Page 20 - Fundamentals of Magnetic Thermonuclear Reactor Design

2 Fundamentals of Magnetic Thermonuclear Reactor Design

generated in the atmosphere by cosmic rays. For power engineering purposes,
tritium must be produced in man-made fusion or fission reactors through the
interaction of neutrons with lithium isotopes:

+
6 Li +→ T + He 4.8 MeV
4
n
4
n
Li6+n→T+He4+4.8 MeVLi7+n2.5Me 7 Li +→ T + He + ′ n – 2.5 MeV
V→T+He4+n′
1.2 PHYSICAL BASIS OF FUSION POWER ENGINEERING
The fusion reaction power is

v
Pfus=∫n n <σv>EfdVp P fus = ∫ n n < σ > EdV p (1.1)
12
f
1 2
where n = n + n ; n , n and ν are the mean plasma concentration, concentration
1
2
1
2
of interacting nuclei and their relative velocity, respectively; σ is the reaction
cross-section depending on v; <σν> is the reaction average intensity per pair
of interacting nuclei; E is energy released at one fusion event; and V is the
p
f
plasma volume.
The reaction maximum output is at n = n . For present-day tokamaks em-
2
1
ploying reaction 3, the highest possible concentration, fulfilling the confinement
−3
20
requirement, is close to 10 m , that is, five orders of magnitude smaller than
the atmospheric air concentration.
For fusion reaction to proceed in a vacuum chamber, a quasi-neutral hydro-
gen plasma is required, which must be kept thermally insulated from the cham-
8
9
ber walls and heated to ∼10 K (fusion reaction 3) or ∼10 K (fusion reactions
1, 2 and 4).
There are two possible approaches to solving the controlled thermonuclear
fusion problem: (1) isolate a relatively rarefied quasi-stationary plasma using an
external magnetic field (fusion reactors with magnetic confinement) and (2) get
28
−3
a dense (n ∼ 10 m ) hydrogen fuel capsule compressed from all sides in a
−8
pulsed mode (∼10 s), then heat the fuel to “fusion” temperatures and burn it
(inertial confinement fusion reactors). During the combustion, fuel particles do
not have time to disperse due to their mechanical inertia [1].
From here on, we shall restrict the discussion to magnetic fusion reactors
and focus on tokamaks.
In tokamaks, a required magnetic field configuration is achieved through
superposition of the poloidal field of the plasma current (the discharge current)
with an external toroidal (longitudinal) field. This field is generated by a set of
magnetic coils embracing the plasma column. The lines of a resultant magnetic
field are helical (corkscrew-shaped). This allows the suppression of a vertical
drift of ions and electrons caused by the radial gradient of the toroidal field
which, in turn, is a result of the longitudinal field being larger on the inside of
the torus, than outside.