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| == Comparison of Tokamaks and Stellarators ==
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| The following table presents a comparative overview of [[tokamak]] and [[stellarator]], based primarily on results and discussions from <ref name="Xu2016" />, together with additional standard literature in magnetic confinement fusion. The comparison highlights key physical properties, transport characteristics, stability behavior, and reactor-relevant challenges of both concepts. The aim is to provide a simplified and coherent picture of the main technical and physical challenges faced by each configuration, and to show how far current experiments are from a practical fusion reactor. <ref name="Xu2016" /><ref name="Spitzer1958" /><ref name="Helander2012" /><ref name="Connor1977" /><ref name="Stroth1998" /><ref name="Xu2013" /><ref name="Stix1973" /><ref name="Helander2007" /><ref name="Feng2011" />
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| {| class="wikitable sortable"
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| |+ Comparison between [[Tokamak]] and [[Stellarator]] plasmas
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| ! Aspect
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| ! [[Tokamak]]
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| ! [[Stellarator]]
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|
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| |-
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| ! colspan="3" style="text-align:center;" | '''Magnetic Geometry and Plasma Confinement'''
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|
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| |-
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| | Magnetic field generation
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| | External toroidal coils + poloidal field from plasma current<ref name="Xu2016" />
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| | Entirely by external non-axisymmetric (helical) coils<ref name="Xu2016" />
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|
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| |-
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| | Axisymmetry
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| | Axisymmetric configuration<ref name="Xu2016" />
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| | Non-axisymmetric (three-dimensional)<ref name="Xu2016" />
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|
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| |-
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| | Plasma volume
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| | Typically large
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| | Usually small
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|
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| |-
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| | Aspect ratio (R/a)
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| | Typically small: 2.5–4
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| | Usually large: 5–12
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|
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| |-
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| | Plasma confinement
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| | High confinement due to helical field lines; prone to instabilities
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| | Slightly lower confinement; more stable without plasma current
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|
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| |-
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| | Rotational transform
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| | Mainly from plasma current<ref name="Spitzer1958" /><ref name="Xu2016" />
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| | From 3D magnetic shaping<ref name="Spitzer1958" /><ref name="Xu2016" />
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|
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| |-
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| ! colspan="3" style="text-align:center;" | '''MHD stability and operational limits'''
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|
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| | MHD instabilities
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| | Many types due to large plasma current
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| | Very few, mostly small tearing modes
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| |-
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| | Plasma current (<math>I_p</math>)
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| | Large toroidal plasma current required<ref name="Xu2016" />
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| | No net toroidal plasma current required<ref name="Xu2016" /><ref name="Helander2012" />
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|
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| |-
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| | Plasma disruptions
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| | Major disruptions possible<ref name="Xu2016" />
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| | Nearly disruption-free<ref name="Xu2016" />
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| | Beta limit (<math>\beta</math>)
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| | Limited by ideal-MHD ballooning modes<ref name="Connor1977" /><ref name="Xu2016" />
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| | Softer beta limit<ref name="Xu2016" />
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| ! colspan="3" style="text-align:center;" | '''Transport and confinement'''
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| | Diffusivity regimes
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| | 3 main regimes: neoclassical, Bohm, turbulent
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| | 4–5 regimes: Classical, neoclassical, turbulent, longitudinal, convective
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| | Neoclassical transport
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| | Generally low<ref name="Xu2016" />
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| | Higher<ref name="Helander2012" /><ref name="Xu2016" />
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| | Turbulent transport
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| | Comparable to stellarators<ref name="Stroth1998" /><ref name="Xu2016" />
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| | Comparable to tokamaks<ref name="Xu2016" />
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| | ITG (Ion Temperature Gradient) modes
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| | Collisionless microturbulence; similar behavior in both devices
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| | Collisionless microturbulence; similar behavior in both devices
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| |-
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| | TEM (Trapped Electron Mode)
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| | Generally unstable; strong electron transport
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| | Often stabilized by 3D magnetic geometry
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| |-
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| | KBM (Kinetic Ballooning Mode)
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| | High growth at high beta
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| | Growth reduced; 3D geometry provides partial stabilization
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| |-
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| | Pressure gradient (<math>\nabla p</math>)
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| | Can be large; may drive strong MHD instabilities
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| | Weaker effect; 3D geometry stabilizes gradients
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|
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| |-
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| | Isotope effect
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| | Clearly observed<ref name="Xu2013" /><ref name="Xu2016" />
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| | Not clearly observed<ref name="Xu2016" />
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| ! colspan="3" style="text-align:center;" | '''Plasma rotation'''
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| | Plasma rotation
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| | Strong toroidal rotation<ref name="Stix1973" /><ref name="Xu2016" />
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| | Weaker rotation<ref name="Helander2007" /><ref name="Xu2016" />
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|
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| |-
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| | Zonal flows
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| | Weaker damping<ref name="Xu2016" />
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| | Stronger damping<ref name="Xu2016" />
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| |-
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| ! colspan="3" style="text-align:center;" | '''Edge and divertor physics'''
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| |-
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| | Divertor concept
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| | Single-null or double-null divertors<ref name="Feng2011" /><ref name="Xu2016" />
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| | Island or helical divertors<ref name="Feng2011" /><ref name="Xu2016" />
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| |-
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| | Impurity control
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| | Ion-temperature-gradient force often dominant<ref name="Feng2011" /><ref name="Xu2016" />
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| | Stronger impurity retention<ref name="Feng2011" /><ref name="Xu2016" />
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| |-
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| | X-point
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| | Common; used in divertor to remove heat and impurities
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| | Less common; 3D geometry often provides natural edge shaping
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| ! colspan="3" style="text-align:center;" | '''Reactor and engineering considerations'''
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| | Engineering complexity
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| | Relatively simpler magnetic geometry<ref name="Xu2016" />
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| | Highly complex coil geometry<ref name="Xu2016" />
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| | Reactor prospects
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| | Clear near-term path but challenged by steady-state operation and disruptions<ref name="Xu2016" />
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| | Attractive long-term option due to steady-state and disruption-free operation<ref name="Xu2016" />
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|
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| |-
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| | Next fusion reactor
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| | DEMO (DEMonstration power plant)
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| | HELIAS (HELIcal Advanced Stellarator)
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| |-
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| | Reactor challenges
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| * Overcome divertor heat load
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| * Handle high-energy neutron bombardment
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| * Tritium breeding blanket
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| * Confine alpha particles at high pressure
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| * Control instabilities driven by alpha particles
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| * Reduce divertor/edge heat load
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| * Handle high-energy neutron bombardment
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| * Tritium breeding blanket
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| * Confine alpha particles at high pressure
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| * Limit impact of instabilities and ripple-driven losses
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| |}
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| == References ==
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| <references>
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| <ref name="Xu2016">Y. Xu, "A general comparison between tokamak and stellarator plasmas", ''Matter and Radiation at Extremes'' '''1''' (2016) 192–200.</ref>
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| <ref name="Spitzer1958">L. Spitzer, "The stellarator concept", ''Physics of Fluids'' '''1''' (1958) 253.</ref>
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| <ref name="Helander2012">P. Helander et al., ''Plasma Physics and Controlled Fusion'' '''54''' (2012) 124009.</ref>
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| <ref name="Connor1977">J.W. Connor and J.B. Taylor, ''Nuclear Fusion'' '''17''' (1977) 1047.</ref>
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| <ref name="Stroth1998">U. Stroth, ''Plasma Physics and Controlled Fusion'' '''40''' (1998) 9.</ref>
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| <ref name="Xu2013">Y. Xu et al., ''Physical Review Letters'' '''110''' (2013) 265005.</ref>
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| <ref name="Stix1973">T.H. Stix, ''Physics of Fluids'' '''16''' (1973) 1260.</ref>
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| <ref name="Helander2007">P. Helander, ''Physics of Plasmas'' '''14''' (2007) 104501.</ref>
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| <ref name="Feng2011">Y. Feng et al., ''Plasma Physics and Controlled Fusion'' '''53''' (2011) 024009.</ref>
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| </references>
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