Internal transport barriers: critical physics issues?

Plasmas regimes with improved core energy confinement properties, i.e. with internal transport barriers (ITB), provide a possible route towards simultaneous high fusion performance and continuous tokamak reactor operation in a non-inductive current drive state. High core confinement regimes should be made compatible with a dominant fraction of the plasma current self-generated (pressure-driven) by the bootstrap effect while operating at high normalized pressure and moderate current. Furthermore, ITB regimes with 'non-stiff' plasma core pressure break the link observed in standard inductive operation between fusion performances and plasma pressure at the edge, thus offering a new degree of freedom in the tokamak operational space. Prospects and critical issues for using plasmas with enhanced thermal core insulation as a basis for steady tokamak reactor operation are reviewed in the light of the encouraging experimental and modelling results obtained recently (typically in the last two years). An extensive set of data from experiments carried out worldwide has been gathered on ITB regimes covering a wide range of parameters (q-profile, Ti/Te, gradient length, shaping, normalized toroidal Larmor radius, collisionality, Mach number, etc). In the light of the progress made recently, the following critical physics issues relevant to the extrapolation of ITB regimes to next-step experiments, such as ITER, are addressed:(i) conditions for ITB formation and existence of a power threshold,(ii) ITB sustainment at Ti ∼ Te, with low toroidal torque injection, low central particle fuelling but at high density and low impurity concentration,(iii) control of confinement for sustaining wide ITBs that encompass a large volume at high βN,(iv) real time profile control (q and pressure) with high bootstrap current and large fraction of alpha-heating and(v) compatibility of core with edge transport barriers or with external core perturbations (such as frozen hydrogen isotope pellets injection).It is shown that the present experimental results provide some valuable and promising answers to these critical issues.