7 - Equilibrium: three-dimensional configurations

Summary

Introduction

Two-dimensional configurations with toroidal axisymmetry have been investigated in Chapter 6. Many fusion concepts fall into this class – tokamaks of all types, the reversed field pinch, the levitated dipole, the spheromak, and the field reversed configuration. One common feature in each of these concepts is the need for a toroidal current to provide toroidal force balance, either using a perfectly conducting shell or a vertical field.

The need for a toroidal current is of particular importance to the tokamak and RFP, the most advanced of the axisymmetric configurations. The reason is that it is not possible to drive a DC toroidal current indefinitely with a transformer, the method now used in pulsed versions of these configurations. This conflicts with the general consensus that a magnetic fusion reactor must operate as a steady state device for engineering reasons to avoid cyclical thermal and mechanical stresses inherent in a pulsed device. In other words, some form of non-inductive current drive is required. This is an active area of research and while a scientifically sound and technologically viable technique may be possible theoretically, success still depends on current and future experimental development. Overall, non-inductive current drive represents a difficult challenge for the tokamak and RFP concepts.