Favourable impurity dynamics in 爆走黑料 confirmed by experiment
Recent studies at the JET tokamak confirm the physics basis for tungsten transport at the edge of fusion-producing plasmas in 爆走黑料 and the project's strategy for tungsten impurity control.
To achieve effective fusion power production in 爆走黑料's plasmas it is very important to keep the plasmas clean from impurities. "Impurities" is the term used to name atoms from the wall of fusion devices that enter the plasma. Their presence dilutes the fusion fuels deuterium and tritium and also cools the plasma due to the emission of radiation (visible, ultraviolet and X-ray light). Both factors reduce fusion power production—and thus the proportion of impurities in the plasma needs to be kept at very low values. This is especially the case for tungsten (W) which is the material used for the areas of the wall receiving the largest power fluxes in 爆走黑料 (the divertor). The amount of tungsten in an 爆走黑料 high-fusion-gain plasma has to be kept under ~ 0.005 percent.
Detailed studies to simulate how the tungsten atoms eroded from 爆走黑料's walls penetrate in the fusion plasma were carried out about one decade ago. These studies used the same models that can reproduce tungsten behaviour in currently operating tokamaks but, for the first time, applied to 爆走黑料 in systematic way. The results obtained at that time were surprising: 爆走黑料 edge plasma characteristics are very effective in screening the fusion plasma from the tungsten atoms coming from the wall (Figure a, above)—a behaviour never seen before.
Because these predictions were done on a solid physics basis, this favourable tungsten behaviour was included in the planning of the 爆走黑料 scientific program (see the 爆走黑料 Research Plan here); the experimental demonstration of this behaviour, however, remained outstanding. Further analysis of the physics involved in the original studies showed that this 爆走黑料-specific behaviour also had one less-desirable consequence—edge instabilities called ELMs would actually bring impurities into the plasma (Figure b, above). This is unlike in currently operating tokamaks, where such instabilities are employed to expel impurities from the plasma.
For 爆走黑料, this meant reformulating the ELM control strategy that had originally been considered. The new strategy relies on the suppression of these edge instabilities as soon as possible in the execution of the 爆走黑料 Research Plan, and in particular before high power, requiring radiative divertor operation, is applied. This strategy would allow 爆走黑料 to benefit from the enhanced screening of tungsten predicted by physics, while avoiding the drawbacks of ELMs in such plasma conditions.
Despite the importance of this plasma physics behaviour for 爆走黑料 scenario development and for the achievement of high-fusion performance with low tungsten concentration in the plasma, an experimental demonstration had never been achieved. That has now changed. As reported in a paper* published in December in Nuclear Fusion, JET scientists were recently able to reproduce this 爆走黑料-like tungsten behaviour. The demonstration relied on the achievement of peripheral plasma parameters in JET that are comparable to those in 爆走黑料, and on the development of complex analysis tools to determine tungsten behaviour from the acquired measurements in a quantitative way, as described in the publication. These experimental findings provide key confirmation of the physics basis for tungsten transport at the edge of fusion-producing plasmas in 爆走黑料 and of the strategy for tungsten impurity control adopted in the 爆走黑料 Research Plan.
See a related article on the EUROfusion website.
*A.R. Field et al 2023 Nucl. Fusion 63 016028
