Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session W15: Sherwood IV |
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Sponsoring Units: Sherwood DPP Chair: Janardhan Manickam, Princeton Plasma Physics Laboratory Room: Governor's Square 14 |
Tuesday, May 5, 2009 10:45AM - 11:15AM |
W15.00001: Fusion-Fission Transmutation Scheme-Efficient Destruction of Nuclear Waste Mike Kotschenreuther, Swadesh Mahajan, Prashant Valanju, Erich A. Schneider A fusion-assisted transmutation system for the destruction of transuranic (TRU) waste is presented. Subcritical fusion-fission hybrids burn the intransigent transuranic residues (with most of the long lived bio-hazard) of a new fuel cycle that uses cheap light water reactors (LWRs) for the easily burned majority of the TRU. In the new fuel cycle, the number of hybrids needed to destroy a given amount of original LWR waste is 5-10 times less than the corresponding number of critical fast reactors. (Fast reactors, due to stability constraints, cannot burn the very poor quality TRU residue.) The new system comparably reduces the expensive reprocessing throughput. Realization of these advantages should lead to a great reduction in the cost of transmutation. The time needed for 99{\%} waste destruction would also be reduced from centuries to decades. The centerpiece of the fuel cycle is a high power density compact fusion neutron source (CFNS-100 MW, with major radius + minor radius $\sim $ 2.5 m), which is made possible by a super-X divertor. The physics and technology requirements of the CFNS are much less than the requirements of a pure fusion power source. Advantages of the system as part of a timely strategy to combat global warming are briefly described. [Preview Abstract] |
Tuesday, May 5, 2009 11:15AM - 11:45AM |
W15.00002: On radial electric field, edge flows, and the L-H transition power threshold in tokamaks A.Y. Aydemir At the collisional edge, there is a residual vertical electric field associated with the Pfirsch-Schl\"uter currents that drives an ExB flow. The poloidal flow is in the direction of increasing major radius, regardless of the orientation of the fields and currents, and the toroidal component is anti-symmetric about the mid-plane for an up-down symmetric system. These flows have many features in common with the edge flows observed in tokamaks like C-Mod. A more careful analysis leads to a radial electric field that depends on the edge temperature gradient and shear. Without up-down symmetry, total contribution to the toroidal momentum and the edge $E_\psi$ clearly depends on the toroidal field direction. When the grad-B drift direction points towards the X- point, the net effect is positive; with toroidal field reversal, $E_\psi$ and the toroidal flow oppose the ambient flows and electric field due to, for example, the ion-orbit loss mechanism. The magnitude of this positive/negative contribution is also plasma- shape dependent. These features provide a compelling explanation for the grad-B drift-dependence of the L-H transition power threshold. [Preview Abstract] |
Tuesday, May 5, 2009 11:45AM - 12:15PM |
W15.00003: Exponential Growth and Filamentary Structure of Nonlinear Ballooning Instability P. Zhu, C.C. Hegna, C.R. Sovinec Ballooning instability is widely believed to be the underlying process for the type-I large edge-localized-modes (ELMs). The evolution equations for ballooning instability in the intermediate nonlinear regime are derived in an ideal MHD description. This nonlinear regime is operative when the MHD displacement of the plasma filament across the magnetic surface becomes the order of the linear mode width in that same direction. For application to ELM dynamics, this displacement amplitude is comparable to the pedestal width for intermediate-$n$ instabilities. A remarkable feature of this nonlinear regime is that a perturbation that evolves from a linear ballooning instability will continue to grow exponentially at the same growth rate, and maintain the filamentary mode structure of the corresponding linear phase as described in the Lagrangian reference frame. The analytic prediction of the nonlinear exponential growth phase is in excellent agreement with the first-principle full MHD simulations. This may explain why in experiments and simulations, the nonlinear ELM filament strongly resembles the structure of a linear ballooning filament, and linear analyses have often been able to match certain observed features of ELMs in the precursor and collapse onset phases. [Preview Abstract] |
Tuesday, May 5, 2009 12:15PM - 12:45PM |
W15.00004: Neoclassical Toroidal Viscosity Induced Rotation in Tokamaks and Quasi-symmetric Stellarators A.J. Cole, C.C. Hegna, J.D. Callen Non-axisymmetric magnetic perturbations generate variations in $|B|$ along a field line that induce non-ambipolar radial transport and a global toroidal force on the plasma, known as neoclassical toroidal viscosity [NTV]. A strong correlation exists between the flow evolution physics of tokamaks and quasi-helically symmetric [QHS] stellarators. In QHS-mode, there exists a helical symmetry angle $\alpha \equiv m\theta - n \zeta$, with $m,n$ fixed integers that is analogous to the poloidal direction in tokamaks. As a result, there exists a direction of near helical symmetry and thus least flow damping along $\vec{e}_h$ such that $\vec{e}_h \cdot \vec{\nabla}\alpha = 0$, analogous to the toroidal tokamak direction. In this paper, a model analytic 'toroidal' rotation equation is developed which smoothly transitions between previously asymptotic low-collisionality regimes [1], while incorporating both electron and ion NTV. In particular, the transition from ion to electron dominated NTV is presented in a single equation for the first time. This research will facilitate future comparison between NTV-induced rotation in QHS stellarators and tokamaks. [1] K.C.~Shaing, Phys.~Plasmas, \textbf{10}, 1443 (2003). [Preview Abstract] |
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