Plasma Science: From Fundamental Research to Technological Applications (1995)

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MAGNETIC CONFINEMENT FUSION 74 TOKAMAK TRANSPORT Introduction and Background Since plasmas of sufficiently high density and temperature must be formed to accomplish thermonuclear ignition, understanding the transport of energy and particles is the key to the design of a fusion reactor. Virtually all tokamak experiments worldwide have demonstrated a common consistent level of energy and particle confinement in the so-called L-mode (L standing for "low confinement") regime of operation. In addition to L-mode, a variety of regimes have been observed and studied that have improved confinement, typically in response to changes in the plasma boundary conditions. The most ubiquitous of these is the H-mode regime (H standing for "high confinement"), which differs from L-mode primarily by having a transport barrier at the plasma edge. In the H-mode, the energy confinement is typically a factor of two better than in the L- Mode. More recently, even better confinement has been achieved in the so- called VH-mode ("very-high'') regime, where confinement times up to four times longer than those of L-mode plasmas have been achieved. Characteristics of the H-mode and VH-mode are illustrated in Figure 4.1. Past Achievements During the past decade, tokamak plasma parameters comparable with those estimated to be required for fusion reactors have been achieved (Figure 4.2), including maximum central ion temperatures of Ti ~37 keV (4 × 108 °C), confinement times of τ ~ 1 s, and central densities n ~ 3 × 1020 m-3. These parameters were not all achieved simultaneously, but simultaneous measurement has been achieved of a fusion triple product nτTi ~ 1.1 × 1021 keV s m-3. Current experiments operate with the same dimensionless parameter values as those expected in reactors, with the exception that reactors will require two to three times greater confinement times and electron temperatures than those obtained in today's machines. Tokamak experiments in the 1980s demonstrated that the transport of thermal plasma energy and momentum across the confining magnetic fields is ~100 times faster than predicted by theory. This theory considers only the effects of Coulomb collisions and essentially is an extension of gas-dynamic models to magnetized plasma transport. These observations have shifted the emphasis in tokamak cross-field transport theory from collisional models to ones that include the effects of plasma turbulence. Dimensionless scaling experiments have begun measuring the scaling of tokamak cross-field thermal transport with respect to the theoretically important dimensionless parameters, in a manner analogous to windtunnel experiments. It has been found that the scaling is unlike that predicted by most theories. In