11.2 Stellarator Projects (Wendelstein 7-X, LHD)
3 min read•july 19, 2024
Stellarators are fusion devices that use twisting magnetic fields to confine plasma without needing a current. They offer and better stability than tokamaks. However, their complex design makes them harder to build and optimize.
in Germany and the Large Helical Device in Japan are leading stellarator experiments. They aim to improve and demonstrate long-pulse operation. While stellarators lag behind tokamaks in some areas, they show promise for future fusion power plants.
Stellarator Concepts and Projects
Principles of stellarator concept
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- Stellarators are fusion devices that use external magnetic coils to create a twisting, toroidal magnetic field to confine plasma in a steady-state manner without requiring a plasma current
- Advantages of stellarators over tokamaks include:
- Inherently steady-state operation due to lack of plasma current eliminates the need for current drive systems or pulsed operation
- Reduced risk of current-driven instabilities (kink modes) and disruptions enhances device safety and reliability
- Greater flexibility in magnetic field configuration and optimization allows for improved plasma confinement and stability
Design of Wendelstein 7-X
- Wendelstein 7-X (W7-X) is an advanced stellarator located in Greifswald, Germany that began operation in 2015 after over a decade of construction
- Unique design features of W7-X include:
- Modular with a complex 3D shape consisting of 50 non-planar coils and 20 planar coils arranged in five identical modules
- Optimized magnetic field configuration for improved confinement and stability by reducing neoclassical transport and bootstrap current and improving equilibrium and stability at high plasma pressure
- Actively cooled divertor system for steady-state heat and particle exhaust enables long-pulse operation
- Large plasma volume (30 m³) and major radius (5.5 m) allow for high-performance plasma experiments
Stellarator Experiments and Comparisons
Goals of Large Helical Device
- Large Helical Device (LHD) is a large stellarator located in Toki, Japan that began operation in 1998 and is operated by the National Institute for Fusion Science (NIFS)
- Research goals of LHD include:
- Investigating plasma confinement and heating in a large-scale stellarator to understand scaling laws and optimize performance
- Studying steady-state operation and high-performance plasmas to demonstrate the viability of stellarators for fusion power plants
- Exploring advanced concepts such as helical divertors and island divertors to improve power and particle exhaust
- Notable achievements of LHD include:
- Achieved record stellarator plasma parameters with ion temperatures exceeding 10 keV, densities exceeding , and stored energy up to 1.6 MJ
- Demonstrated steady-state operation with high-performance plasmas by sustaining plasmas for over 30 minutes with of heating power
- Investigated a wide range of plasma phenomena (turbulence, transport) and advanced concepts (pellet injection, impurity control)
Stellarators vs tokamaks
- Performance comparison:
- Tokamaks have achieved higher plasma temperatures (up to 40 keV), densities (up to ), and confinement times (up to a few seconds) than stellarators (up to 10 keV, , and a few hundred milliseconds, respectively)
- Stellarators have demonstrated longer pulse durations (up to 30 minutes in LHD) and steady-state operation compared to tokamaks (up to a few minutes in EAST and KSTAR)
- Challenges for stellarators:
- Complex and expensive magnet systems due to 3D geometry increase construction and maintenance costs
- Reduced plasma volume and higher aspect ratios compared to tokamaks limit fusion power density
- Historically lower confinement and plasma performance than tokamaks require further optimization
- Challenges for tokamaks:
- Pulsed operation and need for current drive systems increase complexity and reduce efficiency
- Susceptibility to current-driven instabilities (disruptions) poses risks to device integrity
- Difficulty in achieving steady-state operation with high plasma performance limits fusion power plant prospects
Key Terms to Review (18)
David B. Phelps: David B. Phelps is a notable figure in the field of nuclear fusion, particularly recognized for his contributions to the development and analysis of stellarators, including projects like Wendelstein 7-X and the Large Helical Device (LHD). His work focuses on improving the design and efficiency of these fusion devices, which are crucial for advancing sustainable energy solutions through controlled nuclear fusion processes.
Energy Loss Mechanisms: Energy loss mechanisms refer to the various processes through which energy is dissipated or lost in a system, particularly in the context of plasma physics and nuclear fusion. In stellarator designs, understanding these mechanisms is crucial as they affect the overall efficiency and performance of the reactor, impacting plasma confinement and stability, and ultimately influencing the feasibility of sustainable fusion energy production.
Eurofusion: Eurofusion is a European consortium aimed at advancing nuclear fusion research and development, primarily focusing on the ITER project and various other fusion experiments. It brings together numerous research institutes and organizations across Europe to collaborate on the goal of achieving sustainable and commercially viable fusion energy, thus addressing the global energy crisis.
Fusion energy research alliance: A fusion energy research alliance is a collaborative effort among various organizations, institutions, and countries aimed at advancing the development of nuclear fusion as a viable energy source. This collaboration focuses on sharing knowledge, resources, and technologies to overcome the scientific and engineering challenges associated with achieving sustained fusion reactions, ultimately contributing to cleaner and more sustainable energy solutions.
Helical Symmetry: Helical symmetry refers to a specific geometric arrangement in which an object is invariant under a combination of rotation and translation along an axis. In the context of magnetic confinement fusion devices, such as stellarators, helical symmetry plays a crucial role in shaping the magnetic fields that confine plasma and stabilize it for fusion reactions. This symmetry helps to maintain the desired magnetic configuration while allowing for improved confinement and reduced turbulence within the plasma.
International Thermonuclear Experimental Reactor (ITER): ITER is a large-scale international scientific collaboration aimed at demonstrating the feasibility of nuclear fusion as a viable and sustainable energy source. This experimental reactor seeks to mimic the processes that power the sun, using magnetic confinement to achieve the necessary conditions for fusion. ITER is significant not only for its innovative technology and potential energy benefits, but also for its global collaboration efforts, which encompass environmental sustainability and funding strategies for future fusion projects.
LHD (Large Helical Device): The Large Helical Device (LHD) is a stellarator-type fusion reactor designed to confine and control plasma using helical magnetic fields. By utilizing a unique twisted magnetic configuration, it aims to maintain stable plasma conditions for nuclear fusion, distinguishing itself from traditional tokamaks. The LHD contributes to advancing our understanding of plasma behavior and fusion energy generation, making it a significant project in the realm of stellarator technology.
Magnetic Confinement: Magnetic confinement is a method used in nuclear fusion to contain hot plasma through the use of magnetic fields, preventing the plasma from coming into contact with the reactor walls. This technique is crucial for maintaining the conditions necessary for fusion reactions, as it helps stabilize the plasma and reduces energy losses. By leveraging magnetic fields, researchers can achieve the high temperatures and pressures needed to initiate and sustain fusion processes, which are vital for developing practical fusion energy.
Max Planck: Max Planck was a German physicist known as the father of quantum theory, which revolutionized our understanding of atomic and subatomic processes. His work laid the foundation for modern physics and introduced concepts such as energy quanta, essential for advancements in various fields, including nuclear fusion technology. Planck's principles directly influence stellarator designs, affecting how scientists approach magnetic confinement in projects like Wendelstein 7-X and LHD.
National Spherical Torus Experiment: The National Spherical Torus Experiment (NSTX) is a research facility in the United States dedicated to studying plasma physics and nuclear fusion using a unique spherical tokamak design. This approach allows for a more compact and efficient confinement of plasma, which is critical for advancing fusion technology. The NSTX aims to explore innovative concepts in magnetic confinement and investigate the feasibility of spherical tokamaks as a viable path toward sustained nuclear fusion energy production.
Plasma Confinement: Plasma confinement refers to the methods and techniques used to contain and stabilize plasma, the fourth state of matter, within a controlled environment to facilitate nuclear fusion reactions. Effective confinement is essential for achieving the high temperatures and pressures needed for fusion while minimizing energy losses and instabilities.
Plasma confinement time: Plasma confinement time is the duration for which plasma can be effectively maintained in a stable state within a fusion reactor before it loses energy and particles. This time is crucial for sustaining nuclear fusion reactions, as it directly impacts the efficiency and viability of fusion energy production. Longer confinement times allow for more collisions between nuclei, increasing the likelihood of fusion events, which is essential for projects that aim to achieve practical energy output.
Plasma heating methods: Plasma heating methods are techniques used to increase the temperature of plasma to achieve the conditions necessary for nuclear fusion. These methods are critical in maintaining the high energy levels required for fusion reactions and can include various forms of energy transfer such as electromagnetic waves, neutral beams, and radiofrequency waves. Effective plasma heating is essential for sustaining the fusion process, particularly in advanced fusion reactors like stellarators, where complex magnetic confinement systems are employed.
Plasma stability: Plasma stability refers to the ability of a plasma to maintain its confinement and structure without experiencing disruptive instabilities that can lead to loss of containment or energy. Achieving and maintaining stability is critical in fusion systems as it directly impacts plasma performance, energy output, and the longevity of the confinement device.
Reduced Magnetic Turbulence: Reduced magnetic turbulence refers to the minimization of chaotic fluctuations in the magnetic fields within a plasma, which is crucial for maintaining stability in fusion devices. Achieving reduced magnetic turbulence is essential for improving confinement, enhancing the efficiency of energy transfer, and ensuring that the plasma remains stable during operation.
Steady-State Operation: Steady-state operation refers to a condition in which a fusion reactor maintains consistent performance over an extended period, allowing for continuous plasma confinement and energy production. This mode is crucial for the feasibility of nuclear fusion as a practical energy source, emphasizing stability in plasma parameters such as temperature, density, and confinement time while minimizing fluctuations that could lead to disruptions.
Superconducting coils: Superconducting coils are loops of wire made from superconducting materials that exhibit zero electrical resistance below a certain critical temperature. These coils are crucial in the construction of magnetic confinement devices, as they generate strong magnetic fields necessary for stabilizing plasma in fusion reactors, like stellarators.
Wendelstein 7-X: Wendelstein 7-X is a stellarator-type nuclear fusion experiment located in Greifswald, Germany, designed to explore the viability of stellarator configurations for achieving controlled nuclear fusion. This facility is significant in the historical development of fusion research, especially as it represents a major step towards understanding how to maintain stable plasma confinement and sustain nuclear fusion reactions, contributing to the ongoing evolution of fusion technology.