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The theoretical basis for the cyclical generation of macroscopic reconnected magnetic fields in low collisionality plasma regimes is formulated. The relevant process is sustained by the “thermal” energy of the electron population.In particular, an oscillatory mode propagating along and across a confining magnetic field is identified that involves the magnetic reconnection region where the ratio of the longitudinal to transverse electron thermal conductivity is relatively large. A periodic exchange of reconnected magnetic field conductivity energy with electron thermal energy is sustained within a region that remains significant even when the magnetic field configuration from which the mode can emerge involves large- scale distances. The mode growth rate depends on the particle density gradient (aligned with the electron temperature gradient) and the relevant particle diffusion coefficient. A different magnetic reconnection process driven by the plasma pressure gradient was identified explaining the origin of the observed sawtooth oscillations of magnetically confined plasmas.

The introduction of a proper divertor in meaningful fusion burn experiments is at times advocated with the argument that it addresses the rcactor relevant issue of ash and impurity control and it allows easier access to the H-mode regime. On the other hand, a divettor decreases the volume available for the plasnna and introduces structutes that have to withstand high thermal wall loadings and the effects of disruptions in a high magnetic field environment. Other related questions are whether the density profiles that characterize the H-regime, for which a divertor is introduced, are optimal for ignition, whether the divertor has indeed led to cleaner core plasmas than those produced in limiter devices , and whether other means to remove the alfa-particles produced by fusion reactions, e.g. during their slowing down, are more appropriate even in principle. In the Ignitor operating scenario of reference, ignition can be reached by ohmic heating only. The edge conditions are characterized by relatively low temperatures and high densities. In high density regimes , an extensive series of experiments has observed a low level of impurity in the plasma core, thanks to both reduced sputtering from the wall and improved screening properties of the adjacent plasma. Radiation losses gain importance in dissipating the power leaving the main plasma and the whole scrape off layer contributes in dispersing the energy of the particles impinging on the material walls, even without the introduction of additional impurities. Therefore, the extended limiter solution, with the plasma wetting a large fraction of the first wall surface, has been preferred over that of a traditional divertor configuration.

A next major step in the research toward magnetic fusion energy production is to carry out experimental campaigns exploring regimes with relevant amount of fusion power. So far, the theoretical knowledge of the path toward a fusion burning plasma has been acquired mainly by performing numerical studies in 0 or 1-1.5 dimensions. Due to the marked anisotropy of magnetically confined plasmas, however, three-dimensional effects might play a role. In particular, the drastic change in magnetic topology associated with reconnecting modes on selected rational magnetic surfaces [1] may decrease the thermal electron conductivity parallel to the magnetic field lines, with a consequent impact on the electron heating due to fusion products. We describe this new scenario, and present analytical and numerical calculations aimed at verifying the impact of reconnection on fusion heating

An endogenous magnetic reconnection process is characterized by a driving factor that lays within the layer where a drastic change of magnetic field topology occurs. This kind of process is shown to take place in the presence of an electron temperature gradient in a well-confined plasma where, referring to quasi-collisionless regimes, the resulting electron temperature fluctuations can be anisotropic. Then a class of (radially) localized reconnecting modes is identified. These involve a transverse generated field Bx of odd parity (as a function of the radial variable), and have finite (phase) velocities of propagation contrary to commonly considered reconnecting modes. The widths of the relevant reconnection layers remain significant even when large macroscopic distances are considered. Given that there are plasmas in the Universe with considerable electron thermal energy contents, these features can be relied upon in order to produce magnetic field generation, or conversion of magnetic energy into particle energy when the coupling of the localized odd modes to extended even modes can be significant. In any case, the resulting magnetic islands are not symmetric. With their excitation these modes can extract momentum from the main body of the plasma column which should recoil in the opposite direction. The excitation of antisymmetric endogenous modes is shown to be relevant to the electron temperature heating due to the reaction products in a fusion burning plasma as, in this case, the longitudinal thermal conductivity on selected rational magnetic surfaces can be decreased, relative to its collisional value, by the effects of reconnection. This kind of steepening is proposed to have a role in enhancing the growth rate of the instability involved in disruption events of the plasma column. The best agreement between theory and experiments concerning the onset of magnetic reconnection is (probably) represented by the theory of the internal kink mode. The observed accelerated reconnection rate following the onset is suggested as being explained by the formation of a relatively large magnetic island with a local steepening of the electron temperature gradient. A new kind of odd 'thermonuclear heating' mode associated with symmetric reconnection is identified.