The Pedestal

To reach sufficient fusion power gain, a tokamak needs to reach a minimum value of thermal pressure \(p\) and energy confinement time \(\tau_E\) - the former sets the fusion power density, and the latter measures how well the plasma retains its heat (thus how well heat from fusion can maintain the necessary conditions for a self-sustaining reaction). The maximum pressure (relative to magnetic field strength) is set by stability limits: we need to make up the difference with confinement improvements.

By default, both energy and particles are driven rapidly from the plasma core by turbulence - and this transport gets worse with more heating power! The brute force approach is to run in a larger machine with stronger magnetic fields to compensate, but this is expensive. In its default state, this plasma is not suitable for an economical power plant.

Under the right conditions (shaping, heating power) the plasma transitions to "high-confinement" or H-mode (compared to the default "low-confinement" L-mode). The plasma forms a transport barrier, the pedestal, in the edge, where sheared flows cut off turbulent transport - density and temperature profiles "pile up" behind this barrier, leading to much higher core pressure and fusion power density.

Up next: H-Mode