The Center Stack

The center stack is the slender central column that carries the toroidal-field return legs and shaping conductors through the heart of the machine.

The core of a spherical tokamak

Everything in a spherical tokamak is organized around the center stack: the vertical column on the machine axis that the plasma wraps around. It houses the inboard legs of the toroidal-field coils, structural support, coolant paths, and in many designs the inner poloidal-field and solenoid conductors, all packed inside a radius smaller than the plasma minor radius.

What it must do

The center stack carries the full return current of the toroidal-field system, so its conductors see the highest current density in the machine. It reacts the enormous centering forces that push the toroidal-field coils toward the axis. And it sits closest to the plasma, so it absorbs the most intense neutron and heat flux with the least room for shielding.

Design consequences

These demands make the center stack the pacing component of the whole machine. Its conductor choice (REBCO high-temperature superconductor for the field-shaping legs, with the center post itself a distinct engineering problem), its cooling, and its shielding all trade against one another in a very small volume. The center post carries a limited operational lifetime, noted at roughly 0.01 fpy, and is designed as a replaceable module so the machine can be refurbished rather than rebuilt.

• Central column carrying TF return legs and shaping conductors
• Highest current density and neutron loading in the machine
• Designed as a replaceable module (limited center-post life)

This page describes a design and simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030.