The tall, powerful electromagnet at the heart of a tokamak that drives plasma current inductively — functioning as the primary winding of a transformer with the plasma ring as its secondary.
The central solenoid (CS) is a large superconducting electromagnet positioned along the vertical axis at the center of a tokamak. It serves as the primary coil of a transformer circuit in which the toroidal plasma itself acts as a single-turn secondary winding. By ramping the current through the central solenoid, a changing magnetic flux is driven through the plasma, inducing the toroidal plasma current essential for tokamak confinement.[1]
Without a central solenoid or equivalent current-drive system, a tokamak cannot initiate or sustain the plasma current that creates the poloidal magnetic field component needed for stable confinement.
The central solenoid operates on Faraday's law of electromagnetic induction. Before a plasma discharge, the solenoid is charged to its maximum current, establishing a strong magnetic flux through the bore of the tokamak. When the current is then ramped down, the changing flux induces a loop voltage around the torus that breaks down the fill gas into a plasma and drives a large toroidal current through it.[1]
This inductive drive is inherently pulsed — once the solenoid has swung through its full current range, the flux change stops and the plasma current decays. Sustaining the plasma beyond this inductive flat-top requires auxiliary current-drive methods such as neutral beam injection or radiofrequency waves.
The ITER central solenoid is constructed from six stacked modules, each wound from niobium-tin (Nb3Sn) superconducting cable-in-conduit conductor. Nb3Sn was chosen over the more common niobium-titanium because the CS must operate at magnetic fields exceeding 13 T, beyond the capability of NbTi. The conductor is cooled by supercritical helium at 4.5 K flowing through the cable conduit.[2]
The modular design allows each CS module to be fabricated and tested independently before stacking. The enormous electromagnetic forces — including vertical compression forces of hundreds of meganewtons — are managed through a massive precompression structure that clamps the modules together.
In spherical tokamaks with very tight aspect ratios, the limited space at the center of the machine makes it difficult or impossible to accommodate a conventional central solenoid. This has driven interest in solenoid-free startup techniques, including merging-compression and coaxial helicity injection, which could enable spherical tokamak power plants to operate without a central solenoid entirely.[3]
For conventional-aspect-ratio tokamak reactor designs, the central solenoid remains a critical component, though its pulse length and stored energy requirements impose significant constraints on magnet technology, structural design, and operational duty cycle.