ALBERT

Notes: The high-Tc superconductor LaBa2Cu3Oy has been prepared under various conditions, and characterized by x-ray and neutron powder diffraction techniques, dc and ac magnetic susceptibility measurements, and dc resistivity measurement. It has been found that samples with excellent superconducting characteristics and also with good structural properties are obtained in a reproducible way if the following three procedures are performed: preheat treatment of La2O3 powders, sintering in N2, and low-temperature annealing in dried O2. Long-time annealing in undried O2 was sometimes found to degrade the grain boundaries and to lead to a lower zero-resistance temperature.

Notes: The transport of ions through a collisionless rf glow-discharge sheath is simulated with a Monte Carlo method in order to determine the influence of surface topography on ion trajectories. The simulation results indicate that the perturbation of the local electric field by the geometric shape of the trench has very little influence on ion trajectories, and as such cannot be considered as the origin of reactive ion etching lag in low-pressure rf plasmas. These results are in sharp disagreement with the model proposed by Lee and Zhou [J. Electrochem. Soc. 138, 2439 (1991)] and with the calculations by Ingram [J. Appl. Phys. 68, 500 (1990)].

Notes: A new mechanism for Langmuir wave localization is investigated. Typical computer simulation of weak electron beam–plasma instabilities proceeds as follows: As Langmuir waves grow out of the noise, they react back on the beam particles by trapping. The subsequent phase shift puts the electrons out of phase with the waves. Then the instability is locally quenched, resulting in strong wave modulation, even before saturation of the instabilities. The typical size of wave packets is given by 2πvp/ω¯t, where vp and ω¯t denote the phase speed of the dominant wave and the average bounce frequency, respectively. The time scale for this process is considerably shorter than that of parametric instabilities. © 1996 American Institute of Physics.

Notes: Using a quasirealistic generalized continuum Takayama–Liu–Maki (TLM) model including acoustic phonon effects, perturbation theory is applied and a formal method for computing the kink of the TLM model with its perturbative corrections due to acoustic phonon interaction effects is presented. It is found that acoustic effects induce the motion of the soliton. The kink is then constrained to propagate along the molecular chain with a fixed velocity of magnitude close to that of the acoustic phonon velocity. No static solution is consistent with perturbation theory.

Notes: The Feynman rules for the Anderson model with a (2J+1)-component localized spin are formulated by means of a generalized Wick's expansion together with the reduction formulas in the thermo-field-dynamics. In the U→∞ limit, Feynman rules for arbitrary J correspond closely to those for the case J= 1/2 and may be obtained from them by a simple replacement rule.

Notes: We examine the sine–Gordon equation with a perturbation λΔV. We derive necessary conditions on ΔV such that the perturbed equation has solutions with finite energy, analytic in λ, and which reduce to the static soliton when the perturbation is removed (λ→0). Several examples illustrating these conditions are presented.

Notes: At the onset of the L–H transition in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p.159], a fast (≈100 μsec) suppression of microturbulence is observed as the edge transport barrier is formed. This fast edge suppression is followed by a much slower (tens of msec), but substantial (≥50%) reduction in the relative density fluctuation level. This second turbulence suppression phase, which is observed to correlate with growing E×B velocity shear, has been localized to the plasma interior, and may explain why the observed transport reduction in the H mode has been observed to extend deep into the plasma, well beyond the edge transport barrier.

Notes: The L to H transition in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] is associated with two clear signatures: edge density fluctuations are abruptly suppressed (in ≈100 μsec), while the edge poloidal rotation velocity vθ increases, implying that the radial electric field Er becomes more negative. Detailed new spectroscopic profile measurements show that the changes in vθ and Er generate a region of sheared electric field and poloidal flow of width ≈3–5 cm. This region develops simultaneously with, and has the same spatial extent as, the edge fluctuation suppression zone as measured using a reflectometer system. Furthermore, the radial extent of the shear and fluctuation suppression zones encompass the location of the H-mode edge transport barrier. These observations are consistent with recent theoretical models of the L–H transition, and a comparison with these theories is presented. Data are also presented on the evolution of edge parameters and density fluctuations after the transition: the shear and fluctuation suppression layers are maintained for the duration of the quiescent H-mode phase, while relative density fluctuation levels decrease and interior plasma confinement gradually improves. Precursors to several different types of edge localized mode (ELMs) are also discussed.

Notes: The L to H transition in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] is associated with a decrease in the edge density and magnetic fluctuations. In addition, in single-null divertor plasmas, a reduction in the heat flux asymmetry between the inner and outer divertor hit spots occurs. These observations indicate that the L to H transition is associated with the reduction in anomalous, fluctuation-connected transport across the outer midplane of the plasma. Magnetic fluctuations are measured with a poloidally distributed set of Mirnov loops while density fluctuations are detected with multiple fixed-frequency microwave reflectometers. Spectroscopic observations of edge poloidal and toroidal rotation have allowed the inference that the radial electric field just inside the separatrix is negative in the L mode and becomes more negative at the L to H transition. These changes in fluctuations and in the edge electric field occur in plasmas where the H mode is created by neutral beam heating, electron cyclotron heating, and even by Ohmic heating alone. Accordingly, these changes are characteristic of the H mode and are not artifacts of the heating method. Several published theories of the L to H transition involve the effects of the electric field and edge plasma rotation. The observations in this paper have been critically compared with these theories.

Notes: This is a review of what is known about fluctuations and anomalous transport processes in tokamaks. It mostly considers experimental results obtained after, and not included in, the reviews of Liewer [Nucl. Fusion 25, 543 (1985)], Robinson [in Turbulence and Anomalous Transport in Magnetized Plasmas (Ecole Polytechnique, Palaiseau, France, 1986), p. 21], and Surko [in Turbulence and Anomalous Transport in Magnetized Plasmas (Ecole Polytechnique, Palaiseau, France, 1986), p. 93]. Therefore much of the pioneering work in the field is not covered. Emphasis is placed on results where comparisons between fluctuations and transport properties have been attempted, particularly from the tokamak TEXT [Nucl. Technol./Fusion 1, 479 (1981)]. A brief comparison of experimentally measured total fluxes with the predictions of neoclassical theory demonstrates that transport is often anomalous; fluctuations are thought to be the cause.The measurements necessary to determine any such fluctuation-driven fluxes are described. The diagnostics used to measure these quantities, together with some of the statistical techniques employed to analyze the data, are outlined. In the plasma edge detailed measurements of the quantities required to directly determine the fluctuation-driven fluxes are available. The total and fluctuation-driven fluxes are compared: the result emphasizes the importance of edge turbulence. No model adequately describes all the measured properties. In the confinement region experimental observations are presently restricted to measurements of density and potential fluctuations and their correlations. Various distinct turbulence features that have been observed are described, and their characteristics compared with the predictions of various models. Correlations observed between these fluctuations and plasma transport properties are summarized. A separate section on magnetic fluctuations shows there is very little information available inside the plasma, generally prohibiting quantified comparisons between fluctuation levels and transport. Both coherent and turbulent magnetic fluctuations are addressed, and the differences between low and high plasma pressure (low and high beta) are noted. The contributions of alternate confinement devices, such as stellarators and reversed field pinches, to understanding tokamak anomalous transport are discussed. Finally, future directions are proposed.