The significance of medium- or small-size devices in fusion research

The significance of medium and small size devices is reviewed using the example of the ITER Physics R and D. It is noted that the heat flux density expected in a fusioning plasma will become large and it is not self-evident that one can use current energy confinement time scalings to predict ITER performance. The confinement of plasma is considered to be related to heat, momentum, and mass transport problems in a nonlinear, non-equilibrium system with a finite source and dissipation. Therefore, recent progress in fluid dynamics, especially where basic experiments are unveiling modes of heat and mass transport in the nonlinear non-equilibrium system (including conditions of transition to turbulence), is referred to frequently. Hence, systematic and basic research along the same lines is necessary in medium and small devices in order to develop further insight into the physics of plasma confinement.