Is it possible to use a lens to focus sunlight to a point hot enough to fuse hydrogen? [duplicate]

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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?

Answer 5

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