ALBERT

Notes: The effect of Zr content on the microstructure and magnetic properties of Sm(CobalFe0.1Cu0.088Zrx)8.5 magnets with x varying from 0 to 0.1 was systematically studied using magnetic, x-ray diffraction, and transmission electron microscopy measurements. The results have revealed that addition of Zr greatly affects the microstructure and magnetic properties. X-ray diffraction, electron diffraction, and thermomagnetic data showed that the normal microstructure consists of 2:17 cells surrounded by 1:5 cell boundaries and intersected by lamella phase perpendicular to the c axis. The lamella phase appears to have the SmCo3 structure and its amount increases with increasing Zr content. When the Zr content is above 0.08, the microstructure becomes inhomogeneous, showing regions with a considerable amount of mixtures of 1:3 and 2:7 phases. The formation of the cellular microstructure mainly depends on the Cu content, and the existence of the lamellar phase can stabilize a uniform cellular microstructure with larger cells and help the redistribution of Cu at the cell boundaries, improving the coercivity greatly. A room temperature intrinsic coercivity of around 40 kOe has been obtained in magnets with a wide range of Zr contents from x=0.02–0.06. © 2000 American Institute of Physics.

Notes: A fully kinetic assessment of the stability properties of toroidal drift modes has been obtained for a case for the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. This calculation retains the important effects in the linearized gyrokinetic equation, using the lowest-order "ballooning representation" for high toroidal mode number instabilities in the electrostatic limit. Results for toroidal drift waves destabilized by trapped particle dynamics and ion temperature gradients are presented, using three-dimensional magnetohydrodynamic equilibria reconstructed from experimental measurements. The effects of helically trapped particles and helical curvature are investigated. © 2000 American Institute of Physics.

Notes: A fully three-dimensional gyrokinetic particle code using magnetic coordinates for general geometry has been developed and applied to the investigation of zonal flows dynamics in toroidal ion-temperature-gradient turbulence. Full torus simulation results support the important conclusion that turbulence-driven zonal flows significantly reduce the turbulent transport. Linear collisionless simulations for damping of an initial poloidal flow perturbation exhibit an asymptotic residual flow. The collisional damping of this residual causes the dependence of ion thermal transport on the ion–ion collision frequency, even in regimes where the instabilities are collisionless. © 2000 American Institute of Physics.

Notes: Recent developments in gyrokinetic-magnetohydrodynamics (MHD) theory and in electromagnetic gyrokinetic particle simulations raise the question of consistency between the gyrokinetic model and the fluid model. Due to the special characteristics of the guiding center coordinates, it is a nontrivial exercise to show this consistency. In this paper it is shown, in a very general setting, that the gyrokinetic theory and the fluid equations do give an equivalent description of plasma equilibrium (∂/∂t=0). The fluid continuity equation and momentum equation for equilibrium plasmas are recovered entirely from the gyrokinetic theory. However, it was Spitzer who first realized the importance of consistency between guiding-center motion and fluid equations. In particular, he studied the "apparent paradoxical result" regarding the difference between perpendicular particle flow and guiding-center flow, which will be referred to as the Spitzer paradox in this paper. By recovering the fluid equations from the gyrokinetic theory, we automatically resolve the Spitzer paradox, whose essence is how the perpendicular current and flow are microscopically generated from particles' guiding-center motion. The mathematical construction in the gyrokinetic theory which relates observable quantities in the laboratory frame to the distribution function in the guiding-center coordinates is consistent with Spitzer's original physical picture, while today's gyrokinetic-MHD theory covers a much wider range of problems in a much more general and quantitative way. © 2000 American Institute of Physics.

Notes: Gyrocenter-gauge kinetic theory is developed as an extension of the existing gyrokinetic theories. In essence, the formalism introduced here is a kinetic description of magnetized plasmas in the gyrocenter coordinates which is fully equivalent to the Vlasov–Maxwell system in the particle coordinates. In particular, provided the gyroradius is smaller than the scale-length of the magnetic field, it can treat high-frequency range as well as the usual low-frequency range normally associated with gyrokinetic approaches. A significant advantage of this formalism is that it enables the direct particle-in-cell simulations of compressional Alfvén waves for magnetohydrodynamic (MHD) applications and of rf (radio frequency) waves relevant to plasma heating in space and laboratory plasmas. The gyrocenter-gauge kinetic susceptibility for arbitrary wavelength and arbitrary frequency electromagnetic perturbations in a homogeneous magnetized plasma is shown to recover exactly the classical result obtained by integrating the Vlasov–Maxwell system in the particle coordinates. This demonstrates that all the waves supported by the Vlasov–Maxwell system can be studied using the gyrocenter-gauge kinetic model in the gyrocenter coordinates. This theoretical approach is so named to distinguish it from the existing gyrokinetic theory, which has been successfully developed and applied to many important low-frequency and long parallel wavelength problems, where the conventional meaning of "gyrokinetic" has been standardized. Besides the usual gyrokinetic distribution function, the gyrocenter-gauge kinetic theory emphasizes as well the gyrocenter-gauge distribution function, which sometimes contains all the physics of the problems being studied, and whose importance has not been realized previously. The gyrocenter-gauge distribution function enters Maxwell's equations through the pull-back transformation of the gyrocenter transformation, which depends on the perturbed fields. The efficacy of the gyrocenter-gauge kinetic approach is largely due to the fact that it directly decouples particle's gyromotion from its gyrocenter motion in the gyrocenter coordinates. As in the case of kinetic theories using guiding center coordinates, obtaining solutions for this kinetic system involves only following particles along their gyrocenter orbits. However, an added advantage here is that unlike the guiding center formalism, the gyrocenter coordinates used in this theory involves both the equilibrium and the perturbed components of the electromagnetic field. In terms of solving the kinetic system using particle simulation methods, the gyrocenter-gauge kinetic approach enables the reduction of computational complexity without the loss of important physical content. © 2000 American Institute of Physics.

Notes: Scientific simulation in tandem with theory and experiment is an essential tool for understanding complex plasma behavior. In this paper we review recent progress and future directions for advanced simulations in magnetically confined plasmas with illustrative examples chosen from magnetic confinement research areas such as microturbulence, magnetohydrodynamics, magnetic reconnection, and others. Significant recent progress has been made in both particle and fluid simulations of fine-scale turbulence and large-scale dynamics, giving increasingly good agreement between experimental observations and computational modeling. This was made possible by innovative advances in analytic and computational methods for developing reduced descriptions of physics phenomena spanning widely disparate temporal and spatial scales together with access to powerful new computational resources. In particular, the fusion energy science community has made excellent progress in developing advanced codes for which computer run-time and problem size scale well with the number of processors on massively parallel machines (MPP's). A good example is the effective usage of the full power of multi-teraflop (multi-trillion floating point computations per second) MPP's to produce three-dimensional, general geometry, nonlinear particle simulations which have accelerated progress in understanding the nature of turbulence self-regulation by zonal flows. It should be emphasized that these calculations, which typically utilized billions of particles for thousands of time-steps, would not have been possible without access to powerful present generation MPP computers and the associated diagnostic and visualization capabilities. In general, results from advanced simulations provide great encouragement for being able to include increasingly realistic dynamics to enable deeper physics insights into plasmas in both natural and laboratory environments. The associated scientific excitement should serve to stimulate improved cross-cutting collaborations with other fields and also to help attract bright young talent to plasma science. © 2002 American Institute of Physics.

Notes: The effect of Cu and Zr on the microstructure and coercivity of Sm(CobalFe0.1CuyZrx)8.5 magnets has been studied using magnetometry and transmission electron microscopy. For Zr-free samples, when y=0.088, a cellular microstructure is not formed. Instead a rod-like 1:5 phase is observed, distributed in the 2:17 matrix phase. For a sample with y=0.168, the cellular microstructure develops after a short aging. However, the cellular microstructure coarsens and becomes highly nonuniform and finally, breaks down with further aging. This leads to a reduction in coercivity from an optimal value of 5.6–2 kOe. After adding Zr(x=0.04), the cellular microstructure along with a lamellar phase can be formed even with a Cu content as low as 0.048 and the coercivity increases dramatically. Coercivities of up to 8.9 and 38.4 kOe are obtained for the y=0.048 and 0.168 samples, respectively. These results show that the formation of cellular structure strongly depends on the Cu content. But the formation of the lamellar phase can stabilize a uniform cellular microstructure over a wide range of cell size and help the redistribution of Cu at the cell boundaries, which is a key to obtain appropriate microstructures with high coercivity. © 2000 American Institute of Physics.

Notes: Optimization studies in Sm(Co, Cu, Fe, Zr)z magnets for high-temperature applications led to some compositions which develop their high coercivity with simple processing. Homogenized magnets with higher Cu and Zr content acquire a coercivity of above 20 kOe after a short aging (3 h) at 850 °C without the traditional slow cooling to 400 °C which is required for the commercial magnets. Microstructure studies showed that the homogenized magnets consist of a mixture of Sm(Co, Cu)5 precipitates in a disordered 2:17 matrix as compared to a uniform and featureless microstructure of the traditional homogenized magnets. Because of this, the time required for the full development of uniform cellular and lamellar structures with the right microchemistry is much shorter in the magnets. © 2000 American Institute of Physics.

Notes: The "anomalous" nonmonotonic temperature dependence of coercivity, reported in Sm–Zr–Co–Cu magnets, has also been observed in bulk-hardened Y–Zr–Co–Fe–Cu alloys with a similar microstructure. The phenomenon appears to be universal for all R–Co magnets (R=rare earth) having a microstructure consisting of R2Co17 cells surrounded by the RCo5 phase. The effect of R and Cu on the temperature dependence of coercivity cannot be simply explained by traditional domain-wall pinning model based on the difference in a domain wall energy. Possibility that the coercivity is controlled by nucleation of reversed domains in magnetically isolated R2Co17 cells is discussed. © 2001 American Institute of Physics.

Notes: High-Tc Josephson junctions with a graded barrier have been prepared by using a composite target. Such a barrier is synthesized by utilizing Y1−xPrxBa2Cu3Oy with a continually graded concentration of Pr, in which no lattice mismatch and other incompatible problems take place. The structural interfaces are absent in the weak link region and Josephson coupling occurs at the naturally formed superconducting/normal interfaces within the Y1−xPrxBa2Cu3Oy layer. Thus, it can significantly enhance the reproducibilty and performance of these junctions. The temperature dependences of the barrier thickness and Josephson were also studied.© 2001 American Institute of Physics.