Monitoring of the main sources of energy loss in compact fusion plants

A major obstacle to Earth's control of the fusion that powers the sun and the stars is the leakage of energy and particles from the plasma, the hot, charged state of matter composed of free electrons and atomic nuclei that feed the fusion reactions. At the Princeton Plasma Physics Laboratory (PPPL) of the United States Department of Energy (DOE), physicists focused on validating computer simulations to predict energy losses caused by turbulent transport during fusion experiments.

Researchers used codes developed by General Atomics (GA) in San Diego to compare the theoretical predictions of turbulent electron and ion transport with the results of the lab's first test campaign – or "low ratio". "appearance" of the laboratory (NSTX). -U). GA, which operates the DIII-D national fusion facility for DOE, has developed codes that are well-suited for this purpose.

Low-aspect ratio tokamaks are shaped like hollowed-out apples, in contrast to the more commonly used tokamaks that are shaped like donuts.

State codes of the art

"We have advanced codes based on a sophisticated theory to predict transport," said physicist Walter Guttenfelder, lead author of a Nuclear fusion article presenting the results of a team of researchers. "We now need to validate these codes under a wide range of conditions in order to use predictions to optimize current and future experiments."

The transport analysis observed in the NSTX-U experiments revealed that an important factor behind the losses was the turbulence that made the electron transport "abnormal", which meant that they were spreading rapidly in the same way that the milk mixes with the coffee when it is stirred by a spoon. GA codes predict that the cause of these losses will be a complex mixture of three different types of turbulence.

The observed results opened a new chapter in the development of transport forecasts in low aspect ratio tokamaks – a type of fusion facility that can serve as a model for next-generation fusion reactors combining elements light in the form of plasma to produce energy. Scientists around the world are seeking to replicate the fusion on Earth to obtain a virtually inexhaustible source of energy for the production of electricity.

PPPL researchers now aim to identify the mechanisms behind the abnormal transport of electrons in a compact tokamak. The simulations predict that this loss of energy is due to the presence of three distinct types of complex turbulence: two types with relatively long wavelengths and a third with wavelengths of. a fraction smaller than the largest size.

The impact of one of two long wave types, which is usually found in the heart of the low aspect ratio tokamaks, as well as on the edge of the plasma in the classical tokamaks, must be fully taken into account when predicting the low aspect ratio. transport.

Challenge to simulate

However, the combined impact of the three types of turbulence is a challenge to simulate since scientists normally study different wavelengths separately. Massachusetts Institute of Technology (MIT) physicists have recently performed multi-scale simulations and their work highlights the considerable computational time these simulations require.

Researchers now need to test additional simulations to reach a more complete agreement between transport forecasts and plasma experiments in low aspect ratio tokamaks. These comparisons will include turbulence measurements taken by co-authors at the University of Wisconsin-Madison Nuclear fusion paper that will better constrain forecasts. An improved agreement will ensure energy loss forecasts for current and future facilities.