Hello. I was just wondering. What if you build a big vacuum sphere (of glas?) about 1-3 m in diameter and took a BIG parabol to focus the sun in the middle of the sphere. How big must you make it to be able to reach the temperature for fusion? If the diameter is to hugh for one on earth, would it be possible to use many hugh parabols and send the energy in to a system of mirrors and lead the sun beam to the vacuum sphere where the last step of focus is made in to the center. What sun area is needed for fusion? Regards Magi
Interesting idea... Just a few days ago i was brousing around and ran into the National Ignition Facility Project, where they are trying to induce fusion through UV lasers focused into a point in a test chamber. I'm assuming that the 500 terawatt output they mentioned is what you'd need to fry a pellet of fusion fuel to get it to go off. so, 500 terawatts = 500,000,000,000,000 watts. Wikipedia had the figure of solar radiation recieved in earth orbit as about 1367 w/m^2. (this isn't the radiation that reaches the ground, but that is in space itself, assuming this thing was built in space) 500 trillion divided by 1367 watts = 365,764,447,696 m^2 Square root of (365,764,447,696 m^2 / pi) = a radius of 341.2 km A single perfectly aimed parabolic mirror would need to be 683 km/ 424 miles wide. That would swallow quite a few countries... it's so large, you'd probably use more energy getting the damn thing built than you'd get from it when you got fusion to work! but then again if it were much closer to the sun, radiation is more intensive, allowing for a smaller mirror, but having a fusion power source really close to the mother of all fusion power cources seems a tad redundant! We'd probably be better off having fusion power systems that recycle their own power to sustain themselves, which is the struggle they're trying to do now.
With mirrors the beam could be directed from many parabols in to the sphere from many directions. Inthe center there could me a magnetic field with a plasma where you could shoot in dehuterium or something. Im just thinking loud. Too bad the area of parabols is so hugh. Please Register or Log in to view the hidden image! Regards Magi
I agree. What's more, the sun's gravity field obviates the need for fancy shmancy magnetic fields to hold the plasma together. Once you're out in space, just put up some solar panels!
if the light intensity was high enough to cause fusion... or fission.... then yes it would... fusion takes like 50 million degrees.... glass melts at 1000 or less.... the only way is to use a jacket of water to cool the fusion chamber... also... without the means to confine and maintain the pressure.... the glass bottle would explode.... -MT
Welcome, but this idea is impossible. As your target gets hotter, it too begins to radiate. When it is as hot as sun surface aproximately 5000 degrees C, it radiates the same as the sun (per square meter, per uint of solid angle.) It is impossible to heat object B, by radiation from object A, to a temperature greater than the surface temperature of object A. This in slightly different words is sometimes called the "fundamental law of optics". It is easy to prove. If it were possible it would violate some fundamental laws of thermodynamics, make unlimited temperatures available, etc. I will not rigorously prove, but ask you to consider the implications of being able to heat B to 10,000 with radiation from A at 5000 degrees. Use hotter B to heat C to 20,000 etc. Continue to any temperature you wish. The maxim efficiency that any therma engine (Carnot cycle) is (T-t)/T where T is the heat source and t is the exhaust heat temperature. By stepping up the original "T temp" you have now achieved more efficient conversion than Carnot cycle. etc. There are many other violations of physic possible, if one could violate the fundamental law of optics.
Ah, but the energy would be spread evenly over the whole of the lens, but concentrated far away at the point of conversion.
So what you're saying Billy T, is if you had a magnifying glass that you were using to fry ants (or induce fusion, whatever your fancy), the temperature of the spot could only get to be as high as the temperature of the light-emitting parts of the sun? And no i'm not saying that spot under a magnifying glass IS that temperature, I'm wondering if that temperature's the limit.
correct. It also has to do with fact that anything tha absorbes sun light well also emits that same spectrum well, so as target that can absorb from sun gets hot it emits like the sun (per unit area and per unit solid angle) Actually the more normal statement of the fundamental theorm of optics is that no optical system can increase the product of the image area and the solid angle forming the image (usually this product is a constant as you go thru the optical system, but you can screw it up to be smaller with badly designed "stops") For example, When you use a lens to project a bigger image on screen of an object, the solid angle of the light cone going to the screen is smaller than that coming to the lens from the object, so that their product has not changed.
