Development of the elevation drive assembly for orbiting solar observatory I (EYE)

The requirements for pointing accuracy, friction, and power for the elevation drive assembly of an orbiting space observatory are discussed briefly. A description of the components making up the assembly is presented. Special features requiring development testing prior to unit fabrication are more fully described together with a review of the test programs conducted and results obtained

Nonequilibrium Approach to Bloch-Peierls-Berry Dynamics

We examine the Bloch-Peierls-Berry dynamics under a classical nonequilibrium dynamical formulation. In this formulation all coordinates in phase space formed by the position and crystal momentum space are treated on equal footing. Explicitly demonstrations of the no (naive) Liouville theorem and of the validity of Darboux theorem are given. The explicit equilibrium distribution function is obtained. The similarities and differences to previous approaches are discussed. Our results confirm the richness of the Bloch-Peierls-Berry dynamics

Moving Wigner Glasses and Smectics: Dynamics of Disordered Wigner Crystals

We examine the dynamics of driven classical Wigner solids interacting with quenched disorder from charged impurities. For strong disorder, the initial motion is plastic -- in the form of crossing winding channels. For increasing drive, the disordered Wigner glass can reorder to a moving Wigner smectic -- with the electrons moving in non-crossing 1D channels. These different dynamic phases can be related to the conduction noise and I(V) curves. For strong disorder, we show criticality in the voltage onset just above depinning. We also obtain the dynamic phase diagram for driven Wigner solids and prove that there is a finite threshold for transverse sliding, recently found experimentally.Comment: 4 pages, 4 postscript figure

MINNESOTA CURRENT FARM SITUATION

These papers describe the current economic conditions and outline some of the opportunities facing Minnesota farmers as they complete 1998 and develop plans for 1999. The first paper, Prospects For Farm Income in 1998, describes the variation in net farm income farmers in the Southeastern and Southwestern Farm Management Associations have experienced over the past twenty years. The possible effects of 1998's low commodity prices and government program payments on net farm income are discussed in the context of this historical variation. This paper points out that some farmers are experiencing a very good year in 1998, while others may have very low net farm income this year. Those with high incomes in 1998 may be primarily interested in opportunities to enhance their cash flow for 1999, while those experiencing financial stress during 1998 may need to consider a wider range of adjustments as they plan for the coming year. The remaining papers provide information farmers may want to consider as they plan their marketing and financial strategies for the coming year. Financial Management Alternatives outlines a wide range of financial management alternatives that can be used to deal with cash flow problems. Some of these alternatives will be of interest to farmers who have difficulty in projecting a positive net cash flow with the relatively low commodity prices being projected for 1999. Others are of primary interest to farmers who have relatively high debt levels and must find ways to improve both their net cash flow and their debt/asset position. Some observers have suggested that the low commodity prices projected for 1999 may lead to reductions in cash rental rates. The paper, Cropland Rental Market Impacts of Low Crop Prices, discusses a procedure to estimate "fair rental rates", and provides an historical perspective of the impact low commodity prices have had on land rental rates. The remaining two papers, Situation and Outlook For the Livestock Sector, and Considerations in Developing a Corn/Soybean Marketing Plan for 1998/1999, discuss the major factors to evaluate and options to consider in developing a marketing plan for corn, soybeans, hogs and cattle for the coming year.Farm Management,

Theory of orientational ordering in colloidal molecular crystals

Freezing of charged colloids on square or triangular two-dimensional periodic substrates has been recently shown to realize a rich variety of orientational orders. We propose a theoretical framework to analyze the corresponding structures. A fundamental ingredient is that a non spherical charged object in an electrolyte creates a screened electrostatic potential that is anisotropic at any distance. Our approach is in excellent agreement with the known experimental and numerical results, and explains in simple terms the reentrant orientational melting observed in these so called colloidal molecular crystals. We also investigate the case of a rectangular periodic substrate and predict an unusual phase transition between orientationnaly ordered states, as the aspect ratio of the unit cell is changed.Comment: 4 pages, to appear in Phys. Rev. Let

Dynamics and hysteresis in square lattice artificial spin-ice

Dynamical effects under geometrical frustration are considered in a model for artificial spin ice on a square lattice in two dimensions. Each island of the spin ice has a three-component Heisenberg-like dipole moment subject to shape anisotropies that influence its direction. The model has real dynamics, including rotation of the magnetic degrees of freedom, going beyond the Ising-type models of spin ice. The dynamics is studied using a Langevin equation solved via a second order Heun algorithm. Thermodynamic properties such as the specific heat are presented for different couplings. A peak in specific heat is related to a type of melting-like phase transition present in the model. Hysteresis in an applied magnetic field is calculated for model parameters where the system is able to reach thermodynamic equilibrium.Comment: Revised versio

Large-Scale Simulations of Clusters of Galaxies

We discuss some of the computational challenges encountered in simulating the evolution of clusters of galaxies. Eulerian adaptive mesh refinement (AMR) techniques can successfully address these challenges but are currently being used by only a few groups. We describe our publicly available AMR code, FLASH, which uses an object-oriented framework to manage its AMR library, physics modules, and automated verification. We outline the development of the FLASH framework to include collisionless particles, permitting it to be used for cluster simulation.Comment: 3 pages, 3 figures, to appear in Proceedings of the VII International Workshop on Advanced Computing and Analysis Techniques in Physics Research (ACAT 2000), Fermilab, Oct. 16-20, 200

Critical Behavior and Lack of Self Averaging in the Dynamics of the Random Potts Model in Two Dimensions

We study the dynamics of the q-state random bond Potts ferromagnet on the square lattice at its critical point by Monte Carlo simulations with single spin-flip dynamics. We concentrate on q=3 and q=24 and find, in both cases, conventional, rather than activated, dynamics. We also look at the distribution of relaxation times among different samples, finding different results for the two q values. For q=3 the relative variance of the relaxation time tau at the critical point is finite. However, for q=24 this appears to diverge in the thermodynamic limit and it is ln(tau) which has a finite relative variance. We speculate that this difference occurs because the transition of the corresponding pure system is second order for q=3 but first order for q=24.Comment: 9 pages, 13 figures, final published versio