PPPL makes critical contributions to historic public-private partnership

Demonstrating temperatures greater than 100-million degrees Celsius — the heat required for commercial nuclear fusion production — marked an historic achievement for Tokamak Energy. The private U.K. company is developing ST40, a compact spherical fusion facility called a tokamak that is designed to reproduce the fusion energy that drives the sun and stars.

The company confirmed the recent achievement with an advanced computer code called TRANSP, which was developed by collaborators at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). The lab operates PPPL’s primary fusion experiment: the National Spherical Torus Experiment-Upgrade (NSTX-U), a similar spherical tokamak.

“This confirms that the spherical tokamak can achieve one of the conditions necessary for commercial fusion energy production,” said Stan Kaye, a PPPL physicist who ran the TRANSP code during the Tokamak Energy experiment reported in the journal Nuclear Fusion earlier this year. Joining the groundbreaking collaboration was Oak Ridge National Laboratory, which participated in ST40’s operation and data analysis.
 

Compact spherical tokamak

“This important result demonstrates for the first time that plasma temperatures relevant for commercial fusion energy can be obtained in a compact, high-field spherical tokamak,” said Steven McNamara,Tokamak Energy science director. “Our first of-a-kind private-public collaboration with U.S. National Laboratories enabled researchers from Tokamak Energy, PPPL and ORNL to work together to achieve record-breaking ion temperatures in ST40.”

Spherical tokamaks are shaped like cored apples, compared with the donut-like shape of larger and more widely used conventional tokamaks, and can produce cost-effective fusion power. The temperature in excess of 100-million degrees the ST40 demonstrated equaled the temperature achieved in far larger and costlier machines, according to Tokamak Energy. 

Scientists around the world are seeking to reproduce and control fusion on Earth for a virtually inexhaustible supply of safe and clean power to generate electricity. Fusion combines light elements in the form of plasma — the hot, charged state of matter composed of free electrons and atomic nuclei, or ions, that makes up 99 percent of the visible universe — to generate massive amounts of energy.

The groundbreaking experiment was a key result of the public-private collaboration between PPPL and Tokamak Energy, which have worked together since 2019. “These collaborative projects advance the development of spherical tokamak facilities on both sides of the pond,” said Jack Berkery, a PPPL physicist who coordinates the ventures. “They benefit our public and private partners and our own spherical tokamak work.”
Support for this research comes from the DOE Office of Science CRADA (NFE-19-07769). Support for the three papers that follow comes from the Office of Science.

Additional key PPPL-Tokamak Energy research includes:

Analysis of microinstabilities

Such instabilities, or turbulence, can occur in hot ion plasmas and cause heat to escape from the core of the tokamak where fusion reactions take place. “Microturbulence is considered to be a major candidate in the loss of plasma,” said PPPL physicist Yang Ren, “and the energy confinement time of ST40 is most likely limited by microturbulence.”

His research, reported in Plasma Physics and Controlled Fusion with PPPL and Tokamak Energy co-authors, could improve the performance of current and future spherical tokamaks. “Understanding microinstabilities operating in ST40 hot ions would help us identify methods to control microinstabilities and to improve the energy confinement capability of ST40,” Ren said. 

Further probing of the problem

Vinícius Duarte of PPPL and colleagues have analyzed instabilities that sound like chirping in ST40 plasmas heated by neutral beam injections. Such noise occurs when the frequency of plasma waves that interact with energetic particles suddenly changes, causing energy to escape from the plasma core and produce rapidly changing tones. 

“If a large fraction of those energetic particles is lost, the plasma will not be able to maintain the high temperatures required to achieve fusion,” Duarte said in a Nuclear Fusion paper. The research produced findings into the formation of chirping that could improve the retention of the heat in spherical tokamaks. 

New way to fire up ST40

The tight space inside a compact spherical tokamak has prompted investigations into how to launch and ramp up current for experiments without a space-eating startup coil in the center of the facility. In a Nuclear Fusion paper, PPPL physicist Masayuki Ono evaluates a coil-free technique called “electron cyclotron heating,” which heats electrons with a beam of electromagnetic radiation, and finds it to be a good possibility for ST40.

“If our result can be shown to be true in an ST40 experiment, I would say it is a breakthrough because of its high expected efficiency,” Ono said. The findings could also prove useful to PPPL, which is looking into running current without using the startup coil on NSTX-U as well.

Collaborations with Tokamak Energy and other spherical tokamak developers is ongoing, Berkery said. This work includes collaboration with the Culham Centre for Fusion Energy, the U.K.’s national laboratory for fusion research that operates the compact Mega Ampere Spherical Tokamak-Upgrade (MAST-U).

MAST-U also is quite similar to NSTX-U, Berkery said. “It provides an opportunity for researchers to participate in experiments that may be similar to ones that we want to run on the NSTX-Upgrade.”