Peering inside an artificial sun

Jan 29, 2010 David L. Chandler
A tiny chamber made of gold, called a hohlraum, is used to contain the pellet of heavy hydrogen fuel at the center of a fusion reaction at the National Ignition Facility. Laser beams enter through the two open ends of the hohlraum and are reflected in toward the fuel, heating it up to produce the fusion reaction. Image: National Ignition Facility

(PhysOrg.com) -- After more than five decades of research, a major milestone toward the harnessing of fusion power is expected within the next year or two. This milestone, known as "fusion ignition," should take place at an experimental facility built for that purpose in California. Known as the National Ignition Facility, or NIF, it started initial experiments last fall.

Researchers at MIT’s Plasma Science and Fusion Center (PSFC) have played an important part in making this pivotal event possible, and that role is outlined this week in a paper published in the journal Science. In a nutshell, they’ve figured out how to use a second as a kind of backlight, allowing them to see the details of what’s happening inside the primary reaction.

Fusion, the merging of two small atoms into one with a prodigious release of energy, is the process that powers the sun, and is seen as a potential long-term solution to the world’s energy needs because in principle it could supply vast amounts of energy without any . But the practical harnessing of this powerhouse is thought to remain decades away.

Achieving ignition would represent an important and long-sought step in that direction. One problem for the researchers and engineers trying to make it happen, though, is that the actual reactions would be taking place inside a 2-mm diameter fuel capsule whose temperature and pressure, as it implodes to 1/40 its initial diameter, become much greater than those at the very center of the sun. That’s not an easy environment for taking pictures, or any kind of measurements, in order to fine-tune the system to achieve the desired results.

An MIT team led by PSFC Senior Research Scientist Richard Petrasso developed the fusion backlighting method, which was described in a paper in Science in 2008. Now, the team is reporting in Science that they successfully used the method in a test facility at the University of Rochester, and were indeed able to learn important details about the nature of the electric and magnetic fields in and around this tiny capsule.

With the system they devised, “we’re taking a snapshot of what these electric and magnetic fields look like,” Petrasso says. “This is information is very difficult if not impossible to obtain any other way.”

Providing the ‘sparkplug’ for fusion

NIF uses an approach called indirect drive inertial fusion, in which the tiny capsule of heavy hydrogen fuel is centered inside a cavity called a hohlraum. Laser beams bombard the inside walls of the hohlraum, heating it and generating x-rays that cause the capsule to implode. Ignition, the goal of the NIF, means the point at which the energy released by some fusing atoms at the center of the capsule provides the “sparkplug” that causes other surrounding super-dense atoms to fuse, and so on, in a chain reaction.

But to get to the point of ignition, Petrasso explains, diagnostic tools are needed to reveal the details of what actually happens inside the imploding pellet, where temperatures reach 200 million degrees Kelvin and the pressure can reach a trillion times atmospheric pressure. In order for the ignition to work, the capsule of deuterium and tritium — two heavy forms of the element hydrogen — must be nearly perfectly spherical, nearly perfectly placed at the center of the hohlraum cavity, and must implode in a nearly perfectly symmetrical way.

How much room for error is there in these parameters? That’s one of the things that remains to be determined, and that’s why ways of peering inside the system while it’s in action could play an important role, Petrasso says.

To do that, in these experiments at the Laboratory for Laser Energetics in Rochester, a second capsule was placed nearby and hit by another set of laser beams, producing a flash of protons to illuminate the first capsule, inside a hohlraum.

Nelson Hoffman, a physicist at Los Alamos National Laboratory, says the MIT team has developed “several very effective ways” of measuring important aspects of what goes on inside the fusion capsules, which he says are essential to know “as an indicator of how close they are to the ignition goal.” He adds that as a result, the MIT team has already found surprising phenomena in the way the electric and magnetic fields are distributed.

“The quest to achieve is one of the hardest scientific problems ever tackled,” Hoffman says, “so looking at the problem with ‘new eyes,’ like MIT's proton radiographs, is crucial for detecting phenomena that don't show up any other way.”

In the results being reported this week, for example, the MIT team along with collaborators from Lawrence Livermore National Laboratory, the Laboratory for Laser Energetics, and General Atomics, saw results from one experiment that produced a striking “five-pronged, asterisk-like pattern” in the fields surrounding the imploding capsule. The pattern results from the positioning of the incoming laser beams — something that will require further analysis to understand its potential impact on fusion dynamics.

Petrasso expects that at NIF it will take many months from the beginning of the experiments until the point where ignition is achieved. “This has never been done before, so we need to rely in part on empirical knowledge as we bring the experiment up and create the precise conditions that are required,” says Petrasso, who has worked at the PSFC since 1978. “Many parts of this effort have a sound experimental and theoretical basis, and some less so. Because of this, we need to fill in those gaps, to get the conditions just right.” In addition to this work at the Rochester facility, MIT researchers including six doctoral students have had an ongoing role in the work at the NIF.

Not only could ignition be an important step toward perhaps making practical one day, he says, but it will certainly be an important scientific tool for better understanding how the sun and other stars work.

“You create conditions which you really only can find in the centers of stars,” he says, even though in these experiments those conditions only exist for a few billionths of a second. “Astrophysicists, for one, will find these conditions extremely interesting and compelling.”

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User comments : 27

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Yellowdart
2.9 / 5 (7) Jan 29, 2010
Does this work like video games made by Blizzard? They say a year or two, but in reality they mean 5 to 10? :)
sender
not rated yet Jan 29, 2010
AuXe compounds sound perfect for a hohlraum
jj2009
1.3 / 5 (8) Jan 29, 2010
i feel more confident about these experiments, where they at least have some idea what is going to happen than with the LHC, where they are just going to switch it on and see what happens, because nobody really knows.
Rynox77
not rated yet Jan 29, 2010
Does this have a practical use as a weapon of war and alternative to thermonuclear warheads? Does this reaction create radioactive by-products?
LariAnn
1 / 5 (1) Jan 29, 2010
My understanding is that this quest for a method of fusion ignition is precisely to avoid the use of a radioactivity-generating way of igniting it. The hydrogen bomb uses a small fission bomb as an ignitor; were that not necessary, the hydrogen bomb would be radiation-free.
Thrasymachus
5 / 5 (1) Jan 29, 2010
Deuterium/tritium fusion produces helium-4 and a free neutron, both with quite a bit of kinetic energy. I'm pretty sure those count as radiation.
finitesolutions
not rated yet Jan 29, 2010
Perfect! I think we will nail this one down. The future is electric. At last an inexhaustible source of electricity. I do want to fly on an commercial electric plane without pollution guilt.
brant
not rated yet Jan 29, 2010
z-pinch, focus fusion and the Bussard fusion system have already show fusion signatures.

z pinch has already shown 2 BILLION degrees!

You are never going to get that from lasers, Tokamak, NIF or anything else.

Electrically accelerated fusion is the way.
Bob_Kob
1 / 5 (2) Jan 29, 2010
Whys it made out of gold? Why not make it fuelled by childrens tears then lol.
sender
not rated yet Jan 30, 2010
Electrically accelerated fusion is the way.


Infact it might make good shielding systems for the NIF ignition chamber, question left is ways to harness for energy? Boiling Noble gases, Superconducting metals, Thermodynamic systems, Radio senstive piezoceramic polymers, Direct charge convertors, the list continues etc etc.
PMende
5 / 5 (2) Jan 30, 2010
For those who don't know, this project was originally proposed as a way to study the aging nuclear stockpile. The notion of studying it for possible future energy purposes only became a thought after the initial proposal.

The system does NOT use gold as its fuel. Inside the gold cylinders are small spheres that have I believe 3 different different zones: there is the outer layer, which surrounds a layer of deuterium, which itself encases a center sphere of tritium. When the lasers hit the surface, the outer layer contracts extremely rapidly. When the shockwave strikes the deuterium layer, the outer layer bounces off, and completely sheds itself from the sphere. The deuterium then contracts onto the tritium core, which causes an ignition event, which blows the gold canisters.

Brant: why would we want or need a system that achieves a 2billion degree temperature? We don't have the materials to directly harness the 200million degree temp. differentials from NIF, much less 2billion.
Husky
not rated yet Jan 30, 2010
While amazing recent progress has been made towards peta / exawatt lasers, grid input power to output efficiency has to be improved, my bet is still on the slightly lower tech Z-pinch as the first to finish as a commercial attractive reactor, remind me to reread this post in 5 years, maybe i will be proven wrong...
Husky
5 / 5 (1) Jan 30, 2010
Bob, perhaps the gold is chosen for being dense/high inertia, will provide a good squeezing time allowing sufficient reactions to happen before the pellet would fly appart. One of my thoughts is to experiment with depleted uranium hull, even denser, increasing fusion lockup time (or how do they call it?) and perhaps a few added catalysed fission reactions/heat by neutrons hitting the DU, wouldn't be totally clean fusion, but would be an interesting experiment. Likewise how about Z-pinch with DU instead of tungsten wires?
daveib6
not rated yet Jan 30, 2010
Husky, Bob,
My understanding is that the band gap of the ionized gold atoms matches the xray frequency they are after to reflect back the shock wave produced by the initial laser pulse. It is by this means that the implosion takes place. The precise band-gap characteristics of Gold match the needs better than other elements.
Any one else an expert on this?
daveib6
not rated yet Jan 30, 2010
Ignition, the goal of the NIF, means the point at which the energy released by some fusing atoms at the center of the capsule provides the "sparkplug" that causes other surrounding super-dense atoms to fuse, and so on, in a chain reaction.


Wrong: Ignition is the condition in which the resulting reaction releases more energy than the energy that was input in order to cause the reaction. It is most definitly NOT an indefinite chain reaction, which is called a Nuclear Bomb!
StarDust21
not rated yet Jan 30, 2010
Does this work like video games made by Blizzard? They say a year or two, but in reality they mean 5 to 10? :)

hehe, hope not, when was starcraft 2 supposed to be out yet? like a decade ago?
NeilFarbstein
not rated yet Jan 30, 2010
Bob, perhaps the gold is chosen for being dense/high inertia, will provide a good squeezing time allowing sufficient reactions to happen before the pellet would fly appart. One of my thoughts is to experiment with depleted uranium hull, even denser, increasing fusion lockup time (or how do they call it?) and perhaps a few added catalysed fission reactions/heat by neutrons hitting the DU, wouldn't be totally clean fusion, but would be an interesting experiment. Likewise how about Z-pinch with DU instead of tungsten wires?

Dont DU it, it will create a radioactive waste problem.
NeilFarbstein
not rated yet Jan 30, 2010
While amazing recent progress has been made towards peta / exawatt lasers, grid input power to output efficiency has to be improved, my bet is still on the slightly lower tech Z-pinch as the first to finish as a commercial attractive reactor, remind me to reread this post in 5 years, maybe i will be proven wrong...


solaid state picosecond lasers are commercially available at 60% efficiency. I dont know what amount of power that can develop in a picosecond. Does anyone know?
refamat
not rated yet Jan 31, 2010
Some people obviously do not understand what radiation is. The visible spectrum can kill as well. And it's also apparent that some cannot read.
Alburton
not rated yet Jan 31, 2010
Anyone else finds these know-it-all remarks about radiation pointless?
Radioactive waste is a serious problem,and the fact that the word ionizing is not attached to the usual frasings of it does not make it any less harmful.
PMende
5 / 5 (1) Jan 31, 2010
The NIF facility has floated the idea of encasing the ignition chamber in the non-usable portions of spent nuclear fuel to be cycled in and out at appropriate times. The free neutrons would be captured by these long lived isotopes, transforming them into shorter lived ones that can be stored more effectively and with less danger.

The alpha radiation is completely harmless, as I hope you know. I hope no one is eating or breathing in the contents of the ignition chamber while it's hot. It would probably sting a bit.
antialias
not rated yet Feb 01, 2010
The alpha radiation is completely harmless, as I hope you know.

Wrong. Alpha radiation is the most harmful (since an alpha has much more energy than a beta or a gamma because of its much larger mass). The only saving grace is that alpha radiation is the easiest one to block (you can block a sizeable proportion by a piece of paper).

But swallowing or breathing in something that emits alpha radiation is _really_ bad news. Much worse than ingesting a beta or gamma radiation source of similar activity (in becquerel).
broglia
1 / 5 (1) Feb 01, 2010
I don't think, laser fusion is viable source of thermonuclear energy. Indeed, it's a still more safe and perspective research, then the LHC - especially with respect to antimatter production.

http://arxiv.org/pdf/0908.4229
PMende
not rated yet Feb 01, 2010
The alpha radiation is completely harmless, as I hope you know.

Wrong. Alpha radiation is the most harmful (since an alpha has much more energy than a beta or a gamma because of its much larger mass).

You see, it sometimes pays to read the whole of what someone says instead of just reading one thing and responding to that:

I hope no one is eating or breathing in the contents of the ignition chamber while it's hot. It would probably sting a bit.


When I say "sting a bit," I mean "completely obliterate your entire body" since you would immediately combust upon entering the ignition chamber. You'd have more important things to worry about than ingesting or breathing in alpha radiation if you were in there.
LuckyBrandon
1 / 5 (1) Feb 01, 2010
wait wait wait, there was just a story last week ON THIS SITE that they already achieved this...for something like 13 billionths of a second, but it gave off something like 5x more energy in that multi billionth of a second than it took to get the reaction started...
Javinator
not rated yet Feb 05, 2010
Wrong. Alpha radiation is the most harmful (since an alpha has much more energy than a beta or a gamma because of its much larger mass). The only saving grace is that alpha radiation is the easiest one to block (you can block a sizeable proportion by a piece of paper).


Alphas only travel a few centimeters in air. They're really the least of your worries wrt radiation. Most smoke detectors have alpha emitters in them.
zbarlici
not rated yet Feb 06, 2010
NIF fusion is just a fron for weapons development studies. Theyre going this route in hopes to get more funding.