5.3 Stellarator physics

Stellarators are a unique approach to magnetic confinement fusion, using complex 3D magnetic fields to confine plasma. They offer potential advantages over tokamaks in steady-state operation and stability, but face challenges in achieving good confinement due to their 3D nature.

Stellarator optimization focuses on improving plasma confinement through magnetic field design. Techniques like quasi-symmetry and neoclassical transport reduction aim to overcome inherent challenges and achieve fusion-relevant performance comparable to tokamaks.

Stellarator concept

• Stellarators represent an innovative approach to magnetic confinement fusion in High Energy Density Physics
• Utilize complex 3D magnetic field configurations to confine and heat plasma for fusion reactions
• Offer potential advantages over tokamaks in terms of steady-state operation and plasma stability

Basic stellarator design

Top images from around the web for Basic stellarator design

• 

  Basic Principles, Ampère’s Law and Magnetic Fields Strength View original

  Is this image relevant?

• 

  Stellarator - Wikimedia Commons View original

  Is this image relevant?

• 

  Processes of Science and Art Modeled as a Holoflux of Information Using Toroidal Geometry View original

  Is this image relevant?

• 

  Basic Principles, Ampère’s Law and Magnetic Fields Strength View original

  Is this image relevant?

• 

  Stellarator - Wikimedia Commons View original

  Is this image relevant?

1 of 3

Top images from around the web for Basic stellarator design

• 

  Basic Principles, Ampère’s Law and Magnetic Fields Strength View original

  Is this image relevant?

• 

  Stellarator - Wikimedia Commons View original

  Is this image relevant?

• 

  Processes of Science and Art Modeled as a Holoflux of Information Using Toroidal Geometry View original

  Is this image relevant?

• 

  Basic Principles, Ampère’s Law and Magnetic Fields Strength View original

  Is this image relevant?

• 

  Stellarator - Wikimedia Commons View original

  Is this image relevant?

1 of 3

• Consists of a toroidal vacuum vessel surrounded by external magnetic field coils
• Employs non-planar coils to create a helical magnetic field structure
• Requires precise engineering to achieve desired magnetic field geometry
• Aims to confine plasma without relying on internal plasma currents

Magnetic field configuration

• Creates a helical magnetic field path that twists around the torus
• Utilizes a combination of toroidal and poloidal field components
• Achieves rotational transform through external coils rather than plasma current
• Produces magnetic surfaces that form nested flux tubes within the plasma volume

Comparison vs tokamaks

• Stellarators operate without large toroidal plasma currents, reducing risk of disruptions
• Offer potential for true steady-state operation without need for current drive
• Require more complex magnetic field designs and coil systems
• Present challenges in achieving good confinement due to their inherent 3D nature
• Typically have lower plasma pressure limits compared to tokamaks

Plasma confinement in stellarators

• Stellarators rely on carefully designed magnetic fields to confine and control fusion plasmas
• Aim to minimize particle losses and optimize energy confinement for fusion reactions
• Present unique challenges due to their three-dimensional magnetic geometry

Magnetic surfaces

• Form nested toroidal surfaces of constant magnetic flux
• Provide the basic structure for plasma confinement in stellarators
• Can be visualized as concentric "shells" of magnetic field lines
• Ideally form closed surfaces to prevent radial particle transport
• Quality of magnetic surfaces impacts overall confinement performance

Particle orbits

• Charged particles follow complex trajectories along magnetic field lines
• Include bounce motion between magnetic mirror points
• Exhibit drift motions due to magnetic field gradients and curvature
• Can lead to enhanced particle losses in non-optimized stellarator designs
• Optimization aims to reduce unfavorable drift orbits and improve particle confinement

Transport processes

• Include classical, neoclassical, and turbulent transport mechanisms
• Neoclassical transport often dominates in stellarators due to 3D geometry
• Leads to enhanced radial particle and energy losses compared to tokamaks
• Can result in formation of electric fields that affect particle confinement
• Optimization techniques focus on reducing neoclassical transport to improve performance

Stellarator optimization

• Focuses on improving plasma confinement and stability through magnetic field design
• Utilizes advanced computational tools to model and optimize 3D magnetic configurations
• Aims to overcome inherent challenges of stellarator concept to achieve fusion-relevant performance

Quasi-symmetry

• Approximates symmetry in magnetic field strength along field lines
• Reduces neoclassical transport and improves particle confinement
• Can be achieved through careful shaping of magnetic field geometry
• Includes quasi-axisymmetry, quasi-helical symmetry, and quasi-isodynamic configurations
• Aims to combine benefits of stellarator and tokamak magnetic field properties

Neoclassical transport reduction

• Targets minimization of radial particle and energy losses
• Involves optimizing magnetic field structure to reduce unfavorable particle drifts
• Utilizes advanced optimization algorithms to find optimal coil configurations
• Aims to achieve confinement levels comparable to or better than tokamaks
• Crucial for improving overall stellarator performance and fusion relevance

Bootstrap current minimization

• Reduces self-generated plasma currents that can distort magnetic field configuration
• Helps maintain desired magnetic field structure during plasma operation
• Contributes to improved stability and steady-state capabilities
• Achieved through careful tailoring of magnetic field geometry
• Allows for better control of plasma equilibrium and confinement properties

Types of stellarators

• Stellarators encompass a diverse family of magnetic confinement devices
• Different types explore various approaches to optimizing plasma confinement and stability
• Reflect the evolution of stellarator concept and advances in physics understanding

Classical stellarators

• Represent early stellarator designs with simple helical coil configurations
• Utilize a combination of planar toroidal field coils and helical windings
• Suffer from poor confinement due to lack of optimization
• Include historical devices (Wendelstein 7-A, CLEO)
• Provided valuable insights for development of more advanced stellarator concepts

Modular stellarators

• Employ discrete, non-planar coils to create optimized 3D magnetic fields
• Allow for greater flexibility in magnetic field shaping and optimization
• Simplify engineering and maintenance compared to continuous helical coils
• Include modern optimized stellarators (Wendelstein 7-X, HSX)
• Enable implementation of quasi-symmetry and other advanced optimization techniques

Helical axis stellarators

• Feature a non-planar magnetic axis that follows a helical path
• Aim to improve particle confinement through reduced drift orbit losses
• Can achieve quasi-helical symmetry for improved neoclassical transport
• Include devices (HSX, NCSX - cancelled project)
• Explore alternative approaches to stellarator optimization and performance enhancement

Stellarator experiments

• Represent cutting-edge research facilities in fusion science and technology
• Aim to demonstrate feasibility and performance of optimized stellarator concepts
• Provide valuable data for validation of stellarator physics and engineering models

Wendelstein 7-X

• Located in Greifswald, Germany, operated by Max Planck Institute for Plasma Physics
• World's largest and most advanced stellarator experiment
• Utilizes 50 non-planar and 20 planar superconducting coils to create optimized magnetic field
• Designed to demonstrate reactor-relevant plasma performance in quasi-isodynamic configuration
• Achieved world record fusion triple product for stellarators in 2022

Large Helical Device

• Situated in Toki, Japan, operated by National Institute for Fusion Science
• Features continuous helical coils to create twisted magnetic field configuration
• Largest heliotron-type device, closely related to stellarator concept
• Explores long-pulse and high-beta plasma regimes
• Contributes to understanding of 3D plasma physics and stellarator-relevant fusion science

HSX stellarator

• Located at University of Wisconsin-Madison, USA
• Compact stellarator designed to study quasi-helical symmetry
• Utilizes modular coils to create optimized magnetic field configuration
• Focuses on reducing neoclassical transport and improving particle confinement
• Provides valuable data on benefits of quasi-symmetry in stellarator designs

Engineering challenges

• Stellarators present unique engineering hurdles in fusion reactor development
• Require advanced manufacturing and assembly techniques to achieve desired precision
• Push boundaries of current technology in various areas of fusion engineering

Complex coil design

• Demands high-precision manufacturing of non-planar coils with complex 3D shapes
• Requires advanced computational tools for coil optimization and design
• Necessitates development of novel winding techniques for superconducting coils
• Presents challenges in coil support structures to withstand electromagnetic forces
• Impacts overall cost and complexity of stellarator construction

Plasma-facing components

• Must withstand high heat and particle fluxes in 3D geometry
• Require careful design to accommodate complex magnetic field structure
• Present challenges in maintenance and replacement due to limited access
• Demand development of advanced materials capable of withstanding fusion environment
• Necessitate innovative cooling solutions to manage high heat loads

Neutron shielding

• Crucial for protecting superconducting coils and other sensitive components
• Presents unique challenges due to complex 3D geometry of stellarator designs
• Requires careful optimization to balance shielding effectiveness and access for maintenance
• Impacts overall size and cost of stellarator fusion reactors
• Demands development of advanced neutron-resistant materials and shielding concepts

Stellarator performance

• Evaluates the effectiveness of stellarators in achieving fusion-relevant plasma conditions
• Compares stellarator performance to other magnetic confinement concepts (tokamaks)
• Guides future research and development efforts in stellarator optimization

Energy confinement time

• Measures how long energy remains confined within the plasma
• Typically lower in stellarators compared to similarly sized tokamaks
• Improves with optimization techniques and increased device size
• Scales favorably with plasma volume in stellarators
• Critical parameter for achieving fusion ignition and net energy production

Beta limits

• Represent the ratio of plasma pressure to magnetic field pressure
• Generally lower in stellarators compared to advanced tokamaks
• Improve with optimization of magnetic field configuration
• Impact overall fusion power density and reactor economics
• Ongoing research aims to increase beta limits while maintaining stability

Steady-state operation

• Stellarators excel in capability for continuous plasma operation
• Avoid need for pulsed operation or complex current drive systems
• Reduce risk of plasma disruptions common in tokamaks
• Present challenges in managing continuous heat loads on plasma-facing components
• Offer potential advantages for future fusion power plants

Future prospects

• Stellarators show promise as alternative path to fusion energy
• Ongoing research aims to overcome challenges and improve performance
• Advancements in technology and physics understanding drive stellarator development

Reactor concepts

• Explore feasibility of stellarator-based fusion power plants
• Include designs (HELIAS, ARIES-CS) that extrapolate current experiments to reactor scale
• Address integration of fusion technologies with optimized stellarator configurations
• Consider challenges of tritium breeding, neutron shielding, and power extraction
• Evaluate economic competitiveness compared to other fusion concepts

Stellarator vs tokamak debate

• Compares advantages and disadvantages of both confinement concepts
• Considers steady-state operation and disruption risk as key stellarator benefits
• Weighs complexity and cost of stellarator designs against potential performance gains
• Explores possibility of combining favorable features from both concepts
• Influences funding decisions and research priorities in fusion energy development

Hybrid designs

• Investigate fusion concepts that combine stellarator and tokamak features
• Include quasi-axisymmetric stellarators that approximate tokamak-like symmetry
• Explore use of stellarator-like shaping in tokamak devices to improve stability
• Consider designs with both external shaping coils and plasma current
• Aim to leverage strengths of both concepts while mitigating their weaknesses