The 192 lasers of the National Ignition Facility at Lawrence Livermore National Laboratory in California can focus 500 trillion watts of power onto a pellet of hydrogen fuel the size of a pencil eraser. With full-scale experiments slated to begin soon, we’ll learn much about the feasibility of nuclear fusion on Earth, hoping to extract more energy from the process than goes into making it happen. The forms of hydrogen at play here are deuterium and tritium, which fuse to form helium.
Image: All of the energy of NIF’s 192 beams is directed inside a gold cylinder called a hohlraum, which is about the size of a dime. A tiny capsule inside the hohlraum contains atoms of deuterium (hydrogen with one neutron) and tritium (hydrogen with two neutrons) that fuel the ignition process. Credit: National Ignition Facility.
Inertial confinement fusion using lasers is a different approach than the magnetic confinement method used at the International Thermonuclear Experimental Reactor (ITER), currently being built in Cadarache, France. There, super-heated gas is managed via magnetic fields inside a vessel called a tokamak.
But as multi-track fusion studies continue, it’s interesting to see how the National Ignition Facility will also serve as a laboratory for astronomers hoping to understand the physics of exploding stars. At full power, the NIF lasers will throw a 1.8 megajoule punch at the target. Energies like this, according to a recent BBC story on the NIF, will create temperatures of 100 million degrees and pressures billions of times greater than Earth’s atmospheric pressure, forcing hydrogen nuclei to fuse.
But adjusting the elements in the fuel pellet also sets up experiments that mimic a stellar core. The result is a mini-supernova, says Paul Drake (University of Michigan):
“You choose the material and the structures between them to be relevant to what happens when the star explodes. The laser would strike the centre – the analogue of the core of the star – launching a tremendously strong shock wave that would blow the material apart.”
All this occurs, of course, in billionths of a second, so that the results have to be scaled to the actual astrophysical environment. Nonetheless, ‘supernova’ experiments like these could be productive in helping us understand the stellar explosions that produced the elements so crucial for life. That adds a powerful tool to our arsenal, complementing the observing programs that search for supernovae in distant galaxies.
The BBC also talks to David Stevenson (Caltech) about using the NIF to study the formation of gas giant planets. Because the NIF’s lasers can produce pressures equivalent to billions of times what is found at sea level on Earth, we can study conditions that exist inside such planets, where dramatic changes to chemistry occur. The behavior of hydrogen, helium, carbon and water in such a setting should be fascinating.
We already know that hydrogen can become a metallic fluid at much lower pressures. Ray Jeanloz (UC – Berkeley) paints a graphic picture of such materials in a Jupiter-like setting:
“Hydrocarbons would actually decompose to a mixture of hydrogen and a carbon. The end result being that diamonds would actually be hailing out of the atmosphere. That’s the kind of process you would never have guessed unless you had studied the materials themselves.”
The National Ignition Facility will be the world’s largest and highest-energy laser system, delivering more than sixty times the energy of any previous laser facility. But keep your eye as well on the European High Power Laser Energy Research (HIPER) study, which received a funding boost last October. Construction of HIPER isn’t scheduled to begin for a decade or so, but success at NIF could be followed by a HIPER facility aimed at taking inertial confinement fusion to a truly commercial level.