Fusion Reaction Visualizer Beta

Q-value and mass deficit

The Q-value equals the mass difference between reactants and products, converted to energy via E = Δmc². For D–T fusion, the combined rest mass of deuterium (2.01410 u) and tritium (3.01605 u) exceeds that of helium-4 (4.00260 u) and a neutron (1.00866 u) by 0.01889 u, which corresponds to 17.6 MeV.1 This energy appears as kinetic energy of the products, split by conservation of momentum: the lighter neutron carries ~80% (14.1 MeV) and the heavier alpha carries ~20% (3.5 MeV).

The Coulomb barrier

Two positively charged nuclei repel each other via the Coulomb force F = kZ₁Z₂e²/r². The resulting potential barrier E_C = 1.44⋅Z₁Z₂/(R₁+R₂) MeV·fm must be overcome for fusion.2 For D–T (Z₁Z₂ = 1), the barrier is only ~0.4 MeV. For p–¹¹B (Z₁Z₂ = 5), it rises to ~1.9 MeV. In practice, quantum tunneling allows fusion at thermal energies far below the classical barrier. The D–T reaction proceeds efficiently at ~10–20 keV (~100–200 million K), which is why it was the first to be considered for energy production.

Why D–T is the baseline fuel

The fusion reaction rate scales as n₁n₂⟨σv⟩, where ⟨σv⟩ is the Maxwell-averaged reactivity.3 D–T has the highest ⟨σv⟩ at the lowest ion temperature, exceeding D–D by a factor of ~80 at T_i = 10 keV. This means a D–T plasma requires the lowest triple product nTτ_E for ignition (the Lawson criterion).4 ITER is designed to demonstrate Q = 10 (10× more fusion power than heating power) using D–T fuel at T_i ≈ 15 keV.

Energy gain and self-heating

The energy gain factor Q = P_fusion/P_input measures how much energy a plasma produces relative to what is pumped in. At Q = 1 ("scientific breakeven"), the plasma returns as much fusion energy as it absorbs. At Q = 5, alpha particle heating begins to dominate, and at Q → ∞ the plasma reaches ignition: it sustains its own temperature without any external heating.4 A burning plasma is effectively an energy avalanche, with each generation of alpha particles heating the fuel for the next round of fusion reactions. Achieving and controlling this self-sustaining burn is the central goal of ITER and the next generation of fusion devices.

Energy partition and confinement

Charged fusion products (α particles, protons) are confined by the tokamak's magnetic field and deposit their energy in the plasma, sustaining its temperature (self-heating or "burning plasma"). Neutrons, being electrically neutral, stream through the magnetic field and deposit energy in a lithium blanket surrounding the plasma, where they also breed tritium via &sup6;Li + n → T + &sup4;He.5 The fraction of energy in neutrons determines how much power can self-heat vs. how much goes to the blanket.

Aneutronic fusion

Reactions like D–³He and p–¹¹B produce only charged particles as primary products. This eliminates neutron radiation damage, neutron activation of structural materials, and the need for a tritium breeding blanket.6 In principle, charged products could be converted directly to electricity using electrostatic or magnetohydrodynamic converters, bypassing the thermal cycle entirely. The fundamental challenge is that these reactions require much higher plasma temperatures (50–300 keV), where bremsstrahlung radiation losses become significant and harder to overcome.7

The basics

Every atom has a nucleus made of protons and neutrons. Fusion is what happens when two light nuclei collide hard enough to merge into a heavier one. When they do, the new nucleus weighs slightly less than the two that made it. That missing mass becomes energy, a lot of it, following Einstein's E = mc². This is the opposite of fission, which powers today's nuclear plants by splitting heavy atoms like uranium apart. Fusion combines light atoms instead. Pound for pound, it releases several times more energy, and its fuel (hydrogen isotopes) can be extracted from seawater.

Fusion in nature

Fusion is how every star works. The Sun fuses 600 million tons of hydrogen into helium each second, and has been doing so for 4.6 billion years. The catch is that it takes extreme temperatures, over 100 million degrees, to force nuclei close enough to fuse, because their positive charges naturally repel each other. Stars use their enormous gravity to sustain these conditions. On Earth, we have to find another way.

Approaches on Earth

The tokamak is the most mature approach: a doughnut-shaped chamber that confines plasma with a combination of toroidal and poloidal magnetic fields. ITER in France, Japan's JT-60SA, and China's EAST are among the most prominent tokamak experiments today. The stellarator, like the Wendelstein 7-X in Germany, uses a twisted, carefully optimized coil geometry to confine plasma without needing a plasma current, which avoids certain instabilities that tokamaks face.

Inertial confinement takes a completely different approach, using powerful lasers or ion beams to compress a tiny fuel pellet until it implodes and fuses. The National Ignition Facility (NIF) at Lawrence Livermore achieved scientific breakeven in late 2022 with this method. There are also electrostatic devices like fusors and polywells that use electric fields to accelerate and confine ions, magnetized target fusion (General Fusion), field-reversed configurations (TAE Technologies), Z-pinch machines (Zap Energy) that compress plasma with its own magnetic field, and micro-scale fusion concepts (Avalanche Energy) exploring compact, modular reactor designs.

The fuel is abundant, the waste is minimal, and there is no risk of meltdown. The hard part is sustaining the extreme conditions long enough to get net energy out.