Triple Langmuir Probes Measurements of LaB6 Hollow Cathodes Plume

Lanthanum hexaboride hollow cathodes represent a viable option for high power Hall effect thruster applications, under development for the next generation of manned and robotic interplanetary missions. In this scenario, SITAEL and the University of Pisa are actively developing high current hollow cathodes capable of providing discharge current in the range 10–100 A to be coupled with high power Hall effect thrusters. The cathode design is based on an in-house theoretical model of the internal sections of the cathode, recently integrated with a simplified model of the cathode plume. Despite the application of hollow cathodes on flight and laboratory model Hall effect thrusters, many questions remain unsolved. In particular, issues related to onset of instabilities, due to plume mode or ion acoustic turbulence, are still unclear, while it is known that they can affect the overall performance of the cathode and thruster unit. This paper focuses on the experimental investigation of the cathode plume by means of measurements of the main plasma parameters, at different operating conditions and for different cathode geometry. Two cathodes were investigated, namely HC20 and HC60, designed to be coupled with SITAEL's HT5k and HT20k (5 kW- and 20 kW-class) Hall effect thrusters. The cathodes were mounted in stand-alone configuration with an auxiliary cylindrical anode. The experimental campaign was performed using triple Langmuir probes as plasma diagnostic system. The probes were mounted on scanning mechanisms to measure the plume parameters at various radial and axial distances from the keeper exit. General trends of electron temperature, plasma potential and plasma density are reported in terms of discharge current, mass flow rate and cathode orifice geometry. The results highlight that the cathode plate orifice selection affects the plume mode onset, giving the possibility to extend the stable mode of cathode operation in the current range required by the thruster

Au2phen and Auoxo6, Two Dinuclear Oxo-Bridged Gold({III}) Compounds, Induce Apoptotic Signaling in Human Ovarian A2780 Cancer Cells

Au(2)phen ((2,9-dimethyl-1,10-phenanthroline)(2)Au(2)(µ-O)(2))(PF(6))(2) and Auoxo6 ((6,6′-dimethyl-2,2′-bipyridine)(2)Au(2)(µ-O)(2))(PF(6))(2) are two structurally related gold(III) complexes that were previously reported to display relevant and promising anticancer properties in vitro toward a large number of human cancer cell lines. To expand the knowledge on the molecular mechanisms through which these gold(III) complexes trigger apoptosis in cancer cells, further studies have been performed using A2780 ovarian cancer cells as reference models. For comparative purposes, parallel studies were carried out on the gold(III) complex AuL12 (dibromo(ethylsarcosinedithiocarbamate)gold(III)), whose proapoptotic profile had been earlier characterized in several cancer cell lines. Our results pointed out that all these gold(III) compounds manifest a significant degree of similarity in their cellular and proapoptotic effects; the main observed perturbations consist of potent thioredoxin reductase inhibition, disruption of the cell redox balance, impairment of the mitochondrial membrane potential, and induction of associated metabolic changes. In addition, evidence was gained of the remarkable contribution of ASK1 (apoptosis-signal-regulating kinase-1) and AKT pathways to gold(III)-induced apoptotic signaling. Overall, the observed effects may be traced back to gold(III) reduction and subsequent formation and release of gold(I) species that are able to bind and inhibit several enzymes responsible for the intracellular redox homeostasis, in particular the selenoenzyme thioredoxin reductase

Protection from cigarette smoke-induced vascular injury by recombinant human relaxin-2 (serelaxin)

Smoking is regarded as a major risk factor for the development of cardiovascular diseases (CVD). This study investigates whether serelaxin (RLX, recombinant human relaxin‐2) endowed with promising therapeutic properties in CVD, can be credited of a protective effect against cigarette smoke (CS)‐induced vascular damage and dysfunction. Guinea pigs exposed daily to CS for 8 weeks were treated with vehicle or RLX, delivered by osmotic pumps at daily doses of 1 or 10 μg. Controls were non‐smoking animals. Other studies were performed on primary guinea pig aortic endothelial (GPAE) cells, challenged with CS extracts (CSE) in the absence and presence of 100 ng/ml (17 nmol/l) RLX. In aortic specimens from CS‐exposed guinea pigs, both the contractile and the relaxant responses to phenylephrine and acetylcholine, respectively, were significantly reduced in amplitude and delayed, in keeping with the observed adverse remodelling of the aortic wall, endothelial injury and endothelial nitric oxide synthase (eNOS) down‐regulation. RLX at both doses maintained the aortic contractile and relaxant responses to a control‐like pattern and counteracted aortic wall remodelling and endothelial derangement. The experiments with GPAE cells showed that CSE significantly decreased cell viability and eNOS expression and promoted apoptosis by sparkling oxygen free radical‐related cytotoxicity, while RLX counterbalanced the adverse effects of CSE. These findings demonstrate that RLX is capable of counteracting CS‐mediated vascular damage and dysfunction by reducing oxidative stress, thus adding a tile to the growing mosaic of the beneficial effects of RLX in CVD

Oxidative Modification of Fibrinogen Is Associated With Altered Function and Structure in the Subacute Phase of Myocardial Infarction.

Objective— Among plasma proteins, fibrinogen represents a major target of oxidative modifications. In patients with post–acute myocardial infarction (6 months after the acute event), fibrinogen oxidation-induced carbonyls and fibrinogen function were estimated using in vitro and ex vivo approaches. Fibrinogen structural features and clot architecture were also explored. Approach and Results— In 39 patients with post–acute myocardial infarction and 28 age-, sex-, and risk factor-matched controls, oxidative stress markers (in plasma and in purified fibrinogen fractions), thrombin-catalyzed fibrin polymerization, and plasmin-induced fibrin lysis were estimated. Circular dichroism spectra of purified fibrinogen extracts, electron microscopy, and differential interference contrast microscopy analyses of fibrin clots were also performed. Marked signs of oxidative stress in plasma ( P &lt;0.01 versus controls) and, correspondingly, an increased extent of fibrinogen carbonylation (3.5-fold over control values; P &lt;0.01 versus controls) were observed in patients. Furthermore, fibrinogen fractions purified from patients exhibited significantly reduced clotting ability and decreased susceptibility to plasmin-induced lysis ( P &lt;0.01 versus controls). Alterations in fibrinogen secondary structure, as suggested by circular dichroism spectroscopy, and in fibrin clot architecture, as analyzed by electron and differential interference contrast microscopy, were also identified. Conclusions— Here, we report for the first time that patients with post–acute myocardial infarction present with an overall imbalance in redox status and marked fibrinogen carbonylation associated with altered fibrinogen function, thus suggesting a role for carbonylation as a direct mechanism of fibrinogen function. The observed features occur along with modifications in protein structure and in clot architecture. </jats:sec

Cathode & Electromagnet Qualification Status and Power Processing Unit Development Update for the Ascendant Sub-kW Transcelestial Electric Propulsion System

A review of the component-level flight qualification efforts and power processing unit development status of the Ascendant Sub-kW Transcelestial Electric Propulsion System (ASTRAEUS) program is presented. Component-level qualification efforts were undertaken for the system’s ultra-compact heaterless LaB6 hollow cathode and electromagnets, both of which employ designs bespoke to ASTRAEUS, as they represent the highest failure risks for the thruster. Through parallel long-duration wear and ignition tests, the ASTRAEUS cathode demonstrated invariant discharge performance over more than 5000 h of operation at its maximum operating current of 4 A and demonstrated more than 25,000 ignition cycles. The ASTRAEUS electromagnets completed their environmental qualification through a demonstration of more than 1200 deep thermal cycles with no indication of coil degradation (the test articles previously completed qualification-level vibration and shock testing). ASTRAEUS’s prototype power processing unit has demonstrated more than 92% total power conversion efficiency and class-leading power density & specific power density of 4.5 W/cm3 & 1670 W/kg, respectively. The various power converters found in the ASTRAEUS power processing unit are reviewed with a focus on the methods by which such high performance was achieved

Different Antioxidant Efficacy of Two MnII-Containing Superoxide Anion Scavengers on Hypoxia/Reoxygenation-Exposed Cardiac Muscle Cells

Oxidative stress due to excess superoxide anion ([Formula: see text]) produced by dysfunctional mitochondria is a key pathogenic event of aging and ischemia-reperfusion diseases. Here, a new [Formula: see text]-scavenging MnII complex with a new polyamino-polycarboxylate macrocycle (4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diacetate) containing 2 quinoline units (MnQ2), designed to improve complex stability and cell permeability, was compared to parental MnII complex with methyls replacing quinolines (MnM2). MnQ2 was more stable than MnM2 (log K = 19.56(8) vs. 14.73(2) for the equilibrium Mn2+ + L2-, where L = Q2 and M2) due to the involvement of quinoline in metal binding and to the hydrophobic features of the ligand which improve metal desolvation upon complexation. As oxidative stress model, H9c2 rat cardiomyoblasts were subjected to hypoxia-reoxygenation. MnQ2 and MnM2 (10 μmol L-1) were added at reoxygenation for 1 or 2 h. The more lipophilic MnQ2 showed more rapid cell and mitochondrial penetration than MnM2. Both MnQ2 and MnM2 abated endogenous ROS and mitochondrial [Formula: see text], decreased cell lipid peroxidation, reduced mitochondrial dysfunction, in terms of efficiency of the respiratory chain and preservation of membrane potential (Δψ) and permeability, decreased the activation of pro-apoptotic caspases 9 and 3, and increased cell viability. Of note, MnQ2 was more effective than MnM2 to exert cytoprotective anti-oxidant effects in the short term. Compounds with redox-inert ZnII replacing the functional MnII were ineffective. This study provides clues which further our understanding of the structure-activity relationships of MnII-chelates and suggests that MnII-polyamino-polycarboxylate macrocycles could be developed as new anti-oxidant drugs

Plasma instabilities in the plume of high current hollow cathodes

La propulsione al plasma ad alta potenza è diventata una scelta sempre più attraente come sistema di propulsione principale per la prossima generazione di veicoli spaziali, costruiti per esplorare il sistema solare e oltre. Grazie alle elevate velocità di scarico del gas ionizzato e alla spinta elevata resa possibile dalla lavorazione di potenza molto alta, la propulsione elettrica può oggi consentire la possibilità di esplorazioni a lunga distanza e missioni cargo. I catodi cavi ad alta corrente sono le sorgenti di elettroni ampiamente utilizzate nella propulsione elettrica e nella generazione di plasma in laboratorio. Questi dispositivi sfruttano l’emissione termoionica da un materiale con bassa funzione di lavoro per emettere elettroni e sostenere la corrente di scarica richiesta dal sistema propulsivo. Nonostante la loro applicazione in propulsori spaziali dal 1970, molti aspetti che riguardano le operazioni a catodo non sono perfettamente compresi. In particolare, la regione del plasma appena al di fuori del catodo ad alta corrente è caratterizzata dalla presenza di molte fluttuazioni, che possono sviluppare strutture coerenti di plasma che influenzano il funzionamento e la vita operativa del catodo. Inoltre, le precedenti campagne sperimentali hanno evidenziato la presenza di ioni energetici nella regione del plasma esterno, che incrementano direttamente l'erosione delle superfici esterne e colpiscono la vita del catodo. Il progetto di ricerca qui descritto presenta un approccio sperimentale e teorico per studiare le oscillazioni e le instabilità del plasma all’esterno dei catodi cavi ad alta corrente sviluppati per applicazioni di propulsione elettrica nello spazio. In particolare, gli obiettivi di questo progetto comprendono la ricerca dell'insorgenza di instabilità di plasma in diverse condizioni di funzionamento, la loro evoluzione e l'eventuale saturazione, ed esplora la possibilità di una connessione tra la comparsa delle instabilità e la produzione di ioni energetici responsabili dello sputtering e dell'erosione superficiale. La prima parte del progetto di ricerca è stata dedicata all’attività sperimentale. Questo studio caratterizza le proprietà di plasma di due catodi cavi di corrente medio-alta, il primo disegnato per 5-100 A di corrente, mentre il secondo 25-350. L'indagine sperimentale prevede l'utilizzo di un set di diagnostica elettrostatica e magnetica (intrusivo e non). Le proprietà del plasma sono state caratterizzate sia per la mappatura dello stato stazionario che per le oscillazioni dipendenti dal tempo. In particolare, i parametri del plasma (temperatura degli elettroni, densità del plasma e potenziale del plasma) sono stati misurati nella regione all’esterno e all'interno del catodo. La produzione di ioni energetici è stata valutata misurando la funzione di distribuzione dell'energia degli ioni vicino alla superficie esterna del catodo. Le informazioni sulle onde di plasma sono state raccolte misurando le fluttuazioni sia elettrostatiche che elettromagnetiche con l'insieme dei sistemi diagnostici impiegati nell'indagine. Questi dati forniscono una caratterizzazione spaziotemporale delle onde generate. Le proprietà del plasma sono state misurate facendo funzionare i catodi al variare di corrente di scarica, portata di propellente e campo magnetico applicato esternamente. I risultati sperimentali di questa indagine rivelano la presenza di molte instabilità nella regione della plume del catodo a diverse condizioni operative. In particolare, questo plasma risulta essere influenzato da instabilità di ionizzazione che si sviluppano sotto forma di impulsi longitudinali quando il flusso di gas in entrata viene ridotto oltre una certa soglia, in assenza di campo magnetico esterno. La presenza di un campo magnetico assiale stabilizza questi modi longitudinali e favorisce invece lo sviluppo di fluttuazioni elicoidali con caratteristiche tipiche delle instabilità magnetoidrodinamici di tipo kink. La seconda parte del progetto di ricerca ha riguardato l’analisi teorica della instabilità azimutale trovata, con l’obiettivo di identificare la relazione di dispersione di tale modo. L'occorrenza di questi modi, la frequenza e il numero d'onda sono stati studiati applicando il modello magnetoidrodinamico ideale. In particolare, un modello fenomenologico basato su MHD ideale è stato adattato alla regione del plasma a catodo cavo, per predire i risultati sperimentali considerando la condizione limite di un anodo cilindrico caratterizzato dalla presenza di una guaina di plasma e non perfettamente conduttivo. I risultati sperimentali delle proprietà del modo MHD a diverse correnti di scarica e portata massica mostrano caratteristiche aggiuntive che potrebbero essere qualitativamente d'accordo con modi MHD resistivi. L’analisi teorica di questi modi con l’applicazione del modello di MHD resistivo rivelano la presenza inaspettata di un'instabilità elettromagnetica con caratteristiche di tipo resistive kink nella plume di catodi ad alta corrente che operano con un campo magnetico applicato esternamente. Inoltre, le misurazioni sperimentali forniscono evidenze di un collegamento tra la dinamica degli ioni ad alta energia e l'instabilità resistiva, suggerendo la possibilità di una natura comune di tali fenomeni. Questi risultati illuminano la fisica delle instabilità del plasma nella plume dei catodi e la produzione di ioni energetici, facilitando l'applicazione di catodi cavi ad alta corrente in missioni di volo di lunga durata

Thermal and Life Assessment of Lanthanum Hexaboride Hollow Cathodes for High Power Hall Effect Thrusters

Lanthanum Hexaboride (LaB6) hollow cathodes have been found in electric propulsion applications since the 1970’s. They demonstrated high reliability at discharge current level 1 - 10 A, in particular as main discharge providers in Hall effect thrusters. The next generation of high power electric propulsion thrusters requires higher level of discharge current, up to 300 A, and longer life, to be selected for the future manned deep space missions. This thesis project focused on LaB6 hollow cathodes, capable of maintaining discharge current in the range 5 - 300 A. The main goal is the experimental characterization of the behavior of plasma from the inside of the cathode varying the operative conditions of discharge current, propellant flow rate and orifice geometry. From the property of the plasma we can assess the thermal behavior of the cathode, in particular of the emitting surface, and use the thermal analysis to evaluate the cathode operative life. Experiments were conducted using laboratory model of the cathodes designed for the Hall Effect Rocket with Magnetic Shielding (HERMeS) thruster and the nested Hall thruster XR-100. An extensive characterization of the internal of the cathode was made using internal scanning Langmuir probes, which provide on-axis behavior of the plasma parameters, in particular electron temperature, plasma potential and plasma density. These measurements were used as input of a coupled thermal/plasma model, in order to evaluate the temperature profiles of the cathode, at several conditions. This temperature was then necessary to predict quantitatively the expected life of the cathode. Results of thermal and life model were compared with previous experiments done on the cathodes, in particular with a 4000 hours wear test effectuated on the smaller of the cathodes analyzed. Comparison of the theoretical prediction with the experimental results suggests that lanthanum redeposition reduces the net evaporation rate and extends the cathode life. The predicted life of this type of cathodes is found to be more than 30000 hours, thus it qualifies the LaB6 hollow cathode for long deep space missions

Resistive MHD modes in hollow cathodes external plasma

A significant number of plasma instabilities occur in the region just outside of hollow cathodes, depending on the injected gas flow, the current level and the application of an external magnetic field. In particular, the presence of an axial magnetic field induces a helical mode, affecting all the plasma parameters and the total current transported by the plasma. To explore the onset and behavior of this helical mode, the fluctuations in the plasma parameters in the current-carrying plume outside of a hollow cathode discharge have been investigated. The hollow cathode was operated at a current of 25 A, and at variable levels of propellant flow rate and applied magnetic fields. Electromagnetic probes were used to measure the electromagnetic fluctuations, and correlation analysis between each of the probe signals provided spatial-temporal characterization of the generated waves. Time-averaged plasma parameters, such as plasma potential and ion energy distribution function, were also collected in the near-cathode plume region by means of scanning emissive probe and retarding potential analyzer. The results show that the helical mode exists in the cathode plume at sufficiently high applied magnetic field, and is characterized by the presence of a finite electromagnetic component in the axial direction, detectable at discharge currents not less than 25 A. A theoretical analysis of this mode reveals that one possible explanation is consistent with the hypotheses of resistive magnetohydrodynamics, which predicts the presence of helical modes in the forms of resistive kink. The analysis has been carried out by linear perturbation of the resistive MHD equations, from which it is possible to obtain the dispersion relation of the mode and find the $k-omega$ unstable branch associated with the instability. These findings provided the basis for more detailed investigation of resistive MHD modes and their effect in the plume of hollow cathodes developed for electric propulsion application

A scaling methodology for high-power magnetically shielded Hall thrusters

The past two decades have seen an increasing interest in Hall thrusters in space propulsion, thanks to their favorable performance characteristics with respect to a wide variety of missions of current and future interest and to the signifcant extension in operational life potential achievable with magnetic shielding. Nevertheless, the physics underlying their behavior is complex and not yet fully understood, limiting the practical applications of models based on frst principles due to their inability to self-consistently predict the device performance. Fortunately, modern Hall thrusters were developed through a lengthy process of gradual refnement, and thus they represent convenient reference devices to design new thrusters using appropriately defned scaling criteria. The objective of this work is to propose a new scaling methodology, especially intended for magnetically shielded high-power Hall thrusters. To this purpose, a novel phenomenological model for shielded thrusters is presented and discussed. This model includes free coefcients, whose values are chosen based on the agreement with the empirical data collected in a specially created high-power Hall thruster database. The proposed methodology features a new reference thruster and aims at keeping unchanged its main plasma intensive parameters in a scaling transformation. The possibility of creating performance maps at constant discharge power, which show how the scaling results vary with the channel dimensions, is also proposed as a preliminary design tool