LaB6 Cathode Research Articles

Performance of ITER pressure gauges during deuterium operation in the Large Helical Device

Abstract During deuterium campaigns on the heliotron large helical device (LHD), ITER pressure gauges with different cathode materials were used to measure the neutral pressure in the sub-divertor region. Throughout these campaigns, it was observed that the performance of the LaB6 cathodes was unsatisfactory, credited to the generation of neutrons impacting the gauges. Conversely, measurements taken with pressure gauges with a ZrC cathode performed well throughout the deuterium pulses. The ITER pressure gauge with the ZrC cathode could be operated with a very high electron current of 800 µA, thus improving the lower detection limit of the neutral pressure in LHD. With this design it was also possible to avoid jumps in the ion current within strong magnetic fields, improving the accuracy of the measurement from 15% uncertainty to 5%. These features allowed very precise neutral pressure measurements to be made in a fusion device with magnetic confinement. The total running time in the magnetic field was around 60 hours, less than the cathode life time of 350 hours.

Experimental demonstration of a water electrolysis Hall Effect Thruster (WET-HET) operating with a hydrogen cathode

This study presents the first experimental demonstration of a Hall Effect Thruster operating with water electrolysis-derived constituents: oxygen for the anode (thruster) and hydrogen for the cathode, both from pressurised bottles. In contrast to prior research where the thruster was operated with oxygen to the anode and xenon/krypton to the cathode, this investigation uses an advanced set-up that safely allowed the combined use of pressurised oxygen and hydrogen. Firstly, a comparative assessment between the established plasma filament cathode (utilised in previous research) and a novel LaB6 cathode (referred to as Hydrocat), both krypton-fuelled, is conducted. Results show a different IV curve for these cathodes, but negligible disparities in thruster performance for fixed discharge power and anode mass flow. Thrust, anode specific impulse, thrust-to-power ratio, and both, anode and total efficiency of the system using oxygen (anode) and hydrogen (cathode) are then presented and compared against oxygen (anode) and krypton (cathode). The cathode successfully operated on hydrogen, with a reduction in thrust performance of under 10% compared to when krypton is used as the cathode propellant. This performance difference can be attributed to different factors, such as the reduced efficiency of the cathode running with hydrogen due to a higher ionisation potential and the cathode propellant’s contribution to thrust, more significant when using heavier propellants such as xenon/krypton compared to hydrogen. The impact of changing the hydrogen-to-oxygen ratio is also assessed and shown to have no impact on the thruster’s performance. The most optimal performance was obtained at the highest discharge power tested (Pd=1953±1 W), using a hydrogen:oxygen ratio of Φ = 1:25. This operating point corresponded to a thrust of 20.9±0.2 mN, anode specific impulse of 2130±20 s, thrust-to-power ratio of 10.7±0.5 mN/kW, anode thrust efficiency of 11.2±0.3 %, and total efficiency of 10.1±0.3 %.

Study of changes in the surface structure of tungsten irradiated by helium plasma

One of the important aspects is the interaction of plasma with the surface of a material, especially in the conditions of a fusion facility. The current work presents the preliminary results of the study of tungsten surface structure modification under helium plasma irradiation. A small-sized linear simulator KAZ-PSI with a plasma-beam setup was designed and assembled, where helium was used as a working gas. The main elements of the linear plasma simulator are an electron beam gun with a LaB6 cathode, a plasma-beam discharge chamber, an interaction chamber, a target device, and an electromagnetic system consisting of electromagnetic coils. It was revealed that under irradiation on the surface of the samples there is a relief with defective structure consisting of chaotically arranged ledges and pits of various shapes with average size (100‒600) nm and pore sizes (0.1‒1.5) μm with visible areas of flaking and sputtering. It was found that when the negative potential on the target is varied by –500V/–1000V/–1500V, the formation of dislocation with chaotic and cellular structure of tungsten with an average grain size of (1‒25) μm is observed; it was revealed that the total values of elastic and plastic components of deformation across the tungsten grain differ from each other by about 2.5 times.

Linear Plasma Device for the Study of Plasma–Surface Interactions

At the research and production company “PlasmaScience” (Ust-Kamenogorsk, Kazakhstan), a linear plasma generator installation, KAZ-PSI (Kazakhstan Plasma Generator for Plasma Surface Interactions), has been developed and constructed for the study of the interaction of plasma and materials. This article outlines some features of the developed experimental installation designed for the investigation of surface–plasma interactions. The primary components of the linear plasma installation include an electron-beam gun with a LaB6 cathode, a plasma-beam discharge chamber, an interaction chamber, a target device, and an electromagnetic system comprising electromagnetic coils. The KAZ-PSI unit enables continuous plasma generation using hydrogen, deuterium, helium, argon, and nitrogen. The electron density of the plasma is in the range of about 1017–1018 m−3 and the electron temperature is in the range of 1 to 20 eV. The incident ion energy is regulated by applying a negative potential of up to 2 kV to the target. Experiments on the irradiation of tungsten with helium plasma were carried out using the KAZ-PSI installation for the first time. This article presents the research findings on the structure and properties of tungsten relative to the temperature of helium plasma irradiation. Alterations in roughness, microstructure, hardness, modulus of elasticity, and erosion of the tungsten’s surface following helium plasma irradiation at varying temperatures were examined. The study’s results indicate that helium plasma irradiation induces changes in the morphology of the tungsten’s surface, creating surface relief due to sputtering by helium ions, as well as the formation of blisters. Mechanical testing revealed that after irradiation at T = 500 °C, there was an increase in hardness of up to 10%, and a slight decrease in modulus of elasticity. And after irradiation at T = 900 °C and T = 1300 °C, both hardness and elastic modulus decreased with rising temperature. The tungsten surface erosion evaluation results showed that the degrees of surface erosion increase with increasing target temperature.

Linear plasma device for the study of plasma-surface interactions

In the research and production company "PlasmaScience" (Ust-Kamenogorsk, Kazakhstan) a new laboratory linear plasma installation KAZ-PSI (Kazakstan plasma Generator for Plasma Surface Interactions) for studying the interaction of plasma with material has been developed and built. The article describes some features of the developed experimental setup for the study of surface-plasma interactions. The main elements of the linear plasma setup are an electron-beam gun with a LaB6 cathode, a plasma-beam discharge chamber, an interaction chamber, a target device, and an electromagnetic system consisting of electromagnetic coils. The KAZ-PSI facility allows generating continuous plasma of hydrogen, deuterium, helium, argon and nitrogen. The electron density of the plasma is in the range of the order of 1017 -1018 m-3 and the electron temperature is in the range of 1 to 20 eV. The incident ion energy is controlled by applying a negative bias of up to 2 kV to the target. Experiments on irradiation of tungsten with helium plasma were carried out for the first time at the KAZ-PSI facility. The paper presents the results of studies conducted to study changes in the tungsten mixture. The results have shown that after helium plasma irradiation the morphology of tungsten surface changes and relief is created on the surface due to atomisation of the surface by helium ions, as well as due to the formation of blisters.

Development of a large-bore plasma window with an indirectly heated hollow cathode

For plasma window (PW) applications, we developed the cascade arc discharge device with an indirectly heated hollow cathode. The 8-mm channel diameter hollow cathode made of a lanthanum hexaboride (LaB6) was heated by the C/C composite heater surrounding the cathode to increase the thermionic electron emission. The PW developed successfully separated 2.4 kPa and 16 Pa, and the pressure separation capability was sustained for over 1 h. H-β Stark broadening measurement and the Thomson scattering measurement showed that the electron density and temperature inside the channel reached 1019–1020m−3 and 2.0 eV, respectively. The power balance analysis on the electron thermal energy revealed that the neutral density and temperature inside the channel were as high as 1023m−3 and 4000 K, respectively. The relation between the pressure separation capability and the neutral temperature showed that the flow inside the channel of the PW had the molecular flow feature. The SEM-EDX analysis on the LaB6 cathode showed that boron diffused to the molybdenum (Mo) shaft during plasma operation, which supported the LaB6 cathode. Mo shaft became brittle after more than 50 h of operation, exhibiting the necessity of buffer material between the LaB6 cathode and Mo shaft for long-duration operation.

The Features of LaB6 Cathode Operation in a Technological Electron-Beam Gun

The Features of LaB6 Cathode Operation in a Technological Electron-Beam Gun

A simple replacement of tungsten filament hot cathodes by DC heated LaB6 rod and its noise characteristics with laser-induced fluorescence.

Hot cathode discharges are common plasma sources for fundamental plasma physics studies and other applications due to their quiescent and relatively simple properties, and tungsten filaments are commonly used for the ease of heating them. Recently, tungsten filaments are increasingly being replaced by less luminous alternatives, such as barium oxide or lanthanum hexaboride. These materials can emit electrons at temperatures close to 1000K lower than tungsten, greatly reducing their blackbody radiations. This results in significant improvement in signal recovery for active spectral diagnostic, such as laser-induced fluorescence. However, these less luminous cathodes often come in vastly more complicated designs than those of tungsten hot cathodes and are much more expensive to procure and difficult to operate. In this paper, we present a simple, low cost direct current heated designof a LaB6 cathode that is manufactured at suitable dimensions and make a comparison of the laser-induced fluorescence (LIF) signal-to-noise ratio of this LaB6 hot cathode discharge with that of a typical tungsten filament discharge, revealing that LaB6 has, indeed, an improved LIF signal-to-noise ratio compared with the tungsten filament.

Effect of Nitrogen on Heat Flux to a Tungsten Target in He Plasma

Nitrogen (N2) gas was injected into the helium (He) plasma with a LaB6 cathode as an impurity to observe the variations of heat flux and the SOL heat flux width in the Divertor Plasma Simulator-2 (DiPS-2) device. The injection rate of N2 impurities was varied from 2.5% to 30% with a constant neutral pressure (~9 mTorr). To evaluate the plasma heat flux to a tungsten target, three diagnostic methods were used: (1) three thermocouples (TCs) were installed within the tungsten target to estimate the inner temperature of the tungsten, (2) an infrared (IR) camera detected the surface temperature of the tungsten; (3) a triple (electric) probe (TP) was used to determine the electron temperature (Te), plasma density (n0), and heat flux (q) of He plasma. With 30% N2 injection, a 20% decrease in Te and a 60% decrease in n0 were observed at the center of the plasma. The target temperature and heat flux also decreased with N2 seeding rate. The heat fluxes to the tungsten target deduced from the IR and TC methods produce higher values than those of the TP. The width of the heat flux transported from the main plasma to the scrape-off layer (SOL) heat flux width (λq) decreases with the impurity ratio; hence, the heat flux to the tungsten target.

Study of the operating modes of the electrostatic lens of the welding electron gun

The paper presents the results of a study of the operating modes of a technological electron gun used in installations for electron beam welding. The design of an electron gun, which is part of the ELA-15I power complex, is considered, and the results of modeling the accelerating gap for a new design gun with an accelerating voltage of 120 kV are presented. The current-voltage characteristics of the gun operation were experimentally obtained at different temperatures of the main LaB6 cathode. For this, the beam current was recorded at different values of the control electrode potential. A mathematical model of the accelerating gap was implemented, which makes it possible to analyze the shape of the electron beam in the region of the first lens. Using a mathematical model, the shapes of the electron beam were calculated for various operating modes of the gun, and the characteristic transverse size of the beam in the crossover was determined. The beam diameter and the angle of convergence in the area of focusing the beam on the product were determined experimentally. Conclusions were made about the equality of the crossover diameters of the full-scale and mathematical models, as well as about the sufficient coincidence of the experimental and calculated volt-ampere characteristics. The design of the accelerating gap of the ELA-15 gun was optimized with an increase in the accelerating voltage from 60 kV to 120 kV. The optimization results are shown for the original and modified design in the form of a comparison of the patterns of the distribution of the electrostatic field strength and a comparison of the current-voltage characteristics.

Electron gun and collector for RAON EBIS charge breeder

A heavy-ion accelerator facility, called RAON, is being constructed in Korea. RAON is a large scale experimental facility for basic science research through nuclear physics experiments using various rare-isotope (RI) beams. An Electron Beam Ion Source (EBIS) will be utilized as a charge breeder in the isotope separation on-line (ISOL) system of RAON. The electron gun (e-gun) and electron collector are essential components of the EBIS to charge-breed the RI beams. The e-gun, a magnetically immersed type, has been designed to generate electron beams with currents up to 3 A. The material of the cathode is IrCe, which provide a higher current density and longer lifetime than the LaB6 cathodes. In addition, the collector which can dissipate electron beam power up to 20 kW has been designed and built. A test bench has been designed and constructed to commission the e-gun and collector sub-systems. The test bench is equipped with a negative high-voltage platform, called a cathode platform, to provide an independent electron beam current loop. The control system was also designed and implemented to be able to control two different platforms, such as the cathode and ground platforms, using the NI LabVIEW hardware and software. The experimental goals of the tests described in this paper are to generate and transport to collector 2 A electron beam, and to demonstrate a stable operation mode with electron beam width of 50 ms and a repetition rate of 10 Hz. These goals have been determined for the first charge breeding experiment, which is to obtain the charge-bred beam of Cs33+, in the future. This paper presents the electron gun and collector sub-system of RAON EBIS, including the cathode tests, the simulation results, and the electron beam experiment.

Dynamic thermal behavior of polycrystalline LaB6 hollow cathodes

Lanthanum hexaboride (LaB6) hollow cathodes have demonstrated a capability for long life operation, which is critical to many space exploration missions. Thermal characterization of LaB6 hollow cathodes has revealed lower than expected electron emitter temperatures when the cathode reaches a steady state. This phenomenon is observed at discharge currents ranging from 5 to 35 A and xenon mass flow rates of 5–25 SCCM in cathodes with three different orifice diameters. Thus, the currently accepted value of the work function for polycrystalline LaB6, 2.67 eV, does not describe well the emission characteristics of LaB6 hollow cathodes operating with internal gas discharges at a steady state. We use empirically measured temperatures combined with a model of the hollow cathode emitter and xenon discharge to estimate the value of the work function, yielding a value ranging from 2.1 to 2.44 eV. This lower work function value implies that LaB6 hollow cathodes are expected to have even longer lifetimes than previously anticipated, further establishing them as a more suited alternative to other conventional cathode technologies for the task of long duration travel. Direct measurements of the work function as a function of depth on a hollow cathode emitter using x-ray photoelectron spectroscopy and ion beam milling indicate that the work function decreases with depth. We postulate several mechanisms that could explain the observed work function enhancement. Altogether, our results have important implications to the design, study approach, and operation of LaB6 cathodes and potentially other cathodes with hollow configuration. Finally, our work opens the question of why the work function is reduced upon interaction with Xe plasma.

Thermal Analysis and Testing of Different Designs of LaB6 Hollow Cathodes to Be Used in Electric Propulsion Applications

LaB6 emitters are commonly used in hollow cathodes that are utilized in electric space propulsion systems. In order to obtain necessary surface current emission densities of 1–10 A/cm2 for cathode operations, LaB6 emitters require temperatures above 1500 °C. Hence, the design for LaB6 cathodes presents thermal and mechanical challenges. In this paper, several design iterations for LaB6 hollow cathodes are presented and thermal analyses are conducted for each design. Temperature and heat flux distributions that are obtained from thermal analyses are investigated. The designs are evaluated according to the required heat input to the emitter, and the radiative and conductive heat loss mechanisms. In addition to the thermal analyses, experimental tests are conducted for different cathode designs and, based on these tests, various modes of failure are determined. Revising the cathode design and the material selection iteratively to eliminate the encountered failure mechanisms, a novel cathode design is achieved. Experimental tests of this novel cathode are conducted and current-voltage characteristics are presented for different mass flow rates and for discharge currents between 0.5 and 12 A. Tests and analysis results show that the selection of materials and design are crucial for a sturdy and long lifetime cathode.

New low temperature multidipole plasma device with a magnetic X-point and its properties

A new low temperature multidipole plasma device with a magnetic X-point is developed. With a usual multidipole configuration generated by permanent neodymium magnets, a pair of axially flowing electrical currents up to 1.0 kA in the chamber creates figure-eight shaped poloidal magnetic fields with the X-point which separates plasmas into three distinct regions of core, edge and private regions. This new device, magnetic X-point simulator system (MAXIMUS), is equipped with end-plate wall filaments, core filaments and a LaB6 cathode as DC plasma sources. A wide range of plasma densities from 108 to 1012 cm−3 with electron temperatures of 0.4 to 3 eV is achieved. Plasmas in MAXIMUS are highly correlated with the shape of the magnetic fields as electrons are magnetized. Furthermore, electron velocity distribution functions can be significantly modified from usual Maxwellian distributions due to the strong grad-B and curvature drifts of electrons, resulting in high skewness and excess kurtosis. Such a capability of controlling the distribution function as well as having closed circular magnetic fields will allow us to systematically investigate effects of non-Maxwellian distribution functions and curved magnetic fields on various physical phenomena such as cross-field diffusion process, plasma waves and many nonlinear physics including solitons, shock waves and three-wave interactions. Tokamak edge physics correlated with neutral particles is also to be investigated with MAXIMUS.

Discharge mode transition in a Krypton-fed 1 A-class LaB6 cathode for low-power Hall thrusters for small satellites

Thermionic cathodes are essential for the operation of various electrostatic propulsion devices. They strongly influence the performance and lifetime of the propulsion system. In this study, a 1 A-class LaB6 laboratory model hollow cathode has been tested with krypton in diode and triode configurations in order to assess the cathode discharge mode transition behavior. Measurements have been performed over a range of krypton mass flow rates (0.1, 0.15, and 0.21 mg/s, or 1.6, 2.4, and 3.4 sccm), keeper (0.1, 0.15, and 0.2 A), and anode currents (0.1–1 A) at a fixed cathode-to-anode distance. Seven criteria were used to distinguish between the spot and plume mode operations. The results show that the mode transition in low-current cathodes may be a nonlinear phenomenon, and only some of the existing mode transition criteria can be used to accurately predict the spot/plume discharge regions at low emission currents.

Modifications produced on a large magnetized plasma column by a floating end-plate that is partially emissive: Experiment and theory

An experiment is performed on a large plasma device operated by the Basic Plasma Science Facility at the University of California, Los Angeles, in which an electrically floating structure is placed near the end of the 18-m magnetized plasma column. The structure consists of a flat carbon plate that acts as a mask for a smaller, ring-shaped LaB6 emissive surface whose temperature can be externally controlled. This configuration has been previously used to study electron heat transport and pressure-driven avalanches [B. Van Compernolle and G. J. Morales, Phys. Plasmas 24, 112302 (2017)] by biasing the LaB6 ring-cathode with respect to a distant anode in a cold afterglow plasma. In contrast, the present study is performed during the active portion of the steady-state discharge in which the nominal plasma parameters are determined by the injection of an electron beam from a BaO cathode at the opposite end. It is found that, even without an applied bias on the LaB6 cathode, the self-consistent potential and current profiles are modified near the end plate as the LaB6 temperature is increased, resulting in density increases on the field lines in contact with the ring-cathode. In the absence of enhanced ionization, at the largest cathode temperatures, the ambient density can be doubled. A theoretical model is presented that provides a quantitative explanation for the experimental observations.

Boron vacuum-arc ion source with LaB6 cathode.

We describe a pulsed vacuum arc ion source with a lanthanum hexaboride (LaB6) cathode for high-dose selective implantation of 10B and 11B isotope ions. The design and main parameters of the source and its plasma generator are described, and research data are reported on the formation, transport, and magnetic separation of 10B and 11B ions extracted from the erosion plasma of a vacuum arc discharge. Cathode spots are initiated by flashover across an alumina ceramic block of diameter ∼2.5 mm located at the center of the working cathode. The design of the trigger system provides rather uniform cathode erosion, and the cathode dimensions (diameter of 14 mm, length of ∼15 mm) ensure long-term operation of the source. The plasma generator operates at a pulse repetition rate of up to 10 Hz with a discharge current pulse width of 280 µs. At an arc current of 130 A, the source provides an ion beam of a rectangular cross section of 0.6 × 6 cm2 with an energy of 25 keV and amplitude up to 75 mA. A mass analyzer magnet was used for mass-to-charge separation.

Processing windows for Ti-6Al-4V fabricated by selective electron beam melting with improved beam focus and different scan line spacings

Purpose Selective electron beam melting (SEBM) is a highly versatile powder bed fusion additive manufacturing method. SEBM is characterized by high energy densities which can be applied with nearly inertia free beam deflection at high speeds (<8.000 m/s). This paper aims to determine processing maps for Ti-6Al-4V on an Arcam Q10 machine with LaB6 cathode design. Design/methodology/approach Scan line spacings of 100, 50 and 20 µm in a broad parameter range, focusing on high deflection and build speeds are investigated. Findings There are broad processing windows for dense parts without surface flaws for all scan line spacings which are defined by the total energy input and the area melting velocity. Originality/value The differences and limitations are discussed taking into account the beam properties at high beam energy and velocity as well as evaporation related loss of alloying components.

Influence of the Environment on the Operational Safety of a Fluidized Bed Boiler

Abstract The use of fluidized bed boilers in modern power engineering is a promising solution for clean and economically acceptable combustion of various fuels, including coal, biomass and waste, for the generation of electricity. The fluidized bed boilers are nowadays technically advanced and complex combustion facilities where all individual boiler elements are subjected to withstand continuous structural and thermal loads during their operation. Intensive loading of boiler elements can be quite easily linked with boiler operational safety and is quite often one of the main reasons for emergency shutdowns followed by necessary replacements of the damaged items. In case of industrial large-scale circulating fluidized bed boilers the frequency of unplanned shutdowns is also quite often affected by the hydrodynamics of the fluidized bed and some other parameters, such as the intensity of solids circulation, temperature, solids concentration, flue gas composition and velocity, solids accumulation and deposition, as well as the corrosion or erosion of heat transfer surfaces. The present paper briefly reports the results of authors investigation focused on the morphology and structure of some chosen elements (steel anchors) sampled from one of Polish circulating fluidized bed boilers. The anchors were degraded during boiler operation and lost their mechanical durability. In order to determine the reasons of anchors’ degradation chemical composition of the elements was determined with the use of a spark spectrometer SPECTROLAB and sample morphology was investigated with JEOL JSM-6610LV scanning microscope equipped with LaB6 cathode.

Anode position influence on discharge modes of a LaB6 cathode in diode configuration

A laboratory model of a 5 A-class cathode was experimentally studied in diode configuration with a disk anode. The core of the cathode is a flat disk lanthanum hexaboride (LaB6) insert. Electron emission is achieved using a heating element in direct contact with the insert. The paper reports the characterization of the LaB6 cathode operated at xenon mass flow rates between 0.4 and 1.0 mg s−1 with discharge currents ranging from 2 A to 12 A. Apart from the operating envelope and discharge mode (spot versus plume) differentiation, the influence of anode position on cathode discharge mode was studied. For this purpose, the cathode was operated at 4 A and 0.6 mg s−1 and at 10 A and 0.6 mg s−1, while the cathode-anode gap was increased from 20 mm to 70 mm. Both electrical and plasma parameters were collected and analyzed in order to highlight the main changes in cathode discharge when the cathode-anode gap was increased. Particular attention was paid to the identification of the discharge mode and mode transition based on spectral analysis of discharge current waveforms. It was demonstrated that an increase in the cathode-anode gap induces the discharge mode transition from spot mode, corresponding to lower gap values, to plume mode, corresponding to higher gap values. Changes in plasma property were also noticed, the cathode-anode gap increase inducing lower plasma density and higher electron temperature in the cathode plume.