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In the discussion of the question "Nuclear Fusion with extremely high pressure and low temperature", the conclusion is only ultrahigh pressure cannot trigger fusion. But why does NIF try to use 192 laser beams to trigger capsule ignition? There is no source to heat fusion fuel.

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    $\begingroup$ Those 192 lasers pack a lot of spank.. The fuel will get plenty hot and dense. $\endgroup$ – zeta-band Mar 19 at 18:56
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The lasers provide the heat, as the laser energy is absorbed by the outer surface of the fuel pellet. The surface of the pellet is heated enough that it becomes a plasma, and as it expands, it compresses and heats the rest of the pellet. This is aided by a propagating spherically-symmetric shockwave that propagates inward after the initial laser impact; the shockwave, when reaching the center, compresses and heats the center of the pellet even more, ideally to fusion temperatures.

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  • $\begingroup$ You said the surface not the core of D-T gas is heated. The x-ray only heats the shell of pellet, the fusion gas will stay normal temperatures during compression. So this situation is as same as described in the question "Nuclear Fusion with extremely high pressure and low temperature". $\endgroup$ – Darwin Zou Mar 20 at 3:55
  • $\begingroup$ @DarwinZou The compression happens so quickly that the process can be thought of as close to adiabatic (there is no time to exchange heat with the environment). In an adiabatic process, compression leads to heating, for non-degenerate equations of state (and the equation of state of a fuel pellet is non-degenerate; in other words, compression makes it get smaller). $\endgroup$ – probably_someone Mar 20 at 9:37
  • $\begingroup$ Hi, buddy, you mean because of compression happens so quickly that there is no time to exchange heat with pellet outside. And the X-RAY or laser will not directly heat D-T gas. The compression process will lead fuel to get hot and density enough, right? If so, then can take a look my new fusion design idea? physics.stackexchange.com/questions/466190/… $\endgroup$ – Darwin Zou Mar 20 at 12:27
  • $\begingroup$ About my idea (Discussion about a new nuclear fusion design [duplicate]), someone said it will not work, because my design, the piston only compress D-T gas without heating gas. But I think if the compression happens very quickly, maybe the gas will be heated hot and dense enough. $\endgroup$ – Darwin Zou Mar 20 at 12:32
  • $\begingroup$ @DarwinZou The lasers don't directly heat the pellet, they heat an assembly around it called the hohlraum to a few million degrees, at which point it emits x-rays that turn the surface of the pellet into a plasma. X-rays can definitely heat a plasma, for example through Compton scattering off of free electrons and ions. $\endgroup$ – probably_someone Mar 20 at 12:46

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