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1991
The question of the influence of nonaxisymmetric field perturbations on tokamaks is investigated. Recent experiments in the COMPASS-C tokamak [in Proceedings of the 15th Symposium on Fusion Technology, Utrecht (North-Holland, Amsterdam, 1989), Vol. 1, p. 3611 with externally applied helical fields reveal that magnetic islands do not appear until th…
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1991
The purpose of this paper is to investigate the possible relationships between transport properties such as thermal diffusivity and resistivity on the one hand, and the magnetic properties such as q profile and toroidal flux change on the other, in tokamaks that exist under macroscopically quasistationary conditions. It is experimentally well estab…
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1991
This paper compares the form of f(v||) arising from the warm-ion, kinetic models of Emmert et al. [ Phys. Fluids 23, 803 ( 1980) ] and Bissel and Johnson [ Phys. Fluids 30,779 ( 1987) ] with experimentally measured distributions from the DITE tokamak obtained with a retarding field analyzer (RFA). The results show that the commonl…
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1991
The Euler-Lagrange equations for a charged particle in a tokamak magnetic fieid, in the limit of large aspect ratio, are obtained in terms of toroidal coordinates from the exact Lagrangian, which is expressed in terms of the toroidal and poloidal magnetic fluxes. It differs fundamentally from the standard guiding-center approach, where the local ma…
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1991
This is Part II of a study of resonant perturbations, such as resistive tearing and ballooning modes, in a torus. These are described by marginal ideal magnetohydrodynamic (MHD) equations in the regions between resonant surfaces; matching across these surfaces provides the dispersion relation. Part I [Phys. Fluids B3, 1532 (1991)] described how all…
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1991
In a cylindrical plasma, tearing modes can be calculated by asymptotic matching of ideal magnetohydrodynamic (MHD) solutions across a critical layer. This requires a quantity that represents the “discontinuity*’ in the ideal solution across the layer. In a torus, poloidal harmonics are coupled and there are many critical surfaces for each toroi…
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1990
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1990
The set of semicollisional layer equations derived by Fitzpatrick [Phys. Fluids B 1, 2381 (1989)] is used to investigate the effect of trapped particles on the linear stability of long wavelength resistive modes in a large-aspect-ratio tokamak. At 0( 1) shear, destabilization by trapped particles is found to significantly modify previous cylindrica…
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1990
Local extrema in the equilibrium magnetic field strength can occur in tokamak plasmas when the plasma beta is increased. Furthermore, the magnetic field strength experienced by an electromagnetic wave propagating along a vertical chord in lower-beta tokamak plasmas may also pass through an extremum. For both these reasons, it is of interest to calc…
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1989
Magnetic feedback control has been used in the DITE tokamak to substantially reduce saturated m = 2, n = 1 instabilities in both Ohmic discharges and discharges with lower-hybrid current drive (LHCD). Feedback has been used for the first time to significantly increase the disruptive density limit in a tokamak. LHCD on DITE stabilizes the sawtooth i…
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