A major barrier in achieving the control of fusion—which powers stars and the sun—on Earth is leakage of particles and energy from plasma. Plasma is the hot, charged state of matter that fuels fusion reactions and is formed of free electrons and atomic nuclei. Physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), have been concentrating on confirming computer simulations that predict energy losses caused by turbulent transport while performing fusion experiments.
Scientists used codes created at General Atomics (GA) in San Diego to compare theoretical hypotheses related to turbulent transport of ions and electrons with results from the first campaign of the laboratory’s compact—or “low-aspect-ratio”—National Spherical Torus Experiment-Upgrade (NSTX-U). GA runs the DIII-D National Fusion Facility for the DOE and has created codes best suited for this purpose.
Low-aspect-ratio tokamaks resemble cored apples, in contrast to the more extensively used conventional tokamaks that resemble doughnuts.
“We have state-of-the-art codes based on sophisticated theory to predict transport,” said physicist Walter Guttenfelder, lead author of a Nuclear Fusion paper that reports the results of a team of scientists. “We must now validate these codes over a broad range of conditions to be confident that we can use the predictions to optimize present and future experiments.”
Examination of the transport seen in NSTX-U experiments found that a key factor underlying the losses was turbulence that led to the transport of electrons to be “anomalous,” meaning that they spread quickly, similar to the way that milk fuses with coffee when mixed by a spoon. The GA codes forecast the cause of these losses to be a complex mix of three varying types of turbulence.
The observed findings paved way for a new chapter in the progress of predictions of transport in low-aspect-ratio tokamaks — a type of fusion facility that could act as a model for next-generation fusion reactors that integrate light elements in the form of plasma to generate energy. Researchers globally are aiming to mimic fusion on Earth for a virtually inexhaustible supply of power to produce electricity.
Scientists at PPPL currently aim to recognize the mechanisms underlying the anomalous electron transport in a compact tokamak. Simulations predict that such energy loss happens due to the presence of three different types of complex turbulence — two types with comparatively long wavelengths and a third with wavelengths a fraction of the size of the larger two.
The influence of one of the two long-wave types, which is normally found in the center of low-aspect-ratio-tokamaks as well as in the edge of the plasma in conventional tokamaks, must be completely taken into consideration when predicting low-aspect-ratio transport.
Challenge to simulate
However, the joint impact of all three kinds of turbulence is a challenge to mimic since researchers usually study the various wavelengths independently. Physicists at the Massachusetts Institute of Technology (MIT) have recently carried out multi-scale simulations and their work stresses the substantial supercomputer time such simulations require.
Scientists must now test extra simulations to realize more comprehensive agreement between predictions of transport and experiments on plasmas in low-aspect-ratio tokamaks. These comparisons will also have measurements of turbulence performed by University of Wisconsin-Madison co-authors of the Nuclear Fusion paper that will better constrain predictions. Enhanced agreement will offer promise of energy-loss predictions for present and future facilities.
This research received support from the DOE Office of Science. The National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility, facilitated the simulations. NSTX-U diagnostic equipment from the University of Wisconsin-Madison supplied data from experiments.