Thread 4 of 7 for my Fusion study week: How close are we to reaching practical use of fusion for energy production? 
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To measure progress you need metrics. There are three major metrics used by the fusion community: gain factor, fusion triple product, and plasma duration. Gain factor (symbol Q) represents the energy output from fusion, divided by input energy required to create the plasma.
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A value of Q=1 means the reactor produces exactly the same energy that it uses. Due to the energy loss converting reactor heat energy back into electricity (via steam turbine), Q needs to be at least 2-3 before it is truly energy-positive.
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Gain grows exponentially due to heat produced by the reaction, so beyond Q=10 it is not a big leap to reach Q=∞. The highest result from any fusion reactor so far is Q=0.67 from JET. ITER was originally designed for Q=∞, but was scaled back to target Q=10 due to funding cuts.
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Fusion triple product measures efficiency of the plasma in producing fusion reactions. The triple product is calculated by multiplying temperature, density, and confinement time (rate of energy loss). The JT-60 tokamak in Japan holds the record for highest triple product.
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ITER will likely have the highest triple product because increasing reactor size results in higher confinement time. As a torus grows, volume for fusion grows faster than surface area for cooling. Bigger is generally better with MCF reactors.
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Plasma duration measures how long the reactor can operate continuously. The W7-X
#stellarator has the current best expected plasma duration at 30 minutes. ITER is expected to have a plasma duration of 10 minutes.1 reply 0 proslijeđenih tweetova 0 korisnika označava da im se sviđaPrikaži ovu nit -
To summarize the current state, there are reactor designs that are projected to produce net energy, but only for short durations. A working power plant needs to have a plasma duration in days or weeks rather than minutes, so there is still a long way to go.
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Future progress is hard to estimate. Without major new breakthroughs the best estimate is 2050 for a commercial reactor (@FusionInCloseUp roadmap). However, the industry is ripe for disruption. Some private fusion ventures target 2030, but lack concrete evidence or peer review.
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There are many fields where innovation could result in major reactor improvements: plasma physics, materials, fuel sources, superconductors, computer simulation, etc. One or more major advances in these fields could chop years or even decades off the roadmap.
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