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Turbulence studies in Tokamak boundary plasmas with realistic divertor geometry

Description: Results are presented from the 3D nonlocal electromagnetic turbulence code BOUT [1] and the linearized shooting code BAL[2] to study turbulence in tokamak boundary plasmas and its relationship to the L-H transition, in a realistic divertor plasma geometry. The key results include: (1) the identification of the dominant, resistive X-point mode in divertor geometry and (2) turbulence suppression in the L-H transition by shear in the ExB drift speed, ion diamagnetism and finite polarization. Based on the simulation results, a parameterization of the transport is given that includes the dependence on the relevant physical parameters.
Date: October 14, 1998
Creator: Xu, X.Q.
Partner: UNT Libraries Government Documents Department

Reynolds stress of localized toroidal modes

Description: An investigation of the 2D toroidal eigenmode problem reveals the possibility of a new consistent 2D structure, the dissipative BM-II mode. In contrast to the conventional ballooning mode, the new mode is poloidally localized at {pi}/2 (or -{pi}/2), and possesses significant radial asymmetry. The radial asymmetry, in turn, allows the dissipative BM-II to generate considerably larger Reynolds stress as compared to the standard slab drift type modes. It is also shown that a wide class of localized dissipative toroidal modes are likely to be of the dissipative BM-II nature, suggesting that at the tokamak edge, the fluctuation generated Reynolds stress (a possible source of poloidal flow) can be significant.
Date: February 1, 1995
Creator: Zhang, Y.Z. & Mahajan, S.M.
Partner: UNT Libraries Government Documents Department

Electron heat transport in improved confinement discharges in DIII-D

Description: In DIII-D tokamak plasmas with an internal transport barrier (ITB), the comparison of gyrokinetic linear stability (GKS) predictions with experiments in both low and strong negative magnetic shear plasmas provide improved understanding for ion and electron thermal transport within much of the plasma. As previously reported, the region for improved ion transport seems well characterized by the condition OE~B>Y-, where SERB is the ExB flow shear, calculated from measured quantities, and y,, is the maximum linear growth rate for ion temperature gradient (ITG) modes in the absence of flow shear. Within a limited region just inside the ITB, the electron temperature gradient (ETG) modes appear to control the electron temperature gradient and, consequently, the electron thermal transport. The increase in electron temperature gradient with more strongly negative magnetic shear is consistent with the increase in the ETG mode marginal gradient. Closer to the magnetic axis the Te profile flattens and the ETG modes are predicted to be stable. With additional core electron heating, FIR scattering measurements near the axis show the presence of high k fluctuations (12 cm-l), rotating in the electron diamagnetic drift direction. This turbulence could impact electron transport and possibly also ion transport. Thermal diffusivities for electrons, and to a lesser degree ions, increase. The ETG mode can exist at this wavenumber, but it is computed to be robustly stable near the axis.
Date: November 24, 1998
Creator: Stallard, B. W.
Partner: UNT Libraries Government Documents Department

Definition of total bootstrap current in tokamaks

Description: Alternative definitions of the total bootstrap current are compared. An analogous comparison is given for the ohmic and auxiliary currents. It is argued that different definitions than those usually employed lead to simpler analyses of tokamak operating scenarios.
Date: June 16, 1995
Creator: Ross, D.W.
Partner: UNT Libraries Government Documents Department

Tritium Experience in Large Tokamaks: Application to ITER

Description: Recent experience with the use of tritium fuel in the Tokamak Fusion Test Reactor and the Joint European Torus, together with progress in developing the technical design of the International Thermonuclear Experimental Reactor has expanded the technical knowledge base for tritium issues in fusion. This paper reports on an IEA workshop that brought together scientists and engineers to share experience and expertise on all fusion-related tritium issues. Extensive discussion periods were devoted to exploring outstanding issues and identifying potential R{ampersand}D avenues to address them. This paper summarizes the presentations, discussions, and recommendations.
Date: May 1, 1998
Creator: Skinner, C.H.; Gentile, C.; Hosea, J.; Mueller, D; Gentile, C.; Federici, G. et al.
Partner: UNT Libraries Government Documents Department

The Mercier Criterion in Reversed Shear Tokamak Plasmas

Description: A recent numerical study has found that, contrary to conventional theoretical and experimental expectations, reversed shear plasmas are unstable primarily because the term proportional to the shear in the Mercier criterion is destabilizing. In the present study, the role of the magnetic shear, both local and global, is examined for various tokamak configurations with monotonic and non-monotonic safety factor profiles. The enhancement of the local shear due to the outward shift of the magnetic axis suggests that the latter are less susceptible to interchanges. Furthermore, by regrouping the terms in the criterion, the V" term when differentiated instead with respect to the toroidal flux, is shown to absorb the dominant shear term. No Mercier instability is found for similar profiles as in the previous study.
Date: June 1, 1999
Creator: Kessel, C.; Chance, M.S. & Jardin, S.C.
Partner: UNT Libraries Government Documents Department

RF Systems for a Proposed Next Step Option (FIRE)

Description: FIRE (Fusion Ignition Research Experiment) is a high-field, burning-plasma tokamak that is being studied as a possible option for future fusion research. Preliminary parameters for this machine are R<sub>0</sub> approximately equal to 2 m, a approximately equal to 0.5 m, B<sub>0</sub> approximately equal to 10 T, and I<sub>p</sub> approximately equal to 6 MA. Magnetic field coils are to be made of copper and precooled with LN<sub>2</sub> before each shot. The flat-top pulse length desired is greater than or equal to 10s. Ion cyclotron and lower hybrid rf systems will be used for heating and current drive. Present specifications call for 30 MW of ion cyclotron heating power, with 25 MW of lower hybrid power as an upgrade option.
Date: April 12, 1999
Creator: Carter, M.D. & Swain, D.W.
Partner: UNT Libraries Government Documents Department

Divertor characterization experiments

Description: Recent DIII-D experiments with enhanced Scrape-off Layer (SOL) diagnostics permit detailed characterization of the SOL and divertor plasma under various operating conditions. We observe two distinct plasma modes: attached and detached divertor plasmas. Detached plasmas are characterized by plate temperatures of only 1 to 2 eV. Simulation of detached plasmas using the UEDGE code indicate that volume recombination and charge exchange play an important role in achieving detachment. When the power delivered to the plate is reduced by enhanced radiation to the point that recycled neutrals can no longer be efficiently ionized, the plate temperature drops from around 10 eV to 1-2 eV. The low temperature region extends further off the plate as the power continues to be reduced, and charge exchange processes remove momentum, reducing the plasma flow. Volume recombination becomes important when the plasma flow is reduced sufficiently to permit recombination to compete with flow to the plate.
Date: June 18, 1996
Creator: Porter, G.D.; Allen, S.; Fenstermacher, M.; Hill, D.; Brown, M.; Jong, R.A, et al.
Partner: UNT Libraries Government Documents Department

Magnetic confinement experiment -- 1: Tokamaks

Description: This report reviews presentations made at the 15th IAEA Conference on Plasma Physics and Controlled Nuclear Fusion on experimental tokamak physics, particularly on advances in core plasma physics, divertor and edge physics, heating and current drive, and tokamak concept optimization.
Date: December 31, 1994
Creator: Goldston, R.J.
Partner: UNT Libraries Government Documents Department

Theory of self-organized critical transport in tokamak plasmas

Description: A theoretical and computational study of the ion temperature gradient and {eta}{sub i} instabilities in tokamak plasmas has been carried out. In toroidal geometry the modes have a radially extended structure and their eigenfrequencies are constant over many rational surfaces that are coupled through toroidicity. These nonlocal properties of the ITG modes impose strong constraint on the drift mode fluctuations and the amciated transport, showing a self-organized characteristic. As any significant deviation away from marginal stability causes rapid temperature relaxation and intermittent bursts, the modes hover near marginality and exhibit strong kinetic characteristics. As a result, the temperature relaxation is self-semilar and nonlocal, leading to a radially increasing heat diffusivity. The nonlocal transport leads to the Bohm-like diffusion scaling. The heat input regulates the deviation of the temperature gradient away from marginality. The obtained transport scalings and properties are globally consistent with experimental observations of L-mode charges.
Date: July 1, 1995
Creator: Kishimoto, Y.; Tajima, T.; Horton, W.; LeBrun, M.J. & Kim, J.Y.
Partner: UNT Libraries Government Documents Department


Description: This month's issue has the following 3 articles: (1) Kathy Yelick is the new director for the National Energy Research Scientific Computing Center (NERSC); (2) Head of the Class--A cray XT4 named Franklin passes a rigorous test and becomes an official member of the NERSC supercomputing family; and (3) Model Comparisons--Fusion research group published several recent papers examining the results of two types of turbulence simulations and their impact on tokamak designs.
Date: November 25, 2007
Creator: Wang, Ucilia
Partner: UNT Libraries Government Documents Department

Thermal ablation of plasma-facing surfaces in tokamak disruptions: Sensitivity to particle kinetic energy

Description: Ablation damage to solid targets with high heat flux impulses is generally greater high-energy electron beam heat sources compared to low-energy plasma guns. This sensitivity to incoming particle kinetic energy is explored with computer modelling; a fast-running routine (DESIRE) is developed for initial scoping analysis and is found to be in reasonable agreement with several experiments on graphite and tungsten targets. If tokamak disruptions are characterized by particle energies less than {approximately}1 keV, then we expect plasma guns are a better analogue than electron beams for simulating disruption behavior and testing candidate plasma-facing materials.
Date: February 1, 1996
Creator: Ehst, D.A. & Hassanein, A.
Partner: UNT Libraries Government Documents Department

Design assumptions and bases for small D-T-fueled Sperical Tokamak (ST) fusion core

Description: Recent progress in defining the assumptions and clarifying the bases for a small D-T-fueled ST fusion core are presented. The paper covers several issues in the physics of ST plasmas, the technology of neutral beam injection, the engineering design configuration, and the center leg material under intense neutron irradiation. This progress was driven by the exciting data from pioneering ST experiments, a heightened interest in proof-of-principle experiments at the MA level in plasma current, and the initiation of the first conceptual design study of the small ST fusion core. The needs recently identified for a restructured fusion energy sciences program have provided a timely impetus for examining the subject of this paper. Our results, though preliminary in nature, strengthen the case for the potential realism and attractiveness of the ST approach.
Date: December 31, 1996
Creator: Peng, Y.K.M.; Galambos, J.D. & Fogarty, P.J.
Partner: UNT Libraries Government Documents Department


Description: OAK B202 TOKAMAK EQUILIBRIA WITH CENTRAL CURRENT HOLES AND NEGATIVE CURRENT DRIVE. Several tokamak experiments have reported the development of a central region with vanishing currents (the current hole). Straightforward application of results from the work of Greene, Johnson and Weimer [Phys. Fluids, 3, 67 (1971)] on tokamak equilibrium to these plasmas leads to apparent singularities in several physical quantities including the Shafranov shift and casts doubts on the existence of this type of equilibria. In this paper, the above quoted equilibrium theory is re-examined and extended to include equilibria with a current hole. It is shown that singularities can be circumvented and that equilibria with a central current hole do satisfy the magnetohydrodynamic equilibrium condition with regular behavior for all the physical quantities and do not lead to infinitely large Shafranov shifts. Isolated equilibria with negative current in the central region could exist. But equilibria with negative currents in general do not have neighboring equilibria and thus cannot have experimental realization, i.e. no negative currents can be driven in the central region.
Date: June 1, 2002
Creator: CHU, M.S. & PARKS, P.B.
Partner: UNT Libraries Government Documents Department


Description: We present explicit calculations of the complicated geometric objects known as homoclinic tangles that surround magnetic islands in the Poincare mapping of a tokamak's magnetic field. These tangles are shown to exist generically in the magnetic field of all toroidal confinement systems. The geometry of these tangles provides an explanation for the stochasticity known to occur near the X-points of the Poincare mapping. Furthermore, the intersection of homoclinic tangles from different resonances provides an explicit mechanism for the non-diffusive transport of magnetic field lines between these resonance layers.
Date: June 1, 2002
Partner: UNT Libraries Government Documents Department


Description: OAK A271 OPTIMUM PLASMA STATES FOR NEXT STEP TOKAMAKS. The dependence of the ideal ballooning {beta} limit on aspect ratio, A, and elongation {kappa} is systematically explored for nearly 100% bootstrap current driven tokamak equilibria in a wide range of the shape parameters (A = 1.2-7.0, {kappa} = 1.5-6.0 with triangularity {delta} = 0.5). The critical {beta}{sub N} is shown to be optimal at {kappa} = 3.0-4.0 for all A studied and increases as A decreases with a dependence close to A{sup -0.5}. The results obtained can be used as a theoretical basis for the choice of optimum aspect ratio and elongation of next step burning plasma tokamaks or tokamak reactors.
Date: November 1, 2002
Partner: UNT Libraries Government Documents Department

Inertial fusion energy: A clearer view of the environmental and safety perspectives

Description: If fusion energy is to achieve its full potential for safety and environmental (S&E) advantages, the S&E characteristics of fusion power plant designs must be quantified and understood, and the resulting insights must be embodied in the ongoing process of development of fusion energy. As part of this task, the present work compares S&E characteristics of five inertial and two magnetic fusion power plant designs. For each design, a set of radiological hazard indices has been calculated with a system of computer codes and data libraries assembled for this purpose. These indices quantify the radiological hazards associated with the operation of fusion power plants with respect to three classes of hazard: accidents, occupational exposure, and waste disposal. The three classes of hazard have been qualitatively integrated to rank the best and worst fusion power plant designs with respect to S&E characteristics. From these rankings, the specific designs, and other S&E trends, design features that result in S&E advantages have been identified. Additionally, key areas for future fusion research have been identified. Specific experiments needed include the investigation of elemental release rates (expanded to include many more materials) and the verification of sequential charged-particle reactions. Improvements to the calculational methodology are recommended to enable future comparative analyses to represent more accurately the radiological hazards presented by fusion power plants. Finally, future work must consider economic effects. Trade-offs among design features will be decided not by S&E characteristics alone, but also by cost-benefit analyses. 118 refs., 35 figs., 35 tabs.
Date: November 1, 1996
Creator: Latkowski, J.F.
Partner: UNT Libraries Government Documents Department