The thermal energy of plasma electrons, measured in kiloelectronvolts — where 1 keV corresponds to roughly 11.6 million degrees Celsius.
Electron temperature (Te) describes the average kinetic energy of electrons in a plasma. In fusion physics, temperature is conventionally expressed in electronvolts (eV) or kiloelectronvolts (keV) rather than kelvins, using the equivalence 1 eV = 11,604 K. A plasma with Te = 1 keV therefore has electrons at an effective temperature of approximately 11.6 million degrees Celsius — hotter than the core of the Sun.[1]
Electrons play several critical roles in a fusion plasma. They carry the bulk of the plasma current in a tokamak (and thus sustain the poloidal magnetic field), they mediate energy transfer between ions via Coulomb collisions, and they are the primary channel for radiative energy loss. The electron temperature profile therefore influences current drive efficiency, energy confinement, and overall power balance.[2]
Because electrons are much lighter than ions, energy exchange between the two species through Coulomb collisions can be slow at low densities. In many tokamak plasmas, especially those with strong neutral beam injection, the ion temperature can significantly exceed the electron temperature in the plasma core. Conversely, in plasmas heated primarily by radiofrequency waves at the electron cyclotron frequency, Te may exceed Ti. The ratio Te/Ti affects stability, transport, and the fusion reaction rate.[1]
In present-day large tokamaks, central electron temperatures of 5–15 keV are routinely achieved. JET has reached electron temperatures above 10 keV in high-performance D-T discharges, and ITER is designed to operate with central Te in the range of 20–25 keV. Stellarators such as Wendelstein 7-X have demonstrated Te > 8 keV in optimised configurations.[3]
The standard diagnostic for electron temperature is Thomson scattering, which measures the Doppler broadening of laser light scattered by plasma electrons. Electron cyclotron emission (ECE) radiometry provides complementary measurements with excellent temporal resolution, exploiting the fact that the plasma radiates at harmonics of the electron cyclotron frequency whose intensity is proportional to Te.[2]