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.
`0003-6935/79/111824-04$00.50/O.
`© 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 elsewhere5 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.
`
`ASML 1234
`(cid:34)(cid:52)(cid:46)(cid:45)(cid:1)(cid:18)(cid:17)(cid:20)(cid:22)
`ASML 1234
`
`

`
`

`
`where 6 (n) represents the radiative recombination rate
`coefficient into level n.
`
`The solution of Eqs. (5) and (6) in conjunction with
`the appropriate set of 20 population density equations6-8
`yields solutions such as those presented in Fig. 2.
`In
`this instance the initial sodium vapor density was as-
`sumed to be 1016 cm‘3, and the laser irradiance was
`taken to be 1 MW cm‘? at 589 nm consistent with the
`
`experiments of Lucatorto and McIlrath.13 Two-photon
`ionization from the resonance level and single-photon
`ionization from n > 3 levels are taken into account.8
`
`The particularly noteworthy features of this inter-
`action are as follows: The electron temperature jumps,
`within a few nanoseconds of the redistribution of the
`population between the resonance and ground levels,
`toia plateau value that is in essence determined by a
`balance between collisional excitation and superelastic
`quenching of the resonance state.4v8
`Ionization pro-
`ceeds with a rate that is first determined by two—photon
`ionization of the laser maintained resonance state
`population.
`(For lower laser power densities associative
`ionization might be significant.) Once the free electron
`density exceeds 1012 cm”3, electron collisional ionization
`of the resonance level and single—photon ionization of
`the higher levels appear to take over and further ac-
`celerate the rate of ionization. During the final ion-
`ization burnout phase a momentary drop of the electron
`temperature is seen to be predicted.
`Under saturating conditions, the attenuation of the
`laser beam can be expressed in the form”
`
`dI‘(2)/dz = -Q’
`
`(W cm‘3),
`
`(7)
`
`where Q’ represents the net volume rate of power dis-
`sipation for the laser radiation and comprises the laser
`power dissipated in maintaining the resonance state
`population against (1) superelastic quenching [the first
`term on the right—hand side of Eq. (5) and represents the
`primary electron heating mechanism], (2) excitation to
`higher levels, and (3) spontaneous emission. The
`temporal variation of Q’, at a sodium density of 1015
`cm"3, is shown in Fig. 2. Evidently, Q’ increases rapidly
`with increasing electron density reaching a peak just
`prior to ionization burnout. Our computer calculations
`clearly indicate that the superelastic heating term"
`
`Q55 7* NcN2K21E21
`
`(3)
`
`dominates Q’ at this time (i.e., N2 w Ne w N0/2) in
`which case the relation
`
`inax =
`
`K2lE21/4
`
`is closely approximated over several orders of magni-
`tude variation in No, as seen by reference to Fig. 3 where
`G E (g2/g1)/ (1 + gg/g1), and No represents the original
`sodium atom density prior to laser irradiation.
`fnax thus expresses the maximum rate of laser energy
`deposition into the plasma, and as such we wish to
`compare it with the rate of laser energy deposition via
`inverse bremsstrahlung,“ viz.,
`
`1.17 x 10-7N3 A211
`12c2(leTe)3/2
`
`Q” =
`
`(10)
`
`-_—__...
`
`soonum, No= Io'5(cm-
`
`8000
`
`0)§
`
`A8OTEMPERATURE
`
`(cm"3)
`
`
`
`POPULATIONosmsmss
`
`
`
`
`
`ABSORBEDLASERPOWERDENSITY,Q9(wcm-3)
`
`
`
`
`
`0 T
`
`IME from ONSET of LASER RESONANCE SATURATION
`(nsec)
`
`Fig. 2. Temporal variation of electron density N.., electron tem-
`perature Tc, ion temperature T,-, Ng(3p) resonance and N4(3d) ex-
`cited state populations, and absorbed laser power density Q’ as pre-
`dicted by LIBORS code for sodium with N., = 1016 (cm‘3) and I’ =
`105 (W cm‘2).
`
`l
`-1 Qmax (LIBORS EON. 9)
`I.
`0
`Qmax rusons cons)
`——— Q13 INVERSE BREMSSTAHLUNG
`(N¢= No/2)
`
`6 6
`
`(17
`
`IT"
`
`5
`
`I06-
`
`
`
`
`
`MAXIMUMLASERPOWERABSORBEDPERUNITVOLUME(Wcm‘3)
`
`
`
`
`
`
`
`INITIAL SODIUM DENSITY, No(cm'3)
`
`Fig. 3. Comparison of the variation in maximum laser energy de-
`position rate with initial sodium density as predicted by LIBORS
`code, simple LIBORS model, and for inverse bremsstrahlung (the
`latter for three values of laser irradiance).
`
`1826
`
`APPLIED OPTICS / Vol. 18, No. 11 / 1 June 1979