No abstract

Measurements of particle and heat transport have been made in the Helically Symmetric Experiment [F. S. B. Anderson et al., Fusion Technology 27, 273 (1995)]. Experimental differences in the density and temperature profiles are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The electron temperature is higher in the QHS configuration, due to a reduction in electron thermal diffusivity that is comparable to the neoclassical prediction. The density profile in plasmas with the symmetry broken is measured to be hollow, while in QHS plasmas the profile is centrally peaked. Calculations of the radial particle flux using the DEGAS code [D. Heifetz et al., J. Comput. Phys. 46, 309 (1982)] show that the hollow profile observed with the symmetry broken is due to neoclassical thermodiffusion. Thermodiffusion is reduced in the QHS configuration, resulting in a peaked density profile.

No abstract

No abstract

and Technology was established in 1988 by Congress to "assist industry in the development of technology . . . needed to improve product quality, to modernize manufacturing processes, to ensure product reliability . . . and to facilitate rapid commercialization . . . of products based on new scientific discoveries."NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S. industry's competitiveness; advance science and engineering; and improve public health, safety, and the environment. One of the agency's basic functions is to develop, maintain, and retain custody of the national standards of measurement, and provide the means and methods for comparing standards used in science, engineering, manufacturing, commerce, industry, and education with the standards adopted or recognized by the Federal Government.As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic and applied research in the physical sciences and engineering, and develops measurement techniques, test methods, standards, and related services. The Institute does generic and precompetitive work on new and advanced technologies. NIST's research facilities are located at Gaithersburg, MD 20899, and at Boulder, CO 80303. For more information contact the Public Inquiries Desk, 301-975-3058. T Cover:The figure on the cover depicts a time sequence of images showing one cycle of the ringing of a Bose-Einstein condensate (BEC) in the JILA TOP (time-averaged orbiting potential) trap after being driven by strong oscillations of trap potential. The top left image shows the earliest time, with time increasing from left to right. To create each image, a BEC was formed and then strongly driven and finally allowed to expand ballistically, after which a picture was taken. BEC excitations have recently been observed in both the JILA [D. S. Jin et al., Phys. Rev. Lett. 77, 420 (1996)] and MIT [M.-O. Mewes et al., Phys. Rev. Lett. 77, 988 (1996)] traps. The ringing frequencies of a BEC for weak driving agree well with zero-temperature mean-field theory. Cover photographs courtesy Eric Cornell, JILA, Boulder, CO 80309. Bose-Einstein Condensation PrefaceAmong the most remarkable effects that quantum mechanics adds to the catalog of the thermal properties of matter is the "condensation" of an ideal gas of identical particles into a single quantum state, the principle of which was discovered in the theory of statistical mechanics by Bose and Einstein in the 1920s.Bose-Einstein condensation (BEC) is thus a mechanism for producing a macroscopic quantum system, and is exemplary of the macroscopic quantum phenomena of superconductivity and superfluidity. However, it has proven most difficult to attain BEC in the laboratory vs in the theory of the ideal gas. As discussed in the first article of this Special Issue, the basic criterion for BEC is that the typical distance between particles be comparable to or less than their deBroglie wavelengths; and in virtually every real material system, the effects of...