Browsing by Subject "Detachment"
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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 High Resolution Study of Micro-Meter Particle Detachment and Resuspension on Different Surfaces(2012-08-16) Kassab, Asmaa 1983-In an effort to understand the resuspension phenomena, interactions of spherical micro-meter particles (glass beads (GB) and Stainless steel (SS)) were investigated experimentally on different surfaces (glass, ceramic, hardwood, metal and chemical agent resistant coated metal (CARC)). Particles were deposited on the lower surface of a 10 cm square wind tunnel by gravitational settling. Air flows were imposed from an open entrance at average velocities up to 16 m/s. Individual particle trajectories obtained by high-speed imaging reveal three different types of motion: rolling/bouncing, immediate liftoff and complex motion. Surface roughness significantly affects the particle initial motion prior to liftoff. The majority of particle trajectories from the glass substrate were parallel to the surface with complex motion, covering 25% of the total distance traveled in rolling/bouncing motion before liftoff. Hardwood substrates took the longest time for initial particle movement (t >1 s) causing a more rapid liftoff. The ceramic substrate showed the most rolling/bouncing motion, for 80% of the particles. Additionally, single layer detachment showed that the detachment percentage initially follow an exponentially increasing trend for a period of ~ 1 s, followed by a plateau phase for a period of 5 s. Changing velocity, substrate and particle size significantly affects GB particle detachment. Furthermore, detachment from the metal substrate was consistently higher than the CARC substrates. However, particle density is not a significant difference in the bigger particle size studied. Initial 3-D particle tracking showed that particles seem to travel in a constant angle to the left rather than going straight in the flow direction. A detachment mode model showed that the detachment by direct liftoff required a much higher speed than rolling motion with a minimum of 14 m/s for both GB70 and SS70 on glass and metal surface, and the velocity increased to 21 m/s for the smaller particle. Incorporating the different types of particle motion prior to liftoff into resuspension models, and how their relative contributions change with different particle and substrate materials, can potentially yield improved predictive capabilities.