Evolution of radial force balance and radial transport over L-H transition

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Sayer, Min-hee Shin
Stacey, Weston M.
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Understanding of plasma confinement modes is an essential component in development of a fusion reactor. Plasma confinement directly relates to performance of a fusion reactor in terms of energy replacement time requirements on other design parameters. Although a variety of levels of confinement have been achieved under different operating conditions in tokamaks, tokamak confinement is generally identified as being either Low (L-mode--poor confinement) or High (H-mode--good confinement) In operation of a tokamak experiment, the plasma confinement condition generally changes from L-mode to H-mode over a few hundred milliseconds, sometimes quite sharply. Such a difference in transition period seems to be largely due to operating conditions of the plasma. Comparison of experimental data exhibits various distinctions between confinement modes. One noteworthy distinction between confinement modes is development of steep density and temperature gradients of electrons and ions in the plasma edge region of High confinement, H-modes, relative to Low-confinement, L-modes. The fundamental reason for the change for L-mode to H-mode is not understood. Previous studies have suggested i) the development of reduced diffusive transport coefficients that require a steepening of the gradients in a localized region in the edge plasma, the "transport barrier" in H-mode confinement ii) the radial force balance between pressure gradient forces and electromagnetic (radial electric field and VxB) forces require radial particle fluxes to satisfy a pinch-diffusion relation. A recent study suggests that the major difference between L-mode and H-mode are associated with the electromagnetic forces in the "pinch velocity" and the pressure gradient, not in the diffusion coefficients that multiplies the pressure gradient. The research will examine in detail the time evolution of the radial force balance and the particle and energy transport during the L-H transition. For the analysis, DIII-D shot #118897 is selected for transition between L- and H-mode confinements. Plasma conditions in L-mode, near the L-H transition and following the transition are selected for analysis of various parameter profiles. The initial analysis will be based on the four principal equations for plasma: particle balance, momentum balance, force balance and heat conduction. Based on these equations, specific equations have been derived: toroidal and radial momentum balances, diffusion coefficient, pinch velocity and heat conduction relation for calculation of parameters. The analysis of these equations, using the measured data, will establish how various terms in the radial force balance (radial electric field, VXB (electromagnetic) force, and pressure gradient) and the diffusive transport coefficients evolve over the confinement mode transition.
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