Apparatus for and method of operating electron beam attachment stabilized devices for producing controlled discharges and/or visible and UV laser output

Apparatus for and method of operating a laser wherein a discharge is produced preferably in a high pressure lasing gaseous mixture comprising at least one suitable first gaseous species capable of providing an excited state which has a finite probability of being ionized and a molecular second gaseous species having a capability for attaching electrons to form negative ions. The gaseous mixture may, for example, comprise argon, neon, helium, xenon, krypton or a metal vapor such as mercury as the first species and, for example, hydrogen iodide, carbon tetrachloride, bromine, iodine or fluorine as the second species. A buffer gas such as, for example, argon, helium or neon may also be used. The discharge is produced by means of an electron beam and an electric field. The discharge resulting from the application of the electric field heats secondary electrons produced by the electron beam to an energy level sufficient to make excited states. Thus, for a mixture comprising argon, krypton and fluorine, for example, the heated secondary electrons pump at least some of the argon and the krypton to the metastable state. The excited argon transfers energy to the krypton to form additional excited krypton which, in turn, reacts with the fluorine to form excited krypton fluoride molecules. The krypton fluoride then dissociates or decays upon the emission of spontaneous or stimulated radiation. At power input levels where the electron density remains constant in time for a constant electric field, efficient discharge pumping of the excited states is provided when the fractional excited state population is kept small. Stable discharge operation is achieved when the lasing mixture contains an amount of the second species gas sufficient to provide an attachment rate n times the equilibrium ionization rate where n is the number of electron excitations which causes ionization of the first species.

This invention relates to lasers and more particularly to visible and 
ultraviolet lasers which have an electron attaching species present. 
Lasers operating in the visible spectrum are useful in many fields, some of 
which are optical radar, remote atmospheric and meteorological 
measurements using Raman backscatter, oceanographic measurements and 
detection, laser pellet fusion, isotope separation and laser-induced 
chemistry. 
In such lasers, because the photon energy in the visible spectrum is 
greater than 1 eV, the mean discharge electron energy has to be about 3 eV 
and greater, i.e., the more energetic (shorter wavelength light) radiation 
requires a larger energy transition. This compares to a 1/10 eV photon 
energy in a CO.sub.2 laser and a 1 eV discharge electron energy. As a 
result of these higher electron energies, the ionization rate is many 
orders of magnitude greater than in CO.sub.2 laser discharges and 
discharge stability (prevention of arcing) is a far more severe problem. 
So far as is presently known, the only prior art techniques of pumping for 
visible lasers are well stabilized dischargers such as is used in 
helium-neon and argon ion lasers, and avalanche (uncontrolled) discharges, 
such as used in nitrogen lasers. All of the prior art methods are severely 
limited in size, average power capability and efficiency. 
It is an object of the present invention to provide a laser operable in the 
visible or near ultraviolet portion of the spectrum. 
It is another object of the present invention to provide a rare gas halide 
laser. 
It is another object of the present invention to provide apparatus for and 
a method of providing a stable discharge in a gaseous mixture in which to 
produce desired lasing action as in the visible and near ultraviolet 
portion of the spectrum, a large ionization rate is found because electron 
energies of the order of 3 eV and greater are required. 
A still further object of the present invention is to provide a method of 
and apparatus for producing a stable KrF laser discharge wherein electron 
attachment is at least equal to twice the ionization rate.

While the preferred embodiment of the present invention will be described 
in connection with a KrF exciplex laser wherein the lasing medium is a 
mixture of argon, krypton and fluorine, it is to be understood that other 
rare gas/halogen and rare gas/oxygen mixtures are included within the 
scope of the invention as the lasing medium wherein excited state 
ionization and attachment are the dominant electron production and loss 
mechanisms respectively, the attachment rate resulting from a loss of 
electrons by attachment to a constituent gas being at least twice the 
equilibrium ionization rate. Typical rare gases are xenon, argon and 
krypton and typical halides are fluorine, bromine and iodine. Lasers in 
accordance with the present invention can operate at subatmospheric 
pressures as well as at pressures in excess of one atmosphere. 
The physics of rare gas/halogen discharges is dominated by the excited 
species when the fractional population of the rare gas metastables exceeds 
about 10.sup.-9. The dominant ionization mechanism in KrF laser discharges 
(Kr*/Kr+Ar.apprxeq.10.sup.-5) is ionization of argon and krypton 
metastables, i.e., two-step ionization. The ionization rate is 
proportional to the power density deposited by the discharge and thus is 
not just a function of the discharge electric field. Another important 
process in such discharges is the excitation of mestastables to higher 
lying levels. This process strongly influences the secondary electron 
energy distribution function and also the efficiency of producing the rare 
gas metastables in the discharge. 
Two distinct discharge operating regimes are possible. The first is a 
stable regime in which the rapid metastable ionization is balanced by 
F.sub.2 electron attachment so that the electron density reaches a stable 
equilibrium. The second and heretofore less desirable regime is an 
unstable regime seen at higher discharge power inputs where the electron 
density increases faster than exponentially for a constant electric field. 
For purpose of the present invention, the discharge heats the secondary 
electrons produced by the electron beam until they have sufficient energy 
to make argon and krypton metastables. The krypton metastables are formed 
by direct electron impact and/or by collisional transfer from argon 
metastables while the argon metastables are formed by direct electron 
impact. KrF* is created by the following reactions: 
EQU Kr*+F.sub.2 .fwdarw.KrF*+F 
or 
EQU Ar*+F.sub.2 .fwdarw.ArF*+F 
followed by 
EQU ArF*+Kr.fwdarw.KrF*+Ar 
Since KrF is very weakly bound or unbound in the ground state, KrF* decays 
into krypton and fluorine after emitting a photon. 
The discharge physics is strongly affected by electron impact excitation 
and ionization of the rare gas metastables. To model these effects, the 
krypton metastables may be treated as rubidium and the argon metastables 
treated as potassium. This assumption is supported by the fact that it has 
been used successfully in predicting the emission spectra of the excited 
rare gas monohalides and is further justified physically by the atomic 
similarity between rare gas metastables and the alkalis. Some of the 
electron impact cross sections thus obtained are shown in FIG. 1. The 
cross section for excitation from the 5s configuration to the 5p 
configuration in Rb (Kr*) has a peak value of 75 A at 8 eV. Also shown are 
the ionization cross section of Rb and the excitation and ionization cross 
sections of Ar. From FIG. 1 it is clear that the peak value of the 
metastable excitation cross section is 30 times the peak value of the 
argon excitation cross section. More important, however, is that most of 
the electrons can excite the 5s to 5p transitions which have a threshhold 
of 1.6 eV, whereas only the high energy tail of the electron energy 
distribution (those electrons having at least 10 eV of energy) can produce 
metastables from the ground state. 
The above-noted cross sections have been put into a computer code which 
solved the Boltzmann electron transport equation. This Boltzmann code 
takes the cross section data and the electric field and calculates 
self-consistently the electron energy distribution and the partitioning of 
discharge energy amongst the various excited states and ionization. The 
predictions of the code are shown in FIGS. 2, 3, and 4. FIG. 2 shows the 
percentage of energy that goes into producing Kr* as a function of the 
fractional metastable population Kr*/(Kr+Ar) for electric fields of 2-6 kV 
cm atm. It is apparent from FIG. 2 that the efficiency of producing the 
metastables is a strong function of the Kr* population. For example, the 
efficiency of forming Kr* is almost 60% when the fractional population is 
10.sup.-5 and the electric field is 2 kV/cm atm. This efficiency decreases 
to less than 10% when the fractional population is increased to 10.sup.-4. 
This decrease in efficiency can be made up by increasing the electric 
field. However, the ionization rate (see FIG. 3) increases rapidly and may 
quickly become so large that it precludes discharge stabilization by 
F.sub.2 electron attachment for cases where the discharge power 
sufficiently exceeds the E-beam power into the laser medium. FIG. 4 is a 
plot of the average electron energy as a function of the fractional 
metastable population. It is to be noticed that the electrons cool as the 
metastable population increases. The cooling effect is much stronger at 
smaller electric fields. 
Using the rate constants predicted by the Boltzmann code, a self-consistent 
kinetics code has been developed that follows the temporal evolution of 
the secondary electrons, positive and negative ions, Ar*, Kr* and KrF*. 
The kinetics code may be coupled to a simultaneous set of differential 
equations that describe the electrical circuit. The outputs of this code 
may include the temporal evolution of the discharge current and voltage 
and the KrF* fluorescence for a given preionization level, discharge 
capacitor charge voltage and gas mixture. 
The predictions of this discharge model were compared with KrF laser 
discharge experiments. The top trace in FIG. 5 is the E-beam current in 
the discharge cavity and the lower trace is the fluorescence as observed 
by a photomultiplier after the signal passes through a 1/4 meter Jarrel 
Ash monochromator tuned to 2485 A. The cavity was filled with a 2 atm mix 
of 93.7% Ar, 6% Kr and 0.3% F.sub.2. The dashed trace is the prediction of 
the code. The amplitude of this predicted trace was adjusted to closely 
match the measured fluorescence. This amplitude normalization was 
necessary due to the absence of an absolute calibration on the 
fluorescence emanating from the discharge. For subsequent comparisons 
between experiment and theory, no further adjustments were made. 
If the KrF* fluorescence amplitude is normalized, the magnitude and 
efficiency of discharge produced KrF* fluorescence enhancement can be 
measured. FIG. 6 shows the experimental results and theoretical 
predictions when a 0.3 .mu.F capacitor was charged to 10 kV. The top trace 
is the discharge voltage, the second trace is the discharge current, and 
the third trace is the KrF* fluorescence. By the end of the pulse, the 
enhancement in the fluorescence is 3. The metastables are being produced 
with a maximum efficiency of 1.4 times the efficiency of producing the 
metastables by a pure E-beam. FIG. 7 shows the results when the capacitor 
was charged to 16 kV. In this case, the discharge current continually 
increased until the discharge went through the glow to arc transition 
which is marked by an abrupt decrease in KrF* fluorescence. It is believed 
that the initial (almost linear) increase in the discharge current is 
caused by a volumetric discharge instability. The efficiency for producing 
metastables rises rapidly to 1.7 times the efficiency of producing 
metastables in a pure E-beam and them begins to fall despite the fact that 
the voltage is constant. The KrF* production efficiency decreases because 
the metastable density increases and the discharge pumping efficiency of 
Ar* and Kr* falls. 
From the above, it will be seen that, in accordance with the present 
invention, rare gas metastables can be produced with high efficiency 
(70-80%) as long as the fractional metastable population is kept 
sufficiently small (.ltoreq.2-3.times.10.sup.-5). This high metastable 
production efficiency can lead to KrF* production efficiencies as high as 
35% under suitably controlled discharge conditions. It may also be seen 
that large metastable ionization rates accompany fractional metastable 
populations of 2-3.times.10.sup.-5. In accordance with the present 
invention, it has been found that in rare gas/halogen mixtures, this rapid 
ionization can be balanced by the provision of an attachment rate that is 
at least twice the ionization rate so that long, stable discharge pulses 
are possible. 
Direction attention now to FIG. 8 which illustrates laser apparatus for 
carrying out the invention, there is shown in combination, an electrically 
nonconductive duct 11 for receiving a flowing gaseous lasing medium 12 
including an optically resonant cavity 13 having a working region 14, an 
electron beam gun 15 for generating a substantially uniform broad area 
electron beam 16 and introducing it through an electron window 17 into the 
working region 14 to ionize the lasing medium therein, a reticulated 
discharge cathode electrode 18 adjacent the electron beam window 17, a 
discharge Rogowski anode 19 opposite the cathode 18 to produce a discharge 
across the working region and the lasing medium therein when ionized by 
the electron beam 16 from the electron beam gun 15, and a power supply 21 
for applying a discharge voltage across the cathode 18 and anode 19. 
For a more thorough discussion of laser apparatus utilizing an electron 
beam and an electric discharge for producing lasing action, reference is 
made to U.S. Pat. No. 3,702,973 issued Nov. 14, 1972, which is 
incorporated herein as if set out at length. 
Broadly, in the aforementioned patent, albeit that reference is briefly 
made to the possibility of using attachment stabilization at Column 16, 
lines 32-42, the discharge voltage is maintained at a level less than that 
which would cause significant secondary ionization. While the apparatus 
for carrying out the invention is similar in most, but not all, respects, 
to that of the aforementioned patent, the present invention is concerned 
only with the production of laser action in the visible or ultraviolet 
range where, if higher electron beam densities are not required, discharge 
voltages sufficient to produce significant secondary ionization are 
required. In the absence of a mechanism, such as attachment as suggested 
in the aforementioned patent, the production of significant secondary 
ionization results in an uncontrollable discharge or arcing. 
The present invention, while utilizing the basic components of the electron 
beam-sustainer stabilized laser, contemplates the use of a lasing mixture 
requiring pumping by the electric or sustainer discharge at a level to 
produce significant secondary ionization. However, the lasing mixture, in 
addition to containing one rare gas such as krypton, and a second inert 
rare gas (buffer) such as argon, also contains a third constitutent, such 
as fluorine, a portion of which not only participates in the production of 
the stimulated emission, but a further portion is also present in a 
quantity that the attachment rate of secondary electrons to its atoms is 
at least twice the equilibrium ionization rate in the lasing mixture. 
While the type of electron beam gun 15 is not critical, it has been found 
that at the electron beam parameters found useful (for example, 2-5 
amps/cm.sup.2 and 130-150 kV), the cold cathode type electron beam gun is 
preferable because of its efficiency, rapid turn on/turn off 
characteristics, simplicity, and ability to deliver high power pulses of 
nanosecond duration. However, it is to be noted that the present invention 
is not limited to the use of electron beams. The ionization produced by 
the described electron beam may also be produced, for example, by photons, 
nuclear fission, ion beams and the like. 
The power supply 21 for the discharge circuit may be of conventional high 
voltage, low inductance capacitive discharge design and triggered in 
conventional manner as by a triggered spark gap. 
Returning now to FIG. 8, the discharge cathode may be a stainless steel 
screen 18 disposed over and covering the E-beam window 17. The discharge 
anode 19, because of the currents and voltages that may be involved, is 
preferably of the Rogowski type as shown in FIG. 8. Such an anode 
configuration leads to greatly increased uniformity in the discharge. 
The optically resonant cavity is defined by two oppositely disposed mirrors 
in conventional manner. Only one mirror 22 is shown in FIG. 8, the optical 
axis 23 being normal to the direction of the electron beam and normal to 
the direction of gas flow. The output laser beam may be coupled out of the 
working region by a partially transmissive mirror, or for very high power 
operation, by a suitable aerodynamic window. 
The electron beam gun may typically provide 2 amps/cm.sup.2 at 200 kV in 
the working region at a pressure of about 1-3 atmospheres, the discharge 
circuit provides about 3 kV/cm atmospheres in a lasing medium comprising 
about 93.7% argon, 6.0% krypton and 0.3% fluorine. Typical pulse lengths 
may be 300 nanoseconds to 1 microsecond. However, apparatus in accordance 
with the invention may operate in the CW mode where failure of the 
electron beam window can be avoided. 
The discharge circuit may typically provide about 3 kV/cm atmosphere for a 
pulse length of about 250 nanoseconds, the discharge pulse beginning about 
40 nanoseconds after the beginning of the electron beam pulse. Trigger 
circuit means 24 are provided for actuating the electron beam gun and 
discharge circuit in the desired time relationship. The lasing medium may 
comprise about 93.7% argon, 6.0% krypton, and 0.3% fluorine at a pressure 
of about three atmospheres and provide an output laser beam at 2485 A. 
Radiation in accordance with the invention is produced when KrF* emits a 
photon at 2485 A and decays into krypton and fluorine. The KrF* is 
produced first, when excited krypton reacts with fluorine to form excited 
krypton fluoride (Kr*+F.sub.2 .fwdarw.KrF*+F), and second, when excited 
argon reacts with fluorine to produce excited argon fluoride (Ar*+F.sub.2 
.fwdarw.ArF*+F) which then reacts with krypton to produce additional 
krypton fluoride (ArF*+Kr.fwdarw.KrF*+Ar). 
It has been previously pointed out that in a KrF laser, the efficiency of 
producing the metastables is a strong function of the Kr* population, 
i.e., at low metastable densities, the efficiency is high and vice versa. 
As also previously pointed out, one can compensate for a decrease in 
efficiency by increasing the electric field produced by the discharge 
circuit. However, when this is done, the ionization rate rapidly becomes 
so large that it precludes discharge stabilization by attachment for those 
cases where the discharge power at least substantially exceeds the E--beam 
power into the lasing medium. 
Where xenon or krypton is used, argon is necessary as a buffer gas. 
Further, for a mixture of xenon and argon, nitrogen trifluoride may be 
substituted for fluorine. Thus, for operation at four atmospheres, for 
example, the lasing medium may comprise 99.5% argon, 0.4% xenon, and 0.1% 
nitrogen trifluoride. It is to be noted that for a lasing gas mixture of 
only argon and fluorine, for example, a buffer gas is not required. 
In accordance with the present invention, where electron beam-electric 
field pumping is utilized, the power put into the gaseous lasing mixture 
by the application of the electric field should be sufficient to produce 
the lasing radiation desired as discussed earlier, and in any event, not 
less or substantialy less than the power put into the lasing mixture by 
the electron beam. The ionization rate produced by secondary electrons 
should not be greater or substantially greater than one-half the 
attachment rate. 
The various features and advantages of the invention are thought to be 
clear from the foregoing description. Various other features and 
advantages not specifically enumerated will undoubtedly occur to those 
versed in the art, as likewise will many variations and modifications of 
the preferred embodiment illustrated, all of which may be achieved without 
departing from the spirit and scope of the invention as defined by the 
following claims: