UCRL-MI-140140

One of the major highlights of the technologydevelopment for the National Ignition Facility

(NIF) is the optics. NIF is both the largest laser andthe largest optical instrument ever built, requiring7500 large optics (over one foot across) and more than30,000 small optics. The design, manufacture, andassembly of these important pieces have called forinnovative ways to make optics of higher quality thanever before, and to do so at unprecedented speeds.The most obvious role of NIF optics is to steer the 192laser beams through the 700-foot-long building onto adime-size laser-fusion target. The less obvious opticroles are to use NIF laser glass to create laser light outof normal light and to use KDP crystals to convertthat laser light to the correct frequency, both of whichare technically challenging requirements. The OpticsProcessing Laboratory and Optics Assembly Buildingenable the final preparation and mounting of theseamazing optics for their use in NIF.

NIF Laser GlassLaser glass is the heart of the NIF laser system; it is the

material that generates the laser light. Laser glass containsa chemical additive consisting of atoms of neodymium.The glass is installed in the laser in a metal box and sur-rounded by a series of flashlamps, which perform in thesame way as a flashbulb on a camera. When we are readyto fire the laser, we push a button that discharges the elec-trical energy into the flashlamps from a series of capaci-tors, producing an enormous flood of white light. Thiswhite light is absorbed by the neodymium atoms in theglass, causing them to become “excited.” A weak laserbeam then passes through and stimulates all the excitedatoms to give up their energy at exactly the same wave-length as the weak beam. This process amplifies the weakbeam, making it much stronger.

NIF laser system uses about 3100 large plates of laserglass. Each glass plate is about three feet long andabout half as wide. If stacked end-to-end, the 3100plates would form a continuous ribbon of glass nearlytwo miles long. To produce this laser glass, we use anew production method developed by two companies(Hoya Corporation, USA and Schott GlassTechnologies, Inc.) that continuously melts and poursthe glass. Once cooled, the glass is cut into pieces thatare polished to the demanding NIF specifications.

KDP CrystalsNIF’s KDP crystals have two uses: frequency conver-

sion and polarization rotation. The development of tech-nology to more quickly grow high-quality crystals wasa major undertaking and is perhaps the most highlypublicized technological success of the NIF Project.

Frequency ConversionNIF laser beams originate with an infrared frequency,

but the interaction of the beams with the fusion target ismuch more favorable if the beams have an ultravioletfrequency. By passing the laser beams through plates cutfrom large KDP crystals, their frequency is converted toultraviolet before they strike the target.

Polarization Rotation The amplifier section of the NIF’s laser beampath

has an optical device (called a plasma electrode Pockelscell) containing a plate of KDP. This device, in concertwith a polarizer, acts as a switch to let beams in, thenrotates its polarization, trapping the laser beams in theamplifier section. It rotates the polarization back afterthe beams’ fourth pass through, letting them escapetoward the target chamber.

NIF laser glass under inspection prior to precision cleaning inthe Optics Processing Development Laboratory.

National Ignition Facility & Photon Science

The KDP Growth ChallengeThe rapid-growth process for KDP, developed

to keep up with NIF’s aggressive constructionschedule, is amazingly effective: crystals thatwould have taken up to two years to grow by tra-ditional techniques now take only two months. Inaddition, the size of the rapid-growth crystals islarge enough that more plates can be cut from eachcrystal, so a smaller number of crystals can pro-vide NIF with the same amount of KDP.

NIF Optics AssemblyAfter final surface finishing by outside suppliers,

NIF optics undergo precision cleaning and application

of special antireflection coatings in the OpticsProcessing Facility (OPF) here at the LivermoreLaboratory. Without these coatings, much of the laser’senergy would be lost long before it reached the fusiontarget. The optics are then transferred to the OpticsAssembly Building (OAB), which is attached to themain NIF laser building on the end opposite the targetchamber. The OAB is a clean-room facility where thelarge optics and mechanical parts are combined intoline-replaceable units (LRUs) before being deployed tooperating locations within the NIF. The ability of theOAB and the LRU transport vehicles to quickly, safely,and cleanly assemble, test, install, and repair LRUs is akey element of the NIF’s ability to perform itsdemanding experimental duties.

NIF Optics

10-11-07

Questions concerning the National Ignition Facilityat LLNL should be directed to the LLNL PublicAffairs Office, (925) 423-3103

Large rapid-growth KDP crystals for NIF. Approximately 35% ofthe NIF KDP crystal plates can be yielded from these boules.

A laser glass LRU being moved by a transport vehicle in the OAB