Page 402 - High Power Laser Handbook

370 So l i d - S t at e La s e r s The National Ignition Facility Laser 371

14.5 Performance Qualification Shots for
Ignition Target Pulse Shapes

In March 2006, we fired two 1ω PQ shots separated by three hours
and eighteen minutes, an interval that is significantly shorter than the
NIF design requirement of less than or equal to 8 hours between
system shots. Shot intervals of less than 4 hours have been repeated
on a regular basis during the commissioning of the first 40 NIF beam-
lines, with no discernable degradation in either beam wavefront or
near-field modulation.
These PQ shots were taken to validate NIF’s capability to meet its
energy, power, and temporal contrast design goals. One beam from
each shot was routed to the PDS. The other seven beams were mea-
sured in the 1ω calorimeters. We will follow the performance of the
laser, as measured by the diagnostics, through the four sections of the
laser, starting with the 1ω sections (master oscillator room, preampli-
fier module, and main laser) and finishing with the 3ω diagnostics
following the FOA. A detailed discussion of the PDS diagnostics,
main laser diagnostics, and calorimeters can be found in the appen-
dices to Haynam et al. 1

14.5.1 Master Oscillator and Pulse Shaping System
The master oscillator and pulse shaping system, referred to by the
acronym MOR (master oscillator room) (Fig. 14.9) creates the tempo-
ral pulse shape specified by LPOM. The MOR temporal pulse shape
compensates for gain saturation in the rest of the 1ω laser and for
the power dependence of the frequency converter efficiency, so that
the desired 3ω pulse shape is achieved.
The pulse begins in a CW Yb:fiber master oscillator tuned to
1.053-μm wavelength. The CW signal from the oscillator’s output is
chopped by an acousto-optic modulator to a pulse width of 100 ns at
a pulse repetition rate of 960 Hz. The light is phase modulated at a
frequency of 3 GHz to a total bandwidth of 30 GHz in order to sup-
press stimulated Brillouin scattering (SBS) in the main laser optics.
41
A high-reliability fail-safe system is in place to guarantee that the
pulse cannot proceed beyond the MOR unless adequate modulation
34
has been applied to ensure that SBS will be suppressed. A separate
modulator operating at 17 GHz can apply more than 150 GHz of
additional bandwidth at 1ω (450 GHz at 3ω) for beam smoothing by
spectral dispersion (SSD), as will be discussed in Sec. 14.6.2. The pulse
then transits a cascade of fiber splitters and Yb:fiber amplifiers, cul-
minating in 48 fiber outputs, each of about 1-nJ energy. Each output
goes into an amplitude modulator chassis (AMC) that sets the pulse
shape for injection into a preamplifier module (PAM).
In order to account for varying gain/loss characteristics among
the beamlines, and to afford operational flexibility to fire a variety of