\GWfig{0.8}{Tokamak/ToroidalElectricField/SchemaTransformator}{}{}
A change of magnetic flux $\Phi_{tor}$ in the central column of the transformer core, see \see{fig:}{http://buon.fjfi.cvut.cz/golem/?p=Tokamak/IronTransformerCore}, induces an electric field $E_t$ in the chamber. This field accelerates particles, results in plasma breakdown and drives the current. Plasma is the secondary winding of the transformer in this case. $U_l$ measures a voltage per one turn around the chamber in the toroidal direction. 
\begin{equation}
U_l\;=\;\oint_{l_{tor}}~\mathbf{E_t}\cdot \mathrm{d}\mathbf{ l}\;=\;-\frac{\mathrm{d}\Phi_{tor} }{\mathrm{d}t}
\label{Eq:Uloopdefinition}
\end{equation}