3 ms·

> a worthy science project that could one day save the world What is the path from NIF to a practical power plant? Are they providing data that will help ITER

by p1mrx 3y ago

> a worthy science project that could one day save the world What is the path from NIF to a practical power plant? Are they providing data that will help ITER, CFS, Helion and others progress toward that goal? None of them seem to be pursuing the 'lasers lasers n lasers' approach.

credit_guy 3y ago
I don't think they have any concrete plans yet. But don't worry, ITER, CFS and Helion will never achieve economical fusion reactors either. The reason is quite simple: in order to heat a certain volume with matter to a certain temperature, you need to heat both the matter (vibration energy) and the vacuum (radiation energy). The energy to heat matter is proportional to the temperature (it's actually the definition of temperature). The energy to heat the vacuum is proportional to the fourth power of the temperature. If you draw a graph of T and T^4 you'll see that initially T^4 is negligible compared to T, and later T is negligible compared to T^4. If you want to heat 1m3 of vacuum to the temperature needed for sustained fusion, it takes more than 10 kT of TNT equivalent. In an actual nuclear bomb, this is available from the primary fission bomb; plus the volume of the part undergoing fusion is squeezed tremendously, so it's nothing close to 1 m3. ITER plans to have a volume of plasma of 840 m3. At the temperatures they talk about, the energy to heat 840 m3 exceeds the energy released by the Tsar bomba. Bottom line: the only possible way to get controlled fusion is the NIF way. But even that one is very, very far in the future.
- p1mrx 3y ago
  Why does heating the vacuum require any energy at all? If the plasma is radiating at T^4, doesn't that mean they need something to reflect the radiation so it doesn't escape?
  - credit_guy 3y ago
    Radiation is electromagnetic waves. These waves, or photons if you wish, exist in vacuum, and have various frequencies from very low to very high. The energy of a photon or wave depends on the frequency, and is given by the Panck's formula; we don't care about this particular form. A volume of vacuum that is in equilibrium with its surroundings has a certain temperature. If you take the sum of the energies of all the photons inside that volume, you get a number that is proportional to T^4. This is called Stefan's law. If you take a volume of plasma with a certain average kinetic energy per particle (i.e. a certain temperature), then that average kinetic energy per particle may be higher or lower than the average energy of the photons in the same volume. If it's higher, then the plasma will radiate, and the average energy of the photons will go up, if it's lower, the photons will give heat to the plasma, and the plasma will get hotter. When they were working on making the fusion bomb, they initially did not think they can create an equilibrium between the plasma and the vacuum, in other words the vacuum was colder and the plasma will leak energy to the vacuum continuously. The hope was that enough fusion will happen so that the produced energy will make up for this heat loss and the reaction will continue for a while. That didn't happen. Fusion bombs that don't reach an equilibrium between the plasma and the vacuum were made and detonated, but their bang was underwhelming. Ulam and Teller came up with the breakthrough that resulted in an equilibrium, and a sustained fusion. The bomb they made (well, not the two of them, the other scientists and engineers) yielded 10 megatons of TNT, which was many times what one can obtain with either regular fission bombs, or with non-equilibrium fusion bombs. If their discovery was a peaceful one, they would have gotten a Nobel for sure.

by p1mrx 3y ago

credit_guy 3y ago

I don't think they have any concrete plans yet. But don't worry, ITER, CFS and Helion will never achieve economical fusion reactors either. The reason is quite simple: in order to heat a certain volume with matter to a certain temperature, you need to heat both the matter (vibration energy) and the vacuum (radiation energy). The energy to heat matter is proportional to the temperature (it's actually the definition of temperature). The energy to heat the vacuum is proportional to the fourth power of the temperature. If you draw a graph of T and T^4 you'll see that initially T^4 is negligible compared to T, and later T is negligible compared to T^4. If you want to heat 1m3 of vacuum to the temperature needed for sustained fusion, it takes more than 10 kT of TNT equivalent. In an actual nuclear bomb, this is available from the primary fission bomb; plus the volume of the part undergoing fusion is squeezed tremendously, so it's nothing close to 1 m3. ITER plans to have a volume of plasma of 840 m3. At the temperatures they talk about, the energy to heat 840 m3 exceeds the energy released by the Tsar bomba. Bottom line: the only possible way to get controlled fusion is the NIF way. But even that one is very, very far in the future.

p1mrx 3y ago
Why does heating the vacuum require any energy at all? If the plasma is radiating at T^4, doesn't that mean they need something to reflect the radiation so it doesn't escape?
- credit_guy 3y ago
  Radiation is electromagnetic waves. These waves, or photons if you wish, exist in vacuum, and have various frequencies from very low to very high. The energy of a photon or wave depends on the frequency, and is given by the Panck's formula; we don't care about this particular form. A volume of vacuum that is in equilibrium with its surroundings has a certain temperature. If you take the sum of the energies of all the photons inside that volume, you get a number that is proportional to T^4. This is called Stefan's law. If you take a volume of plasma with a certain average kinetic energy per particle (i.e. a certain temperature), then that average kinetic energy per particle may be higher or lower than the average energy of the photons in the same volume. If it's higher, then the plasma will radiate, and the average energy of the photons will go up, if it's lower, the photons will give heat to the plasma, and the plasma will get hotter. When they were working on making the fusion bomb, they initially did not think they can create an equilibrium between the plasma and the vacuum, in other words the vacuum was colder and the plasma will leak energy to the vacuum continuously. The hope was that enough fusion will happen so that the produced energy will make up for this heat loss and the reaction will continue for a while. That didn't happen. Fusion bombs that don't reach an equilibrium between the plasma and the vacuum were made and detonated, but their bang was underwhelming. Ulam and Teller came up with the breakthrough that resulted in an equilibrium, and a sustained fusion. The bomb they made (well, not the two of them, the other scientists and engineers) yielded 10 megatons of TNT, which was many times what one can obtain with either regular fission bombs, or with non-equilibrium fusion bombs. If their discovery was a peaceful one, they would have gotten a Nobel for sure.