\def\descr{Tokamak (GOLEM) basic concept}

\def\spim#1#2{\item {\bf #1}: #2}

\def\default{
\GWif{width=0.8\tw}{Theory/Fusion/FusionIntro/TokamakTechnologyBriefly/fig/Schematic-of-a-tokamak-chamber-and-magnetic-profile.jpg}{Tokamak - Schematic, credit \cite{wiki:GeneralTokamakConcept}}
The basic structure of a tokamak is shown in figure \ref{Fig:Theory/Fusion/FusionIntro/TokamakTechnologyBriefly/fig/Schematic-of-a-tokamak-chamber-and-magnetic-profile.jpg} (more information can be found e.g. in \cite{wesson99science}). Tokamaks comprise three essential parts:
\begin{itemize}
\spim{vacuum chamber}{ has the shape of a torus of the size approximately 1-20 m across; its purpose is to contain the plasma while allowing limited access through diagnostic ports.}
\spim{toroidal field coils}{Around the vacuum chamber are wrapped dozens of \emph{toroidal magnetic field coils}, which generate the confining toroidal magnetic field $B_t$ (0.5-5 T). The confining magnetic field structure is the result of the superposition of the toroidal magnetic field $B_t$ generated by external coils and the poloidal magnetic field $B_p$ generated by a strong toroidal plasma current $I_p$ induced by the transformer coil. In the resulting magnetic geometry, field lines are winding helically around a torus surface, which is called the magnetic surface. The tokamak magnetic field
consists of such nested magnetic surfaces. The helical structure at each magnetic surface is described by the safety factor $q$.}
\spim{transformer}{creates and heats the plasma by inducing a loop voltage $U_{loop}$ (several V) inside the vacuum chamber and then driving a plasma current $I_p$ (kA to MA).}
\end{itemize} 
\par
Because of this structure, the duration of tokamak plasma existence is intrinsically limited. The plasma can only exist so long as the plasma current $I_p$ is driven, because the ohmic heating $P_{OH} = U_{loop}.I_p$ sustains its high temperature in spite of continuous heat losses. (It also ensures plasma stability, but that is outside the scope of this manual.) And since driving a current in the secondary coil (plasma) requires a monotonically changing current in the primary coil (shown in figure \ref{fig:tokamak}), which cannot be done forever, at some point the primary coil current reaches a maximum and the transformer stops transforming. Presently the plasma current dies out, the plasma cools down, electrons and ions recombine into a neutral gas and the plasma ceases to exist. Therefore, tokamak plasmas are created in so called \emph{discharges}, or \emph{shots} for short. Discharge duration strongly depends on the machine --- on GOLEM it is $<20$ ms, on the largest machines it is $>1$ s.
}



\def\GWfigure#1{
\GWifigure{#1}{Theory/Tokamaks/BasicConcept/Sketch.golem/fig/drawing.pdf}{\descr}{fig:Theory/Tokamaks/BasicConcept/Sketch.golem}
}

\def\GWpar{
\GWfigure{width=0.8\tw}
The basic mission of the tokamak technology is to  \emph{heat} and
\emph{confine} for a sufficiently long time the fusion fuel (deuterium-tritium
mixture) up to $\approx$ 100 milions degrees Celsius - a sufficient temperature
to fuse these light nuclei into a heavier Helium nucleus and a neutron. At these
high temperatures the fuel is in a state of matter called plasma: a collection
of ionized nuclei and free electrons. These charged particles are confined by a
strong magnetic field.
The
fusion reaction products have a significantly higher kinetic energy and heat the
surrounding plasma. This fusion-generated heating power can then be used for
electricity production.
\par
The basic structure of a tokamak (see e.g. \cite{wesson99science}) can be seen
in Figure \ref{fig:Theory/Tokamaks/BasicConcept/Sketch.golem}. The confining magnetic field structure is the result of the superposition of the toroidal magnetic field $B_t$ generated by external coils and the poloidal magnetic field $B_p$ generated by a strong toroidal plasma current $I_{p}$ induced by the transformer. 
}


\def\GWparzmb{The tokamak (see Fig. \ref{fig:Theory/Tokamaks/BasicConcept/Sketch.golem}) is a device where in its simplest possible description it is necessary to heat the fuel /deuterium-tritium mixture/ to the plasma state up to 100 milions centigrades while securing the insulation of such a hot media from the reactor vessel. Both requirements are reached providing a very  special configuration of electric and magnetic fields applied on the toroidal geometry of the reactor.}