Internal magnetic field structure and parallel electric field profile evolution during the sawtooth cycle in MST

Temporal dynamics of the magnetic field, parallel current density, parallel electric field and safety factor profiles in the core of a high-temperature reversed-field pinch plasma are experimentally resolved. Measurements are realized using a high-speed polarimeter–interferometer diagnostic which is employed to simultaneously determine the electron density, toroidal current density and poloidal magnetic field profiles. Combined with external magnetic measurements, these data allow determination of the equilibrium profile dynamics during individual sawtooth magnetic relaxation events. At the sawtooth crash, the E∥ profile has a large positive peak on-axis, and a negative peak of slightly smaller value near the reversal surface demonstrating the need for a dynamo to sustain the plasma equilibrium.

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