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  • 1
    Electronic Resource
    Electronic Resource
    Transitionless enhanced confinement and the role of radial electric field shear (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 615-625 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Evidence is presented for the role of radial electric field shear in enhanced confinement regimes attained without sharp bifurcations or transitions. Temperature scans at constant density, created in the reheat phase following deuterium pellet injection into supershot plasmas in the Tokamak Fusion Test Reactor [J. D. Strachan, et al., Phys. Rev. Lett. 58, 1004 (1987)] are simulated using a physics-based transport model. The slow reheat of the ion temperature profile, during which the temperature nearly doubles, is not explained by relatively comprehensive models of transport due to Ion Temperature Gradient Driven Turbulence (ITGDT), which depends primarily on the (unchanging) electron density gradient. An extended model, including the suppression of toroidal ITGDT by self-consistent radial electric field shear, does reproduce the reheat phase. The extended reheat at constant density is observed in supershot but not L-Mode plasmas. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873867
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  • 2
    Electronic Resource
    Electronic Resource
    Energetic particle orbits in the National Spherical Tokamak Experiment (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3667-3675 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The trajectories of neutral beam injected energetic ions in spherical tokamaks are examined. The large poloidal magnetic field in the outboard region of spherical tokamaks causes neutral beam injected ions to be born on trapped orbits even with cotangential injection. Numerical solutions to the equations for particle motion and for guiding center drifts are compared in several magnetic equilibria for a range of particle initial conditions. Even when rL/a∼1/4 the guiding center orbits closely resemble the path of the instantaneous center of gyration of the particle motion; exceptions occur primarily for orbits near the trapped/passing boundary. Finite Larmor radius effects are included in guiding center simulations of prompt orbit loss in the National Spherical Tokamak Experiment (NSTX) [J. Spitzer et al., Fusion Technol. 30, 1337 (1996)]. Orbit loss in the NSTX is caused primarily by collisions with the close fitting conducting shell, and severe losses would be expected with counter directed injection. While most orbits are similar to those found in conventional tokamaks, additional orbit types are possible in spherical tokamaks. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872263
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  • 3
    Electronic Resource
    Electronic Resource
    Deuterium-tritium simulations of the enhanced reversed shear mode in the Tokamak Fusion Test Reactor (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1316-1325 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The potential performance, in deuterium-tritium plasmas, of a new enhanced confinement regime with reversed magnetic shear [enhanced reversed shear (ERS) mode] is assessed. The equilibrium conditions for an ERS mode plasma are estimated by solving the plasma transport equations using the thermal and particle diffusivities measured in a short duration ERS mode discharge in the Tokamak Fusion Test Reactor [F. M. Levinton et al., Phys. Rev. Lett. 75, 4417 (1995)]. The plasma performance depends strongly on Zeff and neutral beam penetration to the core. The steady-state projections typically have a central electron density of ∼2.5×1020 m−3 and nearly equal central electron and ion temperatures of ∼10 keV. In time-dependent simulations the peak fusion power, ∼ 25 MW, is twice the steady-state level. Peak performance occurs during the density rise when the central ion temperature is close to the optimal value of ∼15 keV. The simulated pressure profiles can be stable to ideal magnetohydrodynamic instabilities with toroidal mode number n=1,2,3,4 and ∞ for βnorm up to 2.5; the simulations have βnorm≤2.1. The enhanced reversed shear mode may thus provide an opportunity to conduct alpha physics experiments in conditions similar to those proposed for advanced tokamak reactors. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872307
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  • 4
    Electronic Resource
    Electronic Resource
    Confinement and the safety factor profile (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1348-1355 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The conjecture that the safety factor profile, q(r), controls the improvement in tokamak plasmas from poor confinement in the Low- (L-) mode regime to improved confinement in the supershot regime has been tested in two experiments on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 1, 51 (1987)]. First, helium was puffed into the beam-heated phase of a supershot discharge, which induced a degradation from supershot to L-mode confinement in about 100 ms, far less than the current relaxation time. The q and shear profiles measured by a motional Stark effect polarimeter showed little change during the confinement degradation. Second, rapid current ramps in supershot plasmas altered the q profile, but were observed not to change significantly the energy confinement. Thus, enhanced confinement in supershot plasmas is not due to a particular q profile, which has enhanced stability or transport properties. The discharges making a continuous transition between supershot and L-mode confinement were also used to test the critical-electron-temperature-gradient transport model. It was found that this model could not reproduce the large changes in electron and ion temperature caused by the change in confinement. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871788
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  • 5
    Electronic Resource
    Electronic Resource
    Testing the ρ* scaling of thermal transport models: Predicted and measured temperatures in the Tokamak Fusion Test Reactor dimensionless scaling experiments (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1362-1370 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Theoretical predictions of ion and electron thermal diffusivities are tested by comparing calculated and measured temperatures in low (L) mode plasmas from the Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)] nondimensional scaling experiments. The DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] L-mode ρ* scalings, the transport models of Rebut-Lallia-Watkins (RLW), Boucher's modification of RLW, and the Institute for Fusion Studies-Princeton Plasma Physics Laboratory (IFS-PPPL) model for transport due to ion temperature gradient modes are tested. The predictions use the measured densities in order to include the effects of density profile shape variations on the transport models. The uncertainties in the measured and predicted temperatures are discussed. The predictions based on the DIII-D scalings are within the measurement uncertainties. All the theoretical models predict a more favorable ρ* dependence for the ion temperatures than is seen. Preliminary estimates indicate that sheared flow stabilization is important for some discharges, and that inclusion of its effects may bring the predictions of the IFS-PPPL model into agreement with the experiments. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872311
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  • 6
    Electronic Resource
    Electronic Resource
    Tests of local transport theory and reduced wall impurity influx with highly radiative plasmas in the Tokamak Fusion Test Reactor (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 877-884 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electron temperature (Te) profile in neutral beam-heated supershot plasmas (Te0∼6–7 keV ion temperature Ti0∼15–20 keV, beam power Pb∼16 MW) was remarkably invariant when radiative losses were increased significantly through gas puffing of krypton and xenon in the Tokamak Fusion Test Reactor [McGuire et al., Phys. Plasmas 2, 2176 (1995)]. Trace impurity concentrations (nz/ne∼10−3) generated almost flat and centrally peaked radiation profiles, respectively, and increased the radiative losses to 45%–90% of the input power (from the normal ∼25%). Energy confinement was not degraded at radiated power fractions up to 80%. A 20%–30% increase in Ti, in spite of an increase in ion–electron power loss, implies a factor of ∼3 drop in the local ion thermal diffusivity. These experiments form the basis for a nearly ideal test of transport theory, since the change in the beam heating power profile is modest, while the distribution of power flow between (1) radiation and (2) conduction plus convection changes radically and is locally measurable. The decrease in Te was significantly less than predicted by two transport models and may provide important tests of more complete transport models. At input power levels of 30 MW, the increased radiation eliminated the catastrophic carbon influx (carbon "bloom") and performance (energy confinement and neutron production) was improved significantly relative to that of matched shots without impurity gas puffing. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873327
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  • 7
    Electronic Resource
    Electronic Resource
    Current relaxation time scales in toroidal plasmas (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 333-339 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An approximate normal mode analysis of plasma current diffusion in tokamaks is presented. The work is based on numerical solutions of the current diffusion equation in cylindrical geometry. Eigenvalues and eigenfunctions are shown for a broad range of plasma conductivity profile shapes. Three classes of solutions are considered that correspond to three types of tokamak operation. Convenient approximations to the three lowest eigenvalues in each class are presented, and simple formulas for the current relaxation time scales are given and applied to several cases of simple current relaxation as well as noninductive current drive. Simulations of current relaxation with and without current redistribution caused by a "sawtooth'' model show that sawteeth can considerably shorten the relaxation time if the sawtooth region extends to r∼a/2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859146
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  • 8
    Electronic Resource
    Electronic Resource
    Helium, iron, and electron particle transport and energy transport studies on the Tokamak Fusion Test Reactor (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2215-2228 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results from helium, iron, and electron transport studies on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 26, 11 (1984)] in L-mode and supershot deuterium plasmas with the same toroidal field, plasma current, and neutral beam heating power are presented. They are compared to results from thermal transport analysis based on power balance. Particle diffusivities and thermal conductivities are radially hollow and larger than neoclassical values, except possibly near the magnetic axis. The ion channel dominates over the electron channel in both particle and thermal diffusion. A peaked helium profile, supported by inward convection that is stronger than predicted by neoclassical theory, is measured in the supershot. The helium profile shape is consistent with predictions from quasilinear electrostatic drift-wave theory. While the perturbative particle diffusion coefficients of all three species are similar in the supershot, differences are found in the L mode. Quasilinear theory calculations of the ratios of impurity diffusivities are in good accord with measurements. Theory estimates indicate that the ion heat flux should be larger than the electron heat flux, consistent with power balance analysis. However, theoretical values of the ratio of the ion to electron heat flux can be more than a factor of 3 larger than experimental values. A correlation between helium diffusion and ion thermal transport is observed and has favorable implications for sustained ignition of a tokamak fusion reactor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860755
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  • 9
    Electronic Resource
    Electronic Resource
    Continuous tokamak operation with an internal transformer (1983)
    Springer
    Journal of fusion energy 3 (1983), S. 13-24 
    Details
    ISSN: 1572-9591
    Keywords: fusion reactors ; current drive ; neutral beams ; tokamak ; internal transformer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract A large improvement in efficiency of current drive in a tokamak can be obtained using neutral beam injection to drive the current in a plasma which has low density and high resistivity. The current established under such conditions acts as the primary of a transformer to drive current in an ignited high-density plasma. In the context of a model of plasma confinement and fusion reactor costs, it is shown that such transformer action has substantial advantages over strict steady-state current drive. It is also shown that cycling plasma density and fusion power is essential for effective operation of an internal transformer cycle. Fusion power loading must be periodically reduced for intervals whose duration is comparable to the maximum of the particle confinement and thermal inertia time scales for plasma fueling and heating. The design of neutron absorption blankets which can tolerate reduced power loading for such short intervals is identified as a critical problem in the design of fusion power reactors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01053465
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  • 10
    Electronic Resource
    Electronic Resource
    Techniques for measuring the alpha-particle distribution in magnetically confined plasmas (1981)
    Springer
    Journal of fusion energy 1 (1981), S. 129-142 
    Details
    ISSN: 1572-9591
    Keywords: diagnostics ; alpha particles ; tokamak ; fusion diagnostics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract Methods are proposed for measuring the alpha-particle distribution in magnetically confined fusion plasmas using neutral-atom doping beams, ultraviolet spectroscopy, and neutral particle detectors. In the first method, single charge exchange reactions, A0+He2+→A+ +(He+)*, are used to populate then=2 andn=3 levels of He+. The ultraviolet photons from the decaying excited states are Doppler shifted by 5–10 Å from those produced by the thermalized alpha-particle “ash.” In the second method, double charge exchange reactions, A0+He2+→A2++He0, enable fast neutralized alpha particles to escape from the plasma and be detected by neutral particle analyzers. These methods are distinguished from similar techniques of observing plasma impurities in that, in principle, they allow a determination of the dependence of the distribution function on energy and pitch angle, as well as on spatial position. Detector configurations are analyzed, count rates are estimated, and the detector feasibility is discussed. A preliminary analysis of the feasibility of the required neutral beams is presented, and exploratory experiments on existing devices are suggested.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01050656
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