25.1 Nuclear Fusion and Ignition Conditions

Nuclear fusion is the combination of light nuclei into a heavier nucleus with net energy release. At reaction level, conservation laws still hold: nucleon number and charge are conserved, and the small reactant-product mass difference appears as kinetic energy and radiation. For stars, this microscopic release is the source of macroscopic luminosity.

A deuterium-tritium channel is a useful benchmark because its Q-value is large and its ignition conditions are less severe than many alternatives. But stars are mostly hydrogen, so the proton-proton route dominates in solar-type stars despite a lower effective reaction rate. This distinction between reaction convenience in labs and abundance-driven pathways in stars is a recurring exam theme.

Two nuclei approaching each other are both positively charged, so electrostatic repulsion opposes fusion. For fusion to happen, nuclei must come within strong-force range. High temperature raises average kinetic energy and widens the high-energy tail of the particle distribution, increasing the chance of close approach. Without this thermal energy scale, fusion cross-sections become negligibly small.

In real stellar plasmas, quantum tunneling matters: some nuclei with less-than-classical barrier energy can still fuse. That is why fusion can proceed in stellar cores at temperatures lower than naive barrier-overcoming estimates. In IB reasoning, you should still frame the trend correctly: higher temperature generally improves fusion probability by raising effective barrier crossing opportunities.

Temperature alone is not enough. You also need enough collision opportunities (density) and enough time at fusion-capable conditions (confinement time). In stars, gravity naturally provides confinement over vast timescales. In reactors, plasma confinement is the central engineering challenge. Always articulate these three ingredients together when asked why practical fusion is difficult.