Central solenoid

The large coil through the middle of a tokamak that works as a transformer, with the plasma ring acting as its secondary winding: changing the coil's current induces the current in the plasma and also heats it. A transformer can only keep changing its current for so long, so a tokamak run this way is pulsed. Running without it, called non-inductive current drive, relies on bootstrap current and external current-drive systems, and is what a 'fully non-inductive' result means. Spherical tokamaks have very little room at the centre for a solenoid, which makes other ways of driving current more pressing for them.

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concept:central-solenoid

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