The significance of medium-sized and small devices in fusion research

The significance of medium-sized and small devices is reviewed and discussed taking the case of ITER Physics R&D. It is pointed out that the heat flux density expected in a fusioning plasma will reach 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 a 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 the 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 the scenario and conditions of transition to turbulence. In the field of plasma confinement, a more complex scenario would be expected. Therefore, a systematic and basic research along the same line is highly desired on medium-sized and small devices in order to get insight into the physics of plasma confinement which will provide us the methodology for confinement improvement.