Rapid detection of incipient foil failure and remediation for longevity in repetitively pulsed, electron-beam-pumped excimer lasers for fusion research.

The use of an electron beam to pump an excimer laser has the advantage of being readily scalable to higher laser energies at high efficiency. Typically, a pulsed power driver generates the electron beam in a vacuum diode that consists of an electron emitter and a thin anode foil that holds the vacuum against the atmospheric-pressure laser gas. Even a miniscule leak in the anode foil can lead to an electrical breakdown in the vacuum diode, resulting in the destruction of the foil and evidence of the failure mechanism. The problem is even more onerous at the high voltage, high current, and pulse repetition frequencies needed for the large-area diodes used in excimer lasers for fusion research. Electra is one such laser used at the Naval Research Laboratory to develop excimer laser technologies for inertial fusion energy. To achieve longevity on Electra, it was necessary to instantly detect an incipient foil failure and halt the pulsed power drivers so the physical cause(s) could be studied. This rapid detection was accomplished using an optically filtered photodiode that senses the presence of argon emission from a Penning discharge vessel attached to the vacuum diodes. Details of this "Spectral Penning Leak Detector" device and its operation are presented. The diagnostics allowed the identification of a recurrent pinhole leak in the anode foil induced by cathode spots, which were created by electron emission from the foil during post-pulse voltage reversals. Eliminating the voltage reversals increased the continuous operation of the Electra laser from hundreds of shots to over 90 000 shots.

Direct drive with the argon fluoride laser as a path to high fusion gain with sub-megajoule laser energy.

Argon fluoride (ArF) is currently the shortest wavelength laser that can credibly scale to the energy and power required for high gain inertial fusion. ArF's deep ultraviolet light and capability to provide much wider bandwidth than other contemporary inertial confinement fusion (ICF) laser drivers would drastically improve the laser target coupling efficiency and enable substantially higher pressures to drive an implosion. Our radiation hydrodynamics simulations indicate gains greater than 100 are feasible with a sub-megajoule ArF driver. Our laser kinetics simulations indicate that the electron beam-pumped ArF laser can have intrinsic efficiencies of more than 16%, versus about 12% for the next most efficient krypton fluoride excimer laser. We expect at least 10% 'wall plug' efficiency for delivering ArF light to target should be achievable using solid-state pulsed power and efficient electron beam transport to the laser gas that was demonstrated with the U.S. Naval Research Laboratory's Electra facility. These advantages could enable the development of modest size and lower cost fusion power plant modules. This would drastically change the present view on inertial fusion energy as being too expensive and the power plant size too large. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 1)'.

Measurement of the energy distribution of an intense electron beam in an external magnetic field.

An energy analyzer device has been developed which utilizes a series of stacked foils and Rogowski current monitors to the measure time resolved current of an intense electron beam. The energy distribution of the electron beam is unfolded from measured current ratios using computer simulations. This device is particularly useful where electron beams are guided by external magnetic fields which may make other electron energy measurement techniques difficult. This technique was used to determine the energy distribution of a 550 keV, 95 kA electron beam as it propagates in the gas mixture of a high power KrF laser. The resulting energy distributions at various depths in the gas are in agreement with three-dimensional particle-in-cell simulations providing confidence in the measurement technique.

The transition to high school for academically promising, urban, low-income African American youth.

In nine urban Ohio school systems, low-income minority students identified as academically promising in sixth grade are eligible to participate in an intervention program. In the present study, twenty-two African American students in the program were asked to provide their perceptions of the transition to ninth grade. Specifically, the role of motivating factors, peers, school, teachers, parents, and neighborhood were examined. These students faced similar stressors, yet some were more able to achieve academic success. Results highlight the salience of mothers, the challenges of the ninth-grade curriculum, and adjustment to a bigger, more complex school environment for high and low performers. The implications for improving cooperation between school and family are discussed.

Remote village survey for agents causing hepatosplenic disease in the Republic of Yemen.

The objective of this study was to epidemiologically describe potential infectious agents among rural people in the Republic of Yemen. This would aid clinicians in designing empirical therapy and public health officials in planning disease prevention. We sought to examine evidence for the geographical distribution of pathogens causing human hepatic and splenic disease among villagers and domestic animals living in three remote areas with differing altitudes. In June 1992, a cross-sectional survey was conducted at three survey sites of differing altitudes: 3080, 1440 and 250 m above sea level. Questionnaires, parasitic and serological tests were administered to 627 human volunteers. Additionally 317 domestic animals were studied. Malaria, schistosomiasis, and hepatitis B and C infections were found to be likely causes of human hepatic or splenic disease. Additionally, evidence of human and animal infections with the agents of brucellosis and Q fever was found: IgG antibodies against hepatitis E virus were discovered in two (2.0%) of the 100 volunteers. The prevalence of markers for human and animal disease was often lowest at the village of highest elevation, suggesting that increasing altitude, as a surrogate or a true independent risk factor, was protective against infection with the agents studied.

Analytical electron microscopy of chlorhexidine-induced tooth stain in humans: direct evidence for metal-induced stain.

Using analytical electron microscopy, we directly tested the hypothesis that transition metals and sulfur are the cause of chlorhexidine-induced tooth stain. Plaque scraped from the teeth of water-treated individuals or chlorhexidine-treated "non-stainers" was non- or lightly-stained and contained low concentrations of sulfur and transition metals. Tooth scrapings from heavily-staining chlorhexidine-treated individuals consisted of distinct unstained and stained regions. The stained regions were organic but were in close proximity to mineralized areas. Enhanced levels of sulfur and transition metals, particularly iron, were found in stained regions, whereas unstained regions contained low sulfur and metal levels similar to the water-treated or non-staining individuals. Excluding decreased mineralization, the major elemental change in heavily-stained plaque was an increase in sulfur, and to a lesser extent, iron and other transition metals. Sulphur and iron levels were directly correlated. Following chlorhexidine treatment with iron supplementation, staining was enhanced, the Fe/S ratio increased, and sulfur and iron remained correlated. These data support the hypothesis that transition metals and sulfur are the cause of chlorhexidine-induced tooth stain. The data are consistent with chlorhexidine treatment altering the incorporation into plaque of a natural sulfur-containing organic component of saliva or bacteria. This natural component appears to readily interact with transition metals, particularly iron, producing stained material.

Electron probe analysis of human skin: determination of the water concentration profile.

The water concentration profile across rapidly frozen human epidermis has been measured using electron probe analysis and analytical electron microscopy. Determinations were made within the cytoplasm of individual cells. From the basal layer, the water content remained relatively constant or decreased slightly across the viable tissue and decreased approximately linearly across the stratum corneum. A considerable discontinuity in water content occurred between these two regions over the stratum corneum-stratum granulosum junction and the last granular cell layer. The dominance of the water profile by a discontinuity suggests water loss is governed by a partitioning process, presumably a partitioning into the lipid domain. A water discontinuity offers important functional advantages in the conservation of substances within the body and in protection from intruding molecules.

Electron probe analysis of human skin: element concentration profiles.

Concentration profiles for the major biological inorganic elements Na, P, S, Cl, and K were measured across human skin using electron probe analysis and analytical electron microscopy. Determinations were made within the cytoplasm of individual cells. Uniform element concentrations were present throughout the viable tissue, whereas element profiles in the stratum corneum were considerably diverse. Phosphorus was practically absent from the stratum corneum. Sulfur (per unit volume) continuously increased from the inner to the outer cell layers of the stratum corneum largely as a result of cytoplasmic water loss as cells migrate to the surface. Potassium was essentially excluded from the inner stratum corneum. Very large gradients for K, Na, and Cl occurred from the middle of the stratum corneum to its outer surface; these gradients are likely the result of the inward diffusion of salts from sweat and could play a variety of physiological roles. The paucity of K and P within the inner stratum corneum suggests these important intracellular solutes (and perhaps others, including water) are recycled within the viable tissue, thus providing a virtual nutrient supply immediately underneath the stratum corneum. Alterations in this recirculation could have a regulatory function in the physiology of this tissue.

Reversibility of nephrotoxicity induced in rats by nitrilotriacetate in subchronic feeding studies.

The reversibility of nitrilotriacetate (NTA)-associated nephrotoxicity was investigated by comparing renal tissues from rats fed nephrotoxic levels of NTA for 7 wk with those from rats allowed 5 wk of recovery after the 7-wk exposure. In addition the toxicity of 2% Na3NTA X H2O in the diet (73 mumol/g diet) was compared with that of 1.5% H3NTA (79 mumol/g diet). The two forms of NTA induced comparable renal tubular cell toxicity which was characterized by proximal convoluted cell vacuolation and hyperplasia. These effects were noted in all of the exposed animals although the extent of damage varied. This specific renal tubular cell toxicity was completely reversed during the 5-wk recovery period. Renal pelvic transitional cell toxicity was induced primarily by Na3NTA X H2O. Renal pelvic toxicity was characterized by hydronephrosis, and erosion, ulceration and hyperplasia of the transitional epithelium. All forms of renal toxicity except that accompanying hydronephrosis were reversed when Na3NTA X H2O feeding was discontinued.