Page 416 - High Power Laser Handbook

384 So l i d - S t at e La s e r s The National Ignition Facility Laser 385

14.6 Focal Spot Beam Conditioning and Precision Pulse Shaping
for Ignition Experiments
The PQ demonstrations discussed to this point were shot without
focal spot beam conditioning in order to study the fine-scale charac-
teristics of the NIF focal spots. NIF ignition targets, however, require
spatial and temporal beam conditioning, both to tailor the irradiance
profile in the focal plane and to reduce hot spots that might seed
laser-plasma instabilities. 36,37 Spatial beam conditioning is provided
by phase plates designed to produce elliptical speckle patterns with
about 1- to 1.3-mm average diameter and ellipticity that varies from
beam to beam, depending on the angle of incidence at the target. The
laser speckle contrast is then reduced, both instantaneously and in a
47
time-averaged sense, by the application of polarization smoothing (PS)
and smoothing by spectral dispersion in one dimension (1D SSD). 48,49
Polarization smoothing is limited to a maximum reduction in contrast of
50
/
12. SSD, as implemented on NIF, achieves an additional ~5 times
reduction in speckle contrast on a time scale of a few tens of picoseconds.
The tests described in this section also used precisely shaped pulses
with high temporal contrast (~150:1), single-beam 3ω peak powers in
the range of 1.9 to 2.6 TW, and energies of 5.2 to 9.4 kJ (370 to 500 TW;
1 to 1.8 MJ full NIF equivalents). The two ignition pulse shapes used in
these experiments are shown in Fig. 14.27. The 1-MJ shape is the Rev.
39
1 baseline for the first ignition campaign on NIF. The 1.8-MJ shape is
a slightly updated version of the reference ignition pulse shape that we
assumed for the NIF laser design. For further discussion of pulse-
shaping requirements for the ignition point design, see Refs. 36 to 39.
Table 14.4 summarizes results for two shots: a 1-MJ pulse with a
0.50 mm × 0.95 mm (diameter) elliptical focal spot and 270 GHz of

500 50
450 1 MJ shape 45
40
192-beam power (TW) 300 1.8 MJ shape × 10 30 192-beam power (TW) × 10
400
1.8 MJ shape
35
350
1 MJ shape × 10
25
250
20
200
15
150
100
5
50 10
0 0
0 5 10 15 20
Time (ns)
Figure 14.27 The two shaped pulses used in these experiments, scaled to their
192-beam equivalents. The temporal contrasts are 158:1 and 176:1 for the 1- and
1.8-MJ pulses, respectively.