Page 269 - Fundamentals of Magnetic Thermonuclear Reactor Design

248 Fundamentals of Magnetic Thermonuclear Reactor Design

The methods of correction of the magnetic error fields due to inaccuracies
of tokamak manufacturing and assembly are considered. The problems of the
plasma position and shape reconstruction based on magnetic field measure-
ments are discussed.

8.2 SCOPE OF THE CONTROL SYSTEM DESIGN PROBLEM
The fusion reactor’s operation cycle includes several stages. Actually, it con-
sists of a series of electro-physical and electro-technical operations, which are
characterised by stationary, variable and alternating electromagnetic fields. The
tokamak’s typical operation cycle includes the following stages and plasma
processes:

l initial magnetisation of the poloidal field coils
l D–T gas breakdown in the vacuum vessel and plasma initiation
l plasma current ramp-up
l formation of a divertor configuration
l auxiliary heating of plasma
l plasma burn phase
l plasma current termination and quench of fusion.
We remember that a stationary toroidal field must be provided before the
beginning of the operation cycle.
Topologically, the tokamak plasma magnetic field is a set of closed magnetic
flux surfaces, placed one inside another. It is characterised by the plasma minor
radius a and major radius R . For an elongated plasma, there are the elonga-
p
p
tion k and triangularity δ coefficients. Such magnetic flux surfaces can be
p
p
expressed as a parametric function:
rR + a cos( +θδ p sin ),
θ
=
p
p
=
θ
r = Rp + apcos(θ + δpsinθ),z = apkpsinθ, z ak sin,
pp
where θ is the poloidal angle.
Last closed magnetic flux surfaces are often described using geometric pa-
rameters a , R , k and δ , measured/computed upon an assumption that the
95
95
95
95
magnetic flux surface has 95% of the total poloidal magnetic flux. This is espe-
cially convenient when dealing with plasmas in divertor configurations, where
the separatrix topologically separates magnetic field lines closed within the
plasma region from those closed around the poloidal field coils.
The synthesis of the plasma control system leads to an optimisation solution
of a complex of interconnected engineering problems, such as the following:
l Developing programmed scenarios for different stages of the plasma dis-
charge.