They have to be hotter than the temperature of the Sun
Well they don’t strictly speaking have to but to get fusion you need a combination of pressure and temperature and increasing temperature is way easier than increasing pressure if you don’t happen to have the gravity of the sun to help you out. Compressing things with magnetic fields isn’t exactly easy.
Efficiency in a fusion reactor would be how much of the fusion energy is captured, then how much of it you need to keep the fusion going, everything from plasma heating to cooling down the coils. Fuel costs are very small in comparison to everything else so being a bit wasteful isn’t actually that bad if it doesn’t make the reactor otherwise more expensive.
What’s much more important is to be economical: All the currently-existing reactors are research reactors, they don’t care about operating costs, what the Max Planck people are currently figuring out is exactly that kind of stuff, “do we use a cheap material for the diverters and exchange them regularly, or do we use something fancy and service the reactor less often”: That’s an economical question, one that makes the reactor cheaper to operate so the overall price per kWh is lower. They’re planning on having the first commercial prototype up and running in the early 2030s. If they can achieve per kWh fuel and operating costs lower than gas they’ve won, even though levelised costs (that is, including construction of the plant amortised over time) will definitely still need lowering. Can’t exactly buy superconducting coils off the shelf right now, least of all in those odd shapes that stellerators use.
Well they don’t strictly speaking have to but to get fusion you need a combination of pressure and temperature and increasing temperature is way easier than increasing pressure if you don’t happen to have the gravity of the sun to help you out. Compressing things with magnetic fields isn’t exactly easy.
Efficiency in a fusion reactor would be how much of the fusion energy is captured, then how much of it you need to keep the fusion going, everything from plasma heating to cooling down the coils. Fuel costs are very small in comparison to everything else so being a bit wasteful isn’t actually that bad if it doesn’t make the reactor otherwise more expensive.
What’s much more important is to be economical: All the currently-existing reactors are research reactors, they don’t care about operating costs, what the Max Planck people are currently figuring out is exactly that kind of stuff, “do we use a cheap material for the diverters and exchange them regularly, or do we use something fancy and service the reactor less often”: That’s an economical question, one that makes the reactor cheaper to operate so the overall price per kWh is lower. They’re planning on having the first commercial prototype up and running in the early 2030s. If they can achieve per kWh fuel and operating costs lower than gas they’ve won, even though levelised costs (that is, including construction of the plant amortised over time) will definitely still need lowering. Can’t exactly buy superconducting coils off the shelf right now, least of all in those odd shapes that stellerators use.