Anomalous transport

The best and most complete theory of transport in magnetically confined systems is the Neoclassical theory. However, it is found that transport often exceeds Neoclassical expectations by an order of magnitude or more (also see Non-diffusive transport). The difference between actual transport and the Neoclassical expectation is called "anomalous" transport. It is generally assumed that the anomalous component of transport is generated by turbulence.

How important is anomalous transport?

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 of transport with heating power and machine size. The phenomenon of power degradation, universally observed in all devices, is an indication that standard (diffusive) transport theories are inadequate to explain all transport. Profile consistency indicates that 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.

Can anomalous transport be controlled?

Yes. The impression is that anomalous transport is more difficult to control in tokamaks than in stellarators. However, limited control in tokamaks is possible by making use of edge transport barriers (cf. H-mode) and Internal Transport Barriers (ITBs). This reduces transport to Neoclassical levels, at least transiently.

Particularly in optimised stellarators (W7-AS), transport tends to be close to Neoclassical levels.