Abstract

The NIF, or National Ignition Facility, is a laser-based inertial confinement fusion research device located at the Lawrence Livermore National Laboratory in Livermore, California. National Ignition Facility uses powerful lasers to heat and compress a small amount of hydrogen fuel to the point where nuclear fusion reactions takes place. The Facility is the largest and most energetic inertial confinement fusion device built to date, and expected to be the first to reach the long-sought goal of “ignition” in 2010, producing more energy than was put in to start of the reaction. The National Ignition Facility uses 192 beams to produce pulses that add up to a total power of 500 TW in a 4ns pulse and a total energy of 1.8 MJ at 351nm. The system consists of a multi-pass architecture with a single large amplifier type that provides high gain, high extraction efficiency, and high packing density. This paper analyzes the structural design of the device with emphasis on the laser design itself rather than the science behind the fusion reaction.

1.       Brief History

Lawrence Livermore National Laboratory’s history with the Inertial confinement fusion program starts with physicist John Nuckolls, who predicted in 1972 that ignition could be achieved with laser energies about 1 kJ, while “high gain” would require energies around 1 MJ._[1][2] Although this sounds very low powered compared to modern machines, at the time it was just beyond the state of the art, and led to a number of programs to produce lasers in this power range. Lawrence Livermore National Laboratory decided early on to concentrate on glass lasers, while other facilities studied gas lasers using carbon dioxide (e.g. Antares laser, Los Alamos National Laboratory) or KrF (e.g. Nike laser, Naval Research Laboratory). By the 1980s the advantage of shorter wavelengths in terms of delivering energy to the interior of the targets had been conclusively demonstrated in Lawrence Livermore National Laboratory’s highly successful Shiva laser. This put the glass laser approach pioneered at Lawrence Livermore National Laboratory in the lead for future development _[16].

Figure 1 NATIONAL IGNITION FACILITY target building area obtained from LLNL’s NATIONAL IGNITION FACILITY: The ‘Crown Joule’ of laser Science Article

After the Shiva project, Lawrence Livermore National Laboratory turned to the 20-beam 200 kJ Nova laser design which was expected to reach ignition conditions. During the initial construction phase, Nuckolls found an error in his calculations, and a October 1979 review chaired by John Foster Jr. of the TRW Inc. confirmed that there was no way Nova would reach ignition. The Nova design was then modified into a smaller 10-beam design that added frequency conversion to 351 nm light, which would increase coupling efficiency._[3] In operation, Nova was able to deliver about 20 to 30 kJ of laser energy, about half of what was initially expected, due to various nonlinear optical effects. Throughout these efforts, the amount of energy needed to reach ignition had continually risen and it was unclear whether the current 200 kJ estimate was more reliable than earlier ones. On January 26, 2009, the final line replaceable unit was installed, completing one of the final major milestones of the facility construction project, meaning that construction was unofficially completed. On February 26, 2009, for the first time National Ignition Facility fired all 192 laser beams into the target chamber. On March 10, 2009, National Ignition Facility became the first laser to break the mega joule barrier, firing all 192 beams and delivering 1.1 MJ of ultraviolet light, known as 3?, to the target chamber center in a shaped ignition pulse._[4] The main laser delivered 1.952 MJ

Currently the National Ignition Facility constitutes the largest and most powerful laser system ever built. For the past decade, up to 1000 engineers, scientists, technicians, and laborers have worked at the facility _[5]. It uses 192 laser beam lines that are housed in a building with a volume of 350,000 m³, spanning to three football field (Figure 1). All the optical components to be used have a total area of 3600 m².

2.      Inertial Confinement Fusion

Inertial confinement fusion (ICF) devices use “drivers” to rapidly heat the outer layers of a “target” in order to compress it. The target is a small spherical pellet containing a few milligrams of fusion fuel, typically a mix of deuterium and tritium. The heat of the laser burns the surface of the pellet into plasma, which explodes off the surface. The remaining portion of the target is driven inwards due to Newton’s Third Law, eventually collapsing into a small point of very high density. The rapid blow off also creates a shock wave that travels towards the center of the compressed fuel from all sides. When it reaches the center of the fuel, a small volume is further heated and compressed to a great degree. If the temperature and density of that small spot can be raised high enough, fusion reactions will occur.

The fusion reactions release high-energy particles, some of which (primarily alpha particles) collide with the high density fuel around it and slow down. This heats the fuel further, and can potentially cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel—high enough density and temperature—this heating process can result in a chain reaction, burning outward from the center where the shock wave started the reaction. This is a condition known as “ignition”, which can lead to a significant portion of the fuel in the target undergoing fusion, and the release of significant amounts of energy._[4]

To date most Inertial confinement fusion experiments have used lasers to heat the targets. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles, as well as creating a suitable shock wave. The energy must also be focused extremely evenly across the target’s outer surface in order to collapse the fuel into a symmetric core. When a temperature of 108°C and density of 1000 g/cm³ are reached, ignition of the DT fuel at the center begins. Although other “drivers” have been suggested, notably heavy ions driven in particle accelerators, lasers are currently the only devices with the right combination of features._[4]

Figure 2 Inertial Confinement Fusion steps. Courtesy of Benjamin D. Esham (public domain commons)

The generated power by this small amount of fuel at the center (10% of total fuel) compresses the remaining fuel faster than it can be expelled, propagating the ignition process outwards (In existing fusion experiments the heat produced by the fusion reactions rapidly escapes from the plasma, meaning that external heating must be applied continually in order to keep the reactions going). The resulting fusion reactions yield many times more energy than was absorbed from the driver beams. Calculations estimate that 25 MJ of thermonuclear energy can be extracted from a process requiring 1.8MJ input energy to drive the implosion _[6].

3.       The Design

Every National Ignition Facility experimental shot requires the coordination of up to 60,000 control points for electronic, high voltage, optical and mechanical devices – motorized mirrors and lenses, energy and power sensors, video cameras, laser amplifiers and diagnostic instruments. Achieving this level of precision requires a large-scale computer control system as sophisticated as any in government service or private industry. The meticulous orchestration of these parts will result in the propagation of 192 separate nanosecond (billionth of a second)-long bursts of light over a one-kilometer path length. The 192 separate beams must have optical path lengths equal to within nine millimeters so that the pulses can arrive within 30 picoseconds (trillionths of a second) of each other at the center of the target chamber. Then they must strike within 50 micrometers of their assigned spot on a target the size of a pencil eraser. National Ignition Facility’s pointing accuracy can be compared to standing on the pitcher’s mound at AT&T Park in San Francisco and throwing a strike at Dodger Stadium in Los Angeles, some 350 miles away. Because the precise alignment of National Ignition Facility’s laser beams is extremely important for successful operation, the requirements for vibrational, thermal and seismic stability are unusually demanding. Critical beam path component enclosures (generally for mirrors and lenses), many weighing tens of tons, were located to a precision of 100 microns using a rigorous engineering process for design validation and as-installed verification.

3.1 Hohlraum

The first concern is how to obtain symmetry within the fuel container (The Hohlraum) so that all beams converge onto one point. The difficulty of this task and why there are 192 beams can be described by trying to squash a water balloon with two hands. No matter how hard one tries to spread the fingers evenly over the surface of the balloon, it will still squirt out between ones fingers. Many more fingers would be needed to compress the balloon symmetrically. Earlier high-energy lasers were used to study the conditions required to compress tiny spherical capsules to fractions of their initial diameter while still maintaining the capsule’s symmetry – a crucial requirement of the National Ignition facility is to achieve fusion ignition. National Ignition facility’s designers arrived at 192 focused spots as the optimal number to achieve the conditions that will ignite a target’s hydrogen fuel and start fusion burn.

X-rays from the hohlraum have to exert a pressure with spherical symmetry to the capsule to reduce any hydrodynamic instability in the energy coupling and therefore the implosion process _[6]. This problem is solved by using many laser beams that cover different regions of the hohlraum, with an optimum spot size of 500 µm for all the beams.

Figure 3 Hohlraum Design _[11]

3.2 Power and Energy

In a low power configuration, any energy transferred to the capsule would cause hydrodynamic instabilities. Therefore The National Ignition Facility is designed to give 1.8 MJ of energy with 500TW of power to be safely within the margins that ensure ignition will occur. This is done at the third harmonic of 1053nm pulses from Nd:glass lasers because ultraviolet light is more effective than infrared light in the generation of X-rays radiation when interacting with the hohlraum walls. Furthermore ultraviolet light penetrates the electron shield that the fuel cell produces after being heated up better than infrared light.

Using Type II third harmonic generation (Tripler) The National Ignition Facility is able to generate third harmonics for use in the final optics assembly described in section 4.7 – Final Optics. The total losses in the conversion to the third harmonic (3?) are roughly 40%. A transmission efficiency of 0.9 from the Tripler to the target was estimated in _[6] to determine the energy and power at the first harmonic (1?). These values are: 3.4MJ and 632TW. The levels of power expected at National Ignition Facility have no precedents. National Ignition Facility will be the only terrestrial platform that can concentrate energy and power to levels that approach those in the interiors of stars and nuclear weapons _[13].

Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles (figure 4). The duration of the pulse is 12 ns and the difference between maximum and minimum is two orders of magnitude. Using smoothening techniques such as spectral smoothing the beam can temporally smooth the drive and minimize plasma instabilities caused by interactions between laser intensity and density and temperature of plasma produced inside the hohlraum _[6].

Figure 4 Power vs Time Delivered _[20]

3.4 Beam profile

An important limitation related to power is the maximum ultraviolet f1uence supported at the Tripler. This sets the minimum allowed total aperture. At 3?, the maximum f1uence for the desired pulse shape is 14J/cm². National Ignition Facility is designed to operate at 9.5 J/cm² with an additional filling aperture 80%. There is also a maximum area for a single laser beam. This is limited by many factors, including stimulated Raman scattering in the Tripler, the need to control transverse parasitic in optics, the ability to grow large apertures and the ability use defect-free crystal components. Experimental data fix this maximum at 1600 cm² _[6]. With these two data, the estimated minimum number of laser beams is 160. Symmetry and smoothing requirements demand 192 laser beams, which gives a reasonable design margin of 30 laser beams.

One of the main difficulties in the construction of multiple-pass amplifiers of high power has been the irregularities in the beam profile that are originated in non-linear processes within the optics. Optical phase errors with spatial frequencies higher than about one inverse centimeter, as well as small obscurations due to dirt or damage, seed the nonlinear mechanisms. These variations can grow from acceptable to destructive over a relatively narrow range of beam line intensities, causing a degradation of the ability of the laser to be tightly focused onto the target _[13].

The focal-spot f1uence distribution of these pulses is carefully controlled, through a combination of special optics at the 1? (1053 nm) portion of the laser using continuous phase plates, smoothing by spectral dispersion, and the overlapping of multiple beams with orthogonal polarization (polarization smoothing) _[5].

4.      Laser Design

National Ignition Facility is a very complex optical system. In order to get high amplifications of power, many phases are needed. It is crucial that the 192 laser beams shots are coordinated, and a single seed Yb-fiber master oscillator (CW, 1053nm) is used to feed a cascade of elements that build the pulses, provide temporal, amplitude and bandwidth control and amplification. This unit is called the Injection Laser System. It consists of acousto-optic modulators followed by a cascade of fiber filters that feed 1 nJ pulses to each of 48 Amplitude Modulator Chassis that set the pulse shape. Each of these Amplitude Modulator Chassis is followed by a Pre-Amplifier Module with a gain of 10 and a Multipass Amplifier with a gain of 10³. Each of these 48 resulting pulses of 1J feeds 4 Beam-Lines that will expand the pulse to its full aperture. The output beam from the amplifier is then shaped from a round three-millimeter Gaussian beam to a square 18×18 mm² beam in which the edges of the beam have higher intensity than the center. This shaping compensates for non-uniform gain in the large-slab main and power amplifiers, where the gain is higher in the center than the edges. Then Amplify the pulses to their final energy of 10.4kJ at 1? (gain of 10×10³), convert the pulses to the third harmonic and focus the power into the target. Once multiplied by 192 beams an output of approximately 2MJ can be expected.

4.1 Pre-Amplifier Modules

A schematic of the Injection Laser system regenerative amplifier is shown in Figure 5. Light enters the amplifier from the master oscillator room fiber launch at the right of the Figure. It is collimated, passed through an optical isolator, and injected through a polarizer into the main regenerative amplifier cavity. After passing through the Pockels cell once, the Pockels Cell is switched on, trapping the pulse in the cavity for approximately 30 round trips. During each round trip, the pulse passes twice through a diode pumped rod amplifier. Prior to the final pass, the Pockels Cell is switched off, and the light exits through a second polarizer (short-dashed line). A half-wave plate in combination with a set of polarizers controls the energy transmitted to the next stage of amplification. A second Pockels cell can be used to clip off a trailing portion of the pulse that is meant to saturate the regenerative amplifier for energy stability, but is not required in the rest of the laser. A 20× beam expander in combination with a beam-shaping module sculpts the beam to the desired spatial shape (solid line on left) _[5].

Figure 5 PreAmplifier module _[5]

4.2 Power Amplifier

Figure 6 shows a schematic of the multi-pass amplifier system. Light enters from the regenerative amplifier at the right of the Figure and transmits through the polarizer. The polarization is rotated by a series of half-wave plates and quarter-wave plates so that the pulse passes four times through the 32 mm flash lamp pumped rod amplifier before exiting through the polarizer. Each pass is optically relayed using a set of two vacuum relay telescopes. These vacuum relay telescopes are evacuated to prevent air breakdown at the central focus of the telescope. As the pulse exits the cavity (short-dashed line), it passes through a combination of motorized half wave plates and a polarization-sensitive mirror to allow control of the energy transmitted to the main laser. _[5].