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Nuclear Fusion


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« on: February 13, 2014, 02:54:35 pm »


From the Los Angeles Times....

Nuclear fusion? Laser-wielding physicists find promising hints

By AMINA KHAN | 2:39PM PST - Wednesday, February 29, 2014

An image of the tiny gold can (known as the hohlraum) with a cutaway showing a fuel capsule inside. Scientists at Lawrence Livermore National Laboratory conducting laser-based nuclear fusion experiments have seen more energy come out of their deuterium-tritium fuel than was put in, they report in Nature. — Photo: Lawrence Livermore National Laboratory.
An image of the tiny gold can (known as the hohlraum) with a cutaway showing a fuel capsule inside. Scientists at Lawrence Livermore National Laboratory
conducting laser-based nuclear fusion experiments have seen more energy come out of their deuterium-tritium fuel than was put in, they report in Nature.
 — Photo: Lawrence Livermore National Laboratory.


SCIENTISTS have been trying to harness the secrets of star power since the days of the Cold War. Now, using a 2-millimeter capsule housed in a cavernous building roughly the length of three football fields, researchers have conducted a landmark experiment, using lasers to create the merest hint of controlled nuclear fusion that produced more energy than was put into the fuel alone.

“This is really an important milestone,” said UCLA plasma physicist Warren Mori, who was not involved in the work.

It's a long, long way off from "ignition", the point at which the reaction generates more energy than the powerful laser beams originally poured into the tiny fuel capsule — and there are some significant hurdles to getting to that point, scientists said.

But the findings, described in the journal Nature, give researchers a promising sign that they're on the right path to reaching this goal, one that could one day lead to cleaner nuclear energy, safer weapons and a more profound understanding of powerful astrophysical phenomena such as stars and supernovae.

“We are closer than anyone has gotten before,” said lead author Omar Hurricane, a plasma physicist at Lawrence Livermore National Laboratory.

Nuclear fusion is the process of fusing two atoms’ nuclei together to create a heavier atom, releasing an incredible amount of energy in the process. In some ways it's the reverse of nuclear fission, which breaks large atoms apart into smaller pieces and releases energy (and a significant amount of radiation). Fission is the process that nuclear power plants use today.

Nuclear fusion is what powers the cores of stars and generates their light; all that intense heat and pressure at their centers can force two atoms to join together and become a whole different element. Stars’ cores begin by fusing two hydrogen nuclei into a single helium atom, and then three helium nuclei into a single carbon, and so on, progressing to heavier and heavier elements.

On Earth, scientists hope to do the same thing, but using two “heavy” hydrogen atoms, deuterium and tritium, as nuclear fuel. Deuterium is a hydrogen atom loaded up with a neutron, and tritium packs two extra neutrons. When they fuse together, they end up as a highly energetic helium atom (2 protons plus 2 neutrons) and a separate spare neutron.

In theory, those energetic helium nuclei should further heat the already-hot, pressurized deuterium-tritium fuel and cause more helium nuclei to form, triggering a runaway reaction. It’s a little like lighting a match to some firewood — you don’t have to put a lot of effort into making it burn if you have the right match and the right wood under the right conditions.

The problem for nuclear fusion is, unless you’re inside a star, it's very difficult to create those conditions — high pressure, high temperature — to get the process going.

“Mother Nature is pretty unforgiving — we're trying to stuff a lot of energy in a very small volume,” Hurricane said.

Scientists have been trying for decades to figure out how to get a little bit of mass so compact that they could induce nuclear fusion on Earth. There are two main methods, said Mark Herrmann, a plasma physicist at Sandia National Laboratories in Albuquerque. Magnetic confinement fusion uses a magnetic field to contain and compact a plasma fuel, and inertial confinement fusion uses lasers to compact and heat up a tiny jot of fuel.

The scientists working on the experiment at the Livermore lab’s National Ignition Facility had been using the laser-induced method. It's no easy task: The method requires a cavernous facility holding a formidable array of 192 lasers to shoot high energy beams through two tiny holes on either end of a pill-size gold can that is just one centimeter long and half a centimeter wide. When the lasers go through the entry holes and hit the gold can’s inside walls, the inside surface starts to give off X-rays, which bathe an even tinier plastic capsule just 2 millimeters wide. This tiny plastic grain is lined inside with deuterium-tritium fuel, a lining just 70 microns thick, barely the width of a human hair.

The X-rays beat down on the plastic rind of the 2-millimeter capsule and cause it to explode, pushing outward even as its contents are pushed inward, squeezing down to a tiny fraction of its original size. At this point, it’s very hot and very dense. With just the right series of shocks from the laser beam, the scientists think they can cause the fuel inside to start fusing together.

But there’s a fundamental problem here: The fuel needs to be under extreme pressures to ignite, but Mother Nature “fights you on it,” Hurricane said. Instabilities start to form as the mass is compacted — and the scientists have to be able to control it precisely, to make sure the X-rays are hitting the capsule’s surface in a perfectly even way.

Imagine trying to squeeze a water balloon, Mori said: If you squeeze on one end alone, the water moves to the other side. And this can keep them from getting that fuel to the right state to start fusing.

The scientists had originally tried to deal with this by hitting it with lower-energy laser pulses, to avoid heating the fuel too much and causing it to become unstable. But this hadn’t been working. Instead, they did something counterintuitive: They upped the energy from that first laser blast, which actually seemed to stabilize it, in part because the stronger X-rays seemed to “smooth out” some of the ripples that first formed, even if it made the fuel hotter than was ideal.

After two runs of the experiment, the scientists measured the energy output from the “burnt” fuel. The first one resulted in 14,400 joules of energy released, even though at most the deuterium-tritium fuel had only 12,000 joules to start. Clearly some of the fuel had been fusing into helium and releasing extra energy. The second experiment was even better: 17,300 joules of energy out for just 9,400 joules in.

The pressures at the center of this implosion are in the billions of atmospheres, several times the density at the center of the sun. And it only lasts for about a seventh of a billionth of a second, Hurricane said.

These are just baby steps; the reaction caused only a fraction of the fuel to fuse and did not manage to release more energy than the lasers put into it — they can pour in 1.8 million joules, so 99% of the energy is still being lost in the process. And there may be limits to the hard-and-fast method the scientists used to stabilize the fuel as they compacted it: Because they make it hotter in the beginning, it becomes harder to squeeze it down to the pressures they need to (theoretically) fully ignite the fuel.

But it’s a start — one that points scientists down a path to potentially reaching ignition and perhaps one day making nuclear fusion a practical reality, scientists said.

“I think it’s a promising advance,” said Herrmann, who was not involved in the research. “I was excited.”

Nuclear fusion would be a cleaner source of energy than the current nuclear power plants, scientists said. And it could prove useful for understanding astrophysical phenomena. It could also help scientists understand how to protect and maintain the U.S. nuclear weapons arsenal without performing full scale nuclear tests (which the United States stopped doing in 1992).

As for whether and when they may achieve ignition, the researchers demurred. Hurricane compared it to climbing a mountain whose peak was shrouded in clouds, with no clear, straight path up. For the moment, he said, they'd made it to a scientific base camp, where they could now survey the terrain and plan their next move.


http://www.latimes.com/science/sciencenow/la-sci-sn-nuclear-fusion-lasers-energy-20140212,0,1472661,full.story



From the Los Angeles Times....

Nuclear fusion reactions mark a ‘milestone’

Physicists create nuclear fusion reactions that produce more energy
than was in the fuel involved — the power at work in the sun and
other stars. It may be a step toward cleaner nuclear energy.


By AMINA KHAN | 6:11PM PST - Wednesday, February 29, 2014

IT TOOK 192 lasers and a building big enough to contain three football fields, but physicists have finally produced a pair of nuclear fusion reactions that created more energy than was in the fuel to start with.

The reactions lasted less than a billionth of a second, and they released only a few thousand joules — enough to power a 100-watt light bulb for less than three minutes. But it marks the first time scientists have been able to harness the power of stars here on Earth.

"This is really an important milestone," said Warren Mori, a plasma physicist at UCLA who was not involved in the effort.

The experiment, conducted at Lawrence Livermore National Laboratory in the Bay Area, is still a very long way from "ignition", the point at which the reaction generates more energy than was required to kick it off with lasers. Scientists agree that significant hurdles remain before that goal can be reached.

But the tests, described Wednesday in the journal Nature, give researchers a promising sign that they're finally on the right path to reaching this goal — one that could ultimately lead to cleaner nuclear energy, safer weapons arsenals and a more profound understanding of astrophysics.

"We are closer than anyone has gotten before," said study leader Omar Hurricane, a plasma physicist at Lawrence Livermore National Laboratory.

Nuclear fusion is the process of combining the nuclei of two atoms to create a heavier atom, releasing an incredible amount of energy in the process. It's what powers the stars and generates their light.

In some ways, it's the reverse of nuclear fission, which releases energy — and a significant amount of dangerous radiation — by breaking large atoms into smaller ones. Fission is used in nuclear power plants today.

Hurricane and his colleagues used a comparatively simple fusion recipe. For their fuel, they used two "heavy" hydrogen isotopes: deuterium (which has one proton and one neutron in its nucleus) and tritium (which has one proton and two neutrons). When they fuse together, they create a single atom of helium (two protons and two neutrons), along with a spare neutron — and a massive amount of energy.

In theory, that should prompt more atoms in the deuterium-tritium fuel to merge, triggering a chain reaction. It's similar to dropping a match on a pile of kindling — if you have the right match and the right wood under the right conditions, it's easy to make it burn.

But outside of a star, it's very difficult to create the high pressure and high temperature needed to get the process going.

"Mother Nature is pretty unforgiving," Hurricane said.

The scientists at the Livermore lab's National Ignition Facility tried to simulate the conditions in a star by using a formidable array of lasers to squeeze down and heat up a tiny jot of fuel. The lasers, arrayed around a cavernous building, shot high-energy beams through two tiny holes on either end of a pill-sized gold can.

When the lasers hit the can's interior walls, the surface gave off X-rays that bathed an even tinier plastic sphere about the size of a small bead. The sphere contained a layer of deuterium-tritium fuel that was just 70 microns thick, barely the width of a human hair.

With a precise combination of shocks from the laser beams, the pressure inside the sphere was several times greater than at the center of the sun. It lasted for about a seventh of a billionth of a second, long enough for the fuel inside to start fusing together, Hurricane said.

After two runs of the experiment, the scientists measured the energy output by tracking the energy level of the spare neutrons flying out from the tiny sphere. The first one released 14,400 joules of energy, slightly more than the 12,000 joules contained in the deuterium-tritium fuel to start with. The second run was better: 17,300 joules out for just 9,400 joules in.

Neither of these equations account for the 1.8 million or so joules of energy that streamed in from the lasers; when they're taken into account, 99% of the reaction's total energy was lost.

But it's a start — one that points scientists down a path toward ignition and perhaps making nuclear fusion a practical reality, scientists said.

"I think it's a promising advance," said Mark Herrmann, a plasma physicist at Sandia National Laboratories in Albuquerque who was not involved in the study. "I was excited."

Nuclear fusion would be a cleaner source of energy than the current generation of nuclear power plants because it produces relatively short-lived radioactive byproducts. It could also help scientists understand how to protect and maintain the military's nuclear weapons arsenal without performing full-scale nuclear tests, which the United States stopped doing in 1992.


http://www.latimes.com/science/la-sci-nuclear-fusion-20140213,0,5629190.story
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