The thermal energy of plasma ions — the species that actually fuse, requiring 10–20 keV for significant deuterium-tritium reaction rates.
Ion temperature (Ti) measures the average kinetic energy of the fuel ions (typically deuterium and tritium) in a fusion plasma. Because fusion reactions occur when ions collide at sufficient relative velocity, ion temperature is the most direct determinant of the fusion reaction rate. The D-T fusion cross-section peaks at an ion energy of roughly 64 keV in the centre-of-mass frame, but significant fusion power can be produced at bulk ion temperatures of 10–20 keV because the Maxwellian velocity distribution always contains a population of faster-than-average ions in its high-energy tail.[1]
Below roughly 4–5 keV, the D-T fusion reaction rate is negligible for energy production purposes. The volumetric fusion power density rises steeply with temperature between 5 and 15 keV, and the fusion reactivity ⟨σv⟩ reaches a broad maximum near 60–70 keV. Practical reactor designs target 10–20 keV as an optimal operating range, balancing high reactivity against increasing radiation losses and the difficulty of confining very hot plasmas.[2]
Several heating methods can raise the ion temperature to the required range. Neutral beam injection (NBI) accelerates deuterium atoms to 50–100 keV and injects them into the plasma, where they ionise and transfer energy primarily to the ions through Coulomb collisions. Ion cyclotron resonance heating (ICRH) uses radiofrequency waves at the ion cyclotron frequency (typically 30–80 MHz in tokamaks) to resonantly accelerate ions. Both techniques have demonstrated central ion temperatures above 20 keV in large tokamaks.[1]
JT-60U in Japan achieved the highest ion temperatures in a tokamak, reaching approximately 45 keV (520 million degrees) in dedicated high-Ti experiments with deuterium plasmas. JET produced the current record D-T fusion energy (59 MJ in 2021) with central ion temperatures of roughly 10–12 keV sustained over five seconds. ITER aims for Ti ≈ 20 keV in its burning-plasma scenarios.[3]
Ion temperature is measured by charge-exchange recombination spectroscopy (CXRS), which analyses the Doppler broadening of spectral lines emitted by impurity ions that have captured an electron from injected neutral beam atoms. Neutron spectroscopy provides an independent measure of Ti in D-D and D-T plasmas by analysing the energy spectrum of the emitted fusion neutrons.[1]