Electrode boundary layers in direct-current plasma accelerators

One of the problems that must be faced in the development of direct-current plasma accelerators is that of boundary-layer growth on the electrode surfaces. These surfaces must be maintained at a somewhat lower temperature than is desirable in the bulk of the gas flow. The associated reduction in electrical conductivity near the electrode surface, together with the continuous current through the boundary layer, may result in greatly augmented Joule heating near the surface, and increased heat transfer. This phenomenon is treated within the framework of boundary layer theory. It is found that similar solutions for the thermal and viscous boundary layers exist for a certain class of accelerated flows in which the velocity varies as a power of the streamwise coordinate. The solutions show that the heat-transfer rate at Mach numbers near unity may be as much as ten times that which would be expected for a normal boundary layer. At higher Mach numbers, the similarity is not precisely valid; however, the analysis indicates qualitatively that a stagnation enthalpy overshoot may occur in the high-temperature portion of the boundary layer as a result of the electromagnetic acceleration

The Statistical Mechanics of Dynamic Pathways to Self-assembly

We describe some of the important physical characteristics of the `pathways', i.e. dynamical processes, by which molecular, nanoscale and micron-scale self-assembly occurs. We highlight the fact that there exist features of self-assembly pathways that are common to a wide range of physical systems, even though those systems may be different in respect of their microscopic details. We summarize some existing theoretical descriptions of self-assembly pathways, and highlight areas -- notably, the description of self-assembly pathways that occur `far' from equilibrium -- that are likely to become increasingly important.Comment: To appear in Annual Review of Physical Chemistr

The Law\u27s Mystery

What is the continuing significance of Cohen v. California, the 1971 U.S. Supreme Court decision holding that “Fuck the Draft” is a message protected by the First Amendment? Using Cohen as an exemplar, this article offers a new theory about how to understand the law and judicial opinions. The theory begins in a recognition of the “law” as resting upon mystery and uncertainty, a mystery that is also the source of the law’s enchantment. It is this enchantment that we depend upon for the law to be authoritative rather than authoritarian and reducible to the political and thus to power. In simple terms, the mystery of the law—its being beyond us in this way—constitutes its legitimate authority over us. The law that discloses itself to us does so through the openings that language provides. For our culture, judicial opinions are its primary way of doing this. Having introduced the theory, the article applies it, exploring whether it is possible to bring to the surface the tracings of a “great” judicial performance, using “great” in the sense of revealing an opening through which the law discloses itself. This section describes a reading of Cohen that aims to discover whether through the performance of the opinion, its author has uncovered something that is “of the essence” of our community. The article finally raises questions about what it would mean to legal education and law practice if judicial opinions were evaluated without destroying the law’s mystery. What would it mean if we thought of judges as preservers of this mystery? What would it mean if readers of opinions started thinking in terms of their own experience of the opinion rather than as critics of it? And what would it mean if lawyers saw their task as related to “truth”

Evidence for a disordered critical point in a glass-forming liquid

Using computer simulations of an atomistic glass-forming liquid, we investigate the fluctuations of the overlap between a fluid configuration and a quenched reference system. We find that large fluctuations of the overlap develop as temperature decreases, consistent with the existence of the random critical point that is predicted by effective field theories. We discuss the scaling of fluctuations near the presumed critical point, comparing the observed behaviour with that of the random-field Ising model. We argue that this critical point directly reveals the existence of an interfacial tension between amorphous metastable states, a quantity relevant both for equilibrium relaxation and for nonequilibrium melting of stable glass configurations.Comment: 4 figs, 5 page

Flow instability in particle-bed nuclear reactors

The particle-bed core offers mitigation of some of the problems of solid-core nuclear rocket reactors. Dividing the fuel elements into small spherical particles contained in a cylindrical bed through which the propellant flows radially, may reduce the thermal stress in the fuel elements, allowing higher propellant temperatures to be reached. The high temperature regions of the reactor are confined to the interior of cylindrical fuel assemblies, so most of the reactor can be relatively cool. This enables the use of structural and moderating materials which reduce the minimum critical size and mass of the reactor. One of the unresolved questions about this concept is whether the flow through the particle-bed will be well behaved, or will be subject to destructive flow instabilities. Most of the recent analyses of the stability of the particle-bed reactor have been extensions of the approach of Bussard and Delauer, where the bed is essentially treated as an array of parallel passages, so that the mass flow is continuous from inlet to outlet through any one passage. A more general three dimensional model of the bed is adopted, in which the fluid has mobility in three dimensions. Comparison of results of the earlier approach to the present one shows that the former does not accurately represent the stability at low Re. The more complete model presented should be capable of meeting this deficiency while accurately representing the effects of the cold and hot frits, and of heat conduction and radiation in the particle-bed. It can be extended to apply to the cylindrical geometry of particle-bed reactors without difficulty. From the exemplary calculations which were carried out, it can be concluded that a particle-bed without a cold frit would be subject to instability if operated at the high temperatures desired for nuclear rockets, and at power densities below about 4 megawatts per liter. Since the desired power density is about 40 megawatts per liter, it can be concluded that operation at design exit temperature but at reduced power could be hazardous for such a reactor. But the calculations also show that an appropriate cold frit could very likely cure the instability. More definite conclusions must await calculations for specific designs

Profile of Hired Farmworkers, 1996 Annual Averages

Examines demographic and employment characteristics of the 906,000 persons 15 years of age and older who did hired farmwork in 1996. Approximately 906,000 persons 15 years of age and older were employed as hired farmworkers each week in 1996. An additional 72,000 persons were hired as farmworkers each week as a secondary job. Hired farmworkers were more likely than all U.S. wage and salary workers to be male, Hispanic, younger, less educated, never married, and non-U.S. citizens.hired farmworkers, annual averages, demographic characteristics, hourse worked, median weekly hours, Labor and Human Capital,

Diagonalizing the genome II: toward possible applications

In a previous paper, we showed that the orientable cover of the moduli space of real genus zero algebraic curves with marked points is a compact aspherical manifold tiled by associahedra, which resolves the singularities of the space of phylogenetic trees. In this draft of a sequel, we construct a related (stacky) resolution of a space of real quadratic forms, and suggest, perhaps without much justification, that systems of oscillators parametrized by such objects may may provide useful models in genomics.Comment: 11 pages, 3 figure