Question 1

Why does fusion require such extreme temperatures?

Accepted Answer

Positively charged nuclei repel each other through the electromagnetic (Coulomb) force. To fuse, two nuclei must approach within ~1 femtometer (10^-15 m) where the strong nuclear force takes over. Classically this requires kinetic energies of ~1 MeV, corresponding to 10 billion K. Quantum tunneling allows fusion to occur at lower energies (~100 keV, or ~1 billion K), but even this is 6–7 times the Sun's core temperature. The Sun compensates with enormous mass and confinement time; laboratory fusion must reach higher temperatures to achieve adequate reaction rates in much smaller devices.

Question 2

What is the Lawson criterion?

Accepted Answer

The Lawson criterion, derived by John Lawson in 1955, states that the product of plasma density, energy confinement time, and temperature must exceed a threshold for fusion power to exceed the energy invested in heating: nτT > 3×10^21 keV·s/m³ (for D-T fusion). Increasing any of the three factors can compensate for deficiencies in the others. Tokamaks typically operate at moderate density (n ~ 10^20 m^-3) with long confinement times (τ ~ 1 s), while inertial confinement targets use extremely high densities for very short confinement times (nanoseconds).

Question 3

How does a tokamak confine plasma magnetically?

Accepted Answer

A tokamak uses two magnetic field components: a strong toroidal field (the long way around the torus) generated by superconducting coils wrapped around the vessel, and a poloidal field (the short way around) generated partly by a current driven through the plasma itself. These combine into helical field lines that wind around the torus on nested toroidal surfaces. Charged particles spiral along field lines, and if the field lines close on themselves after winding around the torus, particles are confined. The safety factor q (toroidal/poloidal winding ratio) must exceed 1 everywhere to avoid kink instabilities.

☢️ Fusion Reactor

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