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

The significance of medium- and small-size devices is reviewed and discussed taking the case of ITER Physics R and D. It is pointed out that the heat flux density expected in a fusioning plasma will become as large as an unexperienced high level and it is not self-evident that one could extend the present energy confinement time scaling to predict ITER performances. The confinement of fusion 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, a recent progress of basic experiments and theories in fluid dynamics may greatly be referred, where series of basic experiments are unveiling various modes of heat and mass transport in the nonlinear non-equilibrium system including a scenario and conditions of transition to turbulence. In the field of plasma confinement, the more complex scenario would be expected. Therefore, a systematic and basic research along the same line is highly desired on medium- and small-size devices in order to get insight into the physics of plasma confinement which will provide us the methodology for confinement improvement. (author)