FIFTY years ago, Theodore Maiman first demonstrated a laser, the design of which was originally articulated by Nobel-prize-winning physicist Charles Townes. At that time, how the laser would evolve and its many applications were unknown. Almost simultaneously, Livermore’s John Nuckolls and colleagues invented the idea of inertial confinement fusion (ICF), but they needed a powerful energy source to implode the fuel and create fusion. Scientists at Livermore began pursuing innovative research using lasers with ICF in mind. In 1972, the Laboratory officially formed a laser program to step up efforts to develop large lasers with the goal of achieving fusion in a laboratory setting. Recent experiments at the National Ignition Facility (NIF) move scientists even closer to achieving this goal.
Soon after the NIF dedication in May 2009, scientists ramped up an experimental campaign geared toward creating the necessary environment for ignition and overcoming any challenges associated with the task. Subsequent tests have shown how researchers can fine-tune the laser system to create the necessary conditions for ignition and have yielded valuable data on the timing, pulse shape, energy, and power requirements for ignition experiments.
The first step in obtaining ignition was commissioning the laser. To date, shot results have met or exceeded the laser performance criteria established by the Department of Energy’s National Nuclear Security Administration (NNSA). The initial experimental results have not only shown a robust laser but also established the hohlraum design necessary for ignition experiments. “This accomplishment is a major milestone that demonstrates both the power and reliability of NIF’s integrated laser system,” says Ed Moses, principal associate director of NIF and Photon Science.
NIF is designed to produce well-controlled, precise, repeatable, and flexible shots that provide enhanced capabilities for a variety of applications. Since its completion, the laser system has been used in basic science research for studying the hydrodynamic processes in supernovae and for critical national security programs such as stockpile stewardship.
In December 2009, NIF set a world record by firing more than 1 megajoule of ultraviolet energy at a target—more than 30 times the energy previously delivered to a target by any laser system. This shot combined with data from the experimental campaign suggest that NIF is on track to be the first facility in history to create self-sustaining nuclear burn in a laboratory setting, which is the initial step toward making ICF a feasible source of carbon-free, sustainable energy.
Infrared In, Ultraviolet Out
Thousands of optics and diagnostics, as well as sophisticated mechanical and computer hardware, need to operate perfectly and in proper sequence to create the necessary conditions for ignition. It all begins at the NIF master oscillator, which generates an initial infrared laser pulse containing a few nanojoules of energy. This initial laser pulse is shaped specifically for each experiment and can vary in length and overall energy. The pulse is then split into 48 separate beams, which are directed into individual preamplifier modules that increase the beams’ energies by billions of times—up to a few joules. The beams are then further split to create 192 beams.
Subsequently, each beam passes several times through its own series of glass amplifiers. By the time the beams are redirected to the switchyard, each one contains more than 4 megajoules of 1,053-nanometer-wavelength (1-omega, infrared) light. In the switchyard, mirrors merge the parallel array of beams into 48 groups of 4. Before entering the target chamber, these “quads” pass through 48 final optics assemblies that are symmetrically positioned around the top and bottom halves of the target chamber to precisely orient the beams onto the target.
Inside the optics assemblies, potassium dihydrogen phosphate crystals convert the beams from 1-omega infrared light into the desired 3-omega ultraviolet light. This high-frequency, short-wavelength light improves the coupling of the energy into the target. Several other optics focus the light onto the target.
In total, a NIF laser beam travels approximately 1,500 meters within 5 millionths of a second from its birth at the master oscillator to when it reaches the target chamber. When the beams arrive at the target, they are aligned within 60 micrometers root mean square, less than the width of a human hair. Bruno Van Wonterghem, operations manager for NIF, says, “The precision NIF is designed to achieve is similar to throwing a dime from Livermore to San Francisco [a distance of about 64 kilometers] and landing it perfectly inside the coin slot of a parking meter.”
Tuned to Perfection
All shots are modeled, set up, and analyzed through a computational system called the Laser Performance Operation Model (LPOM). LPOM provides real-time information on the system requirements for meeting a specific set of laser energy, pulse length, and power goals. It defines the configuration for the master oscillator and preamplifier hardware, determines the diagnostic settings for a particular shot, and analyzes shot data. With LPOM, the team can compare predicted shot values with actual data and adjust the settings as necessary for later shots. The data are derived from the many instruments surrounding the target chamber—detectors, oscilloscopes, interferometers, streak cameras, and other diagnostics—that measure the system’s performance and record experimental results.
More than a dozen laser parameters are “tuned” to control and optimize key physics parameters related to ignition capsule performance. For example, in ignition experiments, the master oscillator must produce a pulse consisting of four shocks that are timed to collapse the capsule in a precise sequence. Sudden amplitude (peak power) transitions in the pulse create these shocks, and the timing of them must be exact to create the ignition “hot spot”—which starts fusion burn—at the center of the compressed fuel. The amplitude, duration, timing, and energy of each shock can be manipulated by producing the desired pulse shape from the laser system.
To show how the laser parameters could be adjusted to meet the shock-timing requirements needed for ignition, the team tested a single beamline in a separate unit known as the Precision Diagnostic System (PDS). The team
A Powerful, Shapely Spot
Focal spot size was also tested in PDS. Inside PDS, a beam’s energy can be effectively attenuated while still providing accurate measurements. PDS includes an integrated optics module that converts 1-omega infrared light redirected from the switchyard into 3-omega ultraviolet light and then focuses the pulse in precisely the same way as NIF hardware.
Beam-smoothing techniques are used to reduce the intensity of the energy spikes, lower the contrast of the beam, and spatially shape the beam in a manner that meets target-size and irradiance requirements. A 17-gigahertz frequency modulator in the master oscillator first adjusts the beam’s bandwidth. Then, a grating inside each preamplifier module creates a corresponding high-bandwidth pointing variation that promotes smoothing by spectral dispersion.
The team measured focal spot and beam-smoothing parameters for 1.1- and 1.8-megajoule shots inside PDS. “Both shots simultaneously met ignition requirements for beam conditioning, energy, temporal profile, and peak power,” says Haynam. Using all the necessary smoothing techniques, the team then fired one NIF quad at 3 omega to the target chamber center. This shot demonstrated that the beam-smoothing and pulse-shaping requirements could be met on the main laser system at full energy and power (1.8 megajoules and 500 terawatts when scaled to 192 beams). In December 2009, the energetics shot series culminated in a shot where all of NIF’s beams were fired simultaneously on an ignition-like target at 1.2 megajoules, meeting the beam-smoothing and pulse-shaping requirements for the entire laser.
The Balancing Act
Power balance and synchronization of the beams are also key to achieving this optimal implosion shape. Balancing the power allows the x rays created inside the hohlraum to uniformly compress the target. “The process is analogous to pressing on a balloon,” says Jeff Atherton, director of experiments for the National Ignition Campaign, currently under way. “Unless the balloon is pressed on evenly all the way around, it will bulge out in different directions. The same is true for compressing an ignition capsule. We analyzed the power balance at the start and the peak of the pulse, and it was well within the design requirements.”
Haynam and his team’s efforts are primarily geared toward demonstrating the laser’s flexibility and how it can be tuned to create the conditions needed to occur inside the hohlraum for ignition. However, creating the perfect conditions for ignition also requires the right target. Another group of NIF scientists and engineers is studying target energetics and design requirements. Together, the laser and target experiments will provide the data needed so that operational processes and system components can be designed and engineered to meet optimal performance standards.
A Facility for the Ages
The first set of experiments demonstrated that NIF is a robust platform for the campaign. “The 173 target shots at
Scientists are also experimenting with NIF’s capabilities for other high-energy-density research. For example, using a foil backlighter, they apply NIF’s 1-omega light for x-ray radiography. These experiments allow researchers to see through and analyze materials in greater detail. Additionally, Haynam’s team has demonstrated in PDS that NIF can operate using 527-nanometer-wavelength (2-omega, green) light equal to 3.4 megajoules of energy when scaled to the full NIF equivalent of 192 beams. “These results are exciting because 2-omega operation allows us to go up to higher energy while significantly extending the life of the optics,” says Haynam.
The experimental work being performed at NIF is surpassing the expectations of many people. After a recent tour of the facility, Charles Townes said, “When I was inventing the laser and hoping to build the first one, I was hoping to get milliwatts of power with a small laboratory device. I just never imagined anything like this coming out of it.” Fifty years later, advances in laser technology have brought us closer than ever before to achieving ignition and, with it, the potential to produce a secure, reliable source of limitless energy for future generations.
Key Words: energy gain, hohlraum, ignition, inertial confinement fusion (ICF), laser performance, pulse shape, National Ignition Campaign, National Ignition Facility (NIF), wavelength tuning.
Lawrence Livermore National Laboratory
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