Connecting the power of the stars to geometry

In the pursuit of fusion energy, scientists and engineers meticulously examine plasma, the fourth state of matter, to construct novel power stations. By harnessing the heat generated from the collision of two minute atoms, these facilities aim to produce a sustainable source of electricity, enabling America to attain energy independence.

While scientists concentrate on intricate machinery and temperatures surpassing the Sun's surface, they also strive to determine the optimal designs for these power plants through the lens of geometry.

The design of fusion systems is crucial. The initial device conceived by Lyman Spitzer Jr., the progenitor of the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL), resembled a figure eight. Later, the tokamak, developed in the 1960s, took a doughnut-like form to confine plasma by generating a central electrical current, which formed vital confining magnetic fields.

Other fusion devices assumed various shapes, including straight lines and twisty crullers. Additionally, some fusion systems resemble cored apples, known as spherical tokamaks. These spherical tokamaks have a narrower hole through the center compared to conventional tokamaks.

Recent research indicates that spherical tokamaks might confine plasma energy more efficiently than conventional tokamaks, potentially generating plasma with sufficient temperature and density for an extended period to trigger a fusion reaction, akin to a miniature star on Earth. Furthermore, spherical tokamaks can confine a substantial plasma pressure for a specific magnetic field strength, a ratio known as beta.

Achieving a high beta is a longstanding objective, as it signifies efficient utilization of the confining magnetic field, reducing the need for expensive magnetic fields in fusion power plants.

The National Spherical Torus Experiment-Upgrade (NSTX-U), the most powerful spherical tokamak in the United States, will be utilized by PPPL to study the capabilities of spherical tokamaks. This experiment boasts impressive statistics, including the highest plasma stored energy of any spherical tokamak to date. It will also feature advanced measurement systems, or diagnostics, capturing key plasma features like temperature and density, providing crucial insights to advance fusion energy science.

NSTX-U will serve as an international user facility, allowing public and private institutions and companies to test new materials and components under plasma exposure and establish reliable AI tools for real-time data analysis and control.

Scientists find spherical tokamaks attractive due to their combination of characteristics, falling into three broad categories. Firstly, spherical tokamaks are relatively compact, requiring less building material for construction compared to conventional tokamaks. This smaller size theoretically reduces costs. Secondly, spherical tokamaks confine plasma exceptionally well for the relatively modest magnetic fields they employ.

The spiraling magnetic field lines in a spherical tokamak thread throughout the device but wrap around the central magnet bundle more than they do around the outer edge. This configuration exposes more convex, curved magnetic field lines to the plasma, stabilizing it and reducing large, unwanted motions or instabilities that can dissipate plasma energy.

Lastly, spherical tokamaks have a plasma that rotates relatively quickly, preventing smaller perturbations or wiggles that can move heat out of the plasma, reducing temperature and pressure, and making fusion reactions less likely.