Efficient Fusion = Unity Beta
Power throughput in tokamaks only depends on the toroidal field of the device and its geometrical size, which are engineering parameters, and the ratio of plasma pressure to magnetic pressure, also called beta, a physics parameter linked to the plasma equilibrium inside the device. Modern tokamaks have pushed engineering parameters to their limits, giving rise to sub-gigawatt machines like ITER. However, a fusion power plant needs to generate several gigawatts. While it is possible, with great efforts, to keep on pushing engineering parameters, increasing the plasma beta to unity has gotten very little attention. Yet unity beta has several advantages. First, the fusion throughput of a unity beta machine is similar to the throughput of a low beta machine with a toroidal field higher by two orders of magnitude. Second, unity beta equilibria are highly diamagnetic, forcing particles to drift radially inward, naturally improving confinement. Finally, the magnetic axis is pushed toward the outboard side of the tokamak, helping with core fueling from neutral beams. So why is there no unity beta tokamak around? The Troyon limit! This talk will characterize unity beta equilibria and show why the Troyon limit does not apply in this case. Then the talk will show how unity beta equilibria, which were thought unstable because of their diamagnetism, can be trivially stabilized. Finally, the talk will discuss several scenarios to bring low beta plasmas into the unity beta regime.