The effect of a toroidal current hole on the first orbit (FO) loss and on the collisional loss of alpha particles in JET is investigated. Numerical results of predictive three-dimensional Fokker–Planck modelling of the distribution function of D–T fusion alphas in hollow current JET discharges are presented. If the current hole region is kept reasonably small, it induces only a moderate increase of FO losses as well as of the collisional loss of fast alphas. The current hole effect is shown to be qualitatively equivalent to a reduction of the total plasma current I. Hence, the alpha confinement degradation by the current hole profiles can be compensated by enlarging I.
The Tokamak Fusion Test Reactor ͑TFTR͒ ͑R. J. Hawryluk, to be published in Rev. Mod. Phys.͒ experiments on high-temperature plasmas, that culminated in the study of deuterium-tritium D-T plasmas containing significant populations of energetic alpha particles, spanned over two decades from conception to completion. During the design of TFTR, the key physics issues were magnetohydrodynamic ͑MHD͒ equilibrium and stability, plasma energy transport, impurity effects, and plasma reactivity. Energetic particle physics was given less attention during this phase because, in part, of the necessity to address the issues that would create the conditions for the study of energetic particles and also the lack of diagnostics to study the energetic particles in detail. The worldwide tokamak program including the contributions from TFTR made substantial progress during the past two decades in addressing the fundamental issues affecting the performance of high-temperature plasmas and the behavior of energetic particles. The progress has been the result of the construction of new facilities, which enabled the production of high-temperature well-confined plasmas, development of sophisticated diagnostic techniques to study both the background plasma and the resulting energetic fusion products, and computational techniques to both interpret the experimental results and to predict the outcome of experiments.
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The results of a 3-D (in constants of motion space) Fokker-Planck simulation of the collisional losses of fusion products in axisymmetric DT and DD discharges in TFTR are presented. The distributions of escaped ions over poloidal angle and pitch angle, and their energy spectra, are obtained. Axisymmetric collisional losses of fusion products are found to be less than 2 to 5%. The distribution of confined fusion products is shown to be strongly anisotropic and non-uniform in the radial co-ordinate, mainly for slowed-down fusion products with small longitudinal energy. A comparison between these modelling results and experimental data is made
Transport processes of fast ions in axisymmetric low-aspect-ratio spherical torus (ST) plasmas are investigated, which are induced by the non-conservation of the magnetic moment µ. The reason for non-conservation of µ of fast ions in ST's is the relatively large adiabaticity parameter typically exceeding the value 0.1 ( = ratio of ion gyro-radius to the gradient scale length of the magnetic field). Both analytical and numerical evaluations of the magnitude of nonadiabatic variations of µ are performed. Non-adiabaticity effects are shown to be most significant for fast ions for which the bounce oscillations are in resonance with the gyro-motion, i.e. for ions with ω B -lω b = 0, where ω B and ω b represent the bounce averaged gyro-frequency and the bounce frequency, respectively, and l is an integer. The critical threshold of the adiabaticity parameter, cr , to be exceeded for the transition to stochastic behavior of fast ions in axisymmetric ST's is inspected. Nonadiabatic variations of µ are shown to lead to collisionless transformation of trapped orbits into circulating ones and vice versa. For the case of strong non-adiabaticity, cr > , we assess the transport coefficients describing intense collisionless pitch angle diffusion, whereas, in the case of weak nonadiabaticity, cr < , the more substantial coefficients of enhanced collisional radial diffusion and convection of fast ions gyrating resonantly with the bounce oscillations are estimated.
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