Slowdown of interpenetration of two counterpropagating plasma slabs due to collective effects.

The nonlinear evolution of electromagnetic instabilities driven by the interpenetration of two e^{-},e^{+} plasma clouds is explored using ab initio kinetic plasma simulations. We show that the plasma clouds slow down due to both oblique and Weibel generated electromagnetic fields, which deflect the particle trajectories, transferring bulk forward momentum into transverse momentum and thermal velocity spread. This process causes the flow velocity v_{inst} to decrease approximately by a factor of sqrt[1/3] in a time interval Δt_{αB}ω_{p}∼c/(v_{fl}sqrt[α_{B}]), where α_{B} is the magnetic equipartition parameter determined by the nonlinear saturation of the instabilities, v_{fl} is the initial flow speed, and ω_{p} is the plasma frequency. For the α_{B} measured in our simulations, Δt_{αB} is close to 10 times the instability growth time. We show that as long as the plasma slab length L>v_{fl}Δt_{αB}, the plasma flow is expected to slow down by a factor close to sqrt[1/3].

High-order harmonic generation in an electron-positron-ion plasma.

The laser interaction with an electron-positron-ion mixed plasma is studied from the perspective of the associated high-order harmonic generation. For an idealized mixed plasma which is assumed with a sharp plasma-vacuum interface and uniform density distribution, when it is irradiated by a weakly relativistic laser pulse, well-defined signals at harmonics of the plasma frequency in the harmonic spectrum are observed. These characteristic signals are attributed to the inverse two-plasmon decay of the counterpropagating monochromatic plasma waves which are excited by the energetic electrons and the positron beam accelerated by the laser. Particle-in-cell simulations show the signal at twice the plasma frequency can be observed for a pair density as low as ∼10^{-5} of the plasma density. In the self-consistent scenario of pair production by an ultraintense laser striking a solid target, particle-in-cell simulations, which account for quantum electrodynamic effects (photon emission and pair production), show that dense (greater than the relativistically corrected critical density) and hot pair plasmas can be created. The harmonic spectrum shows weak low-order harmonics, indicating a high laser absorption due to quantum electrodynamic effects. The characteristic signals at harmonics of the plasma frequency are absent, because broadband plasma waves are excited due to the high plasma inhomogeneity introduced by the interaction. However, the high-frequency harmonics are enhanced due to the high-frequency modulations from the direct laser coupling with created pair plasmas.

Fully kinetic Biermann battery and associated generation of pressure anisotropy.

The dynamical evolution of a fully kinetic, collisionless system with imposed background density and temperature gradients is investigated analytically. The temperature gradient leads to the generation of temperature anisotropy, with the temperature along the gradient becoming larger than that in the direction perpendicular to it. This causes the system to become unstable to pressure anisotropy driven instabilities, dominantly to the electron Weibel instability. When both density and temperature gradients are present and nonparallel to each other, we obtain a Biermann-like linear-in-time magnetic field growth. Accompanying particle-in-cell numerical simulations are shown to confirm our analytical results.

Magnetic-field generation and amplification in an expanding plasma.

Particle-in-cell simulations are used to investigate the formation of magnetic fields B in plasmas with perpendicular electron density and temperature gradients. For system sizes L comparable to the ion skin depth d(i), it is shown that B ∼ d(i)/L, consistent with the Biermann battery effect. However, for large L/d(i), it is found that the Weibel instability (due to electron temperature anisotropy) supersedes the Biermann battery as the main producer of B. The Weibel-produced fields saturate at a finite amplitude (plasma ß ≈ 100), independent of L. The magnetic energy spectra below the electron Larmor radius scale are well fitted by the power law with slope -16/3, as predicted by Schekochihin et al. [Astrophys. J. Suppl. Ser. 182, 310 (2009)].

Standards for the electronic health record, emerging from health care's Tower of Babel.

This paper considers the standardization of an Electronic Health Record (EHR). Relations between several distinct medical datasets and information systems are mapped in order to derive a more precise definition of the EHR. Two international efforts to establish standards for the EHR are presented and critiqued. Strategies for standardizing the EHR are analyzed and recommendations are provided for approaching the standardization process.

Barriers to evaluation of clinical vocabularies.

BACKGROUND: The importance of standard vocabularies for representing clinical data is widely accepted. The selection of suitable vocabularies for a given task relies upon criteria for systematically differentiating vocabularies. However, methods and measures for comparing vocabularies are only now emerging. Substantial barriers inhibit progress in the development of these measures. METHOD: As part of a larger project, the authors are developing quantitative measures for characteristics of controlled vocabulary as identified in the literature. RESULTS: Seven barriers have been identified that inhibit the development of quantitative measures of the characteristics of controlled vocabularies: 1) application dependence; 2) empirical v. independent assessment; 3) dichotomous v. continuous measures of characteristics; 4) poor definition of characteristics; 5) number of characteristics; 6) multiple levels of significance of characteristics; and 7) interdependence of characteristics. CONCLUSION: Progress toward quantitative assessment of clinical vocabularies is dependent upon overcoming barriers to the development of appropriate measures. Such progress requires innovative solutions and collaboration among investigators.

Extracting medical knowledge for a coded problem list vocabulary from the UMLS Knowledge Sources.

INTRODUCTION: The Unified Medical Language System (UMLS) Knowledge Sources embody a rich source of medical knowledge. We sought to extract a portion of this knowledge by incorporating information about relationships between UMLS concepts into an existing problem list vocabulary. METHODS: We matched terms from the coded problem list of The Medical Record (TMR), a computer-based patient record system, with those found in the UMLS Metathesaurus. Those UMLS concepts that participate in 'parent' relationships with the matched TMR concepts were translated back into TMR codes and the relationship information was retained for integration into the coded problem list of TMR. RESULTS: Of the coded problems currently in use in TMR, 67% (1627/2436) could be matched by normalized string matches to the UMLS Knowledge Sources. Of these matched TMR concepts, 91% (1488/1627) participated in at least one UMLS-identified parent relationship but only 28% of the matched concepts (454/1627) participated in parent relationships that already matched to a TMR code. As a result, although 67% of TMR codes were matched to UMLS concepts, only 19% of our original problem list (454/2436) could be augmented by relationship information contained in UMLS without improving the rate of matches or adding additional UMLS concepts as coded problems in TMR. CONCLUSION: This study illustrates the rapid decline in overall rates of matching that result from a multiplicative effect of successive matches of terms to concepts, concepts to relationships and concepts back to entry terms. This effect will hamper any effort to extract relationship knowledge from the UMLS for incorporation into an entry vocabulary that is not already one of the source vocabularies of the UMLS Metathesaurus.

Acyclovir in pregnancy registry: six years' experience. The Acyclovir in Pregnancy Registry Advisory Committee.

The Acyclovir in Pregnancy Registry was established to gather data on prenatal exposure to acyclovir. Exposed pregnancies are tracked prospectively to ascertain exposure, risk factors, and pregnancy outcome. Through June 30, 1990, 312 acyclovir-exposed pregnancies had been reported and followed. Of these, 239 were exposed during the first trimester; outcomes included 24 spontaneous fetal losses, 47 induced abortions, 159 live births of infants without congenital abnormalities, and nine outcomes with congenital abnormalities. Among the 73 second- and third-trimester exposures, one infant was born with an abnormality. Exposures are also reported to the registry retrospectively, ie, after the outcome of pregnancy is known. Registry findings to date do not show an increase in the number of birth defects among the prospective reports when compared with that expected in the general population, and there is no consistent pattern of abnormalities among retrospective or prospective reports. These findings should provide some reassurance in counseling women following inadvertent prenatal exposure. The cases accumulated to date represent a sample of insufficient size for reaching reliable and definitive conclusions about the safety of acyclovir for pregnant women and their developing fetuses. Therefore, until further information is available, the Acyclovir in Pregnancy Registry Advisory Committee recommends following the 1989 Centers for Disease Control Sexually Transmitted Diseases Treatment Guidelines for the use of acyclovir in pregnancy, and encourages reporting of all prenatal exposures to the registry (1-800-722-9292, ext. 8465).