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Is it possible to focus the energy of one star to a small enough point and high enough energy to provide the energy to start a fusion reactor?

I'm imagining a large lens placed between the star and its source but I don't even know whether it is possible (ignoring the impracticalities of manufacturing such a lens or whether other methods would be better suited).

A bit of maths showing the size of the lens needed (if it is possible) would be a nice touch if anyone has the time. (In my story I would like there to be some talk by the team involved in the idea when they propose it)

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    $\begingroup$ en.wikipedia.org/wiki/List_of_hottest_stars We need ~100x hotter stars to reach the "temperature" required in "fusion reactors". Maybe it would help if one would know what exactly you want to start and what fusion reactor you are using? $\endgroup$ – Raditz_35 Jul 18 '17 at 13:58
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    $\begingroup$ If this is possible I imagine the lens size will depend on the energy output of the star, the curvature of your lens etc etc $\endgroup$ – Lio Elbammalf Jul 18 '17 at 13:58
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    $\begingroup$ relevant xkcd $\endgroup$ – SilverCookies Jul 18 '17 at 21:07
  • $\begingroup$ The hottest stars (including neutron stars and white dwarfs) we've measured are barely above 200,000 degrees K. Significantly better than the solar surface of 5k K, but significantly below the hundred million degrees required for fusion. $\endgroup$ – Isaac Kotlicky Jul 19 '17 at 14:44
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A Very relevant XKCD

The important part to your problem is that a lens cannot focus light to a higher intensity than its source. So the hottest you could make something using a lot of lenses is as hot as the stars surface. For our sun that works out to ~5000°C, which is pretty hot, but no where near the temperatures in the interior of the sun or providing enough energy to initiate fusion.

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    $\begingroup$ An important component of this answer is that the surface of the sun is cooler than a fusion reactor. $\endgroup$ – BobTheAverage Jul 18 '17 at 15:03
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    $\begingroup$ While the rest of the answer is true, you actually can focus sunlight to a higher intensity than the sun's surface. The Wikipedia page on non-imaging optics notes that the best optical systems that form an image can do is the source intensity, but it then references a paper which notes a design capable of concentrating sunlight to about 120% of the solar flux at the sun's surface. The real reasoning of why this isn't possible really comes purely from thermodynamics. $\endgroup$ – SpectralFlame Jul 18 '17 at 17:52
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    $\begingroup$ SpectralFlame, what is the distinction between intensity and temperature here? If intensity of a blackbody is a function of temperature, how can we manage to get more intensity without increasing temperature? Maybe I am confusing intensity with wavelength. If so, what actually is intensity? $\endgroup$ – Brian Woodbury Jul 19 '17 at 14:55
  • $\begingroup$ @BrianWoodbury Intensity is indeed a function of temperature but it's also a function of the material's emissivity, which is related to how much light it absorbs/reflects. Me and a couple physicist friends are still trying to figure out the exact mechanism which causes the thermodynamic argument to still hold true... we suspect it must get more reflective as it heats up. But yes, looking at just the energy flow it's not immediately apparent that you can't heat it hotter. $\endgroup$ – SpectralFlame Jul 19 '17 at 16:10
  • $\begingroup$ @BobTheAverage The centre of the Sun is cooler than a fusion reactor. The power density at the centre of the Sun is a paltry 300W/m^3. $\endgroup$ – Dan Piponi Jul 19 '17 at 19:05
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No.

This question was on the physics stack. https://physics.stackexchange.com/questions/69652/concentrating-sunlight-to-initiate-fusion-reaction

Here is text copied from the answer from the physics stack, in case there are issues clicking through to read it.

The second law prevents you from using the Sun (or anything) to heat an object to greater than the surface temperature of the Sun. Otherwise you could take a box of gas at equilibrium split it into two halves, use lenses and mirrors to focus the radiation from the left half on the right half, and raise the temperature of the right half. Then you could use that temperature difference to run an engine, thereby extracting work from an equilibriated gas in blatant violation of the 2nd Law.

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    $\begingroup$ Ah, okay...counter intuitive to me but I'll bow out to the knowledge of the physics SE community. Thanks. $\endgroup$ – FreeElk Jul 18 '17 at 14:15
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    $\begingroup$ @FreeElk I'd rather not tell you any ideas a) below poor Will's answer; b) off topic in an vaguely related question; c) out of experience, mostly people get annoyed when confronted with exotic concepts when their mind was already set on conventional approaches. d) because I think you should read a bit first about fusion reactors yourself. If you ask a question like that in another thread, depending on my mood, I might answer - and if I don't someone more competent might (or even if I answer :() $\endgroup$ – Raditz_35 Jul 18 '17 at 14:33
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    $\begingroup$ @FreeElk Yes, terribly counter intuitive. In fact, it still bugs me to this day, even though I can run the numbers that prove it to be true. My opinion on why this is counterintuitive is because most people don't think of the sun as "5000°C hot" they think of it as simply "really really hot" with no numbers attached to it, because we really don't have an intuitive concept of how 5000°C behaves. Thus, the idea that such heat has a limit feels alien and weird. (at least it does for me) $\endgroup$ – Cort Ammon Jul 18 '17 at 15:04
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    $\begingroup$ I knew this rule from the whatif xkcd on a similar question, but I never understood why. This answer finally explains that! $\endgroup$ – Sidney Jul 18 '17 at 19:32
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    $\begingroup$ @jwenting Let me say, as an engineer, "common sense physics" does not always imply intuitive. In fact, the two are often estranged lovers. $\endgroup$ – Cort Ammon Jul 19 '17 at 6:10
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It probably depends. It might be possible for some types of fusion reactors, but you're really focusing on the wrong problem. Getting tritium or deuterium or whatever fusion fuel you have hot enough to initiate fusion is not hard. Sustaining the reaction and extracting energy from it is the hard part.

As observed in other answers, and basically the same question on the Physics Stack, the limitation of a lens is that it cannot heat anything beyond the temperature of the body it's focusing light from, and the surface of a star is not hot enough for a sustained fusion reaction.

Having said that, one type of fusion we humans have explored (and are still exploring, to my knowledge) is called inertial confinement fusion, and basically uses lasers to generate shockwaves in a fuel pellet, compressing and heating it enough to generate a (brief) fusion reaction.

Inertial confinement fusion (ICF) is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium.

To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of laser light, electrons or ions, although for a variety of reasons, almost all ICF devices as of 2015 have used lasers. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating it inwards, compressing the target. This process is designed to create shock waves that travel inward through the target. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur.

The energy released by these reactions will then heat the surrounding fuel, and if the heating is strong enough this could also begin to undergo fusion. The aim of ICF is to produce a condition known as ignition, where this heating process causes a chain reaction that burns a significant portion of the fuel.

I would suggest that with clever enough materials science and engineering of these fuel pellets, it might be possible to achieve the same thing with an intense burst of focused sunlight, rather than a laser (like we do now), though admittedly, it seems like a long way to go do do it that way rather than using lasers like we do now, and a different technique altogether, like magnetic confinement, is probably a better bet for fusion power as an energy source.

The major technical issue with creating a fusion reaction is not getting up to the necessary temperature, it's sustaining the reaction and extracting energy from it. To that point, at least 5 countries (and possibly as many as 9) have successfully tested fusion reactors so far, though the more common term for them is "hydrogen bombs" (a fission bomb is used to heat and compress a tritium payload, which then creates a more powerful fusion explosion).

So I'd suggest that you're focusing on the wrong problem, as far as fusion goes. Getting your fusion fuel hot enough to create fusion is relatively easy, and we've been able to do that for more than 50 years. The part we haven't pulled off yet is sustaining the reaction in a way we can extract usable energy from, and that's probably where your scientists would have trouble too.

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    $\begingroup$ "it seems like a long way to go do do it that way". A redundant "do", perhaps? $\endgroup$ – Faheem Mitha Jul 19 '17 at 10:11
  • $\begingroup$ It is worth noting that the lasers get energy from xenon tubes and other sources which are much cooler than the temperature achieved by the target of the laser, so while the same temperature argument works for reversible optical systems such as lenses and mirrors, it does not prevent all optical systems from achieving the goal. $\endgroup$ – Pete Kirkham Jul 19 '17 at 13:47
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Focus the sun with a large lens and create fusion temperatures? No, as very well answered by XKCD and earlier answers in SE.

However.

Inertial Confinement Fusion using lasers is a real thing. So your simplest (if inelegant) solution is to build something like the National Ignition Facility in space, powered by photovoltaics.

Looking for a more "elegant" solution than photovoltaics? Solar pumped lasers are a real thing, if still a very underdeveloped technology. A sufficiently large and sophisticated system could use concentrated solar energy to "charge" (optically pump) a lasing medium, enabling the delivery of a powerful focused beam to trigger an ICF fusion reaction. Read up on the NIF in Wikipedia and envision the primary beam line being pumped by concentrated solar power instead of flashlamps. How big would it need to be? That is way beyond my level of expertise. But don't limit yourself to envisioning big glass lenses. Think gossamer structures of reflective Mylar. Enormous parabolic surfaces, or troughs hundreds of meters wide and kilometers long with cylinders of gas- or plasma-phase lasing medium running down the center. Also, look into fresnel lenses. Simply etching the right diffraction pattern into a flat transparent sheet can create a great solar concentrator.

And a big shout-out to Larry Niven's Ringworld asteroid defense system consisting of UV lasers powered by solar flares.

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Other answers have shown why a real-time lens is not possible for this setup. However, if you can store the energy of the sun and release it all at the same time then it can be used to start a fusion reaction. In your case, a battery powered by focused light from a lens can be charged over time and used to produce shorter-burst lasers which can start a fusion reaction.

Kengineer's answer touches on this but I didn't think this point was highlighted enough.

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Focusing sunlight to create fusion, no, focusing the power of the sun via the solar wind to create fusion, yes. Fusion through lense focusing has been covered by others, you can't get the temperature over 5,000C.

But you can focus the solar wind using a mass spectrometer with a tokamak at the end for fusion. The solar wind is essentially plasma made of electrons, protons and alpha particles (Helium nuclei) plus a small percent of heavier elements all traveling at 500 km/sec from the sun, though high energy it is very diffuse. First you channel a portion of the solar wind into a mass spectrometer, say 5%, which would be about a 50 million kg/sec to be harvested. Though only a small % of that will be useful for you.

Then using the MS you seperate deuterium (H2), tritium (H3) and He3 nuclei, expelling (or collecting) everything else. At the end of the MS you seperate and collect the deuterium, tritium and He3 into three massive tokamaks (doughnut shaped magnetic confining plasma at very hight temperatures and pressures.) After collecting enough plasma into each of the three tokamaks, you switch the deuterium and tritium streams so that the deuterium is entering the tritium tokamak and vice versa. Now all three tokamaks will eventually reach fusion and create more energy than you put in which you will need to sustain the MS and the tokamaks and use the left over for whatever your civilization needs. Obviously the amount of time and energy into building these structures would be prohibitive and likely create their own problems but theoretically it would be possible.

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There are power system that use a focus light principle : https://en.wikipedia.org/wiki/Concentrated_solar_power

However if it needs to be fusion then there are many methods to start it, with the right components (and shielding) you can build one in your garage such as this person when he was 14 years old : https://en.wikipedia.org/wiki/Taylor_Wilson

The problem with our current understanding of fusion reactors is they require more energy to contain and maintain the reaction then they output.

Some proposals have put large solar arrays into orbit then have the energy sent via microwaves to a receiver. Not sure if that helps you but a classic SimCity disaster was having this beam miss the receiver.

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    $\begingroup$ SimCity had it wrong. The microwave beam would have far lower intensity than normal sunlight and would pass straight through most materials. It certainly wouldn't be a DeathBeam(TM). It's nothing like a "giant microwave oven cooking everything". This should be a given, but... "don't take your physics from games" :P As for Wilson, he did build a fusion reactor, but I think it's fair to assume that the OP wanted a fusion power plant, which a fusor certainly isn't. $\endgroup$ – Luaan Jul 19 '17 at 8:36
  • $\begingroup$ My reference to the simcity disaster was likely not needed, however the principle of wireless power transmission is valid. I was more trying to convey that if you have solar collectors or accumulators for solar energy on that scale you would not necessarily need a fusion power plant as you would be using the fusion power of the sun. $\endgroup$ – Tribmos Jul 20 '17 at 15:40

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