The Central Solenoid

The central solenoid is the transformer coil that ramps the plasma current at startup. Its flux demand sets part of the machine's startup budget.

What

A solenoid acting as the primary of a transformer

Job

Inductively ramp the plasma current

Limit

Finite flux swing → finite pulse without current drive

Analysis

Startup / ramp flux demand (S05)

The central solenoid is a tall coil running up the center of a tokamak that acts as the primary winding of a transformer, with the plasma as its single-turn secondary. Ramping the solenoid's current changes its magnetic flux, which induces and drives the plasma current — the standard way to start a tokamak and drive current inductively.

A solenoid can only swing its flux so far, so inductive drive alone gives a finite pulse; sustained operation needs non-inductive current drive and the self-generated bootstrap current to take over. Kronos MetroVolt's deposited analyses include the startup and ramp flux demand (S05), which sizes the solenoid's role in getting the plasma to its operating point in the space-constrained center stack.