The information provided by different diagnostics has been combined in order to characterize the fast electron distribution function in lower hybrid current drive experiments on JET. In particular, X ray and electron cyclotron emission data are complementary in defining the fast electron energy contents in the directions parallel and perpendicular to the magnetic field. A numerical analysis chain has been developed which identifies the main moments of the distribution function of the current carrying fast electrons and allows simulations based on these moments to be compared with X ray emission, electron cyclotron emission and magnetic data. The method of analysis and the associated diagnostics are described, and results are presented which have been obtained during the lower hybrid current drive campaign on JET.

The optimal conditions under which confined plasmas can reach ignition are identified, referring in particular to the parameters of the Ignitor machine. The key importance of the radial profiles of the particle density, of the associated plasma pressure and of the thermal energy diffusivity, is shown. Peaked density profiles, such as those obtained in the Alcator A and C experiments (at about the same central density and magnetic field as those in Ignitor), characterized by minimal thermal diffusivities and high plasma purity, are best suited for ignition. The H mode regime can be accessed in Ignitor but is not considered a priority because of the typically flat density profiles it involves. The roles of collective modes and their interaction with both high and low energy alpha particle populations are assessed. For the modes generating sawtooth oscillations and involving magnetic reconnection the stabilizing effect of 'shoulder' q(ψ) profiles is pointed out.

Thermonuclear ignition condition for deuterium-tritium plasmas can be achieved in compact, high magnetic field devices such as Ignitor. The main scientific goals, the underlying physics basis, and the most relevant engineering solutions of this experiment are described. Burning plasma conditions can be reached either with ohmic heating only or with small amount of auxiliary power in the form of ICRH waves, and this condition can be sustained for a time considerably longer than all the relevant plasma time scales. In the reference operating scenario, no transport barriers are present, and the resulting thermal loads on the plasma facing component are estimated to be rather modest, thanks to the high edge density and low edge temperature that ensure an effective intrinsic radiating mantle in elongated limiter configurations. Enhanced confinement regimes can also be obtained in configurations with double X-points near the first wall.

The scientific goal of the Ignitor experiment is to approach, for the first time, the ignition conditions of a magnetically confined D-T plasma. The IGNIR collaboration between Italy and Russia is centred on the construction of the core of the Ignitor machine in Italy and its installation and operation within the Triniti site (Troitsk). A parallel initiative has developed that integrates this programme, involving the study of plasmas in which high-energy populations are present, with ongoing research in high-energy astrophysics, with a theory effort involving the National Institute for High Mathematics, and with INFN and the University of Pisa for the development of relevant nuclear and optical diagnostics. The construction of the main components of the machine core has been fully funded by the Italian Government. Therefore, considerable attention has been devoted towards identifying the industrial groups having the facilities necessary to build these components. An important step for the Ignitor programme is the adoption of the superconducting MgB 2 material for the largest poloidal field coils (P14) that is compatible with the He-gas cooling system designed for the entire machine. The progress made in the construction of these coils is described. An important advance has been made in the reconfiguration of the cooling channels of the toroidal magnet that can double the machine duty cycle. A facility has been constructed to test the most important components of the ICRH system at full scale, and the main results of the tests carried out are presented. The main physics issues that the Ignitor experiment is expected to face are analysed considering the most recent developments in both experimental observations and theory for weakly collisional plasma regimes. Of special interest is the I-regime that has been investigated in depth only recently and combines advanced confinement properties with a high degree of plasma purity. This is a promising alternative to the high-density L-regime that had been observed by the Alcator experiment and whose features motivated the Ignitor project. The provisions that are incorporated in the machine design, and in that of the plasma chamber in particular, in order to withstand or prevent the development of macroscopic instabilities with deleterious amplitudes are presented together with relevant analyses.

The scientific objectives of the Ignitor experiment can be achieved owing to the high magnetic field and plasma current planned. The physics projections are here supported by an analysis, carried out by the free boundary 14-D code JETTO, of the plasma evolution during the current ramp up and flat-top phases. The most advanced operating scenario that is envisaged for the machine is considered, while taking the technological constraints of the project into account. The plasma shape and position are checked to agree with the reference magnetic configurations. The density values are always much lower than the Greenwald limit, and the avoidance of disruption boundaries in the (li, q q ) diagram is assured. The influence of the density profile growth on the overall performance is analysed under different assumptions. The results show that ohmic ignition could be reached even assuming transport diffusion coefficients that account for energy confinement times close to the 'ITER89P' scaling, provided that the rampup phase is carefully programmed. The density is the main parameter with which to control the path to ignition, but some other items need attention, such as the plasma shape and dimensions, the current density profile and the impurity content. Even when ignition is not achieved globally, a central ignited core is present.