Producing energy from the deuterium-tritium fusion reaction requires the plasma, and in particular the ions, to reach temperatures in excess of one hundred million degrees. To reach these temperatures, and subsequently a situation in which the plasma is predominantly heated by the energetic helium nuclei produced by the fusion reaction, it is first necessary to use external (or "additional") heating. While this additional heating can be applied to either the electrons or the ions, depending on the method used, heating by collisional energy exchange between the helium nuclei and the plasma will mainly heat its electrons. The following question arises: in a fusion power plant where the dominant heating increases the temperature of the electrons, will the transfer of energy by collision between electrons and ions be sufficiently effective to maintain an adequate ion temperature and sustain the fusion reactions?

This question has been highlighted by recent observations that show, in current machines, tokamaks or stellarators, a saturation of the ion temperature when only electron heating is used. Several mechanisms have been proposed to explain these observations, including (i) the increase in the level of turbulence (the main cause of heat and particle loss) when the electron temperature deviates from the ion temperature (ii) the collisional heat exchange between electrons and ions, which becomes less efficient as the electron temperature increases (lower collision frequency). Does the saturation observed in current experiments threaten the performance of ITER and future fusion power plants?

Experiments and simulations on the WEST tokamak

On the WEST tokamak, where the radio-frequency heating systems mainly heat the electrons, saturation of the central ion temperature has also been observed on a database of several hundred plasmas. Simulations carried out by CEA-IRFM researchers predict the temperature profiles in the radial direction of the plasma using turbulent and collisional transport models, and quantitatively reproduce this saturation of the ion temperature. They highlight the main mechanism, namely the decrease in collisional heat exchange between ions and electrons at higher electron temperatures and lower collisionality. Increasing the electron density in these simulations makes it possible to improve the collisional exchange between electrons and ions and thus increase the ion temperature reached at saturation.

More importantly, simulations show that this saturation or even the ratio of ion temperature to electron temperature are strongly correlated to the competition between two characteristic times: the energy confinement time (characterising the exponential decay of the energy stored in the plasma in the absence of external heating) and the collisional heat exchange time τei depending on the collision frequency and the gap between ion and electron temperature (see figure). When the energy confinement time is sufficiently long compared to the characteristic time τ_ei, the collisional heat exchange between ions and electrons is efficient and the ion temperature remains close to the electron temperature, even in the absence of external ion heating.

Extrapolation to ITER and future fusion power plants

For current machines, the energy confinement time is short, of the order of several tens of milliseconds, which explains the saturation of the ion temperature at low values, incompatible with the operation of a fusion power plant, which requires temperatures in excess of 100 million degrees. For ITER, which is larger and has a higher confinement time of the order of a second, the ratio between this time and the characteristic collisional heat exchange time between ions and electrons is sufficiently large to allow the ion temperature to reach values close to the electron temperature. This was demonstrated in this study [1] using 1D transport simulations, comparing results obtained for the WEST tokamak and a high-performance ITER scenario in which plasma heating is mainly on electrons.

This study provides reassurance that ions will be heated efficiently in ITER and in future power plants, and that fusion reactions will therefore be self-sustained. By shedding light on the physical mechanisms and characteristic times involved, it also explains the observations of saturation of the ion temperature in current, smaller machines.

[1] P. Manas et al, Maximizing the ion temperature in an electron heated plasma: from WEST towards larger devices, Nuclear Fusion 64 036011