The dream of nuclear fusion is on the brink of being realised, but for a technology that stands to revolutionise how we generate clean energy, current nuclear fusion is remarkably leaky. High energy particles can sometimes escape experimental reactors, making the process much less efficient.
Like surfers catching ocean waves, particles within the hot, electrically charged state of matter known as plasma can ride waves that oscillate through the plasma during experiments to investigate the production of fusion energy. The oscillations can displace the particles so far that they escape from the doughnut-shaped tokamak that houses the experiments, cooling the plasma and making fusion reactions less efficient.
But new research may have found a way to keep those particles where they belong. A team of physicists led by the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has devised a method of determining how much this interaction between waves and particles contributes to the efficiency loss in tokamaks.
In a fusion reaction, energy is released when two light atomic nuclei are fused together to form one heavier atom. This is the process that provides the energy powering the Sun and other stars, where hydrogen nuclei are combined to form helium.
The promise of fusion is huge: it represents a zero-carbon, combustion-free source of energy. The problem is that until now every fusion experiment has operated on an energy deficit, making it useless as a form of electricity generation. Decades of disappointment in the field has led to the joke that fusion is the energy of the future – and always will be.
The new work could boost the efficiency of experimental fusion reactors such as ITER, a groundbreaking facility currently under construction in France.
By running simulations on powerful PPPL computers, the researchers learned how a type of plasma vibration known as an eigenmode can deform the resonance and change how it affects plasma particles. "Our research stands out because we took account of the eigenmode shape, which hadn't been done before," White said.
The new DOE research describes complex computer simulations that can track and predict these waves, giving physicists new avenues to prevent them and keep these particles right where they belong.
The way in which eigenmodes change resonance structures and therefore the behavior of plasma particles matters to scientists because the effect could diminish the efficiency of ITER, the multinational facility being built in France to demonstrate the feasibility of fusion power. "The modifications of particle distributions by electromagnetic oscillations is an important problem for ITER," White said. "Studying these phenomena allows scientists to predict how strong the effects of the oscillations will be, and then engineer ways to eliminate the waves, prevent particle loss, and maintain fusion efficiency."
The findings could be used to create a reduced computer model with simplified, yet accurate, code that could simulate plasma behavior with fewer calculations and therefore in much less time than current models take. "The best available simulation of a discharge in DIII-D, the tokamak operated in San Diego by General Atomics, can take a supercomputer several months to complete," said Nikolai Gorelenkov, principal research physicist at PPPL and a co-author of the paper. "That's too long. The ultimate goal is to use simulations of particle-wave interactions in plasma quickly enough to predict where and when losses might occur, and then take action to avoid those losses."
The researchers hope their work will help build ITER, a multinational experimental fusion reactor that’s expected to first go online in 2025 — though doing so will require scaling up the simulation substantially.
“A conservative projection for ITER is that simulations will require approximately 1 million times more calculations than are needed for current tokamaks,” said Nikolai Gorelenkov, a principal research physicist at the Princeton Plasma Physics Laboratory and a co-author of the new paper, in a press release. “It’s an unprecedented amount of computation, so we have to find ways to make the simulation easier to finish.”