3 ms·

For one, it's theoretically impossible to make lasers 102 times more efficient, especially if you consider the current industrial efficiency of up to 20% (30% u

For one, it's theoretically impossible to make lasers 102 times more efficient, especially if you consider the current industrial efficiency of up to 20% (30% under special conditions). For example, the maximum efficiency of a laser is limited by the quantum efficiency of the laser material, which is the ratio of the number of photons emitted by the laser to the number of charge carriers (electrons and holes) injected into the laser material. The quantum efficiency of most laser materials is less than 100%, which means that there is a limit to the efficiency that can be achieved. In addition, the efficiency of a laser is also limited by the thermodynamic laws of thermodynamics, which state that it is not possible to convert heat or any other form of energy into work with 100% efficiency. This means that there is always some amount of waste heat generated when a laser is operated, which limits its efficiency. In other words, fusion is unlikely to be using lasers for the ignitions. UPDATE: People here seem to believe in self-sustained fusion, but this has never been tried or proven to be possible - no one know how much energy is needed for repeated ignition. The current plan is to re-ignite new pellets fed into the containment.

danbruc 4y ago
For one, it's theoretically impossible to make lasers 102 times more efficient, especially if you consider the current industrial efficiency of up to 20% (30% under special conditions). That is not true. You can of course not make a 10 % efficient laser more than 10 times more efficient, but there is no limit to how much more efficient you can make a laser in general, you just have to start with an inefficient enough one. And the NIF lasers are quite inefficient, according to Wikipedia - which might have slightly outdated numbers as someone pointed out to me - they turn 422 MJ stored in capacitors into 1.8 MJ UV laser light, that is an efficiency of 0.43 %. Bringing this up to the 20 % you mentioned would already almost be a factor of 50. Also not all of the gain has to come from more efficient lasers, the fusion process could also be made more efficient. But you are of course correct that there is some general limit to laser efficiency, therefore the question is if the fusion gain can compensate this and all additional losses during the conversion into electricity further down the line.
- TheOtherHobbes 4y ago
  I'm not sure if it's theoretically impossible. But it's certainly an incredible challenge to build a high efficiency laser system that can consistently generate many pulses per second without degrading either the laser head itself or any associated micro-focussing optics. Can we stop pretending this isn't about nuclear weapons research and - basically - continued funding? LLNL is at least a decade behind its initial predictions of ignition. The current lasers may be inefficient but they've been fine-tuned very precisely for optical quality. There's no guarantee more efficient lasers would have the same characteristics and could be focussed in the same way. I'm sure LLNL know the maximum theoretical fusion gain for a realistic pellet design, and it's worth nothing that that number hasn't been mentioned anywhere. Clearly there's only so much energy available for each cycle. If that energy is on the wrong side of what's needed to release it even after efficiency improvements, the entire system isn't workable, no matter how it's re-engineered.
  - danbruc 4y ago
    My point was purely about the claimed impossibility of a hundredfold efficiency increase which is only true if you are already about 1 % efficient. I have no real opinion on what the NIF is doing or not, at best my layman judgment would be that laser fusion is way further from becoming practical than tokamaks or stellerators.
    - tmikaeld 4y ago
      You're right that we don't have enough information here, I've been reading a lot about it's theory and practicality throughout the years. There's also theories about ignition in parallell, by using multiple pellets. Since the first step has been taken, there's a lot of avenues for improvement. However, one also has to take into account the "chaos" created by the ignition in the first place, re-introducing an additional pellet or chain reaction has it's own challenges.
  - scythe 4y ago
    >Can we stop pretending this isn't about nuclear weapons research The laser practicality issue that prevents this from directly becoming a power source would also be a major barrier to its application as a weapon. The laser fires in the UV-B (351 nm), which is scattered and attenuated by air, to say nothing of smoke or dust; it also requires incredibly high targeting precision (<2 mm target diameter) and consequently precise placement of a target weighing only milligrams. Additionally, the various optical components of the laser must be very well aligned, which is difficult to achieve in any battlefield conditions. And the whole thing is a very obvious and vulnerable target. I ascribe a small possibility to its utility as a weapon in space, but practically zero on Earth without other major developments. >I'm sure LLNL know the maximum theoretical fusion gain for a realistic pellet design, and it's worth nothing that that number hasn't been mentioned anywhere. I wouldn't be so sure. Fusion is in general quantum chromodynamics, which is not so well characterized (being the subject of the famous YM mass gap conjecture). Even in this case it was stated that the yield exceeded expectations and damaged the sensors, which was probably not desired.

danbruc 4y ago

For one, it's theoretically impossible to make lasers 102 times more efficient, especially if you consider the current industrial efficiency of up to 20% (30% under special conditions). That is not true. You can of course not make a 10 % efficient laser more than 10 times more efficient, but there is no limit to how much more efficient you can make a laser in general, you just have to start with an inefficient enough one. And the NIF lasers are quite inefficient, according to Wikipedia - which might have slightly outdated numbers as someone pointed out to me - they turn 422 MJ stored in capacitors into 1.8 MJ UV laser light, that is an efficiency of 0.43 %. Bringing this up to the 20 % you mentioned would already almost be a factor of 50. Also not all of the gain has to come from more efficient lasers, the fusion process could also be made more efficient. But you are of course correct that there is some general limit to laser efficiency, therefore the question is if the fusion gain can compensate this and all additional losses during the conversion into electricity further down the line.

TheOtherHobbes 4y ago
I'm not sure if it's theoretically impossible. But it's certainly an incredible challenge to build a high efficiency laser system that can consistently generate many pulses per second without degrading either the laser head itself or any associated micro-focussing optics. Can we stop pretending this isn't about nuclear weapons research and - basically - continued funding? LLNL is at least a decade behind its initial predictions of ignition. The current lasers may be inefficient but they've been fine-tuned very precisely for optical quality. There's no guarantee more efficient lasers would have the same characteristics and could be focussed in the same way. I'm sure LLNL know the maximum theoretical fusion gain for a realistic pellet design, and it's worth nothing that that number hasn't been mentioned anywhere. Clearly there's only so much energy available for each cycle. If that energy is on the wrong side of what's needed to release it even after efficiency improvements, the entire system isn't workable, no matter how it's re-engineered.
- danbruc 4y ago
  My point was purely about the claimed impossibility of a hundredfold efficiency increase which is only true if you are already about 1 % efficient. I have no real opinion on what the NIF is doing or not, at best my layman judgment would be that laser fusion is way further from becoming practical than tokamaks or stellerators.
  - tmikaeld 4y ago
    You're right that we don't have enough information here, I've been reading a lot about it's theory and practicality throughout the years. There's also theories about ignition in parallell, by using multiple pellets. Since the first step has been taken, there's a lot of avenues for improvement. However, one also has to take into account the "chaos" created by the ignition in the first place, re-introducing an additional pellet or chain reaction has it's own challenges.
- scythe 4y ago
  >Can we stop pretending this isn't about nuclear weapons research The laser practicality issue that prevents this from directly becoming a power source would also be a major barrier to its application as a weapon. The laser fires in the UV-B (351 nm), which is scattered and attenuated by air, to say nothing of smoke or dust; it also requires incredibly high targeting precision (<2 mm target diameter) and consequently precise placement of a target weighing only milligrams. Additionally, the various optical components of the laser must be very well aligned, which is difficult to achieve in any battlefield conditions. And the whole thing is a very obvious and vulnerable target. I ascribe a small possibility to its utility as a weapon in space, but practically zero on Earth without other major developments. >I'm sure LLNL know the maximum theoretical fusion gain for a realistic pellet design, and it's worth nothing that that number hasn't been mentioned anywhere. I wouldn't be so sure. Fusion is in general quantum chromodynamics, which is not so well characterized (being the subject of the famous YM mass gap conjecture). Even in this case it was stated that the yield exceeded expectations and damaged the sensors, which was probably not desired.