Mirnov Coils

Fast magnetic pickup coils resolve MHD fluctuations and mode structure, the fabric's earliest window on instability and disruption.

Fast magnetics

Mirnov coils are small pickup coils that measure the time derivative of the local magnetic field. Distributed poloidally and toroidally, they resolve the spatial structure and rotation of MHD modes. Their bandwidth is high, which is why the acquisition chain samples them below the microsecond — the fastest disruption precursors live here.

Mode analysis

• Toroidal and poloidal mode numbers (n, m) are extracted from the phase relationship across the array.
• Mode amplitude and rotation frequency become precursor features.
• A slowing, growing n=1 mode is the classic locked-mode disruption warning at 9.66 MA.
• Phase coherence across coils depends on the single disciplined acquisition clock.

Why phase must be exact

Mode analysis is entirely a phase inference across coils. A per-channel timing skew of even a fraction of a sample would appear as a spurious mode. This is the sharpest reason the fabric enforces one clock and precise timestamping — the Mirnov array is the most timing-sensitive diagnostic in the constellation.

Burner analogue

The burner is a tandem mirror, not a tokamak, but fast magnetics still watch for instabilities of the confined plasma and interchange behavior near the high-field plugs. The coil layout differs, but the fabric's mode-extraction and precursor-feature machinery is shared. Coil positions and calibrations are versioned per machine, and integrated slow flux comes from the complementary flux loops.