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 driven by micro-instabilities. [1]
How important is turbulence?
In spite of lengthy studies into the subject, it is still controversial how important turbulent transport really is. In part, this may be because turbulent transport gives a variable contribution to transport (depending on local and global parameters), whereas Neoclassical transport is always present.
Arguments in favour
An important 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 (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 (non-anomalous) transport theory becomes non-linear.
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.
The suppression of turbulence is possible, either actively (by imposing an external radial electric field), or spontaneously (H-modes, Internal Transport Barriers). 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. [2] 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?
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 can be close to Neoclassical levels. [3]
References
- ↑ J.P. Freidberg, Plasma physics and fusion energy, Cambridge University Press (2007) ISBN 0521851076
- ↑ L.C. Woods, Theory of tokamak transport: new aspects for nuclear fusion reactor design, John Wiley and Sons (2006) ISBN 3527406255
- ↑ M. Hirsch et al, Major results from the stellarator Wendelstein 7-AS, Plasma Phys. Control. Fusion 50 (2008) 053001