Page 431 - High Power Laser Handbook

The National Ignition Facility Laser 399

350

300
Request N091120 (0.83 MJ)
250 Measured N091120 (0.84 MJ)
Total power (TW) 200 Request N091204 (1.04 MJ)

Measured N091204 (1.05 MJ)
Request N091205 (1.19 MJ)
150
100 Measured N091205 (1.20 MJ)

50
0
0 2 4 6 8 10 12 14 16 18 20
Time (ns)

Figure 14.37 Requested (smooth lines) and measured (fluctuating lines) temporal
pulse shapes for three shots fired at different energies on November 20, December 4,
and December 5, 2009. The small periodic variation in the measured power is a
result of the 3-GHz phase modulation applied in the front end to suppress
transverse Brillouin scattering in the large fused silica optics; it has no effect on
target performance. Other than this, achieved power history shows excellent
agreement with the request.

soft x rays) and hence can generate sufficiently high hohlraum tem-
perature to meet the NIF ignition design requirements. Figure 14.37
56
compares the pulse shapes from those two shots, along with that of a
previous shot fired on November 20, with the requests. In all three
cases, the measured temporal pulse agrees with the request to within
a few percent.
A perennial difficulty that has plagued previous ICF lasers is
blooming of the far-field spot size as the power is raised. This bloom-
ing results from self-focusing of localized high-intensity regions in
the beam near field; at the high power at which these lasers operate,
the refractive index of the transmissive optics is modified by the light.
Self-focusing, in turn, introduces high angular content to the beam,
which is reflected in a growing focal spot size. The NIF design was
developed with strict consideration of this effect and with strict limits
both on sources of the fluctuations that seed the effect and on the B
integral that measures its growth rate. The success of this effort is
59
demonstrated in Fig. 14.38, which plots single-beam far-field spot
radius (90 percent enclosed energy), as measured on PDS, against the
single-beam peak power (displayed as equivalent full-NIF power).
The small increase between 0.5 TW per beam and 2.8 TW per beam is
close to the measurement accuracy.