Tungsten and barium transport in the internal plasma of hollow cathodes

The effect of tungsten erosion, transport, and redeposition on the operation of dispenser hollow cathodes was investigated in detailed examinations of the discharge cathode inserts from 8200 h and 30 352 h ion engine wear tests. Erosion and subsequent redeposition of tungsten in the electron emission zone at the downstream end of the insert reduce the porosity of the tungsten matrix, preventing the flow of barium from the interior. This inhibits the interfacial reactions of the barium-calcium-aluminate impregnant with the tungsten in the pores. A numerical model of barium transport in the internal xenon discharge plasma shows that the barium required to reduce the work function in the emission zone can be supplied from upstream through the gas phase. Barium that flows out of the pores of the tungsten insert is rapidly ionized in the xenon discharge and pushed back to the emitter surface by the electric field and drag from the xenon ion flow. This barium ion flux is sufficient to maintain a barium surface coverage at the downstream end greater than 0.6, even if local barium production at that point is inhibited by tungsten deposits. The model also shows that the neutral barium pressure exceeds the equilibrium vapor pressure of the impregnant decomposition reaction over much of the insert length, so the reactions are suppressed. Only a small region upstream of the zone blocked by tungsten deposits is active and supplies the required barium. These results indicate that hollow cathode failure models based on barium depletion rates in vacuum dispenser cathodes are very conservative

An injection and mixing element for delivery and monitoring of inhaled nitric oxide

Background Inhaled nitric oxide (NO) is a selective pulmonary vasodilator used primarily in the critical care setting for patients concurrently supported by invasive or noninvasive positive pressure ventilation. NO delivery devices interface with ventilator breathing circuits to inject NO in proportion with the flow of air/oxygen through the circuit, in order to maintain a constant, target concentration of inhaled NO. Methods In the present article, a NO injection and mixing element is presented. The device borrows from the design of static elements to promote rapid mixing of injected NO-containing gas with breathing circuit gases. Bench experiments are reported to demonstrate the improved mixing afforded by the injection and mixing element, as compared with conventional breathing circuit adapters, for NO injection into breathing circuits. Computational fluid dynamics simulations are also presented to illustrate mixing patterns and nitrogen dioxide production within the element. Results Over the range of air flow rates and target NO concentrations investigated, mixing length, defined as the downstream distance required for NO concentration to reach within ±5 % of the target concentration, was as high as 47 cm for the conventional breathing circuit adapters, but did not exceed 7.8 cm for the injection and mixing element. Conclusion The injection and mixing element has potential to improve ease of use, compatibility and safety of inhaled NO administration with mechanical ventilators and gas delivery devices

Variability in uptake efficiency for pulsed versus constant concentration delivery of inhaled nitric oxide

BACKGROUND: Nitric oxide (NO) is currently administered using devices that maintain constant inspired NO concentrations. Alternatively, devices that deliver a pulse of NO during the early phase of inspiration may have use in optimizing NO dosing efficiency and in extending application of NO to long-term use by ambulatory, spontaneously breathing patients. The extent to which the amount of NO delivered for a given pulse sequence determines alveolar concentrations and uptake, and the extent to which this relationship varies with breathing pattern, physiological, and pathophysiological parameters, warrants investigation. METHODS: A mathematical model was used to analyze inhaled nitric oxide (NO) transport through the conducting airways, and to predict uptake from the alveolar region of the lung. Pulsed delivery was compared with delivery of a constant concentration of NO in the inhaled gas. RESULTS: Pulsed delivery was predicted to offer significant improvement in uptake efficiency compared with constant concentration delivery. Uptake from the alveolar region depended on pulse timing, tidal volume, respiratory rate, lung and dead space volume, and the diffusing capacity of the lung for NO (D(L)NO). It was predicted that variation in uptake efficiency with breathing pattern can be limited using a pulse time of less than 100 ms, with a delay of less than 50 ms between the onset of inhalation and pulse delivery. Nonlinear variation in uptake efficiency with D(L)NO was predicted, with uptake efficiency falling off sharply as D(L)NO decreased below ~50-60 ml/min/mm Hg. Gas mixing in the conducting airways played an important role in determining uptake, such that consideration of bulk convection alone would lead to errors in assessing efficiency of pulsed delivery systems. CONCLUSIONS: Pulsed NO delivery improves uptake efficiency compared with constant concentration delivery. Optimization of pulse timing is critical in limiting intra- and inter-subject variability in dosing

Antigen Presentation and Allogeneic Stimulation by Langerhans Cells

Isolated Langerhans cells were studied for 2 immunologic functions, the ability to present antigen to sensitized T lymphocytes and the ability to act as stimulator cells for mixed lymphocyte reactions. Langerhans cells can perform both of these functions. This fact, with the previous finding that Langerhans cells possess surface la antigens and Fe and C3 receptors, strongly suggests that Langerhans cells act as epidermal macrophages

The effect of cathode geometry on barium transport in hollow cathode plasmas

The effect of barium transport on the operation of dispenser hollow cathodes was investigated in numerical modeling of a cathode with two different orifice sizes. Despite large differences in cathode emitter temperature, emitted electron current density, internal xenon neutral and plasma densities, and size of the plasma-surface interaction region, the barium transport in the two geometries is qualitatively very similar. Barium is produced in the insert and flows to the surface through the porous structure. A buildup of neutral Ba pressure in the plasma over the emitter surface can suppress the reactions supplying the Ba, restricting the net production rate. Neutral Ba flows into the dense Xe plasma and has a high probability of being ionized at the periphery of this zone. The steady state neutral Ba density distribution is determined by a balance between pressure gradient forces and the drag force associated with collisions between neutral Ba and neutral Xe atoms. A small fraction of the neutral Ba is lost upstream. The majority of the neutral Ba is ionized in the high temperature Xe plasma and is pushed back to the emitter surface by the electric field. The steady state Ba^+ ion density distribution results from a balance between electrostatic and pressure forces, neutral Xe drag and Xe^+ ion drag with the dominant forces dependent on location in the discharge. These results indicate that hollow cathodes are very effective at recycling Ba within the discharge and therefore maintain a high coverage of Ba on the emitter surface, which reduces the work function and sustains high electron emission current densities at moderate temperatures. Barium recycling is more effective in the cathode with the smaller orifice because the Ba is ionized in the dense Xe plasma concentrated just upstream of the orifice and pushed back into the hollow cathode. Despite a lower emitter temperature, the large orifice cathode has a higher Ba loss rate through the orifice because the Xe plasma density peaks further upstream

Barium depletion in hollow cathode emitters

Dispenser hollow cathodes rely on a consumable supply of Ba released by BaO-CaO-Al_2O_3 source material in the pores of a tungsten matrix to maintain a low work function surface. The examination of cathode emitters from long duration tests shows deposits of tungsten at the downstream end that appear to block the flow of Ba from the interior. In addition, a numerical model of Ba transport in the cathode plasma indicates that the Ba partial pressure in the insert may exceed the equilibrium vapor pressure of the dominant Ba-producing reaction, and it was postulated previously that this would suppress Ba loss in the upstream part of the emitter. New measurements of the Ba depletion depth from a cathode insert operated for 8200 h reveal that Ba loss is confined to a narrow region near the downstream end, confirming this hypothesis. The Ba transport model was modified to predict the depletion depth with time. A comparison of the calculated and measured depletion depths gives excellent qualitative agreement, and quantitative agreement was obtained assuming an insert temperature 70 °C lower than measured beginning-of-life values

Substance use disorders and the risk of suicide mortality among men and women in the US Veterans Health Administration

Background and AimsLimited information is available regarding links between specific substance use disorders (SUDs) and suicide mortality; however, the preliminary evidence that is available suggests that suicide risk associated with SUDs may differ for men and women. This study aimed to estimate associations between SUDs and suicide for men and women receiving Veterans Health Administration (VHA) care.DesignA cohort study using national administrative health records.SettingNational VHA system, USA.ParticipantsAll VHA users in fiscal year (FY) 2005 who were alive at the beginning of FY 2006 (n = 4 863 086).MeasurementsThe primary outcome of suicide mortality was assessed via FY 2006–2011 National Death Index (NDI) records. Current SUD diagnoses were the primary predictors of interest, and were assessed via FY 2004–2005 VHA National Patient Care Database (NPCD) records.FindingsIn unadjusted analyses, a diagnosis of any current SUD and the specific current diagnoses of alcohol, cocaine, cannabis, opioid, amphetamine and sedative use disorders were all associated significantly with increased risk of suicide for both males and females [hazard ratios (HRs)] ranging from 1.35 for cocaine use disorder to 4.74 for sedative use disorder for men, and 3.89 for cannabis use disorder to 11.36 for sedative use disorder for women]. Further, the HR estimates for the relations between any SUD, alcohol, cocaine and opioid use disorders and suicide were significantly stronger for women than men (P < 0.05). After adjustment for other factors, most notably comorbid psychiatric diagnoses, associations linking SUDs with suicide were attenuated markedly and the greater suicide risk among females was observed for only any SUD and opioid use disorder (P < 0.05).ConclusionsCurrent substance use disorders (SUDs) signal increased suicide risk, especially among women, and may be important markers to consider including in suicide risk assessment strategies. None the less, other co‐occurring psychiatric disorders may partially explain associations between SUDs and suicide, as well as the observed excess suicide risk associated with SUDs among women.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137620/1/add13774.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137620/2/add13774_am.pd

Plasma Heating of Inert Gas Hollow Cathode Inserts

It is shown that in hollow cathodes with a very small-diameter orifice, operating at a low current, the plasma density peaks inside the orifice, and the cathode is heated primarily by plasma bombardment in the orifice and along the orifice plate. As the orifice diameter increases, the peak plasma density moves upstream of the orifice, and ion and electron bombardment heat both the orifice plate and the insert. In hollow cathodes with a large-diameter orifice the plasma extends along much of the insert, the plasma density peaks well within the insert region, and the cathode is heated primarily by ion bombardment of the insert

Ion Engine Plume Interaction Calculations for Prototypical Prometheus 1

Prometheus 1 is a conceptual mission to demonstrate the use of atomic energy for distant space missions. The hypothetical spacecraft design considered in this paper calls for multiple ion thrusters, each with considerably higher beam energy and beam current than have previously flown in space. The engineering challenges posed by such powerful thrusters relate not only to the thrusters themselves, but also to designing the spacecraft to avoid potentially deleterious effects of the thruster plumes. Accommodation of these thrusters requires good prediction of the highest angle portions of the main beam, as well as knowledge of clastically scattered and charge exchange ions, predictions for grid erosion and contamination of surfaces by eroded grid material, and effects of the plasma plume on radio transmissions. Nonlinear interactions of multiple thrusters are also of concern. In this paper we describe two- and three-dimensional calculations for plume structure and effects of conceptual Prometheus 1 ion engines. Many of the techniques used have been validated by application to ground test data for the NSTAR and NEXT ion engines. Predictions for plume structure and possible sputtering and contamination effects will be presented