Magnetic Fields and Magnetic Diagnostics for Tokamak Plasmas

Magnetic fields and tokamak plasmas
Alan Wootton
Equilibrium
We first discuss the equilibrium.
From the basic field measurements themselves we can,
assuming circular straight geometry, reconstruct the current from the equations
j φ = 1
µ 0
r
j θ = − 1 µ 0
d
(
dr rB θ) 18.1
d
(
dr B φ) 18.2
i.e. to obtain j φ (r) we only need the radial dependence of B θ . Unfortunately we have to contend
with non circularity and toroidicity. One technique which has been applied is illustrated by the
results shown in Figure 18.3, where small pick-up coils were used to measure the poloidal
magnetic field at current peak in a small tokamak (TNT inJapan).
Figure 18.3. Equilibrium poloidal fields measured in TNT
We move to the coordinate system of Figure 1.7. The radial component B R (R 0 ,z) is measured
along a vertical line R = R 0 , and the vertical component B z (R,0) is measured along a line z = 0.
The results are fitted to expressions of the form
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