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Non-diffusive transport: Difference between revisions
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It has long been known that the standard model for transport in magnetically confined plasmas ([[Neoclassical transport]]) often fails to provide an accurate description of experimental results: it tends to underestimate transport by one order of magnitude, typically. This is a very disappointing situation with a view to constructing a fusion reactor, since worse confinement means that an eventual reactor will need to be bigger and more expensive. Therefore, the search for the cause of this failure (and for methods to restore transport to its Neoclassical value) is one of the main issues of fusion research. | It has long been known that the standard model for transport in magnetically confined plasmas ([[Neoclassical transport]]) often fails to provide an accurate description of experimental results: it tends to underestimate transport by one order of magnitude, typically. This is a very disappointing situation with a view to constructing a fusion reactor, since worse confinement means that an eventual reactor will need to be bigger and more expensive. Therefore, the search for the cause of this failure (and for methods to restore transport to its Neoclassical value) is one of the main issues of fusion research. | ||
The standard Neoclassical model is a diffusive model, which means that transport is characterised by ''typical scale lengths'', both for space and time, so that the effective diffusion coefficient is essentially the ''mixing length'' value: <math>D = \Delta r^2 / \Delta t</math>, where <math>\Delta r</math> is the typical step size and <math>\Delta t</math> the typical waiting time. | The standard Neoclassical model is a collisional (diffusive) model, which means that transport is characterised by ''typical scale lengths'', both for space and time, so that the effective diffusion coefficient is essentially the ''mixing length'' value: <math>D = \Delta r^2 / \Delta t</math>, where <math>\Delta r</math> is the typical step size and <math>\Delta t</math> the typical waiting time. | ||
In recent years, it has been suggested that the plasma may contain phenomena that invalidate this picture. | In recent years, it has been suggested that the plasma may contain phenomena that invalidate this picture. | ||
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<ref>[http://books.google.es/books?id=Yaupom_qdKIC&lpg=PP1&ots=WmaABP3l92&dq=%22Balescu%22%20%22Aspects%20of%20anomalous%20transport%20in%20plasmas%22%20&lr=&pg=PP1 R. Balescu, Aspects of anomalous transport in plasmas, IOP Publishing (2005)]</ref> | <ref>[http://books.google.es/books?id=Yaupom_qdKIC&lpg=PP1&ots=WmaABP3l92&dq=%22Balescu%22%20%22Aspects%20of%20anomalous%20transport%20in%20plasmas%22%20&lr=&pg=PP1 R. Balescu, Aspects of anomalous transport in plasmas, IOP Publishing (2005)]</ref> | ||
The CTRW model provides a mathematical framework for handling non-diffusive transport, but it does not explain why such non-diffusive transport should arise: answering the latter requires detailed computer simulations of turbulence and experimental observations. | The CTRW model provides a mathematical framework for handling non-diffusive transport, but it does not explain why such non-diffusive transport should arise: answering the latter requires detailed computer simulations of turbulence and experimental observations. | ||
However, even without fully understanding the origin of the non-diffusive behaviour, it is possible to construct models based on these ideas, and see whether these models fare better in predicting the global transport properties of plasmas then the standard diffusive models. | |||
<ref>[http://link.aip.org/link/?PHPAEN/11/2272/1 B.Ph. van Milligen, R. Sánchez, and B.A. Carreras, Phys. Plasmas '''11''', 2272 (2004)]</ref> | |||
<ref>[http://link.aip.org/link/?PHPAEN/11/3787/1 B.Ph. van Milligen, B.A. Carreras, and R. Sánchez, Phys. Plasmas '''11''', 3787 (2004)]</ref> | |||
==References== | ==References== | ||
<references /> | <references /> | ||