Internal inductance

The self-inductance of a current loop is defined as the ratio of the magnetic flux Φ traversing the loop and its current I:

$L=\Phi /I\,$

The flux is found by integrating the field over the loop area:

$\Phi =\int _{S}{{\vec {B}}\cdot d{\vec {S}}}$

On the other hand, the energy contained in the magnetic field produced by the loop is

$W=\int _{V}{{\frac {B^{2}}{2\mu _{0}}}d{\vec {r}}}$

It can be shown that

$W={\frac {1}{2}}LI^{2}$

The internal inductance is defined as the part of the inductance obtained by integrating over the plasma volume P :

${\frac {1}{2}}L_{i}I^{2}=\int _{P}{{\frac {B^{2}}{2\mu _{0}}}d{\vec {r}}}$

Its complement is the external inductance (L = L_i + L_e).

In a tokamak, the field produced by the plasma current is the poloidal magnetic field B_θ, so only this field component enters the definition. In this context, it is common to use the normalized internal inductance per unit length, defined as

$l_{i}={\frac {L_{i}}{2\pi R_{0}}}{\frac {4\pi }{\mu _{0}}}={\frac {2L_{i}}{\mu _{0}R_{0}}}$

and similar for the external inductance. The value of the normalized internal inductance depends on the current density profile in the toroidal plasma.