Optics & Photonics News

The NIF will produce up to 1. 8 MJ of 351-nm laser light—which is higher
than that produced by any previous laser system by a factor of 50.

laser energy required to drive the implosion. The initial ignition capsule will use a Be ablator that contains a 100-µm-thick DT ice shell. The demonstration of ICF ignition on the NIF is the highest priority near-term HED goal.

The pursuit of ignition on the NIF is a project managed through the National Ignition Campaign (NIC), which is executed by a national team including LLNL, LLE, Los Alamos National Laboratory, General Atomics and Sandia National Laboratories. It includes the ignition campaign on the NIF and supporting experiments, primarily carried out on the OMEGA laser facility. The goals of the NIC are to carry out the first “credible” ignition attempt in 2010, leading to the development of robust ignition by 2012.

The NIF laser system

The NIF is the largest, most complex and most expensive optical system ever built. It has 192 beamlines arrayed in 48 quads (four neighboring beamlines) to irradiate the inside of a hohlraum target with cylindrical symmetry to achieve ICF ignition with indirect drive. The resulting X-ray irradiation will compress the capsule to ignition conditions. The NIF will deliver up to 1. 8 MJ of ultraviolet energy in an approximately

20-ns pulse, allowing the ignition hohlraum to reach a radiation temperature of 3,300,000 degrees C (300 eV) and driving the capsule to ignition conditions.

The system requires exquisite pulse shaping, pointing and beam-to-beam co-timing and co-pointing. To meet these stringent requirements, new technologies were developed, and the laser system architecture was completely rethought.

The NIF architecture uses a multipass laser system that extracts a much larger fraction of the energy stored in the amplifiers than was possible with previous high-energy inertial fusion lasers. The NIF laser amplifies light from the master oscillator by a factor of 10.

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The NIF multipass design
is enabled by a number
of major technological
advances, including devel-
opment of large full-beam
aperture (40-cm-square)
plasma-based optical Deformable
mirror (LM1)
switches (Pockels cell) and
deformable mirrors, 48
stable, high-gain preampli- Main
Cavity spatial
amplifier

fiers (one for each of four filter (CSF) beams) capable of high-precision spatial, temporal, and spectral pulseshaping,

and a highly sophisticated computer control system capable of monitoring more than 60,000 individual control points.

The multipass design allows the laser beam to make four passes through the main amplifier while maintaining the spatial uniformity required for effective amplification and, ultimately, frequency conversion to 0.35-mm light at the target chamber. Like all large inertial fusion laser systems in the world, the entire NIF laser—from master oscillator to just outside the target chamber—operates at the fundamental laser wavelength of 1,053 nm. The 351-nm ultraviolet light used in NIF experiments is produced by nonlinear optical frequency conversion of the 1,053-nm fundamental harmonic.

This conversion is performed by using full-aperture potassium dihydrogen phosphate crystals, which are located just outside of the 10-meter-diameter target chamber. This nonlinear frequency conversion scheme was first developed at LLE and is now in use at all major laser fusion facilities worldwide.

The NIF is now complete, with all optics installed. The Department of Energy formally certified completion of the NIF on March 27, 2009. The NIF has fired all 192 beams in the infrared and has demonstrated an equivalent full system energy of more than 4 MJ of infrared laser energy.

LLNL scientists are now in the process of ramping up the laser to full energy ( 1. 8 MJ operation) in the ultraviolet. A single bundle of eight beams has been fired a number of times at an energy of 78 kJ, which is equivalent to 1. 8 MJ for the entire system. The first 192 ultraviolet target shot occurred on February 26, 2009, and a total 192-beam ultraviolet energy of 1. 1 MJ to target chamber center was demonstrated on March 10, 2009. NIF will commence 192 beam target experiments at 1-MJ energy later this year, with operation at 1. 8 MJ expected by 2011. The system has met or exceeded all of its performance requirements, including producing the high-contrast ignition pulse shape.