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Is it possible to use a lens to focus sunlight to a point hot enough to cause atomic fusion?

The point of the diagram below is to create a focused point of light at a point in space that is far from the walls of the chamber and is hot enough to fuse a fuel material. The fused material is very hot at the point of fusion but cools as it mixes with the other unfused fuel. Hopefully enough mixing occurs so that the temperature at the walls of the chamber and piping is low enough to avoid melting them.

The fuel material is actively circulated by a pump to a heat exchanger. Most of the material circulates in a loop, but obviously fused material needs to be removed at some point, and new fuel added.

I used hydrogen in my diagram, but I will accept answers that use another fuel material if its more suitable.

enter image description here

user4574
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  • It is not really that easy, many billions of dollars across decades have been invested with yet very little results. I don't think a lens would work, but I don't know that much. –  Jul 02 '22 at 03:05

5 Answers5

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No. The hottest temperature you could achieve by focusing sunlight is the temperature of the surface of the sun*. That is a few thousand K, whereas fusion requires temperatures in the millions of K.

*heat flows from a hotter object to a cooler object. So if the focal point ever became hotter than the surface of the sun, then heat would flow from the focal point to the sun.

Dale
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    Why is that the hottest temperature? I would think if you took a finite amount of energy and focused into an ever increasingly small dot the temperature could be whatever you want so I think that warrants some explanation. – DKNguyen Jul 02 '22 at 03:42
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    Temperature limits to focused light from xkcd, also discussed here and here. – rob Jul 02 '22 at 03:46
  • @DKNguyen heat flows from hot to cold. I guess I will add a comment to that effect – Dale Jul 02 '22 at 03:51
  • Hmmmm. That new edit is more understandable than the xkcd but is even gnarlier when you think about it. Are objects only heated by the component of the ray perpendicular to the surface? Or does that have nothing to do with it? Because that's kind of what I got from the xkcd. – DKNguyen Jul 02 '22 at 03:54
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    I don’t feel comfortable adding more tangential material to this answer. You should ask your own question, although read what rob linked to first – Dale Jul 02 '22 at 03:56
  • @DKNguyen i'm really baffled by this limit being discussed. in a suitably drawn system boundary, can a mirror be viewed as a 'source' from the perspective of the observer? and can eg. a parabolic mirror start a fire without becoming especially hot itself. i must be missing something here. – antimony Jul 02 '22 at 04:27
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    @antimony a mirror is a hot light source when you're shining a hot light source at it, in the same way and for the same reason that a concrete wall is a high velocity water source when you blast it with a pressure washer. The radiant flux (or the water flux) leaving the interface is a property of the interface and the source, not the material underneath the interface. – g s Jul 02 '22 at 05:04
  • @DKNguyen: Lenses can't focus onto "an increasingly small dot". What they do at "perfect focus" is form a crisp sharp image of the object in question. The size of the image will depend on the size of the lens. This is how a camera works. And your eye, which is a kind of biological camera. The sensor in a camera is right at the place where the light is pretty much as focused as it's ever going to get. Or to put it another way, a perfect lens focuses a perfect point to a perfect point, and focuses a matrix of points to a matrix of points. The problem is your concept of – The_Sympathizer Jul 02 '22 at 05:44
  • lenses as "light squishers" is simply not what they do, but only an imperfect description. At suboptimal focus, a point is not focused to a point, but instead diffused. For an extended object, i.e. a matrix of points, you don't get a crisp image, but a superposition of blurs, which is, itself, a blur. – The_Sympathizer Jul 02 '22 at 05:45
  • @The_Sympathizer When I say dot I am saying an increasingly small image which is why your entire description up there doesn't answer my question. – DKNguyen Jul 02 '22 at 14:07
  • Thanks for the answer. I would like to ask for some clarification. Assuming the hot spot absorbs all the light and then emits it as a "black body". Doesn't the emitted power depend on the on the surface area of the body? In that case, wouldn't focusing the light into a smaller point, result in a smaller surface area for the black body, thus reducing the amount of radiated heat. In that case it would seem that the hot spot would need to be at an increased temperature before the emitted heat equaled the incoming heat. Assuming I am wrong, why is this wrong? – user4574 Jul 03 '22 at 04:03
  • @user4574 the power depends on the surface area, the solid angle of illumination, and the temperature difference. The surface area and solid angle are always positive, only the temperature difference can be positive or negative. So the surface area affects the rate of energy transfer, but not the direction. Only the temperature difference determines the direction of energy transfer. Heat always flows from the hotter object to the cooler one. Only work can move energy the other way – Dale Jul 03 '22 at 11:42
  • @DKNguyen conservation of etendue points to the 2nd Law. Try leveraging your intuition from thermodynamics by imagining particle light instead of geometric optics. Unlike matter particles, photons don't interact with other photons, so our system of particles doesn't gain entropy on its own; it retains the entropy it had at its last scattering surface. A lens can add a little bit of total system entropy in order to reduce the area in which the light does work, but it cannot arrange the photons into a system with less total entropy unless we do work. – g s Jul 03 '22 at 15:41
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No.

Consider looking through the lense (DON'T DO it with the SUN, of course. THINK about it.). The Sun looks bigger. In this sense, the lens "brings the Sun closer to you". Importantly, the Sun's surface looks no brighter. (A safe way to convince yourself of this is to remember how it works when looking at ordinary objects. They get larger, but they don't get brighter.)

The best possible lense will be so big and so magnifying that it will make the Sun look so close that it fills the entire half of your vision. Being under this lense would be being like sitting on the surface of the Sun. You will be getting a Sun-bright surface right up against you.

Your eyes will detonate instantly and you will be rapidly incinerated, at 5800 degrees absolute, but hydrogen will not fuse. Much like a poor ant (please don't torture ants though either :) ). The poor ant dies because it "sees" a Sun that fills a very big part of its field of view, as though it were transported to the Earth 4 billion years in the future near the end of the Sun's lifetime.

That is the limit. To get any hotter, the surface brightness would have to go up. The only way to arrange for that is to add energy actively, and passive optics can't do that.

Now note: this doesn't mean you cannot drive fusion using a solar igniter. Just not passive optics. If you could charge the batteries on something like a tokamak - once we refine it enough to make a working one - using a big array of solar panels, then you can indeed ignite fusion with it, but that's because the solar/tokamak system utilizes the energy in a far different way. Likely, in at least some circumstances, that is how we would ignite such a reactor once we both get them and fully abolish fossil fuels.

The_Sympathizer
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To extend slightly upon Dale's correct answer, when objects (like a sample of gas) get this hot, they begin very strongly radiating away that energy and for this reason it gets progressively more difficult to push on to higher temperatures. In Serber's book The Los Alamos Primer (annotated edition) Serber recounts how Bethe pointed out to Teller that Teller had failed to include re-radiation effects in his calculations on using a fission primary to ignite a fusion reaction, which doomed Teller's earliest "super" bomb designs. I'd cite the page if I still owned a copy of this very handy book.

niels nielsen
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There are many ways to understand why you cannot focus light from a distended ~5700 Kelvin source into a small, hotter point.

First, the premise that the Sun's rays are parallel is only an approximation. Yes, for parallel rays you can focus them into a point, but there is no such thing as a true point source and true parallel rays, and hence no such thing as a true point destination. Try as you might, the physical size of the Sun means that your point cannot be made arbitrarily small. Thus, the energy will be spread out across some area so that the Blackbody temperature will be less than the source temperature.

As Dale mentioned, heat cannot passively flow from colder to hotter.

Conservation of etendue means you cannot make a beam of light narrower than it used to be using a passive optical system (such as a lens).

The same applies to the radiance.

Lastly, if you consider the color of the light, if the color of the photons are not allowed to change, then it's obvious that the blackbody spectrum represented of the light will be associated with a temperature of 5700 Kelvin and cannot be higher. So from the perspective of your source, it is like being bathed (or partially bathed) in 5700 Kelvin material emitting blackbody radiation, so it can only receive energy from that bath if it has a lower temperature.

https://en.wikipedia.org/wiki/Etendue#Conservation_of_etendue

https://en.wikipedia.org/wiki/Radiance#Conservation_of_basic_radiance

Why does conservation of étendue matter when showing one cannot focus light to arbitrary temperatures?

Alwin
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The temperature depends upon at what distance you keep the lens, Sun is probably at infinite distance as compared to the lens' focal length so to cause atomic fusion you need to get nearer to the sun or you need a humongous lens

ItzSwirl
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