Researchers at the University of California, San Diego, have published a new theoretical model that offers a potential explanation for a known discrepancy in nuclear fusion research.
The work suggests that previously underestimated structures, called “voids,” may be responsible for generating greater-than-expected turbulence at the edge of plasmas inside fusion reactors.
The study, authored by physicists Mingyun Cao and Patrick Diamond, addresses a persistent issue in the development of tokamaks, i.e., the primary devices used in the effort to generate controlled fusion energy.
“The dynamics of edge-core coupling is critically important to the optimization of magnetically confined fusion plasmas,” said the researchers in a new study.
Plasma boundary and “shortfall problem”
The findings focus on the physics of the plasma boundary, a complex region that is key to sustaining a fusion reaction.
In fusion research, tokamaks use magnetic fields to confine plasma heated to millions of degrees Fahrenheit. Scientists use complex computer simulations to predict how this plasma will behave.
However, these simulations have historically been unable to fully account for the width of the turbulent layer observed at the plasma’s edge.
This issue, known as the “shortfall problem,” creates uncertainty in predictive modeling. An accurate understanding of the plasma edge is required to maintain the conditions for fusion and to protect the interior components of the reactor from the intense heat.
The discrepancy between simulations and experimental results has been a topic of ongoing study.
Role of “voids”
The UC San Diego research re-examines the processes that occur at the plasma’s outer boundary. This boundary is not static; it undergoes gradient relaxation events, where the edge of the plasma fragments into distinct structures.
These include outward-moving, density-enhanced filaments called “blobs” and inward-moving, density-depleted structures called “voids.”
Past research has largely concentrated on the blobs, as their movement toward the reactor walls is a more direct and observable interaction. The role of the inward-moving voids has been less understood.
“Since early proposals, there has been persistent speculation that inward propagation of turbulence from the boundary is a possible means to energize the edge-core coupling region,” remarked the study.
Cao and Diamond developed a new model based on first principles that treats these voids as coherent, particle-like entities to analyze their effect on the plasma.
New model for turbulence generation
“The detailed mechanism of this process has remained a mystery until recent experiments observed that regular, intense gradient relaxation events generated blob-void pairs very close to the last closed flux surface,” highlighted the researchers.
The model indicates that as a void moves from the cooler plasma edge toward the hotter core, its passage through the plasma’s steep temperature and density gradients generates plasma drift waves. These waves transfer energy and momentum, which in turn creates additional local turbulence.
According to the team’s calculations, this newly identified mechanism could be responsible for the extra turbulence seen in experiments but missing from earlier models.
The model proposed by Cao and Diamond is currently theoretical. If the model is validated, this would allow for more reliable predictions of plasma behavior, which could inform the design of future reactors and the development of new techniques for plasma control.
“The model shows promise to resolve several questions surrounding the shortfall problem and the strong turbulence in the edge-core coupling region,” concluded the study.
