The primary sources of damage on the National Ignition Facility (NIF) Grating Debris Shield (GDS) are attributed to
two independent types of laser-induced particulates. The first comes from the eruptions of bulk damage in a
disposable debris shield downstream of the GDS. The second particle source comes from stray light focusing on
absorbing glass armor at higher than expected fluences. We show that the composition of the particles is
secondary to the energetics of their delivery, such that particles from either source are essentially benign if they
arrive at the GDS with low temperatures and velocities.
We describe the cleaning processes, treatment methods, facilities, and cleanliness verification techniques developed to
achieve and maintain the demanding cleanliness requirements for both hardware and optics used in the National
Ignition Facility (NIF).
Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will
review important optical damage precursors in silica up to UV fluences as high as 45J/cm<sup>2</sup> (3ns) along with studies of
the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface
damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of
a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily
responsible for optic damage below about 10J/cm<sup>2</sup>; they can be mitigated by an optimized oxide etch processes. At
fluences above about 10J/cm<sup>2</sup>, precipitates of trace impurities are responsible for damage; they can be mitigated by
eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica
damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating
fluences these optics see.
A system of customized spatial light modulators has been installed onto the front end of the laser system at the National
Ignition Facility (NIF). The devices are capable of shaping the beam profile at a low-fluence relay plane upstream of the
amplifier chain. Their primary function is to introduce "blocker" obscurations at programmed locations within the beam
profile. These obscurations are positioned to shadow small, isolated flaws on downstream optical components that might
otherwise limit the system operating energy. The modulators were designed to enable a drop-in retrofit of each of the 48
existing Pre Amplifier Modules (PAMs) without compromising their original performance specifications. This was
accomplished by use of transmissive Optically Addressable Light Valves (OALV) based on a Bismuth Silicon Oxide
photoconductive layer in series with a twisted nematic liquid crystal (LC) layer. These Programmable Spatial Shaper
packages in combination with a flaw inspection system and optic registration strategy have provided a robust approach
for extending the operational lifetime of high fluence laser optics on NIF.
Current methods for the manufacture of optical components inevitably leaves a variety of sub-surface imperfections
including scratches of varying lengths and widths on even the finest finishes. It has recently been determined that these
finishing imperfections are responsible for the majority of laser-induced damage for fluences typically used in ICF class
lasers. We have developed methods of engineering subscale parts with a distribution of scratches mimicking those found
on full scale fused silica parts. This much higher density of scratches provides a platform to measure low damage
initiation probabilities sufficient to describe damage on large scale optics. In this work, damage probability per unit
scratch length was characterized as a function of initial scratch width and post fabrication processing including acidbased
etch mitigation processes. The susceptibility of damage initiation density along scratches was found to be strongly
affected by the post etching material removal and initial scratch width. We have developed an automated processing
procedure to document the damage initiations per width and per length of theses scratches. We show here how these
tools can be employed to provide predictions of the performance of full size optics in laser systems operating at 351 nm.
In addition we use these tools to measure the growth rate of a damage site initiated along a scratch and compare this to
the growth measured on an isolated damage site.
Customized spatial light modulators have been designed and fabricated for use as precision beam shaping devices in
fusion class laser systems. By inserting this device in a low-fluence relay plane upstream of the amplifier chain,
"blocker" obscurations can be programmed into the beam profile to shadow small isolated flaws on downstream optical
components that might otherwise limit the system operating energy. In this two stage system, 1920 × 1080 bitmap
images are first imprinted on incoherent, 470 nm address beams via pixelated liquid crystal on silicon (LCoS)
modulators. To realize defined masking functions with smooth apodized shapes and no pixelization artifacts, address
beam images are projected onto custom fabricated
optically-addressable light valves. Each valve consists of a large,
single pixel liquid cell in series with a photoconductive Bismuth silicon Oxide (BSO) crystal. The BSO crystal enables
bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in
turn modulates the amplitude of the coherent beams at 1053 nm. Valves as large as 24 mm × 36 mm have been
fabricated with low wavefront distortion (<0.5 waves) and antireflection coatings for high transmission (>90%) and
etalon suppression to avoid spectral and temporal ripple. This device in combination with a flaw inspection system and
optic registration strategy represents a new approach for extending the operational lifetime of high fluence laser optics.
In many high energy laser systems, optics with HMDS sol gel antireflective coatings are placed in close proximity to
each other making them particularly susceptible to certain types of strong optical interactions. During the coating
process, halo shaped coating flaws develop around surface digs and particles. Depending on the shape and size of the
flaw, the extent of laser light intensity modulation and consequent probability of damaging downstream optics may
increase significantly. To prevent these defects from causing damage, a coating flaw removal tool was developed that
deploys a spot of decane with a syringe and dissolves away the coating flaw. The residual liquid is evacuated leaving an
uncoated circular spot approximately 1mm in diameter. The resulting uncoated region causes little light intensity
modulation and thus has a low probability of causing damage in optics downstream from the mitigated flaw site.