Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session H15: Sherwood II |
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Sponsoring Units: Sherwood DPP Chair: James Myra, Lodestar Research Corporation Room: Governor's Square 14 |
Sunday, May 3, 2009 10:30AM - 11:00AM |
H15.00001: COFFEE BREAK
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Sunday, May 3, 2009 11:00AM - 11:30AM |
H15.00002: Magnetic field threshold for runaway generation in tokamak disruptions T. F\"ul\"op, G. Pokol, H.M. Smith, P. Helander Due to a sudden cooling of the plasma in tokamak disruptions a beam of relativistic runaway electrons is sometimes generated, which may cause damage on plasma facing components. Experimental observations on large tokamaks show that the number of runaway electrons produced in disruptions depends on the magnetic field strength. In this work, two possible reasons for this threshold are studied. The first possible explanation for these observations is that the runaway beam excites whistler waves that scatter the electrons in velocity space and prevents the beam from growing. The growth rates of the most unstable whistler waves are inversely proportional to the magnetic field strength and it is possible to derive a magnetic field threshold below which no runaways are expected. The second possible explanation is the magnetic field dependence of the criterion for substantial runaway production determined by the induced electric field available and by the efficiency of the generation mechanisms. It is shown, that even in rapidly cooling plasmas, where hot-tail generation is expected to give rise to substantial runaway population, the whistler waves can stop the runaway formation below a certain magnetic field unless the post-disruption temperature is very low. [Preview Abstract] |
Sunday, May 3, 2009 11:30AM - 12:00PM |
H15.00003: Revisiting MHD stability comparison theorems: Some surprising new results Antoine Cerfon, Jeffrey Freidberg The classic MHD stability comparison theorems (Kruskal-Oberman, Rosenbluth-Rostoker) show that ideal MHD yields the most stringent stability limits according to the hierarchy $\delta W_{CGL}>\delta W_{KIN}>\delta W_{MHD}$. This has long justified the use of ideal MHD for conservative predictions of MHD stability boundaries. We reexamine these theorems, with the following conclusions:(1) It is crucial to distinguish between ergodic and closed field line systems.(2) It is essential to account for resonant particles in the kinetic MHD model.(3) For ergodic systems the original kinetic MHD analysis over-estimates stability: $\delta W_{KIN}>\delta W_{MHD}$. Our new result predicts $\delta W_{KIN}=\delta W_{MHD}$.(4) For closed line systems plasma compressibility effects become important, and resonant particle effects vanish. Both the original and new analysis predict $\delta W_{KIN}>\delta W_{MHD}$. However, using a Vlasov-Fluid model with Vlasov ions and fluid electrons we show that both $\delta W_{KIN}$ and $\delta W_{MHD}$, while mathematically correct, yield the wrong physical result. The V-F model shows that at marginal stability the compressibility stabilization term vanishes identically! For ergodic systems, marginal stability is always incompressible, so $\delta W_{KIN}=\delta W_{MHD}=\delta W_{VF}$. For compressible modes in closed line systems, however, perpendicular resonant particle effects cancel the stabilizing effect of plasma compressibility predicted by ideal and kinetic MHD: $\delta W_{KIN}>\delta W_{MHD}>\delta W_{VF}$. [Preview Abstract] |
Sunday, May 3, 2009 12:00PM - 12:30PM |
H15.00004: Edge Plasma Characteristics in a Snowflake Magnetic Configuration M.V. Umansky, R.H. Bulmer, R.H. Cohen, T.D. Rognlien, D.D. Ryutov A snowflake configuration for a diverted tokamak uses a 2$^{nd}$ order null of the poloidal field instead of the standard 1$^{st}$ order null. Geometrical properties of snowflake divertor are favorable for reducing heat flux on divertor surfaces, due to stronger fanning of the poloidal flux, larger radiating volume, and larger connection length in the scrape-off layer. Additional potential benefits include better control of ELM activity via the effect on the q-profile just inside the separatrix, and blob dynamics via the stronger magnetic shear near the second-order null point. This study presents a quantitative assessment of performance of snowflake divertor for a high-power tokamak. The analysis utilizes the MHD equilibrium code Corsica and edge transport code UEDGE. Divertor performance is compared for a high-power tokamak with standard and snowflake-like configurations for the same core plasma parameters. For a range of studied cases, the snowflake divertor peak heat-load on the target plates is significantly reduced compared to the standard divertor due to larger plasma-wetted area and larger fraction of power radiated in the edge. [Preview Abstract] |
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