4,424
edits
Anomalous transport: Difference between revisions
→How important is anomalous transport?
| Line 7: | Line 7: | ||
In spite of lengthy studies into the subject, it is still controversial how important anomalous transport really is. | In spite of lengthy studies into the subject, it is still controversial how important anomalous transport really is. | ||
The main argument suggesting that anomalous transport is important to the degree that it often dominates the total transport is the [[Scaling law|scaling]] of transport with heating power and machine size. The phenomenon of [[Scaling law|power degradation]], universally observed in all devices, is an indication that standard (diffusive) transport theories are inadequate to explain all transport. | |||
=== Arguments in favour === | |||
The main argument suggesting that anomalous transport is important to the degree that it often dominates the total transport is the [[Scaling law|scaling]] of transport with heating power and machine size. The phenomenon of [[Scaling law|power degradation]], universally observed in all devices, is an indication that standard (linear, diffusive) transport theories are inadequate to explain all transport, since these would not predict power degradation. | |||
However, the matter is complicated by the fact that the transport coefficients themselves are functions of the (local) plasma parameters, so that the transport theory becomes non-linear. | |||
[[Profile consistency]] indicates that [[Self-Organised Criticality|self-organisation]] plays an important role in transport, and this can only be the case when instabilities or turbulence are able to regulate the profiles, i.e., when they carry an important fraction of transport. | [[Profile consistency]] indicates that [[Self-Organised Criticality|self-organisation]] plays an important role in transport, and this can only be the case when instabilities or turbulence are able to regulate the profiles, i.e., when they carry an important fraction of transport. | ||
The suppression of turbulence is possible, either actively (by imposing an external radial electric field), or spontaneously ([[H-mode]]s, [[Internal Transport Barrier]]s). As a consequence, transport is reduced significantly (to Neoclassical levels). This is a clear indication that turbulence is responsible for the main fraction of anomalous transport. | |||
=== Arguments against === | |||
It has been argued that turbulence cannot be responsible for a significant fraction of the anomalous component of transport, since that would lead to high resistivity (due to collisions), which contradicts experimental observation. | |||
<ref>L.C. Woods, ''Theory of tokamak transport: new aspects for nuclear fusion reactor design'', John Wiley and Sons (2006) ISBN 3527406255</ref> | |||
However, this argument fails to note that transport events may be collective (e.g., via ''streamers''), which do not require an enhanced collisionality. | |||
== Can anomalous transport be controlled? == | == Can anomalous transport be controlled? == | ||