Magnetic Fields and Magnetic Diagnostics for Tokamak Plasmas

Magnetic fields and tokamak plasmas
Alan Wootton
18. INTERNAL PLASMA MEASUREMENTS
So far we have been concerned with measurements of fields taken outside the plasma. In
comparatively low temperature plasmas (say T e < 50 to 100 ev), we can design pickup coils to
make internal measurements. There is always the worry that the insertion of such a coil changes
the very plasma characteristics we would like to determine. This fear is usually allayed by
monitoring certain characteristic plasma features (sawtooth activity, Mirnov activity, loop
voltage) to make sure they do not change significantly during probe insertion. Figure 18.1 and
Figure 18.2 show a possible coil set up which might be used. The coil itself must be protected
from the plasma, typically by a stainless steel case, possibly surrounded by a carbon shield. The
geometry of the surrounding materials must be carefully chosen if we are looking at high
frequencies so as not to cut off the very signals we want to measure.
Figure 18.1. A possible internal coil
construction.
Figure 18.2. A typical coil placement,
showing the flexible belows.
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