Fusion driver study. Final technical report, April 1, 1978-March 31, 1980 Page: 71 of 339

                                
physical understanding of the systems implications for various
extraction schemes. The kinetics code is used primarily to
calculate the pumping dynamics; namely, the input energy den-
sity, small signal gain, time to optimum pumping and upper
laser level population density for various gas mixtures, cavity
pressures and discharge parameters. A separate analytical
model is used to calculate extraction efficiency and output
energy density as a function of quantities such as number of
(multi) passing and gas composition. In a select number of
cases, the kinetics code was run in the pumping-multipass ex-
traction mode to verify the validity of the model.
C. PUMPING KINETICS
An example of the kind of pumping information which is
available form the kinetics code and the way in which it can be
interpreted to aid in system design is shown in Figure 26. The
upper state pumping efficiency is plotted vs N2/CO2 mixture
ratio for a typical value of electron density (2 x 1()12
cm-3). The quantity Eu represents the sum of the popula-
tions of the upper laser level of CO2 and the v=l nitrogen
level. This quantity represents the total energy available for
lasing in a multipass configuration. The assumption is made
here that once the (combined) upper state is populated and
multipass energy extraction begins, no repopulation of that
upper level occurs. When the ratio of Ey to Ejn, where
Ein is the input energy density, is multiplied by the 44%
quantum efficiency of the CO2 laser system, that quantity
becomes the highest possible intrinsic energy efficiency for
that mixture. At each value of the nitrogen fraction,
the computer code runs have been scanned to choose the optimum
value of discharge electric field, E/N. The optimum value of
E/N is rather insensitive to ^ and ranges from ~ 4 x
10-16 < E/N < 5 x 10"16 V-cm2.
The effect of the increased pumping efficiency for nitro-
gen rich mixtures is evident in Figure 26 and for = 0.5, ~
80% of the input energy resides in the combined upper state.
When this figure is multiplied by the 44% quantum efficiency, a
maximum possible extraction efficiency of ~ 36% is indicated.
There are at least two other physical effects which impact the
final choice Of the nitrogen fraction. The first has to do
with the amount of lower laser level population and the frac-
tion of CO2 in the combined upper state. The second effect
involves the number of pulses required to extract the energy.
In Figure 27 the first effect is displayed as the maximum
amount of energy as the population inversion in CO2. At low
values of '/'n* the laser gas is almost totally CO2 and N2
plays a minor role in the energy transfer. The difference in
the upper and lower level pumping rates by electron impact is
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