Researchers examine the performance of a fusion pilot plant to generate electricity

The U.S. fusion community has actively called for an immediate design effort for a cost-effective pilot plant to generate electricity in the 2040s. This effort and related community recommendations are documented in the 2020 report of the Fusion Energy Sciences Advisory Committee entitled "Powering the Future: Fusion & Plasmas."

Now Jon Menard, deputy director for research at the U.S. Department of Energy (DOE)'s Princeton Plasma Physics Laboratory (PPPL), has led a detailed study of the scientific and engineering challenges that such a pilot plant will face. The study also defines performance requirements for a complementary research facility that the community is proposing to close key gaps to a pilot plant.

'Rigor and insight'

"Jon has driven forward ideas to bring down the cost and scale of fusion with his usual rigor and insight," said PPPL Director Steve Cowley. "As we accelerate the delivery of fusion this work becomes more and more important."

The dedicated research facility would have a sustained high-power density (SHPD) capability to address the challenges of integrating the pilot plant core and the edge exhaust region. This task has been identified by the community and embodied in a proposed new tokamak facility named the "EXhaust and Confinement Integration Tokamak Experiment (EXCITE)."

A SHPD-EXCITE device could complement and inform the pilot plant and smooth its operation, said Menard, lead author of a comprehensive analysis in the journal Nuclear Fusion that projects the performance of varied pilot plant designs. "The paper wants to make an apples-to-apples comparison between all options," he said.

The analysis focuses on fusion facilities called "tokamaks" ranging from compact cored-apple shaped devices like the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL to broader and more widely used doughnut-shaped facilities. The research device would serve as a satellite to test ideas during the pilot plant's construction and operation. "It would be overlapping," Menard said.

Vast energy

Fusion, which powers the sun and stars, produces vast energy by combining light elements in the form of plasma, the hot, charged state of matter composed of free electrons and atomic nuclei, or ions. Scientists around the world are seeking to recreate this power for a safe and clean source of energy to produce the world's electricity.

The proposed pilot plant will need to address the delivery of heat from the high-density plasma core to the exhaust region at the tokamak's edge. "In a high-power compact pilot plant, this heat would be significant and we want to make sure we understand how to handle it properly," Menard said.

PPPL research has produced differing models of the impact of the heat. "No one knows for sure which model is correct and there's as yet no machine to test this on," Menard said. "So the community has proposed that a research facility be built to investigate these key questions."

Other topics to be developed in the compact pilot plant include the need to integrate a largely self-driven plasma current with the high-density plasma core. Today's tokamaks use a central magnet called a "solenoid" to produce the current, which creates a magnetic field to bottle up the plasma so that fusion reactions can take place. However, there would be less room for a solenoid in the compact pilot plant the community envisions, producing the need for an internally generated plasma current.

The design and construction of the EXCITE facility will not necessarily delay the arrival of the pilot plant so long as both facilities are flexible enough. "These facilities should not be completely serial but should be overlapping and their roles should be well-defined," Menard said. "The smaller tokamak could be used to test ideas faster and more cheaply without dealing with the nuclear environment that the pilot plant would focus on."

Studies for ITER

Studies underway for ITER, which is being built to demonstrate the feasibility of harvesting fusion power, provide examples of collaborative research relationships, Menard said. "The entire world tokamak program is continuing to do R&D to figure out how best to operate ITER," he said. The international tokamak is scheduled to start operating in 2025 and to move to full power operations in 2035.

Menard noted that a particularly relevant example of collaboration is the implementation of the fully metallic "ITER-like wall" in the Joint European Torus (JET) facility in the United Kingdom over a decade ago to learn how to operate ITER with such a wall prior to ITER operation. JET recently achieved a record fusion energy with the ITER-like wall after extensive experimentation and operational development that should ultimately accelerate research on ITER.

Meanwhile, the U.S. fusion community seeks to pursue innovative ways to refine the design and key features of the proposed pilot plant that would generate low capital-cost electricity in the 2040s—a demanding task that calls for resolving major gaps in the projected fusion facility. "The next step is to confront all the challenges that the pilot plant will face," Menard said.