Magnetic Fields and Magnetic Diagnostics for Tokamak Plasmas

Magnetic fields and tokamak plasmas
Alan Wootton
where the values of α s and α f , derived from the expressions in table 1, are given in table 2.
Without the correction factors α s and α f , z app = z. Moving the sensors from inside to outside
changes the sign of α s . When the plasma (I p > 0, into plane) moves upwards (z > 0) it will
induce a negative I s . For sensors outside the vessel, the vessel currents then produce an apparent
displacement smaller than the real displacement (i.e the vacuum vessel shields the sensing coils).
For sensors inside the vessel, the apparent displacement is larger than the true displacement. The
feedback windings complicate this simple process. If we wanted to, the effects of the feedback
coupling to the sensors (i.e. finite α f ) can be removed by adding to each signal a term
proportional to I f itself. This cannot be done for the coupling from the vessel to the sensors,
because we do not measure the vessel dipole currents.
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