Hollow Cathode Thruster for All-Electric Spacecraft

The Air-breathing Microwave Plasma CAThode (AMPCAT) has been developed for air-breathing electric propulsion in very-low Earth orbit. In this study, the standalone AMPCAT plasma characteristics are analyzed by means of several diagnostic tools and operation on xenon is compared to a conventional hollow cathode. A transition of AMPCAT extracted current from a lower (<0.1 A) to higher-current (>0.5 A) mode, triggered by increasing the negative cathode bias voltage, is accompanied by a significant rise in internal electron density and external electron temperature. The AMPCAT is coupled with a cylindrical Hall thruster in the 100–300 W power-level running on 0.5–0.7 mg/s of xenon, and the thrust is directly measured for cathode operation with both xenon and air. Stable thruster operation is demonstrated for the AMPCAT running on both propellants. For xenon, the performance is compared to a hollow cathode, which reveals matching discharge current profiles but a significantly higher thrust for the AMPCAT at low discharge voltages, approximately two times higher at 200 V. Langmuir probe measurements highlight a 30–40 V lower plasma potential in the cathode vicinity for the AMPCAT with xenon compared to both the hollow cathode and AMPCAT with air. This indicates a significantly improved coupling of cathode electrons to the thruster discharge, yielding an increased degree of ionization. Faraday probe and Wien filter results show that a larger current utilization efficiency drives the observed performance difference at low discharge voltages, rather than a significant change in ion acceleration or plume divergence.

As part of an effort to develop efficient hollow cathodes for the International Space Station and low power electric propulsion systems, the effects of orifice length-to-diameter aspect-ratio on the performance of 0.5 to 2 A hollow cathodes were examined to compare with previous experiments and a recently developed model. Cathodes were constructed with nominally identical orifice diameters and lengths that varied from 1 to 6 diameters. The performance of the cathodes was evaluated at the beginning of life and after at least 50 hours of operation. The data generally followed the trends predicted by the model with a few exceptions which appear to be related to the thermal environment of the cathode. Power consumption scaled with the orifice aspect-ratio, while the minimum spot mode flow rate was inversely related to the length-to-diameter ratio. At currents below 1.0 A, the cathode operation became more complex than that which is assumed in the orifice model; low operating temperature caused instabilities in several of the devices. Nomenclature AR = aspect-ratio of the orifice c = speed of light, 3 x 10 m/s h = Planck's Constant, 6.63 x 10' J-s k = Boltzmann's Constant, 1.38 x 10' J/K n = index of refraction T = temperature, K e = emissivity X = wavelength, m Introduction A parametric investigation has been conducted at the NASA Lewis Research Center in order to evaluate the effect of orifice aspect-ratio on the performance of low current hollow cathodes. The aspect-ratio is defined as the orifice length divided by its diameter. The NSTAR derivative ion thruster for the Deep Space 4 mission, the plasma contactor for the International Space Station, and the NASA Lewis 8 cm ion thruster program all have requirements for low flow rate, low power cathodes.' Experimental data' exist both to support and to refute the hypothesis that cathode performance is inversely proportional to the orifice aspect-ratio. A recent hollow cathode model developed by Mandell and Katz describes the physics leading to the conclusion that cathode performance scales inversely with orifice aspect-ratio. Ions entering a finite length orifice will undergo collisions as they traverse the channel. Ions lost to collisions with the walls must be balanced by ions created in the orifice in order to satisfy the condition of a quasineutral plasma at the exit of the orifice. As orifice aspect-ratio increases, the number of ionizations occurring increases, and the conductivity of the plasma in the orifice decreases. The energy transport within the orifice also varies inversely with the aspect-ratio, and modeling the orifice processes becomes increasingly important to making an accurate prediction about the performance of the cathode. The physical arguments for the importance of orifice aspect-ratio to cathode performance are sufficiently compelling to warrant a new evaluation of the experimentally observed effects, particularly when trying to minimize the power consumption during low current operation. This report summarizes the experimental findings concerning the impact of * Graduate Student Researcher, Student Member AIAA 1 Associate Professor, Senior Member AIAA * Research Engineer, Member AIAA Copyright © 1998 by Matthew T. Domonkos. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. 1 American Institute of Aeronautics and Astronautics orifice aspect-ratio on the power consumption of low current hollow cathodes. Although the current density in the orifice far exceeds that found anywhere else in a plasma contactor, ion thruster, or Hall thruster, previous modeling efforts' have ignored the effects of the orifice processes on the overall performance. Mandell and Katz have proposed a model which conserves both energy and ions necessary for quasineutrality. The model assumes the plasma properties at the entrance to the orifice are known. The ion loss to the walls of the orifice is balanced by electron-neutral collisional ionization. The Ohmic heating in the plasma is balanced by the ionization, radiative, and convective losses. With the orifice region taken as a control volume, the flux of ions incident at the entrance will either collide with the walls or be transmitted through the orifice. Since ion-wall collisions represent a loss in the energy balance, optimization of power consumption requires minimization of the orifice aspect-ratio. Typically in orificed hollow cathodes, the maximum required current determines the orifice diameter. While the theoretical picture indicates that orifice length should then be minimized, practical considerations force a finite length orifice. The machining and welding methods currently employed to fabricate the cathode tubes require a certain minimum overall thickness of the orifice plate. Rawlin and Kerslake9 were among the first investigators to observe the erosion of the orifice plates on hollow cathodes, and increasing the thickness of the orifice plate would, to first order, mitigate the problem of erosion. The goal of the investigation reported here was to provide experimental data for comparison with Mandell and Katz's5 model while meeting the practical considerations for cathode design. This paper presents the results of a parametric experimental investigation of the effects of the orifice aspect-ratio on the power consumption of 3.2 mm diameter orificed hollow cathodes. The experimental apparatus and techniques are described first. Next, the performance data are presented. The results are then analyzed with an emphasis on the performance, and the implications for cathode lifetime are also discussed. Finally, the major conclusions of this investigation are summarized. Experimental Apparatus Four 3.2 mm outer-diameter hollow cathodes with different orifice aspect-ratios were assembled as shown in Figure 1. The construction of the hollow cathodes closely followed the methods developed previously at the NASA Lewis Research Center.10-11 The orifice plates were machined on a lathe. Figure 2 illustrates the variation in the aspect-ratios for three of the cathodes tested. Although some dimensions of this schematic were exaggerated for clarity, the orifice plate dimensions, the cathode-keeper gap (Lck), and the keeper orifice diameter (dj are all drawn to scale. For convenience, the cathodes are referred to in this paper as AR1, AR3, and AR6 for the orifices with aspect-ratios of 1, 3, and 6, respectively. The fourth cathode had an orifice diameter forty percent smaller than the other cathodes and an aspect-ratio of approximately 3; it will be referred to as AR3' here. The orifice plates were electron beam welded to a refractory metal alloy tube with a 3.2 mm outer diameter. The electron emitter was a cylinder of sintered porous tungsten impregnated with 4BaOCaO-Al2O3 and was located at the downstream end of the tube. The emitter outer diameter was nominally 0.38 mm smaller than the inner diameter of the tube. Cathode Tube with Insert Propellant Isolator Keeper Thermocouple — Heater Electrical Stand-Offs Mounting Bracket a) Schematic for the Hollow Cathode Assemblies b) IPhotbgraph Figure 1 The Hollow Cathode Assembly Refractory metal electrical leads attached to the upstream end of the emitter were swaged into electrical contact the with cathode tube. This also fixed the position of the emitter within the cathode American Institute of Aeronautics and Astronautics tube. A sheathed heater was wound and friction fit to the end of the cathode tube. Metal foil was wrapped tightly around the outside of the heater to function as a radiation shield.

Effects of Noradrenaline, Isoproterenol and Acetylcholine on Ureteral Resistance

We investigate the electron acceleration behind dipolarization fronts (DFs) in the magnetotail from −25 R E to −10R E through the examination of the energetic electron energy flux (>30 keV) with the observations from THEMIS satellites. Statistical results of 133 DF events are presented based on the dataset from January to April of the years 2008 and 2009. As the DFs propagate earthward, the acceleration of energetic electrons behind the DFs is found to take place over several R E along the tail. The increase in energetic electron energy flux can reach 2∼4 orders of magnitude. The dominant acceleration mechanisms are different in the mid-tail (X GSM ≤−15R E ) and the near-Earth region (−15 GSM ≤−10R E ). In the mid-tail, the majority of DF events show that the dominant electron acceleration mechanism is betatron acceleration. In the near-Earth region, betatron acceleration is dominant in ∼46% DF events while Fermi acceleration is dominant in ∼39% DF events.

Subchannel analysis of critical heat flux in radially Non-uniform heated rod assembly at low flow rate

This Paper explores keeper ignition and discharge characteristic of a hollow cathode center mounted on a Hall thruster to discuss the optimum startup sequence. A strong magnetic field along the hollow cathode in the center-mount configuration is considered to play an important role in the ignition and discharge; however, the characteristics in this configuration has never been reported. The effect of cathode mass flow rate on the ignition and discharge with 3 A keeper ignition is examined both with and without magnetic field. The influence of cathode heater power and flow from the anode is also investigated. The lower flow rate causes strong ionization instability, while the higher flow rate seems to induce ion acoustic turbulence. Applied magnetic field improves the ionization instability due to the decrease in electron temperature through plasma production. These results indicate ignition sequences over flow rate with magnetic field appear promising. On the other hand, the confined electrons around the magnetic field line prevent the initiation of the discharge. The ignition and instability are improved with high heat-up power because more thermionic emission is obtained. The anode flow provides slight improvement to the characteristics. The Fourier transform also demonstrates the presence of ionization instability and ion acoustic turbulence. The behavior of peak frequencies of ionization instability can be explained based on the predator–prey theory.

Hollow cathodes have been proposed as reaction control thrusters for all-electric and small spacecraft. This paper makes an assessment of modified T5 and T6 hollow cathodes for use as millinewton range thrusters. The influence of terminal parameters such as discharge current, mass flow rate, and cathode/anode geometry on thrust production is discussed. The data indicate that the T5 cathode may be able to develop specific impulses in the range of 150–250 s with argon at reasonable thrust efficiencies of up to 14%. As such, hollow-cathode thrusters may meet some limited applications. However, it is unlikely that this type of thruster could be improved significantly or could compete with similar thrusters in the same operating range. High specific impulse operation is also shown to develop large discharge voltage fluctuations, which may significantly limit the lifetime of such a device.

Water cluster ion intensity and distribution is affected by source conditions in direct sample analysis (DSA) ionization. Parameters investigated in this paper include source nozzle diameter, gas flow rate, and source positions relative to the mass spectrometer inlet. Schlieren photography was used to image the gas flow profile exiting the nozzle. Smaller nozzle diameters and higher flow rates produced clusters of the type [H + (H(2)O)(n)](+) with greater n and higher intensity than larger nozzles and lower gas flow rates. At high gas flow rates, the gas flow profile widened compared with the original nozzle diameter. At lower flow rates, the amount of expansion was reduced, which suggests that lowering the flow rate may allow for improvements in sampling spatial resolution.

To analyze the association between low saliva flow rates and the use of psychoactive drugs among the elderly. A cross-sectional study was carried out with 267 elderly people from 60 to 74 years of age who lived in a borough of the city of Londrina, Paraná State, Brazil. Individuals with high functional dependence or restricted to bed were excluded. Saliva flow rate was the dependent variable with values under the first tercile being considered as low flow rates (less than 0.44 ml/min). The continuous use of psychoactive drugs (antidepressant, antiepileptic, sedative, antipsychotic, hypnotic or sedative-hypnotic drugs) was the independent variable. Multivariate analysis was performed taking into account gender, age and smoking status. The majority of the elderly were women (80.5%), with a mean age of 66.5 years. Use of psychoactive drugs was observed among 31 elderly (11.6%). Mean saliva flow rate was 0.76 ml/min, lower among users of psychoactive drugs (0.67 ml/min). In the multivariate analysis, use of psychoactive drugs was associated with low saliva flow rates (<0.44 ml/min), independent of gender, age or smoking. Results show that there is an association between use of psychoactive drugs and low saliva flow rates in this group of independent and non-institutionalized elderly. These conclusions stress the need of a rational use of these drugs, particularly among the elderly.

The renal safety of sevoflurane.

The performance of molten salt pump is affected by its geometrical structure and the physical properties of delivered fluids. In order to reveal the effects of these factors on pump performance, the performance curves were analyzed theoretically, the flow field was measured accurately by using high-speed photography, and the external characteristics were tested by using water as test medium. The numerical model was verified by the path lines, absolute velocity and performance curves obtained by experiment. Based on the verified model, the relationship among the fluid viscosity, external characteristics and flow field was investigated in the view of energy loss. The results show that reducing the blade outlet angle and outlet width as well as increasing the blade outlet thickness can avoid the unstable running. When delivering low viscosity fluids, since the impact loss is relatively large at low flow rate, the hump is easy to appear in the Head-Flow rate curve. When delivering high viscosity fluids, the opposite happens. With the increase of viscosity, the head and efficiency decrease monotonously, and the maximum efficiency point moves towards low flow rate. At the low flow rate, the shaft power decreases as the viscosity increases, while at the high flow rate, the shaft power increases as the viscosity increases. Compared with delivering water, when the viscosity is lower than 0.01453Pa⋅s, the drop of head and efficiency is less than 5%. The results of the study have reference significance on the development of high-performance molten salt pump and stable operation.

Development of a portable grain mass flow sensor test rig

We present a 43-year old female patient, admitted to the intensive care unit, recovering from a Bentall procedure because of a supra-coronary aneurysm and unicuspid aortic valve. Few hours following surgery,a marked lactic acidosis, hypoxemia and increased central venous pressure developed. Pulmonary angio-CT was performed to exclude pulmonary embolism. Because of severe peripheral vasoconstriction, iodine contrast could only be administered through a narrow lumen central venous catheter, which limited the maximum flow rate to 1.5 ml/s. Images were performed with triggering in the pulmonary trunk. Review of the images showed no evidence for pulmonary embolism. Despite the low flow rate, a clear enhancement of the pulmonary artery was observed (up to 460 HU) (Fig. 1). Such an enhancement of the pulmonary artery is normally only achieved when injecting contrast at a flow rate of 4-5 ml/s. These perfect images, obtained at a low contrast flow rate in fact indicate severely impaired right ventricular function.

Experimental investigation of bubble growth from orifice plates submerged in pools of viscous liquids has been carried out using high speed videography. Conflicting effects of viscosity on ebullience have been reported in the literature. These are addressed in the present study and their range of applicability has been identified. Furthermore, the effects of chamber volume on bubble dynamics in viscous media are examined. Orifice plates made of Acrylic glass (a hydrophilic surface) with varying orifice diameters from 0.813 mm to 1.500 mm, have been utilized. Additionally, bubble dynamics from a stainless steel capillary nozzle was captured and compared with that from orifice plates. The six different liquid pools were used, viz., pure distilled water, ethylene glycol, propylene glycol, and three different aqueous glycerol solutions. The aqueous glycerol solutions varied in viscosity from 48 cP to 128 cP. The flow rate was regulated such that the isolated bubble regime was encountered. For the smaller orifices, viscosity effects were present at all flow rates and the bubbles in water-glycerol solutions were much larger than those in pure water. However, for the larger orifice sizes, water-glycerol solutions produced bubbles that were larger than those in water only at high air flow rates. For larger orifice sizes, at low flow rates, there was no increase in bubble size in highly viscous water-glycerol solutions compared to pure water. In fact, with 1.5 mm diameter plate orifice, the bubbles for 128 cP water-glycerol solution were smaller than those in pure water at low air flow rates. When chamber effects were present, the bubbles in the more viscous medium differed in shape and size from those in pure water.

The aim of the study is to investigate the feasibility of using dual-source computed tomography (CT) combined with low flow rate and low tube voltage for postchemotherapy image assessment in cancer patients. Ninety patients undergoing contrast-enhanced CT scans of the upper abdomen were prospectively enrolled and randomly assigned to groups A, B, and C (n = 30 each). In group A, patients underwent scans at 120 kVp with 448 mgI/kg. Patients in group B underwent scans at 100 kVp with 336 mgI/kg. Patient in group C underwent scans at 70 kVp with of 224 mgI/kg. Quantitative measurements including the CT number, standard deviation of CT number, signal-to-noise ratio, contrast-to-noise ratio, subjective reader scores, and the volume and flow rate of contrast agent were evaluated for each group. There was no statistically significant difference in the subjective image scores within the three groups except for the kidney (all P > 0.05). Group C showed significantly higher CT values, lower noise levels, and higher signal-to-noise ratio and contrast-to-noise ratio values in the majority of the regions of interest compared to the other groups ( P < 0.05). In group C, the contrast agent dose was decreased by 46% compared to group A (79.48 ± 12.24 vs 42.7 ± 8.6, P < 0.01), and the contrast agent injection rate was reduced by 22% (2.7 ± 0.41 vs 2.1 ± 0.4, P < 0.01). The use of 70 kVp tube voltage combined with low iodine flow rates prove to be a more effective approach in solving the challenge of compromised blood vessels in postchemotherapy tumor patients, without reducing image quality and diagnostic confidence.