Magnetic Fields and Magnetic Diagnostics for Tokamak Plasmas

Magnetic fields and tokamak plasmas
Alan Wootton
Flux gates
This is intended for weak fields. A B-H curve below is shown below.
Figure 2.3 a A simple flux gate and the B-H curve
An applied field H to the core induces a magnetic flux B = mH. For high B the material saturates
and µ is very small. There is hysterisis, and the path is different for increasing and decreasing H.
When the core is not saturated the core acts as a low impedance path to lines of magnetic flux in
the surrounding space. When the core is saturated the magnetic field lines are no more affected
by the core. Each time the core passes from saturated to unsaturated and backwards, there is a
change to the magnetic field lines. A pickup coil around the core will generate a spike. Flux
lines drawn out of core implies positive spike, lines drawn into core, a negative spike. The
amplitude of the spike is proportional to the intensity of the flux vector parallel to the sensing
coil. The pulse polarity gives the field direction.
The core must be driven in and out of saturation by a second coil. The excitation current will
induce a corresponding current in the sensor coil, but this can be allowed for.
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