fusion power density is the fusion power produced per unit of plasma volume — the figure of merit that determines how much machine must be built per megawatt, and one that rises steeply with magnetic field strength.
Fusion power density scales with the square of the plasma pressure: doubling the pressure of a reacting plasma quadruples the power from the same volume. In a magnetically confined plasma, the pressure a machine can hold is set by its magnetic field, and at a fixed ratio of plasma pressure to magnetic pressure — the quantity called beta — power density rises as the fourth power of the field. The arithmetic is dramatic: doubling the field multiplies the power from a given volume of 📝plasma by roughly sixteen, or delivers the same power from a far smaller machine.
That scaling divides the two routes to fusion power. The conventional route accepts the moderate fields of low-temperature superconducting magnets and buys performance with size, which is how ITER came to be a 23,000-tonne machine. The high-field route holds the machine small and buys the same performance with field strength — an option opened by 📝High-Temperature Superconducting (HTS) Magnets capable of roughly 20 tesla, which is why compact, high-field devices in a 📝Tokamak geometry can target power-plant-relevant output from plasmas a fraction of ITER's volume.