Maintaining the energy inside a plasma at a high level for a sufficient amount of time to achieve thermonuclear conditions has been an obsession of scientists in the field since the Sixties.
The global energy confinement time τE is defined by the energy in the plasma divided by the power P needed to sustain it, or in terms of ion and electron temperature and average density n
τE=1.5n(Ti+Te)/P
No
theory so far devised can account for the confinement time observed in tokamaks around the world. Instead
empirical scaling laws have been formulated.
Classical transport theory applies to a cylindrical plasma. The toroidal shape of a tokamak results in drifts in the particle orbits (and, hence, enhanced transport). Neoclassical transport accounts for these toroidal effects. However, the observed transport is much greater than neoclassical or classical transport and is known as anomalous transport.
It is thought that this turbulence in energy confinement is due to microinstabilities and the formation of magnetic island structures.
Several confinement modes have emerged. L-mode confinement occurs when the neutral beam heating or radio frequency heating commences. The plasma will then, often, make an L-H transition and go into H-mode confinement (H and L refer simply to high and low confinement). A marked improvement in energy confinement time is observed. Though the onset of the H-mode is not fully understood, it is related to an improvement in conditions at the plasma edge.
More recently, another boon to energy confinement known as the internal transport barrier has been discovered. The barrier is a small volume in the middle of the plasma where anomalous transport is suppressed. Temperatures and densities inside the barrier increase dramatically while outside barrier they remain low.
When a new, larger tokamak is built (ITER), it is to be hoped that new confinement modes will reveal themselves thus bringing the promise of fusion power closer to fulfillment.