Recent research in JET tokaka supports both the project's impurity management technique and the underlying physics of tungsten transport at the edge of fusion-producing plasmas in ITER. It is essential to maintain impurity-free plasmas in the ITER to efficiently generate fusion energy. Atoms penetrating the plasma through the wall of fusion devices are called "impurities". Due to the radiation they emit, their presence dilutes the fusion fuels deuterium and tritium, and also cools the plasma (visible, ultraviolet, and X-ray light). Both variables lower the fusion power output, so contaminants in the plasma must be controlled at extremely low levels.
This is especially true for tungsten (W), which is used to construct the parts of the ITER wall that receive the highest power fluxes. An ITER high fusion gain plasma should contain less than 0.005 percent tungsten.
About a decade ago, extensive work was undertaken to mimic how tungsten atoms eroded from the walls of ITER penetrate the fusion plasma. These experiments are the first to methodically use ITER for the first time, using the same models that can mimic the behavior of tungsten in currently used tokamaks.
The findings at the time were unexpected: ITER edge plasma properties are extremely successful in separating fusion plasma from tungsten atoms coming from the wall – a behavior never observed before.
This positive tungsten behavior was taken into account in the formulation of the ITER scientific program because these predictions were made on the basis of solid physics; however, experimental evidence of this behavior remains unconfirmed. The physics behind the original research were further examined, and it was discovered that this ITER-specific behavior had a less desirable side effect – edge instabilities known as ELMs would actually introduce contaminants into the plasma. Unlike contemporary tokamaks, which use these instabilities to remove contaminants from the plasma, this is not the case.
This required a rewrite of the ELM control method originally considered for ITER. The new approach is based on suppressing these edge instabilities as soon as possible during the execution of the ITER Research Plan, especially before high power application requiring radiative deflector operation. With this approach, ITER will be able to benefit from the enhanced tungsten shielding predicted by physics while avoiding the adverse effects of ELMs in these plasma conditions.
Although the creation of the ITER scenario and the achievement of excellent fusion performance with low tungsten concentration in plasma depended on this plasma physics behavior, no experimental proof was ever made. The situation is different now. Scientists working on JET have recently succeeded in replicating this ITER-like tungsten behavior, as described in a paper* published in Nuclear Fusion in December. Obtaining environmental plasma parameters at JET comparable to those at ITER and the development of sophisticated analysis tools to quantitatively predict tungsten behavior from the resulting measurements were prerequisites for the demonstration detailed in the publication.
These experimental results provide important support for the physics supporting tungsten transport at the edge of fusion producing plasmas in ITER, as well as for the tungsten impurity management method chosen in the ITER Research Plan.
📩 25/01/2023 15:25