system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone explain how Q is calculated with different fusion systems and why some parts of the energy inputs can be ignored?

I am trying to understand more about fusion energy given the recent news. I don’t really understand how they calculate the Q scores and why some of the energy input is ignored. For example with the laser system they used at LLNL I read they used 500MJ for the lasers to deliver 1.8MJ to the target. Why isn’t that 500MJ part of the Q calculation?
gpt
Because you want to know how much goes into the *fuel itself* (deuterium/tritium) vs how much comes out. You don't count light lost along the way that doesn't go into the fuel. So you don't count conversion efficiency at the laser, light lost through absorption and scattering along the way, light that misses the target, conversion to X-rays, X-rays that miss the target, etc.

It's like computing MPG on a car, we don't count crude that leaked from ships, crude lost refining the gas, gas that evaporated at the station, gas spilled on the ground at filling, and gas that leaked from the fuel lines.  We only count the gas that actually makes it to the engine vs the miles you get out.