Page 115 - Fundamentals of Magnetic Thermonuclear Reactor Design

Simulation of Electromagnetic Fields Chapter | 4 99

(
[
t
2
current is It() =Φ ()] / µ R − R 2 − r 2 ) , where Ф(t) is the time dependence I(t)=[Φ(t)]/µ R−R −r ,
2
0
0
of the toroidal flux variations in DINA simulations.
Currents in Toroidal Field Coils: Evolutions of total currents in the TFCs are
taken to be known. The coils are modelled via a set of straight and arc-shaped
conductors with rectangular cross-sections, assuming a uniform current density
over the cross-sections.
Currents in Central Solenoid and Poloidal Field Coils: Central solenoid sec-
tions and PFCs are modelled as ring-shaped conductors with rectangular cross-
sections, assuming a uniform current density over the cross-sections. Evolutions
of the coil currents are given for every plasma scenario.
Halo Currents: Halo currents [10] come from the plasma to the discharge
chamber first wall and close in its conducting components producing high EM
loads due to the interaction of the halo current poloidal components with the
toroidal magnetic field.
DINA provides data on halo currents during plasma current disruptions in
the following forms:
l A drift of positions of the plasma contact with the conducting components
during modelled scenarios.
l Full currents passing between the plasma and conducting components in the
contact regions at every time point.
DINA data are used to create a phenomenological model for the eddy currents
induced in the tokamak’s conductive components during transients. To evaluate
the halo current effect, we use the following technique. Additional conducting
regions are introduced in the computational model to connect the pre-determined
inlet and outlet areas of the halo current. These regions model the halo currents
passing through the plasma to form closed electric circuits at every time point of a
plasma scenario. A special feature of such algorithm is that eddy currents cannot
appear in the additional conducting regions, unlike real conducting components
described in a global model. These specific conducting regions connect so-called
inlets and outlets of the halo current, that is, zones for the inward and outward
currents on conducting components in the phenomenological model, and provide
continuous halo currents. This means that the continuity equation is satisfied for
the current density vector in the quasi-stationary approximation. This is important
to enable physically based results. Neglect of the continuity condition in some
studies has led to essential errors. Full currents through the additional regions are
described in conformity with the halo current variations. According to the degree
of elaboration of this region, detailed or rough halo current models can be built
(Fig. A.4.2.1). In simulations of the components located far away from the halo
current inlets/outlets, a rough description is suitable.
For components located near the halo current inlets/outlets, a detailed model
is required with an extensive description of the current variations in space and
time.