How scientists brought the power of the Sun to Earth
The super-amplified light impacts the target area with an intensity and ferocity only found in the hottest places in the universe. Imagine 500 times the energy of the entire United States being used at any given moment focused on a target smaller than a pinhead!
In a few billionths of a second, the target reaches temperatures well over 100 million degrees and pressures billions of time greater than Earth’s atmosphere. There’s only one other place in our solar system this extreme, and that’s at the very core of our own Sun.
This last Thursday, Jan. 28th, 2010, for the briefest of moments, “the Sun” came to the Earth in Livermore, California.
Well, almost! The jubilant scientists at Lawrence Livermore National Laboratory didn’t create a self-sustained nuclear fusion reaction, but they are closer than ever to this goal, which would give humanity virtually unlimited, pollution-free energy.
The star-making process in a nutshell
So, what exactly happened? How did the scientists recreate the conditions of at the center of a star here on Earth? Here’s what the Press Release from the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory said:
In order to demonstrate fusion, the energy that powers the sun and the stars, NIF focuses the energy of 192 powerful laser beams into a pencil-eraser-sized cylinder containing a tiny spherical target filled with deuterium and tritium, two isotopes of hydrogen. Inside the cylinder, the laser energy is converted to X-rays, which compress the fuel until it reaches temperatures of more than 200 million degrees Fahrenheit and pressures billions of times greater than Earth’s atmospheric pressure. The rapid compression of the fuel capsule forces the hydrogen nuclei to fuse and release many times more energy than the laser energy that was required to initiate the reaction.
Whew, OK! That’s quite a mouthful! And that’s a lot of scientific jargon. For a lot of people, their image of what physicists and engineers do actually do owes more to science fiction than science fact:
But actually, what scientists do is way more cool than popular imagination. Let’s look at that press release again, but let’s “unpack” it and add some helpful pictures. It’s really interesting to understand just how the scientists accomplished this amazing feat of science and engineering.
[Note—There's simply not space to define every scientific concept, but anyone with a knowledge of high-school physics should be able to grasp the fundamentals. I've also make Wikipedia links to keywords when it's helpful.]
Fusion—that’s what it’s all about!
“In order to demonstrate fusion…”
Nuclear fusion is, as the name implies, the fusing together of atoms. In simplistic terms, it’s the opposite of nuclear fission, the splitting of atoms. The first atomic reactor, and the first atomic bomb, all relied on the fission of uranium atoms.
The first hydrogen bomb used the heat and pressure of an uranium bomb to create the heat and pressure necessary to fuse together isotopes of hydrogen, creating a form of helium and releasing a neutron and a lot of energy.
Simple enough, but the scientists want to create controlled fusion, and not blow themselves up in the process. How are they going to get enough heat without a rather messy fission explosion? With light. Lots of light.
How did they do it? Light, lots of light!
So how did the scientists at Livermore achieve such incredible temperatures and pressures? With light. But, isn’t light, well, kind of light? After all, we’re not all knocked flat when the Sun rises, even if the sunlight does hit us at 186,000 miles per second!
But light does create pressure. This is easy to grasp if we think of light as particles, or photons, instead of waves (light is both, but let’s not get into quantum physics). Photons transfer their momentum to what they strike, not unlike a ball hitting a wall. At a temperature of boiling water, (100 °C or 212 °F), light exerts about 2 pounds of force per square mile.
Now, imagine temperatures of hundreds of millions of degrees, instead of 212 degrees. And instead being spread over a square mile, let’s focus all that energy on a pinhead. That’s how the Livermore scientists got so much pressure, and so much, heat, in one spot.
So the next question is: how do you get that much souped-up light and focus it on one tiny spot? Answer: lasers. But not just any lasers; these are the some of most powerful individual lasers on the planet. And at NIF, they combine 192 of them to create one super-laser.
The most powerful lasers on Earth
“…NIF focuses the energy of 192 powerful laser beams…”
Again, there’s not enough space to explain how lasers work, but recall that laser is an acronym for Light Amplification by Stimulated Emission of Radiation.
The NIF laser is described as “the world’s largest and highest-energy laser,” but in fact, it is 192 individual lasers working as one perfectly synchronized unit. This photo from NIF shows the inside of the enormous chamber where all the individual lasers converge on the tip of what looks like a giant pencil:
Here’s what happens to the light before it enters this chamber (again, I won’t attempt to explain the physics of a laser here.)
A single initial pulse of light is sent down 192 separate lasers a distance of 1,500 meters (over 4,900 feet). All along the light guides, the laser light is repeated given powerful boosts of energy (the “amplified” part of L-A-SER). From beginning to end, the laser beams’ total energy is boosted more than a quadrillion times—that’s—1,000,000,000,000,000,000,000,000 times—in about 5 millionths of a second.
All that energy smashes into a pinhead-sized target
“… into a pencil-eraser-sized cylinder containing a tiny spherical target filled with deuterium and tritium, two isotopes of hydrogen…”
This incredible avalanche of cascading photons all comes to a focus in the enormous target chamber, converging on a little cylinder made of gold called a hohlraum:
The hohlraum is a tiny gold capsule containing a pellet made up of atoms of deuterium (hydrogen with one neutron) and tritium (hydrogen with two neutrons.) These atoms are what fuel the fusion ignition process.
Sounds easy right, just fire up those lasers up and let her rip! But in fact, it takes incredible accuracy to get all that light focused and synchronized to arrive at the same time.
After passing through nearly 60 miles of fiber optics, mirrors, crystals, and amplifiers, all the photons have to arrive within a trillionth of a second of each other. They also have to strike within 50 micrometers of the target. As the NIF website says:
“NIF’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.”
That deserves some kind of physics Cy Young award, don’t you think? And now that all that energy has arrived, the fun really begins. It’s fusion time!
A star is born—almost!
“…Inside the cylinder, the laser energy is converted to X-rays, which compress the fuel until it reaches temperatures of more than 200 million degrees Fahrenheit and pressures billions of times greater than Earth’s atmospheric pressure…”
Here’s what’s different about the NIF method. In the past, laser fusion was obtained by having multiple lasers directly strike the target and crushing it into fusion. This was called the “direct drive” method.
At NIF, they are doing something called “indirect drive.” Instead of directly hitting the tiny pellet, the lasers hit the sides of the gold hohlraum.
This creates a super-hot plasma that uniformly radiates the target with powerful x-rays. The x-rays instantly heat the outer surface of the pellet, causing a high-speed blow-off called ablation:
As we know from high-school physics, for every action, there is a reaction, equal in force and opposite in direction. As the surface of the pellets blows outward, the interior of the pellet is accelerated inward. The pellet implodes under star-like pressures.
Before the compressed pellet can be vaporized by the plasma, the deuterium and tritium atoms fuse, or “ignite,” as physicists like to say. Enormous energies are released. A star is born!
The pay-off—the promise of unlimited clean energy
“The rapid compression of the fuel capsule forces the hydrogen nuclei to fuse and release many times more energy than the laser energy that was required to initiate the reaction.”
More energy out than you put in—that’s been the “holy grail” of fusion research from the beginning. And last Thursday, in Lawrence, California, scientist attained that goal. For the first time, a laser produced more energy than the energy it took to produce the fusion. Experiments in the “indirect method” of fusion were key to this breakthrough.
Of course, a huge amount of research and development lie ahead. The super-laser at NIF wasn’t designed to produce electricity. It was meant to solve engineering and physics problems and show what is feasible. It cost tens of millions of dollars and pushed engineering to the limits of present knowledge.
Significant engineering problems remain in terms of sustained energy production. A future fusion power plant would need to “ignite” several pellets per second for significant power production.
Nonetheless, the future looks bright. There are almost unlimited amounts of deuterium in seawater, and tritium can be created in nuclear reactors by transmutation of lithium, a common element in rocks and soil.
Given the problems of fossil fuels, pollution, climate change, and mankind’s near insatiable demand for energy, viable fusion energy can come none to soon!
So kudos to the creative, brilliant men and women at NIF and the Lawrence Livermore National Laboratory. Future generations may well look back on them as heroes whose breakthroughs helped mankind step back from the brink of environmental and economic disaster. They are certainly heroes in my book!
NOTE: You can watch great video animations of the NIF super-laser at the Lawrence Livermore National Laboratory website:
Wired Science Video has an animation at this link: