Frontier physics
Nuclear fusion
Fusion combines light nuclei into heavier products. A positive mass defect can release energy, while reaction rates depend strongly on temperature and nuclear cross-sections.
Two positively charged nuclei must approach closely enough for the strong nuclear force to dominate Coulomb repulsion. High temperature populates the energetic tail of the velocity distribution; quantum tunnelling also matters, so not every reacting pair must classically surmount the Coulomb barrier.
Definition: Reaction Q-value
For Q>0 the products have less total rest mass than the reactants; the difference becomes product kinetic energy and, depending on the reaction, radiation. Positive Q alone does not guarantee a system delivers more useful energy than it consumes.
Temperature and confinement
In a fusion plasma, particle density n, temperature T, and energy-confinement time τ_E jointly govern ignition conditions. The Lawson criterion is expressed through nτ_E (or nTτ_E), with a threshold that depends on fuel and temperature. Heat loss, radiation, and impurities all raise the required input power.
Example: Reaction energy
For D + T → ⁴He + n, Q is about 17.6 MeV. If one reaction occurs, what is the approximate total kinetic energy of the two products in the center-of-mass frame?
Solution
About 17.6 MeV, by energy-momentum conservation when initial relative kinetic energy is small compared with Q. The energy is not shared equally: the neutron receives about 14.1 MeV and the alpha about 3.5 MeV.
An integrated energy criterion must distinguish fusion power from external power delivered to the plasma. In a deuterium–tritium mixture, charged alpha particles can self-heat the plasma if their energy is confined long enough; neutral neutrons escape magnetic confinement and deposit energy in a blanket. The balance is also shaped by bremsstrahlung, turbulent transport, wall exchange, and the efficiency of converting heat to electricity. Thus the Lawson condition is necessary in an idealized model, not a guarantee of a power plant's net energy gain. Current research must couple particle kinetics, MHD stability, plasma–material interaction, and the tritium fuel cycle.
A further design constraint is heat and particle loading on the chamber wall and blanket. Energetic neutrons displace atoms, generate helium through nuclear reactions, and alter material properties; the blanket must also remove heat and breed tritium. Lifetime predictions combine irradiation experiments, neutron-transport models, and materials data, with uncertainties in extrapolation stated explicitly.
Quick check
What does Q>0 mean for a fusion reaction?
Why does plasma temperature not specify a single collision energy?
References
- T. J. M. Boyd, J. J. Sanderson (2003). The Physics of Plasmas
- A. A. Harms et al. (2000). Principles of Fusion Energy