Page 397 - High Power Laser Handbook

366 So l i d - S t at e La s e r s The National Ignition Facility Laser 367

through the PA and TSF. After the TSF, a beam splitter reflects a small
sample of the output pulse back to the central TSF area, where it is
collimated and directed to an output sensor package (OSP) located
under the TSF. OSP diagnostics record the beam energy, temporal
pulse shape, and near-field profiles. The main pulse proceeds to the
30
switchyard, where four or five transport mirrors direct it to one of a
number of final optics assemblies (FOAs) symmetrically located
about and mounted on the target chamber. Each FOA contains a 1ω
vacuum window, focal spot beam-conditioning optics, two frequency-
conversion crystals to reach 351-nm wavelength, a focusing lens, a
main debris shield that also serves as a beam diagnostic pickoff to mea-
sure energy and power, and a 3-mm-thick disposable debris shield.
The debris shields protect the upstream optics from target debris.
For the experiments reported in Secs. 14.4 to 14.6, the beam was
not transported to the target chamber. Instead, an array of either
seven or eight calorimeters was inserted at the TSF output to both
measure and absorb the 1ω laser energy. When the eighth calorimeter
was absent, it was replaced by a pickoff that routed that beam to an
extensive suite of diagnostics for 1ω, 2ω, and 3ω light called the preci-
sion diagnostic system (PDS). The PDS instruments can diagnose one
beamline in great detail, whereas the OSP diagnostics can acquire 1ω
data on all 192 beams during a shot. In PDS, the laser was frequency
converted to the second or third harmonic using typical NIF final
optics, and detailed studies of the 1ω beam entering the FOA, as well
as the 1ω, 2ω, and 3ω beams exiting it were performed. 1
The MA contains eleven Nd:glass laser slabs. The PA is configured
to have as many as seven slabs, though it typically contains only five.
Some NIF shots have had one, three, or seven slabs in the PA to explore
the full range of operating conditions. As an indication of scale, the CSF
is 22 m long, the TSF is 60 m, the path length from the TSF output to the
target chamber is 60 to 75 m, and the target chamber is 5 m in radius.

14.4 1v Bundle Performance and 1v/3v NIF
Operating Envelopes
Each of NIF’s eight-beam laser bundles undergoes 1ω operational and
performance qualification (OQ and PQ) before being used in any
experiments. The OQ-PQ consists of firing 8 to 10 shots, using all 8 beams,
into a bank of full-aperture calorimeters. These calorimeters measure
absolute beam energy and calibrate a system of diodes in the OSP that
serve as energy diagnostics during routine operations. Beam energies
at 1ω for these shots range from 1 to 19 kJ, and pulse shapes are either
flat in time (FIT) at the output or shaped to match user specifications.
In addition to verifying the bundle performance, these shots are used
to calibrate and validate the laser performance operations model
(LPOM) description of these beamlines. LPOM is then used to predict
laser performance and to set up the ILS for all NIF shots.