Self-Organised Criticality: Difference between revisions
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But the interaction of such feedback mechanisms on various radial locations would lead to ''avalanche'' behaviour and a true (scale-free) self-organised state. | But the interaction of such feedback mechanisms on various radial locations would lead to ''avalanche'' behaviour and a true (scale-free) self-organised state. | ||
Indeed, there is direct evidence for avalanching behaviour in numerical simulations, but experimental evidence is scarce. | Indeed, there is direct evidence for avalanching behaviour in numerical simulations | ||
<ref>[http://link.aip.org/link/?PHPAEN/12/092305/1 L. García and B.A. Carreras, Phys. Plasmas '''12''', 092305 (2005)]</ref>, | |||
but experimental evidence is scarce. | |||
<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.84.1192 P.A. Politzer, Phys. Rev. Lett. '''84''', 1192 (2000)]</ref> | <ref>[http://link.aps.org/doi/10.1103/PhysRevLett.84.1192 P.A. Politzer, Phys. Rev. Lett. '''84''', 1192 (2000)]</ref> | ||
However, some indirect evidence exists. Typically, such evidence involves the detection of long-range correlations in fluctuations. | However, some indirect evidence exists. Typically, such evidence involves the detection of long-range correlations in fluctuations. | ||
Revision as of 12:44, 20 July 2009
Self-Organised Criticality (SOC) is a generic concept, applicable to a host of complex systems [1]. A system is said to be in this state when it is at an attractive critical point at which it behaves as in a phase transition (i.e., the spatial and temporal scales are scale-invariant, or nearly so).
In magnetically confined plasmas, this state is thought to be responsible for the global transport phenomena of profile consistency, the Bohm scaling of confinement (in L-mode), and power degradation. Profile consistency is the observation that profiles tend to have roughly the same shape, regardless of the power and location of the applied heating. [2] Power degradation shows up in global transport scaling laws, and implies a sub-linear scaling of the plasma energy content with the injected power.
The basic explanation for this phenomenon is self-regulation of the profiles by turbulence. The strong temperature and density gradients in fusion-grade plasmas provide free energy that may drive turbulence. The turbulence then enhances transport locally, leading to a local reduction of gradients and a consequential damping of the turbulence amplitude. This feedback could be responsible for keeping the gradients below a critical value. Considered locally, the former is a description of a simple marginal state. But the interaction of such feedback mechanisms on various radial locations would lead to avalanche behaviour and a true (scale-free) self-organised state.
Indeed, there is direct evidence for avalanching behaviour in numerical simulations [3], but experimental evidence is scarce. [4] However, some indirect evidence exists. Typically, such evidence involves the detection of long-range correlations in fluctuations. [5]