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How does a solenoid induce an electric current in tokamak plasma?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

In a tokamak, a solenoid positioned in the central hole of the torus is used to generate magnetic flux that increases over time. According to Faraday’s law of induction, this time-varying magnetic flux induces a toroidal electric field within the vacuum chamber. This electric field acts upon the charged particles in the plasma, driving a toroidal electric current that is essential for creating the poloidal magnetic field required for stable plasma equilibrium.1

While this method is an efficient way to initiate and build up a plasma current, it is inherently limited to pulsed operation because it cannot sustain a steady-state current, which would require the magnetic flux to increase indefinitely. Due to the engineering challenges associated with pulsed reactors, such as materials fatigue, other techniques like radio-frequency current drive and neutral-beam current drive are often employed to maintain steady-state operation.1