1 - Nuclear Sciences and Applications - IAEA

740 PEREVERZEV
current drive producing a force, for plasma equilibrium only. In the equilibrium configuration
that is produced a plasma flow determining the particle and energy lifetime
in the system emerges. The transport model developed on the basis of this theory of
equilibrium is used in this paper for explaining some experimental results and also
for selecting optimal conditions for the power input into a tokamak.
In Section 3 current ramp-up in a tokamak is considered. It is shown that the
current ramp-up rate, as a function of input power and plasma density, has a maximum.
This maximum is a universal parameter of the tokamak, since it depends only
on its dimensions and on the energy confinement time. The efficiency of the conversion
of external source energy into the energy of a poloidal magnetic field has a similar
dependence. Optimized scenarios of current ramp-up are suggested. These
scenarios allow either current rise time or external source energy consumption to be
minimized at this stage. Even in this case the non-inductive current rise stage in a
tokamak of the reactor type appears to be rather long, about a few tens of minutes.
2. IONIZATION STAGE
Let us assume a strong toroidal magnetic field Bv in the tokamak chamber and
a much weaker transverse field Bx with a vertical component Bvert. If the neutral
gas density at the beginning of the ionization stage has a value N, the ionization
process will be represented by a zero dimensional set of equations for the energy and
particle balance:
— = -— + n(N - n) (1)
dt r
2 dt " 2 T V
} ^ | + Pei (3)
2 dt 2 T
Here n is the density and Te and T, are the temperatures of the electrons and the
ions. The source of particles in Eq. (1) is written on the assumption that there is complete
recycling, i.e. conservation of the amount of particles in the plasma volume.
Qinp is the total input power, V is the plasma volume, Pei is the heat exchange
between electrons and ions, and T is the confinement time of particles and energy.
The last quantity expresses the main peculiarity of the given model and is chosen
from the following considerations.