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

Materials Characterization of Impregnated W and W-Ir Cathodes after Oxygen Poisoning

Electric thrusters use hollow cathodes as the electron source for generating the plasma discharge and for beam neutralization. These cathodes contain porous tungsten emitters impregnated with BaO material to achieve a lower surface work function and are operated with xenon propellant. Oxygen contaminants in the xenon plasma can poison the emitter surface, resulting in a higher work function and increased operating temperature. This could lead directly to cathode failure by preventing discharge ignition or could accelerate evaporation of the BaO material. Exposures over hundreds of hours to very high levels of oxygen can result in increased temperatures, oxidation of the tungsten substrate, and the formation of surface layers of barium tungstates. In this work, we present results of a cathode test in which impregnated tungsten and tungsten-iridium emitters were operated with 100 ppm of oxygen in the xenon plasma for several hundred hours. The chemical and morphological changes were studied using scanning electron microscopy, energy dispersive spectroscopy, and laser profilometry. The results provide strong evidence that high concentrations of oxygen accelerate the formation of tungstate layers in both types of emitters, a phenomenon not inherent to normal cathode operation. Deposits of pure tungsten were observed on the W-Ir emitter, indicating that tungsten is preferentially removed from the surface and transported in the insert plasma. A W-Ir cathode surface will therefore evolve to a pure W composition, eliminating the work function benefit of W-Ir. However, the W-Ir emitter exhibited less erosion and redeposition at the upstream end than the pure W emitter

Solid and Effective Upper Limb Segmentation in Egocentric Vision

Upper limb segmentation in egocentric vision is a challenging and nearly unexplored task that extends the well-known hand localization problem and can be crucial for a realistic representation of users' limbs in immersive and interactive environments, such as VR/MR applications designed for web browsers that are a general-purpose solution suitable for any device. Existing hand and arm segmentation approaches require a large amount of well-annotated data. Then different annotation techniques were designed, and several datasets were created. Such datasets are often limited to synthetic and semi-synthetic data that do not include the whole limb and differ significantly from real data, leading to poor performance in many realistic cases. To overcome the limitations of previous methods and the challenges inherent in both egocentric vision and segmentation, we trained several segmentation networks based on the state-of-the-art DeepLabv3+ model, collecting a large-scale comprehensive dataset. It consists of 46 thousand real-life and well-labeled RGB images with a great variety of skin colors, clothes, occlusions, and lighting conditions. In particular, we carefully selected the best data from existing datasets and added our EgoCam dataset, which includes new images with accurate labels. Finally, we extensively evaluated the trained networks in unconstrained real-world environments to find the best model configuration for this task, achieving promising and remarkable results in diverse scenarios. The code, the collected egocentric upper limb segmentation dataset, and a video demo of our work will be available on the project page1

Human segmentation in surveillance video with deep learning

Advanced intelligent surveillance systems are able to automatically analyze video of surveillance data without human intervention. These systems allow high accuracy of human activity recognition and then a high-level activity evaluation. To provide such features, an intelligent surveillance system requires a background subtraction scheme for human segmentation that captures a sequence of images containing moving humans from the reference background image. This paper proposes an alternative approach for human segmentation in videos through the use of a deep convolutional neural network. Two specific datasets were created to train our network, using the shapes of 35 different moving actors arranged on background images related to the area where the camera is located, allowing the network to take advantage of the entire site chosen for video surveillance. To assess the proposed approach, we compare our results with an Adobe Photoshop tool called Select Subject, the conditional generative adversarial network Pix2Pix, and the fully-convolutional model for real-time instance segmentation Yolact. The results show that the main benefit of our method is the possibility to automatically recognize and segment people in videos without constraints on camera and people movements in the scene (Video, code and datasets are available at http://graphics.unibas.it/www/HumanSegmentation/index.md.html)

A Preliminary Investigation into a Deep Learning Implementation for Hand Tracking on Mobile Devices

Hand tracking is an essential component of computer graphics and human-computer interaction applications. The use of RGB camera without specific hardware and sensors (e.g., depth cameras) allows developing solutions for a plethora of devices and platforms. Although various methods were proposed, hand tracking from a single RGB camera is still a challenging research area due to occlusions, complex backgrounds, and various hand poses and gestures. We present a mobile application for 2D hand tracking from RGB images captured by the smartphone camera. The images are processed by a deep neural network, modified specifically to tackle this task and run on mobile devices, looking for a compromise between performance and computational time. Network output is used to show a 2D skeleton on the user's hand. We tested our system on several scenarios, showing an interactive hand tracking level and achieving promising results in the case of variable brightness and backgrounds and small occlusions

Implicazioni di web design per Malesia e Paesi Scandinavi attraverso uno studio interculturale

Persone di culture differenti preferiscono pagine web caratterizzate dai valori della loro cultura e tendono quindi a preferire differenti caratteristiche del sito web a seconda delle loro esigenze in termini di navigazione, sicurezza, informazioni sui prodotti, servizio al cliente, strumenti di shopping e design. Per un’impresa che intende globalizzare il proprio mercato è utile implementare interfacce culturali country specific, ovvero utilizzare pagine web diverse per paesi con culture differenti. Il presente studio si fonda sulle conclusioni proposte da due modelli di analisi culturale, Hall (1976) e Hofstede (1980) e attraverso l’applicazione degli studi di Marcus e Gould (2000) e l’analisi di un campione di siti web commerciali, arriva a definire le linee guida di web design per i Paesi Scandinavi e Malesi

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

Freehand-Steering Locomotion Techniques for Immersive Virtual Environments: A Comparative Evaluation

Virtual reality has achieved significant popularity in recent years, and allowing users to move freely within an immersive virtual world has become an important factor critical to realize. The user’s interactions are generally designed to increase the perceived realism, but the locomotion techniques and how these affect the user’s task performance still represent an open issue, much discussed in the literature. In this article, we evaluate the efficiency and effectiveness of, and user preferences relating to, freehand locomotion techniques designed for an immersive virtual environment performed through hand gestures tracked by a sensor placed in the egocentric position and experienced through a head-mounted display. Three freehand locomotion techniques have been implemented and compared with each other, and with a baseline technique based on a controller, through qualitative and quantitative measures. An extensive user study conducted with 60 subjects shows that the proposed methods have a performance comparable to the use of the controller, further revealing the users’ preference for decoupling the locomotion in sub-tasks, even if this means renouncing precision and adapting the interaction to the possibilities of the tracker sensor

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