Recent results are presented for turbulence in tokamak boundary plasmas and its relationship to the low-to-high confinement (L–H) transition in a realistic divertor geometry. These results are obtained from a three-dimensional (3D) nonlocal electromagnetic turbulence code, which models the boundary plasma using fluid equations for plasma vorticity, density, electron and ion temperatures and parallel momenta. With sources added in the core-edge region and sinks in the scrape-off layer (SOL), the code follows the self-consistent profile evolution together with turbulence. Under DIII-D [Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] tokamak L-mode conditions, the dominant source of turbulence is pressure-gradient-driven resistive X-point modes. These modes are electromagnetic and curvature-driven at the outside mid-plane region but become electrostatic near X-points due to magnetic shear and collisionality. Classical resistive ballooning modes at high toroidal mode number, n, coexist with these modes but are sub-dominant. Results indicate that, as the power is increased, these modes are stabilized by increased turbulence-generated velocity shear, resulting in an abrupt suppression of high-n turbulence and the formation of a pedestal in density and temperature, as is characteristic of the H-mode transition. The sensitivity of the boundary turbulence to the direction of the toroidal field Bt is discussed.

The nonlinear evolution of plane hydromagnetic fluctuations propagating along the unperturbed magnetic field direction is considered. From an expansion of the ideal magnetohydrodynamic equations and the hydromagnetic shock jump conditions, it is shown that a wave in which the magnitude of the magnetic field is nonconstant steepens into a shock and subsequently evolves toward a purely Alfvénic fluctuations of lower mean energy density. Explicit expressions are derived for the asymptotic state and for the characteristic lines which describe the evolution toward that state. A class of fluctuations which includes linearly polarized waves is shown to evolve into rotational discontinuities. The results are applied to observations of hydromagnetic fluctuations in the solar wind.

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The basic problem of particle end-loss in a magnetic mirror field with an electrostatic confining potential is considered. The analytic treatments of Pastukhov and Chernin and Rosenbluth have been generalized to apply to any electrostatically confined species in a multi-species plasma, and the Pastukhov analysis has been extended to apply to arbitrary magnetic-field profiles (instead of square-well). The analytic results are compared with results obtained from one-and two-dimensional Fokker-Planck codes. In particular the scaling with potential, mirror ratio, and effective charge is considered. The closest agreement (within 20%) is between the 2-D Fokker-Planck and generalized Pastukhov results.

Articles you may be interested inComment on "Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake" [Phys.The snowflake divertor exploits a tokamak geometry in which the poloidal magnetic field null approaches second order; the name stems from the characteristic hexagonal, snowflakelike shape of the separatrix for an exact second-order null. The proximity of the poloidal field structure to that of a second-order null substantially modifies edge magnetic properties compared to the standard X-point geometry ͑with a first-order null͒; this, in turn, affects the edge plasma behavior. Modifications include: ͑1͒ The flux expansion near the null-point becomes 2-3 times larger. ͑2͒ The connection length between the equatorial plane and divertor plate increases. ͑3͒ Magnetic shear just inside the separatrix becomes much larger. ͑4͒ In the open-field-line region, the squeezing of the flux-tubes near the null-point increases, thereby causing stronger decoupling of the plasma turbulence in the divertor legs and in the main scrape-off layer. These effects can be used to reduce the power load on the divertor plates and/or to suppress the "bursty" component of the heat flux. It is emphasized that the snowflake divertor can be created by a relatively simple set of poloidal field coils situated outside the toroidal field coils. Analysis of the robustness of the proposed divertor configuration with respect to changes of the plasma current distribution is presented and it is concluded that, even if the null is close to the second order, the configuration is robust.