Browsing by Subject "Tokamak"
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Item 2-D magnetic equilibrium and transport modeling of the X-divertor and super X-divertor for scrape-off layer heat flux mitigation in tokamaks(2014-08) Covele, Brent Michael; Mahajan, Swadesh M.; Gentle, Kenneth W.Intense heat fluxes from the divertor incident on material surfaces represent a “bottleneck” problem for the next generation of tokamaks. Advanced divertors, such as the X-Divertor (XD) and Super X-Divertor (SXD), offer a magnetic solution to the heat flux problem by (a) increasing the plasma-wetted area via flux expansion at the targets, and (b) possibly opening regimes of stable, detached operation of the divertor via flux tube flaring, as quantified by the Divertor Index. The benefits of the XD and SXD are derived from their unique magnetic geometries, foregoing the need for excessive gas puffing or impurity injection to mitigate divertor heat fluxes. Using the CORSICA magnetic equilibrium code, XDs and SXDs appear feasible on current- and next-generation tokamaks, with no required changes to the tokamak hardware, and respecting coil conductor limits. Divertor heat and particle transport modeling is performed in SOLPS 5.1 for XD or SXD designs in NSTX-Upgrade, Alcator C-Mod, and CFNS/FNSF. Incident heat fluxes at the targets are kept well below 10 MW/m², even for narrow SOL widths in high-power scenarios. In C-Mod and CFNS, parallel temperature profiles imply the arrestment of the detachment front near the targets. Finally, an X-Divertor for ITER is presented.Item The macro- and micro-instabilities in the pedestal region of the Tokamak(2015-05) Ma, Jingfei; Morrison, Philip J.; Horton, C. W. (Claude Wendell), 1942-; Berk, Herbert; Fitzpatrick, Richard; Hallock, GaryIn this paper, we present the theoretical and numerical studies of the linear characteristics and nonlinear transport features of the instabilities driven by the steep profile gradient and edge current in the pedestal region of the tokamak. Two important instabilities, the peeling-ballooning (P-B) modes (macro-instability) and the drift-Alfven modes (micro-instability), are studied using the fluid analysis and the BOUT++ codes. In particular, the edge-localized modes (ELMs), which appear to be the energy burst in the nonlinear stage of the peeling-ballooning mode, are numerically studied and the results are compared with the experimental measurement. In addition, the features of the impurity transport in the edge region of the tokamak are theoretically analyzed. Firstly, we explore the fundamental characteristics of the P-B modes and the ELM bursts numerically using the three-field reduced MHD model under the BOUT++ framework, in the shifted-circular geometry, i.e. the limiter tokamak geometry. In the linear simulations, the growth rate and real frequency and the mode structure versus the toroidal mode number (n) are shown. The features of the ELM bursts are shown in the nonlinear simulations, including the time evolution of the relative energy loss (ELM size) and the pedestal profile. Secondly, two original research projects related to the P-B modes and the ELM burst are described. One is the study of the scaling law between the relative energy loss of ELMs and the edge collisionality. We generate a sequence of shifted-circular equilibria with different edge collisionality varying over four orders of magnitude using EFIT. The simulation results are in good agreement with the multi-tokamak experimental data. Another is the study of the differences of the linear behaviors of the P-B modes between the standard and snowflake divertor configurations. Using DIII-D H-mode ElMing equilibria, we found that the differences are due to the local magnetic shear change at the outboard midplane, which is the result of the realization of the snowflake configuration. Finally, the micro-instability, the drift-Alfven instability in the pedestal region of the DIII-D tokamak is studied. A modified six-field Landau fluid model under BOUT++ framework is used to study the linear characteristics and transport features of the drift-Alfven modes. Based on the DIII-D H-mode discharge, a sequence of divertor tokamak equilibria with different pedestal height is generated by the ’VARYPED’ tool for our studies. Qualitative agreement is obtained between theoretical analysis and the simulation results in the linear regime. Moreover, the heat transport induced by the drift-Alfven turbulence is explored and the convection level is estimated for both ions and electrons.Item Resistive instabilities in magnetically confined fusion plasmas with velocity shear and rotation(2015-08) White, Ryan Lee; Fitzpatrick, Richard, 1963-; Hazeltine, Richard; Breizman, Boris; Waelbroeck, Francois; Gonzalez, OscarUsing a resistive generalization of the Frieman-Rotenberg formalism for Lagrangian magnetohydrodynamic stability with equilibrium velocity, the leading-order effects of velocity shear and rotation on linear tearing layer stability are studied for tokamak equilibria. The separation-of-time-scales formalism needed for a proper formulation of ideal and resistive stability calculations is presented. Using this formalism, a dispersion relation is first obtained for marginal ideal modes in plane-symmetric equilibria. It is demonstrated how resistive modes arise as a natural continuation of marginal ideal modes. The dispersion relation for resistive modes in slab geometry is derived and used to demonstrate the resistive stability boundary. The widely misrepresented constant-Ψ limit is explained in detail, and used to obtain a dispersion rela- tion for tearing modes. Nyquist techniques are used to compare the Glasser effect in slab and cylindrical models. The resistive layer equations are also obtained in cylindrical geometry, allowing direct verification of the limited validity of gravity-curvature equivalence heuristic for resistive modes. Numerical complications that arise from velocity shear are discussed. Layer equations are also derived in the constant-Ψ limit. The constant-Ψ dispersion relation is obtained for cylindrical equilibria, and used to study the leading-order effects of rotation and velocity shear on the critical value of ∆′ required for tearing instability. It is found that rotation and velocity shear can couple with the parallel current and the current gradient in the layer to reduce ∆′[subscript crit]. If parallel currents are sufficiently weak to compete with second-order effects, velocity shear can be stabilizing, while rotation is found to have a destabilizing effect. Second-order coupling of velocity shear and rotation can have either sign, and thus can affect stability in either direction.