Laser interaction based on resonance saturation (LIBORS):
`an alternative to inverse bremsstrahlung for coupling
`laser energy into a plasma
`
`R. M. Measures, N. Drewell, and P. Cardinal
`
`Resonance saturation represents an efficient and rapid method of coupling laser energy into a gaseous medi-
`um.
`In the case of a plasma superelastic collision quenching of the laser maintained resonance state popula-
`tion effectively converts the laser beam energy into translational energy of the free electrons. Subsequently,
`ionization of the laser pumped species rapidly ensues as a result of both the elevated electron temperature
`and the effective reduction of the ionization energy for those atoms maintained in the resonance state by the
`laser radiation. This method of coupling laser energy into a plasma has several advantages over inverse
`bremsstrahlung and could therefore be applicable to several areas of current interest including plasma chan-
`nel formation for transportation of electron and ion beams, x-ray laser development, laser fusion, negative
`ion beam production, and the conversion of laser energy to electricity.
`
`Introduction
`
`In many diverse applications lasers are used to heat
`and ionize a medium.
`In the plasma field the most
`well-known examples are:
`laser fusion; x-ray laser de-
`velopment; and laser heating of magnetically confined
`plasmas. There is also some interest in the possibility
`of converting laser energy into electrical energy for space
`probes via a thermoelectric process} In almost all cases
`inverse bremsstrahlung plays the important role of
`converting laser energy into plasma energy.
`The purpose of this paper is to show that laser satu-
`ration of an atomic resonance transition of some major
`constituent of a gaseous medium represents an attrac-
`tive alternative mechanism for coupling laser energy
`into the medium, whether it be a plasma or cold and
`un-ionized. Measures? was the first to suggest that this
`approach could be used to enhance substantially the
`ionization of a plasma. According to Measures? laser
`resonance saturation leads to both a heating of the
`electrons via superelastic collision quenching of the
`overpopulated resonance level and an effective reduc-
`tion of the ionization energy of such laser excited atoms
`
`When this work was done all authors were with University of To-
`ronto, Institute for Aerospace Studies, Downsview, Ontario M3H 5T6.
`N. Drewell is now at Atomic Energy of Canada, Chalk River, On-
`tario.
`Received 16 August 1978.
`0003-6935/79/111824-045500.50/0.
`© 1979 Optical Society of America.
`
`1824
`
`APPLIED OPTICS / Vol. 18, No. 11 / 1 June 1979
`
`by the photon energy. These two effects lead to a very
`rapid and almost complete ionization of the medium,
`once the electron density exceeds some threshold value.
`Just prior to ionization burnout, however, there is a very
`rapid rate of laser energy deposition into the plasma.
`If the medium is cold initially, there are several
`mechanisms for generating the seed electrons.
`If the
`laser irradiance is high (several orders of magnitude
`greater than needed to saturate the transition), multi-
`photon ionization from the resonance level is likely to
`predominate in creating these initial free electrons.3v4
`On the other hand, for more modest values of the laser
`irradiance, associative ionization can (for many atoms)
`lead to the formation of such free electrons.5
`In principle, this approach can be used with all ele-
`ments.
`In reality, however, current laser technology
`imposes some restriction on the range of elements that
`are actually amenable to this laser ionization based on
`resonance saturation (LIBORS) technique. The larger
`the energy gap being pumped the greater is the energy
`fed directly into the free electrons via superelastic col-
`lisions. Consequently, multiphoton saturation may be
`worth considering in certain instances.
`We have shown elsewheres that LIBORS appears to
`be particularly well suited for creating long plasma
`channels that will be needed for electron (or ion) beam
`transportation in certain future inertial fusion schemes.
`We have estimated that plasma channels of 5-m length
`with an electron density of about 1015 cm’3 could be
`created with less than 1 J of laser energy. We also be-
`lieve that LIBORS could be used to create a charge ex-
`change plasma that would be ideal for negative ion beam
`formation.
`
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`ASML 1235
`ASML 1235