fusion question

[chebby]

Hi, I have two questions about fusion:

1. I've always thought that hydrogen bomb need a fission device as an initiator. But can't the same pressure be reached using a Voitenko compressor (shaped charge)? On the wiki page it states that it has been used to initiate fusion.
http://en.wikipedia.org/wiki/Voitenko_compressor

2. I heard about the 17 year old that made fusion in a steel chamber with deterium gas. However I don't understand how he was able to reach the required pressure and tempterature using an electrical charge. Could someone explain or give me a link?

thanks in advance,
Chebby

 

[origin]

Hi, I have two questions about fusion:

1. I've always thought that hydrogen bomb need a fission device as an initiator. But can't the same pressure be reached using a Voitenko compressor (shaped charge)? On the wiki page it states that it has been used to initiate fusion.
http://en.wikipedia.org/wiki/Voitenko_compressor

2. I heard about the 17 year old that made fusion in a steel chamber with deterium gas. However I don't understand how he was able to reach the required pressure and tempterature using an electrical charge. Could someone explain or give me a link?

thanks in advance,
Chebby

The big picture is that you need to overcome the electrostatic repulsion of the atomic nuclei to get them close enough to allow fusion. High temperature and high pressure accomplish this.

1. No a fission device is not needed. The sun is a good indication that fission is not needed. There are many current projects that result in fusion at low pressures (you must have very high temperatures), the problem is that the reaction cannot be sustained and more energy is put into the system than comes out. I don't know anything about a Voitenko_compressor but it seems reasonable that the temps and pressure from a shaped conventional charge could cause some level of fusion but of course if the 'shaped charge' is a fission bomb the yield is going to be much higher!

2. Interesting read about the 17 year old. But again it is not that hard to cause low levels of fusion but it takes much, much more energy to cause the fusion than is produced by the fusion.

 

[chebby]

thanks origin

although I don't understand why low level of fusion reached is not enough to heat and compress the gas to have a continuos reaction (at least until the container blows up) Isn't it still millions of degrees hot? Or it's much lower than that in low levels of fusion?

 

[vogon13]

IIRC, if you take the density (I forget the units) times the temperature (ditto) needed for fusion you get a constant.

In the case of the sun, the density is really high, so the temp needed is only 25,000,000.

In our Ulam-Teller H-bombs, the density is not so high, but the fission reactions zap the temp up a great deal higher than the solar interior. Recall in the 'classic' design you have a torus of D and T that is simultaneously heated from within by a stick of fissioning plutonium and from the outside by a shell of depleted U that is spontaneously fissioning from the increasing N flux from the initial fusion reactions. The D and T is instantaneously drastically heated and compressed by two intense walls of energy. Surprisingly, the excess N from the D/T fusion releases more energy in the depleted U in the form of fission than does the fusion reaction itself. The ratio can go up to 75% to 25%. The fusion reactions generate heat in the U which further heats the D and T. This 'bootstrap' effect is quite efficient, and a high percentage of the D, T, and U can be consumed in the bomb. This is all quite different from how the sun works, despite both utilizing the strong nuclear force for their power.


Probably not practical, but it has been suggested to tap fusion power commercially, one needs merely to detonate a device very deep, and then circulate a fluid through the hot constrained debris to make vapor to spin a turbo-alternator. Not pretty, and probably not terribly efficient . . . .

 

[chebby]

I see. So the main purpose of fusion here is to produce extra neutrons to feed the fission reaction. This is a two stage design, so I suppose 3 stage designs could probably reach higher temperatures*density constant and increase the ratio of energy recieved from fusion? I am starting to see why a useful fusion reaction could be hard to achieve for purposes of gathering energy.

 

[vogon13]

The Teller-Ulam device uses a fission primary to generate and intense x-ray flux that implodes a cylindrical fusion device. This part contains a stick of Pu in it's center (which may be boosted further with a small amount of D and T inside). When the Pu fissions it heats the D and T (or decomposes LiD making T) and it starts fusing. The fusion release N which 'burns' an outer layer of depleted U. This layer throws more heat back into the D and T and promotes a thorough burn. As the D and T burn, the N flux consumes the remainder of the depleted U.

Most US H bombs derived most of their power from fission reactions in cheap and plentiful depleted U. So to the Soviet era devices, with the remarkable exception of their Tsar Bomba. In that device, to limit fallout, the depleted U was omitted and the device's yield was mostly from fusion. 50 megatons worth. IIRC, for a few microseconds the device fused H faster than the sun does.

 

[yevaud]

Interestingly, the early Fusion devices utilized a core of Lithium Hydride as the source for the D and T.

 

[UncertainH]

Instead of using a shaped charge there is a company called General Fusion which is attempting to use a spherical container filled with a rotating Lead-Lithium liquid which they smack with pneumonic hammers creating a spherical shock wave. The shock wave moves towards the center of the sphere where there is some deuterium waiting. The force is supposed to be great enough to cause a small fusion reaction releasing neutrons into the liquid and heating it up. The heat is then used to power a normal steam turbine. They hope to repeat the cycle once a second. Check it out at http://www.generalfusion.com - interesting approach.

 

[yevaud]

Sounds like a variation on SonoFusion.

 

[chebby]

Exploding bubbles causing fusion? That's awesome. But I read that they disproved SonoFusion.

UncertainH, very interesing site, thanks. Not sure if it'll work, considering all I've learned. I wonder if they can pattent something likes this, if they can't actually produce it yet?

One interesting thing I've stumbled upon is that force to increase pressure is proportional to temperature (duh, right?) All this shocking and fission bombing unfortunately quickly heats up fusion fuel, but if there was a way to slowly compress the fuel first while keeping it cool, the fusion reaction rate could be dramatically increased.

 

[justinc210]

There are many different ideas about how to make a fusion reactor. The most prevalent right now is the Tokamak reactor, which uses high strength magnetic fields to contain a plasma in which the fusion reaction takes place. Some others are the Dense Plasma Focus, and the Inertial Electrostatic Confinement method. Eric Lerner gave a good presentation about these methods at the Google Tech Talks: http://video.google.com/videoplay?docid ... pr=goog-sl

 

[dangineer]

As far as I know, tokamaks are currently the designs that produce the most energy relative to how much was put in. However, the current problem is that there is too much plasma leakage to sustain a reaction (due to non-uniformities in the magnetic field). The hope is that if you build a big enough reactor, you'll reach ignition, where the energy output is high enough for the reaction to sustain itself. Unfortunately, many believe this will not work and a larger reactor will create larger instabilities and just continue the process.

I wonder who will be right...