4,427
edits
Neoclassical transport: Difference between revisions
Jump to navigation
Jump to search
m
Reverted edits by Otihizuv (Talk) to last revision by Admin
No edit summary |
|||
| Line 1: | Line 1: | ||
The Neoclassical Transport Model is one of the pillars of the physics of magnetically confined plasmas. | The Neoclassical Transport Model is one of the pillars of the physics of magnetically confined plasmas. | ||
<ref>[http://link.aps.org/doi/10.1103/RevModPhys.48.239 F.L. Hinton and R.D. Hazeltine, Rev. Mod. Phys. '''48''', 239 (1976)]</ref> | |||
<ref>P. Helander and D.J. Sigmar, ''Collisional Transport in Magnetized Plasmas'', Cambridge University Press (2001) ISBN 0521807980</ref> | |||
It provides a model for the transport of particles, momentum, and heat in complex magnetic geometries. | It provides a model for the transport of particles, momentum, and heat in complex magnetic geometries. | ||
The difference between the Neoclassical and the Classical models lies in the incorporation of geometrical effects, which give rise to complex particle orbits and drifts that were ignored in the latter. | The difference between the Neoclassical and the Classical models lies in the incorporation of geometrical effects, which give rise to complex particle orbits and drifts that were ignored in the latter. | ||
| Line 8: | Line 7: | ||
== Brief summary of the theory == | == Brief summary of the theory == | ||
The theory starts from the (Markovian) [http://en.wikipedia.org/wiki/Fokker-planck Fokker-Planck Equation] for the particle distribution function | The theory starts from the (Markovian) [http://en.wikipedia.org/wiki/Fokker-planck Fokker-Planck Equation] for the particle distribution function <math>f_\alpha(x,v,t)</math>: | ||
: | :<math> | ||
\frac{\partial f_\alpha}{\partial t} + v\cdot \nabla f_\alpha + F \frac{\partial f_\alpha}{\partial v} = C_\alpha(f) | \frac{\partial f_\alpha}{\partial t} + v\cdot \nabla f_\alpha + F \frac{\partial f_\alpha}{\partial v} = C_\alpha(f) | ||
</math> | |||
where | where <math>\alpha</math> indicates the particle species, <math>v</math> is the velocity, | ||
<math>F</math> is a force (the [http://en.wikipedia.org/wiki/Lorentz_force Lorentz force] acting on the particle) and <math>C_\alpha</math> the Fokker-Planck collision operator. | |||
The derivation of this collision operator is highly non-trivial and requires making specific assumptions; | The derivation of this collision operator is highly non-trivial and requires making specific assumptions; | ||
in particular it must be assumed that a single collision has a small random effect on the particle velocity, | in particular it must be assumed that a single collision has a small random effect on the particle velocity, | ||
| Line 23: | Line 22: | ||
Once the collision operator is decided, the moments of the Fokker-Planck equation can be computed. First, some definitions are needed: | Once the collision operator is decided, the moments of the Fokker-Planck equation can be computed. First, some definitions are needed: | ||
: | :<math> | ||
n = \int{f d^3v} | n = \int{f d^3v} | ||
</math> | |||
(particle density) | (particle density) | ||
: | :<math> | ||
n u = \int{v f d^3v} | n u = \int{v f d^3v} | ||
</math> | |||
(particle flux) | (particle flux) | ||
: | :<math> | ||
P = \int{m v \cdot v f d^3v} | P = \int{m v \cdot v f d^3v} | ||
</math> | |||
(stress tensor) | (stress tensor) | ||
: | :<math> | ||
Q = \int{\frac{m v^2}{2} v f d^3v} | Q = \int{\frac{m v^2}{2} v f d^3v} | ||
</math> | |||
(energy flux) | (energy flux) | ||
: | :<math> | ||
P' = \int{m (v-u) \cdot (v-u) f d^3v} | P' = \int{m (v-u) \cdot (v-u) f d^3v} | ||
</math> | |||
(pressure tensor) | (pressure tensor) | ||
: | :<math> | ||
q = \int{\frac{m (v-u)^2}{2} (v-u) f d^3v} | q = \int{\frac{m (v-u)^2}{2} (v-u) f d^3v} | ||
</math> | |||
(heat flux) | (heat flux) | ||
The main goal of Neoclassical transport theory is to provide a closed set of equations for the time evolution of these moments, for each particle species. Since the determination of any moment requieres knowledge of the next order moment, this requires truncating the set of moments (''closure'' of the set of equations). | The main goal of Neoclassical transport theory is to provide a closed set of equations for the time evolution of these moments, for each particle species. Since the determination of any moment requieres knowledge of the next order moment, this requires truncating the set of moments (''closure'' of the set of equations). | ||
<ref>T.J.M. Boyd and J.J. Sanderson, ''The physics of plasmas'', Cambridge University Press (2003) ISBN 0521459125</ref> | |||
The theory takes account of all particle motion associated with toroidal geometry; specifically, ''& | The theory takes account of all particle motion associated with toroidal geometry; specifically, ''∇ B'' and curvature drifts, and passing and trapped particles (banana orbits). | ||
The theory is valid for all [[Collisionality|collisionality]] regimes, and includes effects due to resistivity and viscosity. An important prediction of the theory is the [[Bootstrap current|bootstrap current]]. | The theory is valid for all [[Collisionality|collisionality]] regimes, and includes effects due to resistivity and viscosity. An important prediction of the theory is the [[Bootstrap current|bootstrap current]]. | ||
| Line 72: | Line 71: | ||
The [[Bootstrap current|bootstrap current]] predicted by the theory is confirmed experimentally, both qualitatively and quantitatively. | The [[Bootstrap current|bootstrap current]] predicted by the theory is confirmed experimentally, both qualitatively and quantitatively. | ||
''(Citation needed)'' | ''(Citation needed)'' | ||
In experimental studies, Neoclassical transport estimates are often used as a | In experimental studies, Neoclassical transport estimates are often used as a "baseline" transport level - | ||
even though experimental values often exceed Neoclassical estimates by an order of magnitude or more. | even though experimental values often exceed Neoclassical estimates by an order of magnitude or more. | ||
In any case, this | In any case, this "baseline" level facilitates the comparison between devices. | ||
Neoclassical theory is also used in the process of machine design and optimisation. | Neoclassical theory is also used in the process of machine design and optimisation. | ||
<ref>[http://dx.doi.org/10.1088/0741-3335/50/5/053001 M. Hirsch et al. ''Major results from the stellarator Wendelstein 7-AS'', Plasma Phys. Control. Fusion '''50''', 5 (2008) 053001]</ref> | |||
== Limitations == | == Limitations == | ||
| Line 84: | Line 83: | ||
* A fixed geometry. Neoclassical transport is calculated in a static magnetic geometry. In actual experiments (especially Tokamaks), the magnetic field evolves along with the plasma itself, leading to a modification of net transport. While a slow evolution (with respect to typical transport time scales) should not be problematic, rapid changes (such as magnetic reconnections) are outside of the scope of the theory. | * A fixed geometry. Neoclassical transport is calculated in a static magnetic geometry. In actual experiments (especially Tokamaks), the magnetic field evolves along with the plasma itself, leading to a modification of net transport. While a slow evolution (with respect to typical transport time scales) should not be problematic, rapid changes (such as magnetic reconnections) are outside of the scope of the theory. | ||
* The linearity of the model. Neoclassical theory is a linear theory in which profiles are computed from sources, boundary conditions, and transport coefficients (that depend linearly on the profiles). No non-linear feedback of the profiles on the transport coefficients is contemplated. However, there are many experimental studies that show that the profiles feed back non-linearly on transport (via [[TJ-II:Turbulence|turbulence]]), leading to some degree of [[Self-Organised Criticality|self-organisation]]. | * The linearity of the model. Neoclassical theory is a linear theory in which profiles are computed from sources, boundary conditions, and transport coefficients (that depend linearly on the profiles). No non-linear feedback of the profiles on the transport coefficients is contemplated. However, there are many experimental studies that show that the profiles feed back non-linearly on transport (via [[TJ-II:Turbulence|turbulence]]), leading to some degree of [[Self-Organised Criticality|self-organisation]]. | ||
* Locality. Neoclassical theory is a theory of diffusion, and therefore it assumes that particle motion between collisions is small with respect to any other relevant spatial scales. This assumption then allows writing down differential equations, expressing the fluxes in terms of ''local'' gradients. This basic assumption is violated under specific conditions, which may include: (a) the low-collisionality limit, (b) any situation in which the gradient scale length is very small (e.g., [[Internal Transport Barrier]]s), (c) locations close to the plasma edge | * Locality. Neoclassical theory is a theory of diffusion, and therefore it assumes that particle motion between collisions is small with respect to any other relevant spatial scales. This assumption then allows writing down differential equations, expressing the fluxes in terms of ''local'' gradients. This basic assumption is violated under specific conditions, which may include: (a) the low-collisionality limit, (b) any situation in which the gradient scale length is very small (e.g., [[Internal Transport Barrier]]s), (c) locations close to the plasma edge<ref>[http://link.aip.org/link/?PHPAEN/8/3305/1 T. Fülöp, P. Helander, Phys. Plasmas 8, 3305 (2001)]</ref><ref>[http://dx.doi.org/10.1088/0741-3335/47/3/010 V. Tribaldos and J. Guasp, ''Neoclassical global flux simulations in stellarators'', Plasma Phys. Control. Fusion '''47''' (2005) 545]</ref>, and (d) particles transported in ''streamers''. Such phenomena could give rise to [[Non-diffusive transport|super-diffusion]]. Points (a) through (c) can be handled by using a Monte Carlo or Master Equation approach instead of deriving differential equations. | ||
* Markovianity. A second assumption underlying diffusive models (including Neoclassics) is Markovianity, implying that the motion of any particle is only determined by its current velocity and position. However, there are situations, such as stochastic magnetic field regions, persistent turbulent eddies, or transport barriers, where this assumption may be violated (due to trapping effects, so that the preceding history of the particle trajectory becomes important). Typically, this would then give rise to [[Non-diffusive transport|sub-diffusion]]. | * Markovianity. A second assumption underlying diffusive models (including Neoclassics) is Markovianity, implying that the motion of any particle is only determined by its current velocity and position. However, there are situations, such as stochastic magnetic field regions, persistent turbulent eddies, or transport barriers, where this assumption may be violated (due to trapping effects, so that the preceding history of the particle trajectory becomes important). Typically, this would then give rise to [[Non-diffusive transport|sub-diffusion]]. | ||
== References == | == References == | ||
<references /> | |||