ITER and the prospects for commercial fusion

The article by Richard Hawryluk and Hartmut Zohm addresses several interesting issues in ITER’s march toward successful plasma burning. In particular, ITER’s design, as presented in the article, relies on the high-confinement mode—the H-mode—that may be achieved with a thermal barrier believed to arise from turbulence-generated zonal flow. ¹ As the discoverer of that type of flow, I highlight here a fundamental issue in the H-mode tokamak reactor. If operation depends on the formation of zonal flow possibly created by the self-organization of a type of Hasegawa–Mima turbulence, it is crucial that such turbulence is continuously generated. The scheme may be considered to be dynamic confinement, as compared with the classic static confinement scheme based on a magnetic bottle.

Dynamic confinement requires continuous injection of free energy via RF or neutral beams to sustain the turbulence and hence the thermal barrier—and, in effect, the pressure profile. For that process to take place on a steady-state basis, one must assume that the injected energy is lost continuously. Therefore, if the ITER design is based on dynamic confinement, ITER should be viewed as a power amplifier—that is, the fusion energy output should be regarded as amplified injected free energy. Since the injected power should be considered lost through an inverse cascade of turbulent energy, ignition criteria, such as the well-known Lawson criterion, that are based on energy confinement time become irrelevant: The energy is not confined. Here the crucial time scale is not of energy confinement but of maintaining the plasma pressure profile sustained by the zonal flow.