Mirnov coil most frequently used in a set of Mirnov coils or Mirnov array - an inductive sensor (search coil) in form of a cylindrical coil used for detection of Mirnov oscillations or position of plasma in tokamaks. The Mirnov coils can be placed in a poloidal or toroidal configuration.
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A paper published in 1971 by Mirnov and Semenov refers to “magnetic probes”, which were used for detection of “large-scale instabilities in the tokamak”.1)
The coils are wound on a non-magnetic former to detect plasma instabilities. Plasma contains moving charges, hence electric current, which generates flux density around itself. This varying magnetic flux density induces voltage signal in the coils, according to the Faraday's law of induction.
For instance, the GOLEM tokamak uses a set of four poloidal Mirnov coils, placed 90° around the plasma chamber. A 0.34 mm wire is wound in two layers: 6 mm and 8.4 mm diameter, respectively, giving the total of 91 turns, inductance 14 μH and resistance 1.055 Ω. Each coil is placed inside a ceramic porolite cylinder for protection against plasma particles.2)
The coils are used in sets or “arrays”, which are located around the plasma chamber, each array comprising from four to tens of individual coils.
The coils are located inside of the plasma chamber, but close to the walls (outside of the so-called limiter radius) in order to minimise the destructive effects of hot plasma, which is contained mainly around the centre of the chamber cross-section.