Anthonares

When fully operational, the US Department of Energy’s National Ignition Facility (NIF) will focus 192 of the most powerful lasers ever constructed onto a target no larger than a BB. The lasers will combine to produce 2 MJ of energy in a split second–a very split second. For the 4 nanoseconds that the lasers illuminate their tiny target, they will produce 500 TW, roughly 40 times the total energy consumption of the entire world! All of this energy focused on such a tiny target will literally vaporize it, via nuclear fusion.

The NIF is currently under construction at Lawrence Livermore National Laboratories (click for Google Earth placemark), where 4 of the lasers are complete. They expect to finish construction by the end of this year and see “first light” a year later. NIF will be used to study nuclear fusion for both commercial energy generation and nuclear stockpile maintenance purposes. Because of this highly sensitive mission, it is unlikely that you or I will soon see the actual facility itself. So I thought I would take us all on a tour of this fascinating project to see how many laser beams it takes to implode a BB (sounds like the intro of a very nerdy joke).

Inertial Confinement Fusion

Before we get going, let’s take a moment to understand the type of nuclear fusion that the NIF lasers will be causing: inertial confinement fusion. For a review of fusion energy and fusion in general, see my post from January. Inertial confinement fusion works on the basic principle of igniting fusion by squeezing a lot of very hot mass (relatively) into a very tiny space.

The diagram on the right illustrates the process. First, the lasers strike an object called a hohlraum (see below) that converts their energy to x-rays. These x-rays rapidly heat the outside of the beryllium-covered deuterium/tritium target BB. The outer layer ablates and rockets away from the remainder of the target. By Newton’s Third Law, there is a force inward on the target equal to the rocket force outward. The outer layer also heats to extremely high temperatures, heating the remainder of the target. This process continues (for a very brief period of time), squeezing the target to an extreme density and heating it to approximately 500 million degrees kelvin. Then, the deuterium and tritium in the target begin to fuse. From there, it’s all over; the rest of the target rapidly fuses, imploding in a fireball that destroys the hohlraum and releases more energy than was used to destroy the target (at least theoretically).

Tour of the NIF

Now that you know the basics of what the nuclear physicists at the Department of Energy are trying to accomplish, let’s take a look at the facility that will be required. On the left is a model of the National Ignition Facility building, with cutaways showing the locations of the various components I’ll describe below. Note that the cutaway shows only 1/2 of the full NIF system, the other half is symmetrically placed opposite the central trunk of the building.

First, you may want to take a look at this very cool animation of the beampath. On the left, in a sub-basement level the laser beams are first created by the master oscillators and preamplifiers that make up the injection laser system (see the labeled diagram above as well). The master oscillators output a nanojoule strength laser (about one million-billion times weaker than the final beam) that is immediately amplified by a factor of a billion in the preamplifier.

From the preamplifier, the beam travels into one of the two laser bays (above and to the right of the injection laser system) where it will travel over 300 meters before finally striking its target. The laser bays are made up of thousands of independently-climate-controlled optics modules that contain the amplifiers and switches that will boost the beams to their full strength.

Those optics modules will contain a total of 3072 neodynium-doped phosphate glass slabs, like the one on the left. These slabs are designed to provide exceptionally low loss of total beam strength. Each beam will pass through an amplifier slab between 16 and 18 times (depending on the final beam energy) on each of its four passes through the amplifier system.

The neodynium-doped glass amplifier slabs are only the substrate upon which the largest commercial flashlamps ever created do their stuff. 7,680 of these enormous flashlamps will each output 30,000 Joules of energy, each, exciting the amplifier slabs. When the beam strikes the excited slabs, the resulting cascade of electrons from high to low quantum states (see the Wikipedia article on the laser) amplifies the laser beam from an initial 10 J to a final energy of 0.1 MJ (a factor of 10,000 or so).

In many ways the lynchpin of this entire system is the world’s largest optical switch, like the one shown on the left. The beam first passes through the switch while it is open and into the amplifier. Then, the switch is closed to reflect the beam back through the amplifiers for a 3rd and 4th pass. The Plasma Electrode Pockels Cell optical switch is made up of two extremely thin crystals of potassium di-hydrogen phosphate (KDP) with gas plasmas in between. When charged, the plasmas effectively turn the two plates of KDP into a nearly perfect mirror. While neutral, the cells are transparent. They can make this transition in about 100 nanoseconds.

From the amplifiers, the 192 beams travel all the way back through their beam paths toward the target chamber. At this point, the beams enter the final optics assemblies (the exterior of which is the red accordion-like structure shown jutting up diagonally from the ground in this picture). The final optics assemblies are responsible for converting the infrared laser beam (at 1.053 microns) into an ultraviolet beam (0.351 microns), and do so with an efficiency greater than 80%. This achieved in two steps. First, the beam passes through a KDP crystal that when illuminated at 1.053 microns emits a beam at 0.53 microns. This beam then shines on a second KDP crystal that shortens the wavelength to the final 0.351 microns.

Now an ultraviolet laser, the beams enter the target chamber (the blue sphere surrounded by red pipes in the lower right of the cutaway model above) via 48 entry ports that are shown above on the right as the large blocky white-ended structure jutting out at various angles from the bottom of the central blue sphere. This target chamber (as a picture below shows) is enormous; it is over 30 meters in diameter. Nevertheless, the positioning system shown protruding into the target chamber on the left precisely places the target (inside the hohlraum) at the focus of the 192 beams.

The hohlraum is a vital feature of the NIF, but not of all inertial confinement fusion systems. The gold-plated holhraum both holds the target and converts the ultraviolet laser beams into x-rays. These x-rays are capable of heating the target much more rapidly and efficiently than the laser alone. They also absorb some of the energy of the lasers, and from the resultant implosion. They are one of the (many) features that stand in the way of commercially-viable inertial confinement fusion. However, for the purposes of the NIF, which is largely designed to study fusion processes in order to maintain the nation’s nuclear stockpile, the hohlraum design is the last crucial engineering step.

Big (and Small) Science

Millions of joules of laser energy generated by a system the size of several football fields focus down on one millimeter-sized target at the NIF. This is definitely big science. The project was initially estimated to run about $700 million, but is now expected to cost between $3.5 and $6 billion when complete.

Much of the cost is consumed by the enormously expensive amplifier system, but large sums have been necessary in order to create such a massive system in which everything is so precisely timed. Early tests have shown that the beams will be capable of arriving within 6 trillionths of a second of each other (6 picoseconds). This means that the variation in beam path length must not be greater than 180 microns (or 0.180 millimeters) out of a total length of over 300 meters! Also, the timing of the initial laser injection system oscillators must be fantastically precise. This demands precision electronics, engineering, construction, and operation. As with most things in life, that extra 1%, or 0.001% in this case, consumes 99% of the costs.

If the NIF works as planned, it will have shattered all sorts of records and led to groundbreaking new developments in the fields of optics and lasers. Practical applications will no doubt result, but its most important objective is currently the most controversial. It may be a key element in helping to maintain a nuclear test ban treaty, while also assuring our stockpile remains safe. However, the second stage of a thermonuclear weapon is also the most stable (the first stage is the fission detonator than compresses the second stage D-T fusor). So questioning the constructing the NIF at such tremendous cost seems entirely valid. That does not, however, need to prevent us from being impressed by it.