Finite-β regimes and transport processes in thermonuclear plasmas

A ''second stability region'' for internal kink modes, characterized by poloidal wave numbers m0=1, has been found in finite-β axisymmetric configurations. This makes it possible to envision a broad class of heating cycles from low-β to high-β regimes that maintain complete ideal MHD stability for both ballooning and internal kink modes. These heating cycles can be viewed as a sequence of equilibria of which the first part, which involves a relatively fast temperature rise, preserves the magnetic flux. In the second part the flux conservation is allowed to relax and the field ''twist parameter'' slash-l(psi) to become larger than unity over a considerable portion of the plasma column. The problem of confinement of energetic fusion reaction products and of the possible influence of the collective modes that can be excited by them in finite-β regimes where ideal MHD ballooning modes are stable also leads to rather optimistic conclusions. In fact, the ''shear Alfven'' type of perturbations from which ballooning modes evolve do not appear as normal modes in realistic toroidal equilibrium configurations. The rate of electron energy transport found in Ohmic-heated experiments is compared with that observed in experiments where injected heating prevails. An analytic expression for the relevant diffusion coefficient is given which is found to reproduce well both the electron temperature profiles and the limited number of experiments whose energy balance has been analysed so far. A 1-D transport code that reproduces the energy confinement times for temperature and density oscillations corresponding to the well known sawtooth oscillations observed in current experiments has been developed. This code has been used to predict the time-dependent rate of fusion reactions for near-term experiments such as Ignitor which should test the α-particle heating of D-T plasmas. (author)