Table of contents

The laser-induced fluorescence at H_alpha line was used to study the neutral hydrogen behaviour in the FT-1 Tokamak. Absolute measurements of the hydrogen density profiles were carried out, and the influence of recycling and gas puffing on the particle balance was studied.

It is shown that a beam of tritons, whose energy is of the order of 200 keV striking a deuterium plasma can produce fusion energy in excess of their kinetic energy provided the plasma temperature is higher than 2.3.10⁷ (K). In very dense Z-pinches the range of such T-beams is compatible with the length of the pinch. In particular if the pinch becomes m=0 unstable and produces D and T beams in situ one may expect a very large neutron output as a result of beam-plasma interaction.

A system consisting of a Multiple Mirror device and a radio frequency generator of 600 W were designed and constructed to study characteristics of plasma produced and heated by radio frequency waves. By using Langmuir probes, the authors have detected large spatial variations of density and temperature along the axis of symmetry of the system. These spatial variations have characteristics similar to those striations waves found in glow discharges, in the presence of an electric current.

The heating of a completely ionized plasma by inverse Bremsstrahlung absorption (IB) is studied by numerically integrating the time varying kinetic equation for electrons, and the resulting distribution functions are used to calculate the continuum X-ray emission. The shape of the distribution functions is determined by the competition between IB and electron-electron collisions, and, in all the uniform plasma simulations, the authors find that the distributions are well fitted by the formula C_mexp(-( nu / nu _m)^m) where m is between 2 and 5. They find the dependence of m on alpha =Z² nu _osc/ nu _th², where Z is the atomic number, nu _osc is the oscillation velocity in the laser field and nu _th is the thermal velocity: m( alpha )=2+3/(1+1.66/ alpha ^0.724). More importantly, in the realistic case of a space and time dependent kinetic simulation of a Be plasma heated by a Nd laser, including heat flow and ion motion, this fit remains valid for electron densities below 0.75 N_c (with constant m), although other effects come into play closer to the critical surface. Thus, these simple results can be used to determine the continuum X-ray spectrum due to Bremsstrahlung (BR) and to direct radiative recombination (DRR).

Transport code simulations of m=2, n=1 mode control experiments by local electron cyclotron heating (ECH) in TFR are presented. The mode activity is calculated by means of the quasi-linear theory for tearing modes. Starting from an ohmic target plasma which is unstable to the m=2, n=1 tearing mode, the evolution of the m=2, n=1 magnetic island during a 100 ms ECH pulse is calculated for various positions of the electron cyclotron resonance and levels of injected power. The best results, i.e. a complete suppression of the island, are obtained for heating almost exactly on the q=2 surface. In contrast to experimental results, the suppression of the island is found to be only temporary. It is concluded that the suppression of the MHD activity obtained in the experiment cannot be explained by profile tailoring alone.

The auxiliary heated Tokamak is considered as an open thermodynamic system in which the auxiliary power is applied externally to a resistively relaxed state of maximum entropy. It results in the magnetic configuration of the plasma maintaining, under the external action, the same form of the profiles as in the purely ohmic state with maximum entropy. The sensitivity of the plasma, with respect to slight resistive perturbations at the edge, is studied, exploiting its tendency to adhere to the structure of an ideally conductive equilibrium as far as the local dependence of the current is linear in time. Assuming the existence of a q-dependent region at the edge with very high thermal diffusivity, a q=const boundary surface exists with constant temperature. This situation results in a link between the temperature and the q=const surfaces of the slightly resistive equilibrium (or between the resistivity and the current) and in the existence of bifurcating states related to a resistive instability whose nonlinear evolution and possible saturation is sensitive on whether the initial disturbance is heating or cooling the edge.

It is shown how the concept of local power deposition can be identified with the time rate of increase of the particle kinetic energy density at second order in the linearization procedure. The secular behaviour of the second order part of the distribution function is also explicitly pointed out.

In the experimental identification of Tokamak equilibria from external magnetic measurements, the addition of integral constraints to the full equilibrium analysis provides extra information on the toroidal current distribution, improving the stability of the solution against measurement errors, and increasing the accuracy of its determination. It is shown that the use of the toroidal flux value measured by the diamagnetic loop as an additional constraint to the free-boundary equilibrium solver increases to four the number of independent parameters that can be identified in the current density profile, providing a more precise evaluation of the relative weight of the pressure and paramagnetic terms.

It is pointed out that there is a remarkable effect of wave-driven radial motion on the relaxation process, the stationary state, the wave-particle coupling mechanism and the wave-driven radial transport for the strong-wave case in a lower-hybrid current drive Tokamak.

The confinement of iron injected by the laser ablation technique in the low field and low current Tokamak TJ-1, has been studied as a function of the toroidal field and plasma density. Results of these scans seem to fit within a neoclassical picture of plasma confinement. The impurity confinement time follows the ion energy confinement time to which it is compared.

The magnetic field induced by a HF electric field E(E_x,E_y,E_z) via the ponderomotive force is studied in a collisionless magnetized plasma in the three-dimensional case. A steady-state magnetic field can be generated in the plane perpendicular to the external magnetic field B₀ if electric field gradients exist along B₀ and across B₀ at the same time. The generated magnetic field is opposite to B₀ whenever there is no space variation along B₀. Simple explicit equations for the magnetic field components are given for the cases of an extraordinary wave and an oblique electrostatic wave.

In relating the density scrape-off layer thickness lambda _n to D_{perpendicular to} , account is taken of two aspects of the JET configuration: (a) the poloidally-varying cross-section of plasma flux tubes in the scrape-off layer, and (b) the different limiter configurations-discrete/continuous. Edge measurements of lambda _n(0) in JET made by Langmuir probes and limiter-viewing cameras are used to deduce edge values of D_{perpendicular to} for ohmic discharges, over a range of conditions, I_p=1-5 MA, n_e=0.7-6*10¹⁹ m^-3. Generally the values are found to be close to D_{perpendicular to} =10¹⁹/n_e m² s^-1, that is, Intor-Alcator scaling.