PPPL, a leader in the drive to reproduce the clean, carbon-free, and climate-benefitting fusion energy that drives the sun and stars, is extending its global reach. PPPL is strengthening a long-term collaboration with the Culham Centre for Fusion Energy in the United Kingdom (UK), whose Mega Ampere Spherical Tokamak-Upgrade (MAST-U) fusion device complements the capabilities of the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL. 

The two cored-apple-shaped facilities are the most prominent spherical devices in the world. Their design could produce cost-effective fusion power and become the model for a fusion pilot plant as an attractive economic alternative to the design of larger and more widely used doughnut-shaped standard tokamak facilities operating around the world.

Fusion produces vast energy by combining light elements such as hydrogen in the form of plasma, the hot, charged state of matter that makes up 99 percent of the visible universe and consists of free electrons and atomic nuclei, or ions. All tokamaks confine the plasma in strong magnetic fields and heat it to tens of million-degree temperatures to force the ions to merge.

“MAST-U is exciting because it’s essentially a new machine and provides an opportunity to participate in experiments that may be similar to ones that we want to run on NSTX-U,” said PPPL physicist Jack Berkery, coordinator of spherical research collaborations. “There’s definitely an advantage to having two machines that are similar but not exactly the same. People can do experiments on one machine and repeat them on the other to see if there’s a difference and try to figure out why.” 

The two spherical tokamaks explore different capacities. “MAST-U is concentrating on the divertor,” he said, the tokamak region that exhausts waste heat, whereas with NSTX-U “we want to see if plasma confinement improves when the temperature grows hotter.” Teams of PPPL researchers will investigate different aspects of MAST-U capabilities, he said, and British researchers are happy to have that kind of collaboration.

"Hugely beneficial"

“The collaboration between the NSTX-U and MAST-U teams is hugely beneficial for spherical tokamak research,” concurred physicist James Harrison of the UK Atomic Energy Authority. “Through our joint efforts to understand key physics issues, such as plasma confinement, stability and exhaust, using the unique capabilities of our researchers and facilities, we are preparing firm foundations for developing future economical fusion energy sources.” 

The approaches complement one another. For example, PPPL physicist Máté Lampert is traveling to the UK to lead MAST-U’s analysis of fast video diagnostic data and gain understanding of crucial plasma structures near the edge of the tokamak. “NSTX-U will not have the capability to do such wide-angle camera measurements,” Lampert said, and the findings will enhance operation of both facilities and of future tokamaks as well.” 

PPPL physicists are preparing for numerous collaborations, and participation in MAST-U experiments. Theoreticians Nate Ferraro and Andreas Kleiner will model large-scale plasma stability in both the core and edge of MAST-U. The aim will be to test the ability of PPPL models to describe and predict large-scale instabilities in MAST-U. The findings will boost the ability to predict stability thresholds in larger-scale spherical torus facilities, including potential pilot plants.

PPPL postdoctoral researcher Domenica Corona is analyzing MAST-U plasmas with the aim of improving their control and the ramp-down of the plasma current at the end of a discharge. These efforts could lead to improved operation of the device.

In another project, PPPL’s Stefano Munaretto will lead a MAST-U experiment to apply 3D fields to the plasma to see how it reacts. Such fields have suppressed heat bursts called edge-localized modes (ELMs) that can halt fusion reactions on conventional tokamaks and damage the facilities. But suppression has not yet been achieved on spherical tokamaks and the same analytical experiment cannot be performed on NSTX-U.

“The goal of this experiment is to understand more in detail how the plasma responds to different geometry of the applied field to improve understanding of the phenomenon,” Munaretto said. “And it’s trying to find a parallel to conventional tokamaks where ELM suppression with 3D fields has been achieved.”

Said Jon Menard, PPPL deputy director for research: “Working closely with the MAST-U team on important issues for spherical tokamaks is very important for the upcoming NSTX-U program and for next-step spherical tokamaks including pilot plants, and we are grateful to DOE Fusion Energy Sciences and MAST-U for their support and assistance.”

MAST-U completed its first experimental campaign last year and is resuming operations this fall. “PPPL has plenty of interest in joining MAST-U experiments and data analysis and calculations,” Berkery said. “A bunch of us are scheduled to go over there for experiments and theoretical work and the findings should be quite rewarding.”