The D-shaped superconducting electromagnets arranged around a tokamak that generate the strong toroidal magnetic field essential for confining a fusion plasma.
A toroidal field (TF) coil is one of a set of large electromagnets spaced equally around the circumference of a tokamak. Together, the TF coils produce the dominant component of the magnetic field — the toroidal field — which wraps around the torus in the long way and prevents the superheated plasma from expanding outward. Without this field, the plasma would strike the vessel wall in microseconds.[1]
Early tokamaks used circular TF coils, but modern machines almost universally adopt a D-shaped cross-section. The D shape is not an aesthetic choice; it is the geometry that minimizes bending stresses in the coil under the enormous electromagnetic forces generated during operation. When current flows through the coil in the presence of its own magnetic field, each conductor segment experiences a radially inward force. The D profile — technically a "constant-tension" curve — distributes these forces as pure tension throughout the winding, eliminating the bending moments that would fatigue and ultimately crack a circular coil.[2]
All modern large tokamaks use superconducting TF coils to sustain the multi-tesla fields required for confinement without prohibitive resistive power losses. Two superconductor families dominate:
Low-temperature superconductors (LTS). Nb3Sn and NbTi have been the workhorses of fusion magnet design. ITER's 18 TF coils use Nb3Sn cable-in-conduit conductors cooled to 4.5 K with supercritical helium, each producing a peak field of 11.8 T on the conductor.[1]
High-temperature superconductors (HTS). Rare-earth barium copper oxide (REBCO) tapes operate at higher temperatures (10–20 K) and can sustain much stronger fields — above 20 T — enabling compact, high-field tokamak designs. Commonwealth Fusion Systems demonstrated a 20 T large-bore HTS magnet in 2021, a milestone for the compact fusion approach.[3]
The centering force pulling each TF coil inward toward the machine axis can exceed 400 meganewtons in a large tokamak. These forces are reacted by an inner cylinder or vault structure where the straight legs of adjacent D-shaped coils press against one another, forming a self-supporting wedged assembly. The overturning torques produced by interaction with the poloidal field add further complexity, requiring massive inter-coil structures to maintain alignment.[2]
Achieving and maintaining the dimensional tolerances needed — often within a few millimeters over a 17-meter span — is one of the premier engineering challenges in fusion device construction.