Heating by injection of fast neutrals and study of the ion energy balance in TFR

In TFR the fast neutrals are injected quasi-perpendicularly, i.e. at an angle of 800 with respect to the magnetic axis. With two injection lines, each consisting of five ion sources, the total power extracted reaches 2 MW, and the power injected into TFR is of the order of 650 kW. The confinement of fast ions was studied from the spectrum of the charge-exchange neutrals and the neutron flux and seems to be classical. The injection efficiencies calculated by means of a numerical code are of the order of 20% for the ratio (Psub(Ni)+Psub(Ne))/Psub(E) and 57% for the ratio (Psub(Ni)+Psub(Ne))/Psub(N), where Psub(E), Psub(N), Psub(Ni) and Psub(Ne) are the extracted power, the power of injected neutrals, and the powers transferred to plasma ions and electrons, respectively. The central ion temperature increase is proportional to the power transmitted to the ions according to a law of the type ΔTsub(i)(r=0)[eV]=20Psub(Ni)[kW]/nsub(e)[1013 cm-3]. In the experiments at low densities (nsub(e)=3X1013 cm-3), the ion temperature doubles and reaches 1.9 keV in the centre. Two series of experiments were analysed in greater detail: the injection of a deuterium beam into a deuterium plasma and the injection of a hydrogen beam into a hydrogen plasma. During the injection of neutrals the impurity radiation is observed to grow. The increase of central density of molybdenum is more important than that of the electron density, at least for the discharges in deuterium. An investigation of the change of ion temperature as a function of the injected power and the temperature variation after beam turn-off yields information on the behaviour of the ions and the energy confinement time (tausub(Ei) approximately equal to 10 ms). When all losses other than those through conduction are neglected, and uncorrected values are used for Tsub(i), the coefficient of ion thermal conductivity can be determined at r=12 cm. This coefficient is slightly anomalous (by a factor of 2-3) compared with the neoclassical value, and seems to be independent of the collision frequency, even in the collisonless regime which has been reached in these experiments. (author)