Gain measurements on a prototype NIF/LMJ amplifier pump cavity Page: 4 of 13
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vs four slabs high for NIF. Since there is a strong vertical symmetry in the amplifier due to the presence
of top and bottom reflectors, we believe that the results obtained with our two-high amplifier will not be
significantly different than those obtained with a four-high NIF prototype. Another difference is the slab
thickness: our MBA used 3.4 cm-thick slabs, whereas the NIF will use 4.1 cm-thick slabs. This
difference is taken into account with our ray-trace codes when predicting NIF amplifier performance.
Another feature of our MBA is the ability to test one or two-wide amplifiers. This is important
when testing the amplifier with -two central arrays, as our pulse-power system limits the number of lamps
we may fire to 20. To test central-central pumping, we placed absorbing architectural glass and a diffuse
metal reflector behind the central lamp array to simulate an adjacent module. By comparing the aperture-
averaged gain coefficients with two-wide and one-wide modules, we have found that the two
configurations produce the same gain coefficient to within 0.1 %/cm.
3. Experimenal results and discussion
In a laser chain, a given slab can occupy one of two positions: 1) an interior position, where the
slab is located between two others, or 2) an end position, where the slab is located at the end of the chain.
An end slab may take on two further configurations, "X" or "Diamond," which refers to the shape the two
end slabs make (see Fig. 2). An interior position is difficult to access experimentally, thus to obtain the
gain coefficient for this location, we take data in the "Diamond," "X," and "V" configurations as shown in
Fig. 2. We use the aperture-averaged gain coefficient obtained in these three configurations in our bulk-
gain model1 to determine a relative cavity transfer efficiency for each configuraton. The cavity transfer
efficiency is a measure of the amount of light leaving the flashlamps that is incident on the laser slab. It
may be shown that the cavity transfer efficiency for an interior configuration may be given as:
Ili=ld+lx - Iv (1)
where ld, lx, and 11v are the cavity transfer efficiencies for the "Diamond," "X," and "V" configurations
respectively. It should be noted that a "V" configuration does not exist in a real laser chain, but is used
here only as a step in determining the cavity transfer efficiency for an interior slab. The value of mi
obtained from Eq. (1) is then used in our bulk-gain model to determine the gain coefficient for an interior
slab.
Once we find the gain coefficients for the interior, "Diamond," and "X" configurations, the average
gain coefficient for a chain N (odd) slabs long is given by:
aN = [(N-2)ai + ad + ax] / N (2)
where ai, ad, and ax are the gain coefficients for the interior, "Diamond," and "X" configurations
respectively.
In Fig. 3, we show the two-dimensional gain coefficient contours for an interior slab at an
explosion fraction of 0.2. For this case, the aperture-averaged gain coefficient is 5.1 %/cm but the peak-
to-average ratio of the gain coefficient, apk / <a>, is 1.09:1, indicating a large degree of gain non-
uniformity. It is desirable to have a uniform gain profile since the energy requirements for the front-end of
the laser system are lower than for amplifiers with a high degree of gain non-uniformity. The gain non-
uniformity depicted in Fig. 3 is due primarily to amplified spontaneous emission (ASE) that is trapped
within the laser slab. Amplified spontaneous emission tends to depump the upper laser level, leading to
lower gain near the edges of the aperture. This is clearly shown in Fig. 4, which shows the horizontal
gain profile at various explosion fractions. At low explosion fractions, ASE effects are small and the gain
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Rotter, M.D.; McCracken, R.; Erlandson, A. & Guenet, M. Gain measurements on a prototype NIF/LMJ amplifier pump cavity, article, December 1, 1996; California. (https://digital.library.unt.edu/ark:/67531/metadc676721/m1/4/: accessed March 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.