Table of contents

The confinement characteristics and behaviour of energetic ions injected during tangential neutral beam injection (NBI) experiments on the Advanced Toroidal Facility (ATF) are examined using several experimental and computational methods. Measurements by a two dimensional scanning neutral particle analyser of the fast ion, slowing down spectra of the injected ions have been used to examine the characteristics of the fast ion distribution in low density plasmas (n_e ⩽ 1.0 × 10¹⁹ m^-3). The energetic ions are found to behave in a manner consistent with the classical slowing down process embodied in the fast ion Fokker-Planck equation. Neutron measurements have been used to extend these results to higher density plasmas (n_e ≈ 7.5 × 10¹⁹ m^-3). Because of experimental uncertainties, it is difficult to determine whether the measured neutron rates are consistent with classical predictions; however, there are indications in some ATF magnetic configurations that the measured neutron source rates are much lower than expected, indicating an enhanced orbit loss of fast ions in these configurations. In addition, computational studies suggest that the effectiveness of tangential NBI in certain operating regimes in ATF may be limited owing to the presence of loss regions at the trapping boundary

The canonical profiles transport model is extended to describe various modes with improved confinement in tokamaks. A generalized profile consistency principle is proposed and a corresponding mathematical formalism is formulated. This formalism is used for the modelling of various regimes, such as H modes in DIII-D, JET and ASDEX, the hot ion mode and enhanced performance after pellet injection (PEP mode) in JET. This modelling, together with a known global scaling law, allowed the dependence of the internal model parameters on the plasma geometry and other physical variables to be established. This makes the model predictive. The approximate analytical criteria for the L to H and L to hot ion mode transitions are also obtained

The small-inverse-aspect-ratio boundary layer approximation, which has been used previously to describe the analytic structure of a driven non-ideal toroidal Alfven eigenmodes (TAEs), is applied to a numerical stability calculation for the TAEs in ITER and TFTR plasmas. Away from TAE gaps (singular layers), zero beta cylindrical magnetohydrodynamics (MHDs) determines the generic structure of the outer solutions. Within each gap, a detailed kinetic treatment is used to include (i) modifications to the fluid equations arising from E// and finite Larmor radius, (ii) collisional damping from trapped electrons, (iii) collisionless (Landau) damping from passing ions and (iv) drive from finite-orbit-width fusion alpha particles and beam ions. The model is valid for arbitrary toroidal mode number and predicts the growth/damping rate of both the MHD-like TAE (that which is predicted by MHD theory) and the relevant kinetic TAEs

Various DD and DT plasmas are analysed for effects of fast ion transport with a time dependent, 1 1/2 -D transport simulation code (TRANSP). The sensitivity of the simulations to fast ion diffusion modelling is tested against numerous parameters. Strong correlations are found with beam power and plasma stored energy. The neutron emission sensitivity is mostly affected by the fraction of beam-beam neutrons. Wall recycling is essential in interpreting the results for DT plasmas heated with pure deuterium or pure tritium beams. The decay of the 14 MeV neutron emission following a short DT beam pulse implies a small fast ion diffusion coefficient (D_f<0.05 m²/s). The agreement of the measured neutron emission and diamagnetic flux with the simulations in DT plasmas heated with various numbers of tritium and deuterium beams, and power, implies that D_f<or=0.2 m²/s

Impurities are injected into JET and Tore Supra by the laser blow-off ablation technique for a variety of experimental situations. The impurity confinement times τ_p, which reflect the impurity transport, have been measured by fitting exponential curves to the decays of the central brightnesses. Two databases are built up, including ohmic and L mode discharges, with the aim of determining a τ_p scaling law common to both devices. Different monomial scaling laws are tested and the best one is chosen on the basis of minimization of the standard deviation of the individual regressions. Moreover, the impurity confinement times are compared with the energy confinement times τ_E for the same data. The energy confinement times are larger than the impurity confinement times, the average ratio tau E/τ_p being approximately 2.5

The baseline configuration of a radiation driven target for heavy ion fusion is a quasi-cylindrical hohlraum containing a fusion capsule and with radiation converters placed at opposite ends of the hohlraum. Ion beams enter each converter from opposite directions and are stopped by the material inside the converters. For a capsule that absorbs about 1 MJ of radiation energy and has an initial radius of 0.234 cm, calculations using the viewfactor code GERTIE indicate that the maximum initial capsule-to-hohlraum surface area ratio that can provide adequate time dependent symmetry for a capsule implosion is about 0.075. These calculations take into account the substantial inward expansion of the hohlraum wall. The capsule implosion and the wall motion are calculated using the one dimensional radiation hydrodynamics code HYADES and the conversion efficiencies of the radiation converters are studied using the two dimensional radiation hydrodynamics code LASNEX. The efficiency of energy coupling between the hohlraum and the capsule is 21%, the peak hohlraum temperature is 0.26 keV, and the energy gain can be as high as 80. If the converters are placed outside the hohlraum, their radii can be varied according to the beam focusing requirements, while the hohlraum dimensions remain unchanged. This provides a convenient way of calculating target gain as a function of converter radius. By bending the radiation converters by 90 degrees , we can obtain a hohlraum configuration in which the ion beams can enter the target from a single direction, eliminating the need for bending the beams by 180 degrees

The characteristics of ergodic divertor (ED) deuterium plasmas in the Tore Supra tokamak are discussed. The divertor volume is a relatively well limited region between the ED modules and ρ = τ/a equivalent to 0.8. The core (ρ ⩽ 0.8) plasma confinement is not degraded by the presence of the ED (by comparison with similar limiter, non-ED plasmas): the electron density and temperature profiles are not modified. The ED configuration compares well with a classical X point divertor: intrinsic impurities are screened out, the power load is deflected to dedicated surfaces, and density control through particle pumping is possible. Up to 5.5 MW of ion cyclotron resonance power and 3 MW of lower hybrid power have been successfully used, power exhaust being mainly by radiation (80%)

It is shown that a residual kink, which is often observed between the crashes in modern tokamaks, leads to the formation of a new kind of fast particle orbit, namely toroidally trapped superbanana orbits. The width of the banana guiding centre trajectory for such orbits scales as Δr/r ~ (ξ/R₀)^1/2, where ξ is the amplitude of the kink distortion of the magnetic axis and R₀ is the major radius of the tokamak. The superbanana diffusion regime, which is realized for energetic beam ions, leads to a characteristic time of the fast ion losses outside the q=1 surface that is comparable with, or even smaller than, the corresponding thermalization time