ne, whereas the rate of stepwise ioniza-
`tion is proportional to N,
`(we are considering the colli-
`sional-kinetics regime typical of the experimental condi-
`tions, when the excited atoms are destroyed mainly by
`electron-atom collisions, and the role of the radiative pro-
`cesses is negligible). The criteria for realization of such a
`regime have been discussed in detail, e.g., in Refs. 10 and 1 1.
`Rougly
`speaking,
`these
`criteria
`are
`of
`the
`form
`Te<fiw, Ne 21013-10” cm'3 (fia) is the atom-excitation
`potential). The electron density Ne, above which the elli-
`ciency of stepwise ionization exceeds the efliciency of associ-
`ative ionization is determined from the condition
`
`Nel~N1 kass/kst?
`
`where ks, is the rate constant of the stepwise ionization of the
`excited atoms. It will be shown below that ks, ~10” cm3/s.
`It follows hence that, for example in the case of Na, the main
`
`920
`
`Sov. Phys. JETP 67(5), May 1988
`
`OO38-5646/88/050920-O5$O4.00
`
`© 1988 American lnstitute of Physics
`(cid:34)(cid:52)(cid:46)(cid:45)(cid:1)(cid:18)(cid:17)(cid:20)(cid:21)
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