Following the successful completion and operation of the preliminary engineering validation facility SUNIST-2, which met its key design objectives, Startorus Fusion is ready to construct Negative Triangularity Spherical Tokamak (NTST). This device is on track to become the world’s first spherical tokamak with negative triangularity, and potentially the first of its kind globally.
The operation of the NTST will pave the way for the construction of the next generation of fusion-grade devices.
The magnetic coil fabrication, vacuum chamber design, and cryogenic cooling systems of the NTST closely resemble those of the upcoming CTRFR-1, a next-generation spherical tokamak intended for fusion-grade operations. By swiftly constructing and operating the NTST, Startorus Fusion aims to validate key technologies related to magnets, vacuum systems, cryogenics, power supplies, control systems, and heat removal for fusion reactors, laying the groundwork for the CTRFR-1 project. The NTST will incorporate numerous innovative designs from Startorus Fusion, ensuring that it meets its design specifications while reducing costs and facilitating maintenance. This approach will continue to drive iterative progress towards the goal of achieving fusion energy in a rapid and cost-effective manner.
All magnets in the NTST will be powered by the second-generation standardized modular power supply system, independently developed by Startorus Fusion, which can be combined in series and parallel to meet the varying current and voltage requirements of different magnets, akin to SpaceX’s approach of using the same engine type to provide thrust for all their rockets.
The operating system of NTST will also be reconstructed on the basis of the existing Tokamak operating system of Startorus Fusion to form a standardized Tokamak operating system with more reasonable architecture, more convenient transplantation, more flexible expansion and more natural human-computer interaction.
NTST will introduce an end-to-end framework based on the existing plasma control system of Startorus Fusion, and try a new Tokamak “autopilot” method.
Startorus Fusion's Negative Triangularity Spherical Tokamak( NTST）
Research Found: Negative Triangular Plasma is More Compatible with Fusion Reactors
In pursuit of enhanced performance, the cross-section of the toroidal plasma in a tokamak is not a perfect circle but often exhibits a certain degree of triangular deformation. When the vertex of the plasma cross-section is near the symmetry axis of the torus, this deformation is referred to as a positive triangular variation; conversely, if the vertex is far from the axis, it is known as a negative triangular variation.
In the early days, due to limited control capabilities over tokamak plasmas, the positively shaped triangular plasma with good self-stability was widely adopted and achieved remarkable success. For instance, the plasma that achieved 69 MJ of fusion energy in the JET facility is an example of a positively shaped triangular plasma.
However, positively shaped triangular plasmas also have certain issues, such as the necessity to enter a high-confinement mode to achieve good confinement performance and the difficulty in avoiding the thermal shock from edge-localized modes (ELMs) reaching up to 1 GW/m², which poses significant challenges to operation and the first-wall materials.
In recent years, with advancements in control technology, tokamaks originally designed for positively shaped triangular plasmas, such as TCV in Switzerland, DIII-D in the United States, JET in Europe, and ASDEX-U in Germany, have conducted research on negatively shaped triangular plasmas by significantly adjusting coil currents. The findings reveal that negatively shaped triangular plasmas generally offer advantages such as reduced first-wall heat loads, elimination of edge-localized modes, improved confinement, and higher density, which align well with the needs of fusion reactors. It is noteworthy that these results were achieved without optimization for negatively shaped plasmas; if the tokamaks were designed inherently for negative triangular plasmas, their performance could be even more promising.
Negative Triangularity Spherical Tokamak: Anticipated to address the challenges in the engineering construction of fusion reactors
Compared to conventional tokamaks, the spherical tokamak significantly enhances confinement by compressing the central space of the plasma torus, keeping the plasma in regions with better confinement as much as possible. However, this compression in the center of the spherical tokamak results in a strong magnetic field, high current density, and a concentration of outgoing cables and joints, posing a series of engineering challenges for the spherical tokamak.
To maintain the confinement performance of the spherical tokamak while reducing the challenges of engineering construction, Startorus Fusion has innovatively proposed a solution that combines the spherical tokamak with a negatively shaped triangular plasma.
Advantage 1: Increasing Center Column Space
The central region of a Negative Triangularity Spherical Tokamak can be transformed from a slender, straight cylindrical form into a more spacious hourglass configuration. This transformation significantly amplifies the available space, markedly diminishes the current density, and facilitates the layout of various outgoing cables and connectors, making them much easier to manage. Consequently, the primary engineering challenges that have traditionally constrained the development of spherical tokamaks can be nearly entirely mitigated. The figure below compares the central regions of the positive triangularity spherical tokamak (left) and the negative triangularity spherical tokamak (right).
The positive triangularity spherical tokamak (left) and The negative triangularity spherical tokamak (right)
Advantage 2: Increasing the Tritium Breeding Ratio and Reducing the Burden on the First Wall
For fusion reactors, the hourglass-shaped central region is an ideal location to install a tritium breeding blanket, which can improve the tritium breeding ratio and, consequently, improve the economic viability of the reactor. Additionally, the striking area of the negative triangular plasma divertor is significantly expanded, which substantially alleviates the thermal load on the first wall, reducing the stress on this critical component of the reactor.
Advantage 3: Reducing the Burden on the Power Supply
In order to permeate the magnetic flux throughout the entire positive triangular plasma, the straight cylindrical solenoid must forcibly shape the magnetic field lines into a vertical orientation, leading to a significant accumulation of magnetic field energy. In contrast, the hourglass-shaped solenoid aligns more naturally with the magnetic field lines, enabling the magnetic flux to cover the entire negative triangular plasma without excessive constriction, thereby significantly reducing the burden on the power supply.
As illustrated in the figure, despite connecting nearly identical magnetic fluxes, the straight cylindrical solenoid for the positive triangular plasma stores 818 kJ of energy, whereas the hourglass-shaped solenoid for the negative triangular plasma stores only 263 kJ.
Naturally, the negative triangularity spherical tokamak plasma will be distant from some regions of strong magnetic fields, demanding higher control over plasma instability, which will be issues that NTST needs to investigate.
The construction of NTST marks not only a milestone in the research and development of Startorus Fusion but also a strategic move towards the future commercialization of fusion energy. Through NTST, Startorus Fusion will validate core fusion technologies, accelerate technological innovation, and is expected to significantly reduce construction and operational costs. Research on negative triangular plasma will provide solutions to the engineering challenges of building spherical tokamaks, laying a solid foundation for the construction of more efficient and economical fusion reactors, and offering robust support for the economic viability and competitiveness of future fusion energy.
※Startorus Fusion will commence construction of the NTST unit after confirming the site for the unit and obtaining relevant qualifications such as EIA certification.
Progress on previous phases of the Startorus Fusion project:
World's First! Startorus Fusion's New Breakthrough in Device Operation and Control
Significant Progress in Engineering Validation of Startorus Fusion's High-temperature Supercond
Magnetic Reconnection Heating! Startorus Fusion Achieves Another Breakthrough Within a Month
Startorus Successfully Operates the First Experimental Fusion Device
