Page 270 - Fundamentals of Magnetic Thermonuclear Reactor Design

Plasma Control System Chapter | 8 249

l Control of plasma current and plasma integral parameters (R , a , k ,
p
p
p
and δ ), plasma magnetic configuration (e.g. gaps between the first wall and
p
the plasma boundary), and magnetic configuration in the divertor region
(including the X-point position, the separatrix branches trajectory in the di-
vertor channels, and the location of strike points on divertor plates). This
also includes the correction of error fields disturbing the magnetic axis sym-
metry, and the corpuscular and magnetic diagnostics of the plasma column
position and shape.
l Control of divertor plasmas (density, fusion power, impurity content and
radiation power, gas puffing to the divertor chamber, etc.).
l Optimisation of the magnet configuration to improve the divertor performance.
l Fast plasma termination by an impurity/hydrogen injection.

This list of engineering problems reflects their phenomenological differenc-
es and hierarchy of concepts. Developing of plasma scenarios is the highest pri-
ority problem. Next comes control of the magnet configuration, because it has
to be adapted to a scenario algorithm. For the control of plasma configuration,
the magnetic measurements are sufficient without kinetic plasma parameters.
As to the quench of fusion, it is quite a stand-alone problem.
The discussed control systems have certain parametric and model uncertain-
ties, complicating their design and optimisation, namely, the ‘dynamic instabil-
ity’ of the plasma itself, the non-linearity of plasma processes and a host of other
factors to be accounted for. The latter include the following:
l Fast changes in the current (I ) and the plasma major radius (R ), internal
p
p
inductance (l ), and poloidal beta (β ).
i
p
l Currents induced in the complex metallic structures surrounding the plasma.
l Electromagnetic coupling between the plasma column, coils and induced
currents at the high power (tens of megawatt) required for the plasma control.
Hence, an optimisation design of control systems with feedback loops (also
called closed-loop control systems, CLCSs) is only possible using numerical
simulation that describes plasma behaviour and response to external perturba-
tions with the control actions.

8.3 BASIC DESIGN METHODOLOGY

Plasma control systems are often designed using the following sequence of logi-
cal and mathematical procedures:

l Providing a set of plasma basic static equilibrium states describing its evolu-
tion for the given variations of the plasma physical parameters. The latter,
generally, are integral parameters I , R , a , k , l , β , and ψ (ψ are mag-
res
res
p
p
p
p
i
p
netic flux losses due to plasma ohmic resistance).