Page 200 - Fundamentals of Magnetic Thermonuclear Reactor Design

182 Fundamentals of Magnetic Thermonuclear Reactor Design

for nuclear fusion applications. All four fusion reactions take place in it concur-
rently, although at different speeds.
The D–T reaction produces 14 MeV neutrons and ‘ash’ (α-particles and
20
protons) at a ratio of around 100:1. (2–4) × 10 of He nuclei are synthesised
each second in a tokamak reactor with a fusion power of 500–1000 MW [8].
The plasma magnetic confinement is not absolute. There is some probability
of ion diffusion across the magnetic field. In this case, the diffusion-induced ion
lifetime τ that characterises this feature of the ions’ behaviour, is of the same
i
order as the energy confinement time. Using the τ , one can express the particle
i
flow from plasma onto wall N as
nV
ip
N = ,
N=niVpτi, τ i
where n is the average ion concentration in the plasma and V is the plasma
i
p
volume.
Plasma is transparent to neutrons, and neutron energy is utilised in the blan-
ket. Alpha particles, deuterium and tritium ions and neutral particles diffusing
towards the chamber walls transfer their energy to plasma ions and the FW.
While some ions penetrate the material, others re-enter the plasma in the form
of ‘cold’ atoms after neutralisation and re-emission from the walls. In plasma,
they get re-ionised. As a result, a more or less the same ion flux with energy
corresponding to the temperature of the plasma column’s outer layers gets back
to the wall. These particles cause sputtering of the walls. However, most of the
particles get back into the plasma and maintain the fusion fuel recycling. The
wall also has to withstand an electron flux.
Plasma is a source of not only corpuscular but also electromagnetic radia-
tion, which is mostly due to the slow-down of electrons by nuclei electric field
and electron movement in a magnetic field. Plasma electromagnetic radiation
is sharply intensified by the entry of impurity ions, especially heavy ones. The
total energy of radiation losses is proportional to the impurity concentration, n ,
Z
and electron concentration, n . The radiation flux density is expressed as
e
(,
Qrad=nenZ⋅f(Z,Te). Q rad = nn ⋅ fZ T ).
e Z
e
Even at a small Z, the intensity of impurity radiation is several orders of
magnitude higher than that of hydrogen. At a ‘lethal’ concentration of impu-
rity ions,

Cleth=2.34⋅Z−1.6, C leth = 2.34 ⋅ Z −1.6 ,
a self-sustained fusion reaction becomes impossible. For example, the lethal
concentration for molybdenum and tungsten is 0.6% and 0.2%, respectively.
In experimental tokamaks, where wall loads are far and away lower than
in projected fusion reactors, the concentrations of heavy ions are close to
0.1%–0.2%.