Numerical simulation of the plasma acceleration process in a magnetically enhanced micro-cathode vacuum arc thruster

A particle-in-cell simulation is conducted to investigate the plasma acceleration process in a micro-cathode vacuum arc thruster.A coaxial electrode structure thruster with an applied magnetic field configuration is used to investigate the effects of the distribution of the magnetic field on the acceleration process and the mechanism of electrons and ions.The modeling results show that due to the small Larmor radius of electrons,they are magnetized and bound by the magnetic field lines to form a narrow electron channel.Heavy ions with a large Larmor radius take a long time to keep up with the electron movement.The presence of a magnetic field strengthens the charge separation phenomenon.The electric field caused by the charge separation is mainly responsible for the ion acceleration downstream of the computation.The impact of variations in the distribution of the magnetic field on the acceleration of the plasma is also investigated in this study,and it is found that the position of the magnetic coil relative to the thruster exit has an important impact on the acceleration of ions.In order to increase the axial velocity of heavy ions,the design should be considered to reduce the confinement of the magnetic field on the electrons in the downstream divergent part of the applied magnetic field.

Comparative analysis of the arc characteristics inside the convergingdiverging and cylindrical plasma torches

A two-temperature thermal non-equilibrium model is used to simulate and compare the arc characteristics within the converging-diverging and traditional cylindrical plasma torches.The modeling results show that the presence of the constrictor within the converging-diverging torch makes the evolution characteristics of the arc significantly different from that of cylindrical torch.Compared with a cylindrical geometrical torch,a much higher plasma flow velocity and relatively longer high temperature region can be generated and maintained inside the converging-diverging torch.In the constrictor of converging-diverging torch,the normalized radius of arc column increases and the degree of thermodynamic equilibrium of the plasma is significantly improved with the increase of axial distance.The radial momentum balance analysis shows that for the cylindrical torch,the pressure gradient that drives the arc expansion and the Lorentz force that drives the arc contraction dominate the radial evolution of the arc.While at the converging and constrictor region of a converging-diverging plasma torch,the radial gas dynamic forces in arc fringes pointing toward the arc center enhance the mixing of the cold gas of boundary layer with the high temperature gas of the arc center,increasing the average gas temperature and decreasing the thickness of cold boundary layer,thereby facilitating the formation of diffusion type arc anode attachment at the diverging section of torch.

Numerical simulation of the effects of protrusion on DC arc anode attachment

The attachment of the DC arc on the anode is usually affected by surface morphology such as protrusions due to ablation or melting deformation.A three-dimensional thermodynamic and chemical non-equilibrium model is used to numerically simulate the effect of artificially assumed surface protrusions on the arc anode attachment.The numerical simulation results show that the arc deflects toward the protrusions on the anode and attaches to them in a constricted mode,resulting in an increase in the temperature of the arc attachment region.The analysis shows that the presence of protrusion on the anode surface changes the electric field distribution,intensifies the degree of thermodynamic and chemical non-equilibrium in its vicinity,further influences the chemical kinetic process of the plasma around it,which is the main reason for the deflection of the arc toward the protrusions and the arc anode attachment in a constricted mode.In order to verify the numerical simulation results,verification experiments are also performed using similar size scale anode protrusion,and the results showed that the presence of protrusion can indeed cause the deflection of the arc and even cause the ablation of the protrusion.

Numerical simulation of constricted and diffusive arc–anode attachments in wall-stabilized transferred argon arcs

A numerical simulation is conducted to investigate arc-anode attachment behavior, especially the formation mechanism of the constricted arc attachment mode for the water-cooled anode of wall-stabilized transferred argon arcs. Argon molecular ions and the corresponding kinetic processes are included to the finite-rate chemistry model in order to capture the chemical nonequilibrium characteristics of the arc near the anode region. Modeling results show that constricted and diffusive arc–anode attachments can be self-consistently obtained at different arc currents while keeping other parameters unchanged. The dominant kinetic processes contributing to ionization and recombination in the arc center and fringes are presented. The results show that in arc fringes and the arc attachment region, molecular ion recombination plays an important role which leads to the rapid loss of electrons. The radial evolution of the production, loss and transport processes of electrons is further analyzed. It is found that for the constricted arc attachment mode, both the recombination and convection transport caused by the anode jet result in the loss of electrons at the arc fringes, which leads to the shrinkage of the arc column at the anode. The formation of the anode jet is due to the combined action of radial and axial Lorentz forces in the anode region.

Numerical simulation of anode heat transfer of nitrogen arc utilizing two-temperature chemical non-equilibrium model

A detailed understanding of anode heat transfer is important for the optimization of arc processing technology.In this paper,a two-temperature chemical non-equilibrium model considering the collisionless space charge sheath is developed to investigate the anode heat transfer of nitrogen free-burning arc.The temperature,total heat flux and different heat flux components are analyzed in detail under different arc currents and anode materials.It is found that the arc current can affect the parameter distributions of anode region by changing plasma characteristics in arc column.As the arc current increases from 100 A to 200 A,the total anode heat flux increases,however,the maximum electron condensation heat flux decreases due to the arc expansion.The anode materials have a significant effect on the temperature and heat flux distributions in the anode region.The total heat flux on thoriated tungsten anode is lower than that on copper anode,while the maximum temperature is higher.The power transferred to thoriated tungsten anode,ranked in descending order,is heat flux from heavy-species,electron condensation heat,heat flux from electrons and ion recombination heat.However,the electron condensation heat makes the largest contribution for power transferred to copper anode.

Special issue on selected papers from CEPC 2020

The development of electric propulsion has taken decades and in fact, began flight testing as early as the 1960s. However, it was initially slow to develop as an alternative to chemical propulsion systems due to the lack of available on-board electrical power for space vehicles. Since the turn of this century, the booming satellite market, and in particular for small satellites, has driven the demand for propulsion systems with high specific impulse, precisely adjustable thrust, long lifetime, and adaptable for different kinds of small vehicles.

Genomic dissection of gastrointestinal and lung neuroendocrine neoplasm

Objective:Neuroendocrine neoplasms(NENs)are relatively rare and heterogeneous malignancies with two major subtypes:low-grade neuroendocrine tumor(NET)and high-grade neuroendocrine carcinoma(NEC).Comprehensive molecular characterization of NENs is needed to refine our understanding of the biological underpinnings of different NEN subtypes and to predict disease progression more accurately.Methods:We performed whole-exome sequencing(WES)of NEN samples from 49 patients(25 NETs and 24 NECs)arising from the stomach,intestines or lung.Clinicopathologic features were assessed and associated with molecular events.Results:NENs generally harbor a low mutation burden,with TP53 being the top mutated gene found in 31%of patients.Consistent with other studies,p53 signaling pathway dysfunction is significantly enriched in NECs compared to NETs(P<0.01).Other than TP53,tissue type-specific mutation profiles of NENs were observed in our cohort compared to those reported in pancreatic NETs.Importantly,we observed significant genomic instability,with increased copy number alterations observed across the NEN genome,which was more profound in NECs and independently correlated with poor overall survival(OS)(P<0.001).NECs could be further stratified into two molecular subtypes based on OS(P<0.001)and the chromosomal instability score(CIS).Interestingly,we discovered that the gain of whole chromosome 5 occurred at the early stage of NEN development,followed by the loss of 5 q exclusively in NECs(P<0.001).Conclusions:These findings provide novel insights into the molecular characteristics of NENs and highlight the association of genomic stability with clinical outcomes.

Application of artificial intelligence in medicine: analysis of Chinese legal and regulatory landscapes

Medical artificial intelligence (AI) in China is now on the eve of a massive surge. Despite great promise from this emerging field, there remain formidable pitfalls and obstacles, especially the challenges to current legal rules. We review the development of legal rules concerning medical AI in China, discuss risks of liability of commercial organizations and health care providers and whether current rules should be amended, and if so, what are the key issues to be addressed. Finally, we close by sketching possible ways forward for the legal and regulatory system.

Enhancing cartilage repair with optimized supramolecular hydrogel-based scaffold and pulsed electromagnetic field

Functional tissue engineering strategies provide innovative approach for the repair and regeneration of damaged cartilage.Hydrogel is widely used because it could provide rapid defect filling and proper structure support,and is biocompatible for cell aggregation and matrix deposition.Efforts have been made to seek suitable scaffolds for cartilage tissue engineering.Here Alg-DA/Ac-β-CD/gelatin hydrogel was designed with the features of physical and chemical multiple crosslinking and self-healing properties.Gelation time,swelling ratio,biodegradability and biocompatibility of the hydrogels were systematically characterized,and the injectable self-healing adhesive hydrogel were demonstrated to exhibit ideal properties for cartilage repair.Furthermore,the new hydrogel design introduces a pre-gel state before photo-crosslinking,where increased viscosity and decreased fluidity allow the gel to remain in a semi-solid condition.This granted multiple administration routes to the hydrogels,which brings hydrogels the ability to adapt to complex clinical situations.Pulsed electromagnetic fields(PEMF)have been recognized as a promising solution to various health problems owing to their noninvasive properties and therapeutic potentials.PEMF treatment offers a better clinical outcome with fewer,if any,side effects,and wildly used in musculoskeletal tissue repair.Thereby we propose PEMF as an effective biophysical stimulation to be 4th key element in cartilage tissue engineering.In this study,the as-prepared Alg-DA/Ac-β-CD/gelatin hydrogels were utilized in the rat osteochondral defect model,and the potential application of PEMF in cartilage tissue engineering were investigated.PEMF treatment were proven to enhance the quality of engineered chondrogenic constructs in vitro,and facilitate chondrogenesis and cartilage repair in vivo.All of the results suggested that with the injectable self-healing adhesive hydrogel and PEMF treatment,this newly proposed tissue engineering strategy revealed superior clinical potential for cartilage defect treatment.

O-alg-THAM/gel hydrogels functionalized with engineered microspheres based on mesenchymal stem cell secretion recruit endogenous stem cells for cartilage repair

Lacking self-repair abilities,injuries to articular cartilage can lead to cartilage degeneration and ultimately result in osteoarthritis.Tissue engineering based on functional bioactive scaffolds are emerging as promising approaches for articular cartilage regeneration and repair.Although the use of cell-laden scaffolds prior to implantation can regenerate and repair cartilage lesions to some extent,these approaches are still restricted by limited cell sources,excessive costs,risks of disease transmission and complex manufacturing practices.Acellular approaches through the recruitment of endogenous cells offer great promise for in situ articular cartilage regeneration.In this study,we propose an endogenous stem cell recruitment strategy for cartilage repair.Based on an injectable,adhesive and self-healable o-alg-THAM/gel hydrogel system as scaffolds and a biophysio-enhanced bioactive microspheres engineered based on hBMSCs secretion during chondrogenic differentiation as bioactive supplement,the as proposed functional material effectively and specifically recruit endogenous stem cells for cartilage repair,providing new insights into in situ articular cartilage regeneration.

Bisphosphonate-based hydrogel mediates biomimetic negative feedback regulation of osteoclastic activity to promote bone regeneration

The intricate dynamic feedback mechanisms involved in bone homeostasis provide valuable inspiration for the design of smart biomaterial scaffolds to enhance in situ bone regeneration.In this work,we assembled a biomimetic hyaluronic acid nanocomposite hydrogel(HA-BP hydrogel)by coordination bonds with bisphosphonates(BPs),which are antiosteoclastic drugs.The HA-BP hydrogel exhibited expedited release of the loaded BP in response to an acidic environment.Our in vitro studies showed that the HA-BP hydrogel inhibits mature osteoclastic differentiation of macrophage-like RAW264.7 cells via the released BP.Furthermore,the HA-BP hydrogel can support the initial differentiation of primary macrophages to preosteoclasts,which are considered essential during bone regeneration,whereas further differentiation to mature osteoclasts is effectively inhibited by the HA-BP hydrogel via the released BP.The in vivo evaluation showed that the HA-BP hydrogel can enhance the in situ regeneration of bone.Our work demonstrates a promising strategy to design biomimetic biomaterial scaffolds capable of regulating bone homeostasis to promote bone regeneration.