Nuclear Fusion

Increasing any of these three quantities will increase the fusion-causing collision frequency, and thus the overall rate of fusion. The cross section is also itself a function of thermal energy in the nuclei. Cross section increases from virtually zero at room temperatures up to meaningful magnitudes at temperatures of 10 - 100 keV. At these temperatures, well above typical ionization energies, the fusion reactants exist in a plasma state.

For any given amount of fuel in a particular state, the rate of fusion in the fuel, f, is constant. Thus the measure of the actual net energy being released is a function of f (and in turn, the temperature), the number of particles in a particular area (its density), and the amount of time they remain together (the confinement time). This can be quantified by what is commonly called the fusion triple product, nTτ or pτ where p=nT.

Releasing useful energy from a fuel can thus take place at a low value of f. For instance, the conditions inside the sun are actually quite "poor", and the nuclei only undergo fusion once in every 10²⁹ seconds. However, the fact that the sun contains 10⁵⁹ nuclei means that the net reaction rate is actually quite high, and since the sun is around for billions of years, eventually the fuel is used up and the total energy released is huge.

On Earth, where fusion fuel is expensive and we have significantly less than a solar mass of available fuel, the rate of fusion must be considerably greater, and thus the temperatures much higher. The higher the temperature, the higher the pressure and the more difficult it is to confine the fuel plasma.

For any particular fuel there is a particular value of nTτ that will result in more energy being released than is required to heat the fuel to start the reaction, this is known as the Lawson Criterion. For the easiest reaction in D-T fuel, nTτ is about 10¹⁴ sec/cm³, a figure that has proven extremely difficult to achieve even after 50 years of trying.

The Lawson Criterion essentially defines a minimum lower bound where net power will be produced from the fusion reaction, often referred to as break even. Another important energy is the ignition point, where the heat generated in the reactions is enough to heat the fuel to fuse. This might sound like it would be the same number, but in fact it tends to be considerably higher because much of the energy generated will tend to "escape" any reasonably sized machine. This is not a concern in a star, where the particles will eventually react with other parts of the star, but in a Earth-bound machine keeping all of the energy in the system is much more difficult. A reactor does not have to reach the ignition point in order to be a useful power generator. However, ignition remains one of the main goals of most research systems.

D + D → T (1.01 MeV) + p (3.02 MeV)

D + D → He³ (0.82 MeV) + n (2.45 MeV)

T + T -> He⁴ + 2 n (11.3 MeV)

He³ reactions

He³ + He³-> He⁴ + 2 p

D + He³ → He⁴ (3.6 MeV) + p (14.7 MeV)

T + He³ → He⁴ (0.5 MeV) + n (1.9 MeV) + p (11.9 MeV) (51%)

T + He³ → He⁴ (4.8 MeV) + D (9.5 MeV) (43%)

T + He³ → He⁵ (2.4 MeV) + p (11.9 MeV) (6%)

p + Li⁶ → He⁴ (1.7 MeV) + He³ (2.3 MeV)

D + Li⁶ → 2 He⁴ (22.4 MeV)

He³ + Li⁶ → 2 He⁴ + p (16.9 MeV)

n + Li⁶ → T + He⁴

n + Li⁷ → T + He⁴ + n

p + B¹¹ → 3 He⁴ (8.7 MeV)

• Joint European Torus JET
• ITER

Inertial containment

• NIF
• inertial electrostatic confinement

External links