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  • 1
    Electronic Resource
    Electronic Resource
    Gas puff fueled H-mode discharges with good energy confinement above the Greenwald density limit on DIII-D : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2017-2022 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: ELMing (edge-localized) H-mode discharges with densities as high as 40% above the Greenwald density and good energy confinement, HITER-89P=2, were obtained with D2 gas puffing on DIII-D [Chan et al., Proceedings of the 16th IAEA Conference, Montreal (International Atomic Energy Agency, Vienna, 1996), Vol. 1, p. 95]. These discharges have performance comparable to the best pellet fueled DIII-D discharges. Spontaneous peaking of the density profile was an important factor in obtaining high energy confinement. Without density profile peaking, the energy confinement at high density degraded with reduction in the H-mode pedestal pressure under the stiff temperature profile conditions observed at high density on DIII-D. Reduction in the pedestal pressure was associated with loss of access to the second stable regime for ideal ballooning modes at the edge, and change in the edge-localized mode (ELM) instability from a low to high toroidal mode number. Gyrokinetic stability calculations indicate that the core of the high-density discharges is dominated by ion temperature gradient mode turbulence. A turbulent transport simulation with the GLF23 [Waltz et al., Phys. Plasmas 4, 2482 (1997)] code produced stiff temperature profiles in agreement with the experiment and did not indicate the formation of an internal transport barrier. Helium transport studies showed an anomalous inward particle pinch at high density. The highest density discharges were terminated by onset of a magnetohydromagnetic instability, which is consistent with destabilization of neoclassical tearing modes through peaking of the pressure profile. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1345504
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  • 2
    Electronic Resource
    Electronic Resource
    Radiative divertor plasmas with convection in DIII-D : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1736-1743 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The radiation of divertor heat flux on DIII-D [J. Luxon et al., in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1987), p. 159] is shown to greatly exceed the limits imposed by assumptions of energy transport dominated by electron thermal conduction parallel to the magnetic field. Approximately 90% of the power flowing into the divertor is dissipated through low-Z radiation and plasma recombination. The dissipation is made possible by an extended region of low electron temperature in the divertor. A one-dimensional analysis of the parallel heat flux finds that the electron temperature profile is incompatible with conduction-dominated parallel transport. Plasma flow at up to the ion acoustic speed, produced by upstream ionization, can account for the parallel heat flux. Modeling with the two-dimensional fluid code UEDGE [T. Rognlien, J. L. Milovich, M. E. Rensink, and G. D. Porter, J. Nucl. Mater. 196–198, 347 (1992)] has reproduced many of the observed experimental features. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872842
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  • 3
    Electronic Resource
    Electronic Resource
    Investigation of physical processes limiting plasma density in high confinement mode discharges on DIII-D : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1752-1760 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A series of experiments was conducted on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] to investigate the physical processes which limit density in high confinement mode (H-mode) discharges. The typical H-mode to low confinement mode (L-mode) transition limit at high density near the empirical Greenwald density limit [M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)] was avoided by divertor pumping, which reduced divertor neutral pressure and prevented formation of a high density, intense radiation zone (MARFE) near the X-point. It was determined that the density decay time after pellet injection was independent of density relative to the Greenwald limit and increased nonlinearly with the plasma current. Magnetohydrodynamic (MHD) activity in pellet-fueled plasmas was observed at all power levels, and often caused unacceptable confinement degradation, except when the neutral beam injected (NBI) power was ≤3 MW. Formation of MARFEs on closed field lines was avoided with low safety factor (q) operation but was observed at high q, qualitatively consistent with theory. By using pellet fueling and optimizing discharge parameters to avoid each of these limits, an operational space was accessed in which density ∼1.5×Greenwald limit was achieved for 600 ms, and good H-mode confinement was maintained for 300 ms of the density flat-top. More significantly, the density was successfully increased to the limit where a central radiative collapse was observed, the most fundamental density limit in tokamaks. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872277
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  • 4
    Electronic Resource
    Electronic Resource
    The two-dimensional structure of radiative divertor plasmas in the DIII-D tokamak : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1761-1773 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent measurements of the two-dimensional (2-D) spatial profiles of divertor plasma density, temperature, and emissivity in the DIII-D tokamak [J. Luxon et al., in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1987), p. 159] under highly radiating conditions are presented. Data are obtained using a divertor Thomson scattering system and other diagnostics optimized for measuring the high electron densities and low temperatures in these detached divertor plasmas (ne≤1021 m−3, 0.5 eV≤Te). D2 gas injection in the divertor increases the plasma radiation and lowers Te to less than 2 eV in most of the divertor volume. Modeling shows that this temperature is low enough to allow ion–neutral collisions, charge exchange, and volume recombination to play significant roles in reducing the plasma pressure along the magnetic separatrix by a factor of 3–5, consistent with the measurements. Absolutely calibrated vacuum ultraviolet spectroscopy and 2-D images of impurity emission show that carbon radiation near the X-point, and deuterium radiation near the target plates contribute to the reduction in Te. Uniformity of radiated power (Prad) (within a factor of 2) along the outer divertor leg, with peak heat flux on the divertor target reduced fourfold, was obtained. A comparison with 2-D fluid simulations shows good agreement when physical sputtering and an ad hoc chemical sputtering source (0.5%) from the private flux region surface are used. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872278
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  • 5
    Electronic Resource
    Electronic Resource
    Simulation of experimentally achieved DIII-D detached plasmas using the UEDGE code (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1967-1975 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The introduction of a divertor Thomson scattering system in DIII-D [J. Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] has enabled accurate determination of the plasma properties in the divertor region. Two plasma regimes are identified: detached and attached. The electron temperature in the detached regime is about 2 eV, much lower than 5–10 eV determined earlier. Fluid models of the DIII-D scrape-off layer plasma successfully reproduce many of the features of these two regimes, including the boundaries for transition between them. Detailed comparison between the results obtained from the fluid models and experiment suggest the models underestimate the spatial extent of the low-temperature region associated with the detached plasma mode. Low-temperature atomic physics processes that are not included in the present models may account for this discrepancy. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871993
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  • 6
    Electronic Resource
    Electronic Resource
    Very high- and high-confinement mode limited discharges in DIII-D (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1005-1011 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The first observations of marginally limited very high confinement mode (VH-mode) discharges have been achieved in DIII-D [Nucl. Fusion Special Supplement: World Survey of Activities in Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1990)] with significant reductions in peak heat flux conducted to plasma facing surfaces. In addition, quasistationary well limited high confinement-mode (H-mode) discharges have been obtained in DIII-D, also with reduced peak heat flux. This demonstration of reduced peak heat flux while maintaining high performance, i.e., high energy confinement time, can be important for the design of fusion ignition devices. Energy confinement enhancements in these high triangularity discharges are comparable to diverted discharges with similar parameters: τE/τITER-89P=2.9 for VH-mode and τE/τITER-89P=1.8 for quasistationary high confinement mode (H mode), where τITER-89P is the empirically derived low confinement mode (L-mode) energy confinement scaling relation [Nucl. Fusion 30, 1999 (1990)]. Comparisons of the conducted heat flux, particle flux, and radiated power profiles show a shift toward the inner wall as the discharge configuration becomes more limited. In addition to the advantage of reduced peak heat flux in these limiter discharges, such configurations also allow more effective use of the internal vessel volume, providing the potential for higher performance, i.e., higher plasma current at a fixed safety factor, q95. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871811
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  • 7
    Electronic Resource
    Electronic Resource
    Effects of plasma flow on error field islands (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1650-1656 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The subject of magnetic islands, due to error fields in devices such as tokamaks, is of importance for understanding energy and angular momentum confinement observed in existing devices and for a rational determination of the accuracy required for the coils of future large devices. Simple arguments show that, for parameter values of interest, the viscous drag on plasma inside an island, caused by a toroidal flow of plasma outside the island, can affect the islands significantly. This subject is studied using a numerical implementation of a simple, almost ideal magnetohydrodynamical, two-dimensional model. A simplified version of the model is also studied analytically. The study suggests that the islands shrink somewhat as the drag force is applied and that there is an upper limit to the total drag force that a surface of islands can sustain; when this maximum drag force is approached, large gradients in the current density are formed, which may give rise to instabilities that make invalid the two-dimensional assumption.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859685
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  • 8
    Electronic Resource
    Electronic Resource
    The role of neutrals in high-mode (H-mode) pedestal formation : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 2134-2140 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analytic model, derived from coupled continuity equations for the electron and neutral deuterium densities, is consistent with many features of edge electron density profiles in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research, 1986, Vol. I (International Atomic Energy Agency, Vienna, 1987), p. 159]. For an assumed constant particle diffusion coefficient, the model shows that particle transport and neutral fueling produce electron and neutral density profiles that have the same characteristic scale lengths at the plasma edge. For systematic variations of density in high-mode (H-mode) discharges, the model predicts that the width of the electron density transport barrier decreases and the maximum gradient increases, as observed in the experiments. The widths computed from the model agree quantitatively with the experimental widths for conditions in which the model is valid. These results support models of transport barrier formation in which the H-mode particle barrier is driven by the edge particle flux and the width of the barrier is approximately the neutral penetration length. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1462032
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  • 9
    Electronic Resource
    Electronic Resource
    Trapping of gun-injected plasma by a tokamak (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 2877-2884 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It has been seen that a plasma produced by a Marshall gun can be injected into and trapped by a tokamak plasma. This trapping of a gun-injected plasma is explained in terms of a depolarization current mechanism. A model is developed that describes the slowing of a plasma beam crossing into the magnetic field of a tokamak. The slowing down time is shown to go as τs∝T3/2eL2/nbα20, where nb and Te are the density and temperature of the plasma beam and α0/L is the pitch of the field lines per unit length in the direction in which the beam is traveling. Experimental tests of this model are consistent with the scaling predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866052
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  • 10
    Electronic Resource
    Electronic Resource
    Control of rotation velocity profile of tokamaks by application of slow electromagnetic waves (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 3422-3428 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Transport of a tokamak plasma may depend sensitively on its rotation velocity profile. Therefore a means for controlling the rotation profile may be useful. The paper describes a study of utilizing rotating islands, established deliberately by currents in external coils, as "stirrers'' of the plasma and thereby influence the velocity profile. One of the main issues of the study is how large a stirring force rotating islands can provide. It is suggested that even relatively narrow islands can provide substantial stirring forces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859772
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