User:Hasan Ghotme
Comparison of Tokamaks and Stellarators
The following table presents a comparative overview of tokamak and stellarator, based primarily on results and discussions from [1], 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. [1][2][3][4][5][6][7][8][9]
| Aspect | Tokamak | Stellarator |
|---|---|---|
| Magnetic Geometry and Plasma Confinement | ||
| Magnetic field generation | External toroidal coils + poloidal field from plasma current[1] | Entirely by external non-axisymmetric (helical) coils[1] |
| Axisymmetry | Axisymmetric configuration[1] | Non-axisymmetric (three-dimensional)[1] |
| Plasma volume | Typically large | Usually small |
| Aspect ratio (R/a) | Typically small: 2.5–4 | Usually large: 5–12 |
| Plasma confinement | High confinement due to helical field lines; prone to instabilities | Slightly lower confinement; more stable without plasma current |
| Rotational transform | Mainly from plasma current[2][1] | From 3D magnetic shaping[2][1] |
| MHD stability and operational limits | ||
| MHD instabilities | Many types due to large plasma current | Very few, mostly small tearing modes |
| Plasma current () | Large toroidal plasma current required[1] | No net toroidal plasma current required[1][3] |
| Plasma disruptions | Major disruptions possible[1] | Nearly disruption-free[1] |
| Beta limit () | Limited by ideal-MHD ballooning modes[4][1] | Softer beta limit[1] |
| Transport and confinement | ||
| Diffusivity regimes | 3 main regimes: neoclassical, Bohm, turbulent | 4–5 regimes: Classical, neoclassical, turbulent, longitudinal, convective |
| Neoclassical transport | Generally low[1] | Higher[3][1] |
| Turbulent transport | Comparable to stellarators[5][1] | Comparable to tokamaks[1] |
| ITG (Ion Temperature Gradient) modes | Collisionless microturbulence; similar behavior in both devices | Collisionless microturbulence; similar behavior in both devices |
| TEM (Trapped Electron Mode) | Generally unstable; strong electron transport | Often stabilized by 3D magnetic geometry |
| KBM (Kinetic Ballooning Mode) | High growth at high beta | Growth reduced; 3D geometry provides partial stabilization |
| Pressure gradient () | Can be large; may drive strong MHD instabilities | Weaker effect; 3D geometry stabilizes gradients |
| Isotope effect | Clearly observed[6][1] | Not clearly observed[1] |
| Plasma rotation | ||
| Plasma rotation | Strong toroidal rotation[7][1] | Weaker rotation[8][1] |
| Zonal flows | Weaker damping[1] | Stronger damping[1] |
| Edge and divertor physics | ||
| Divertor concept | Single-null or double-null divertors[9][1] | Island or helical divertors[9][1] |
| Impurity control | Ion-temperature-gradient force often dominant[9][1] | Stronger impurity retention[9][1] |
| X-point | Common; used in divertor to remove heat and impurities | Less common; 3D geometry often provides natural edge shaping |
| Reactor and engineering considerations | ||
| Engineering complexity | Relatively simpler magnetic geometry[1] | Highly complex coil geometry[1] |
| Reactor prospects | Clear near-term path but challenged by steady-state operation and disruptions[1] | Attractive long-term option due to steady-state and disruption-free operation[1] |
| Next fusion reactor | DEMO (DEMonstration power plant) | HELIAS (HELIcal Advanced Stellarator) |
| Reactor challenges |
|
|
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.30 1.31 Y. Xu, "A general comparison between tokamak and stellarator plasmas", Matter and Radiation at Extremes 1 (2016) 192–200.
- ↑ 2.0 2.1 2.2 L. Spitzer, "The stellarator concept", Physics of Fluids 1 (1958) 253.
- ↑ 3.0 3.1 3.2 P. Helander et al., Plasma Physics and Controlled Fusion 54 (2012) 124009.
- ↑ 4.0 4.1 J.W. Connor and J.B. Taylor, Nuclear Fusion 17 (1977) 1047.
- ↑ 5.0 5.1 U. Stroth, Plasma Physics and Controlled Fusion 40 (1998) 9.
- ↑ 6.0 6.1 Y. Xu et al., Physical Review Letters 110 (2013) 265005.
- ↑ 7.0 7.1 T.H. Stix, Physics of Fluids 16 (1973) 1260.
- ↑ 8.0 8.1 P. Helander, Physics of Plasmas 14 (2007) 104501.
- ↑ 9.0 9.1 9.2 9.3 9.4 Y. Feng et al., Plasma Physics and Controlled Fusion 53 (2011) 024009.