Scientists with the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory announced today that they have achieved a critical step in fusion research: For the first time, their hydrogen fuel has given off more energy than it took in.
Though an important milestone, the result does not mean that your Delorean is soon going to sport a Mr. Fusion reactor. NIF would need to sustain temperatures and pressures much greater than they are currently capable of before they can harness fusion energy.
Nuclear fusion is the energy source of the stars. Deep in our sun’s belly, hydrogen atoms slam into one another at high speed, getting mashed together to form helium atoms and releasing copious amounts of energy. Creating viable fusion energy here on Earth has been a dream since the dawn of the Atomic Age. With true fusion power, the amount of water you use in a single shower could provide all your energy needs for a year. But for six decades, fusion has remained a far-off dream.
To create fusion reactions at NIF, scientists shoot 192 lasers simultaneously with a peak power of 500 trillion Watts, roughly the energy the U.S. consumes every six minutes. This heats up a 1 centimeter gold cylinder to millions of degrees, producing X-rays that get focused at a plastic shell the size of a BB pellet. The X-rays blast the shell, creating an implosion that shrinks the gas inside pellet to 1/35th of its size, compressing isotopes of hydrogen known as deuterium and tritium to incredible densities. At the center of this hydrogen plasma, in an area smaller than the width of a human hair, the atoms fuse. This gives off energy, which should in theory set off a chain reaction that ignites the rest of the hydrogen and creates a self-sustaining ball of fusion.
Because of this convoluted process, only 1/200th of the energy that the lasers generate is imparted to the hydrogen fuel, compressing it enough to produce a small amount of fusion. Until now, the energy given off by the fusing hydrogen hasn’t been enough to set off a chain reaction. The hydrogen fuel also always consumed more energy than it put out. But during experiments late last year, NIF researchers were finally able to get the hydrogen to give off as much as 1.7 times more energy than it had taken in, a result that appears today in Nature. In subsequent experiments last month, the team was able to produce as much as 2.6 times more energy than was put into the hydrogen fuel.
“The physics is a breakthrough,” said physicist Riccardo Betti of the University of Rochester, who was not involved in the work. “If fusion will ever become a viable source of energy, we may look back and say that in 2013, for the first time, a plasma produced more energy out than it took in.”
But the dream of fusion energy isn’t yet a reality. “In terms of making energy to power the grid, it’s still light-years away,” Betti said.
NIF is a $3.5-billion facility that was built to study the dynamics of nuclear explosions for the National Nuclear Security Administration and to test the integrity of the country’s nuclear stockpile without exploding any bombs. After the 1963 Partial Test Ban Treaty, the U.S., Russia, and many other countries agreed to only test atomic bombs underground, and since 1992 the U.S. has placed a moratorium on any nuclear testing. But not being able to physically test the bombs “is like having a car that you’re studying but not allowed to start,” said Livermore Lab physicist Paul Springer, a co-author of the recent fusion results. NIF was the answer to this problem.
When NIF was first being built, researchers were confident that it would produce fusion reactions fairly quickly. The point when fusion becomes self-sustaining is known as ignition. The fusing hydrogen atoms at the fuel center send out helium nuclei, which knock into other hydrogen atoms, setting off a cascading chain-reaction of expansion fusion that should produce more energy than the entire experiment consumes. While ignition requires extremely high temperatures and pressures, computer simulations in 2009 predicted that NIF would achieve the energies to generate it by 2012. Of course, reality doesn’t work as well as a digital model, and the deadline passed without achieving ignition.
Troubles came when scientists found it was extremely difficult to get their hydrogen fuel to compress in the right way. In order to generate the intense pressure and temperatures inside the hydrogen gas needed for fusion, the tiny pellet had to collapse perfectly symmetrically. But small instabilities appearing in the pellet meant that the plasma imploded unevenly, sending fingers of cold gas into the center that doused the fusion reactions.
Over the years, NIF scientists learned from their experiments. They studied the way that the pellet collapsed, and tweaked their designs. They also learned how to time their laser pulses to give the hydrogen the perfect kick. With this knowledge, researchers have been able to go back and improve their simulations, which are now in better agreement with what is physically seen in experiments.
But the latest achievements are still a long way from creating self-sustaining fusion reactions, which would require the hydrogen to reach temperatures of hundreds of millions of degrees and pressures a thousand times more than what is currently possible. The implosion continues to be more of an amorphous blob than a perfect spherical cave-in. To go forward, researchers will have to “make the collapse rounder and more stable against things that cause distortions,” said Springer.
Still, a future with fusion power is starting to look more possible. A European team is also attempting to generate fusion energy at the $20 billion International Thermonuclear Experimental Reactor (ITER) currently under construction in France. That facility will trap superheated hydrogen plasma in a donut-shaped magnetic chamber, an entirely different technique than what has been achieved at NIF, meaning that the lessons from the Livermore Lab won’t be entirely applicable. Rather than set unrealistic deadlines, ITER is moving forward at a very slow and steady pace.
In the meantime, the NIF team is happy with their achievements and cautiously optimistic of their future prospects. “We’ve all been extremely excited about the results that we’ve been getting,” said physicist Denise Hinkel of Livermore Lab, another co-author. “Many people have been waiting for something like this to happen.”
0 comments:
Post a Comment