Filament Cathode Research Articles

A low-energy MHz repetition rate short-pulse electron gun

An electron gun that can produce MHz repetition rates and nanosecond pulses is described. The gun uses a Pierce grid in combination with an anode to extract electrons from a tungsten filament cathode. The electrons emerging from the anode are accelerated and focused using two triple-aperture lenses to form a beam. By applying a high slew rate grid pulse that transitions through the extraction voltage region of the grid/anode combination, pulses of electrons are produced from the gun that have temporal widths less than 5 ns. The pulsed beams are produced at both the rising and falling edges of the driving pulse. The characteristics of the emerging electron beams have been determined using an (e, 2e) coincidence spectrometer, and examples where they are used for time of flight decay measurements are presented.

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 %.

Magnetic field configuration effects on a miniature DC ion thruster with an inverted potential structure

To improve the mass utilization and total efficiency of miniature direct current (DC) ion thrusters, this article proposes a graphene–oxide–semiconductor (GOS) electron source as a cathode in the discharge chamber and neutralizer. To apply the GOS device to the discharge chamber, multiple electron sources were positioned downstream to provide sufficient electrons for plasma generation. Accordingly, the anode was placed upstream of the discharge chamber; thus, the potential structure was inverted from that of a conventional device. Based on this concept, two discharge chamber types were developed with different magnetic field configurations (1- and 2-Cusp types) using a filament cathode and their performance was experimentally evaluated. Probe diagnostics results showed that plasma was generated mainly in the center of the discharge chamber in 1-Cusp, whereas it was generated in the entire discharge chamber in 2-Cusp. Regarding ion beam extraction with 211-hole grids, 1-Cusp exhibited a higher screen current than 2-Cusp. However, using 391-hole grids with an expanded ion-extraction region, 2-Cusp outperformed 1-Cusp with 211-hole grids, attaining a minimum discharge loss of 273 W/A (excluding filament power) and a maximum mass utilization efficiency of 0.48.

Fast breakdown process and characteristics diagnosis of nanosecond pin–pin discharge

In this paper, the characteristics of a nanosecond spark discharge with a pin–pin electrode configuration have been systematically studied. Both a streak camera with high temporal resolution and an intensified charge-coupled device (ICCD) camera are employed together to investigate the breakdown and evolution process of the discharge. The formation of initial breakdown and mode transition from streamer to spark in the electrode gap are clearly observed on the time scale of several nanoseconds with a temporal resolution of 100 ps. In addition, the time-resolved spectra technology is also used to analyze the generation and quenching mechanisms of reactive species, the electron density, and the electron temperature. The results show that there is a 1.25 ns initial discharge breakdown and that a bright cathode spot exists before the transformation from streamer to spark channel. After a faster cathode filament and a slower anode filament propagate and merge at the electrode gap, the spark discharge phase begins. The generation processes of different reactive species depend on the discharge phase to a great extent. The N2* is first generated during the streamer phase while the O*, N*, and N+ are mainly generated under the spark phase, in which the electron temperature calculated by Boltzmann plots is 2.74 eV, and the electron density determined from the Stark broadening of O lines is on the order of 1016 cm−3.

Effect of complex ion-plasma treatment on mechanical properties of high-speed steel R6M5 surface

The paper describes the effect of complex ion-plasma treatment consisting of ion nitriding and subsequent application of a wear-resistant coating using a plasma source with a filament cathode on the physical and mechanical properties of the surface of high-speed steel P6M5. Two different technologies, a complex ion plasma technology and a protective coating of the (TiAl)N system, were compared. The results showed that the samples subjected to complex ion-plasma treatment have higher physical and mechanical properties.

Thermionic coating method with preliminary bombardment of the substrate surface with a stream of low energy ions

Relevance. Vacuum-plasma methods of applying functional coatings are widely used to increase the reliability and durability of machine and mechanism parts, cutting tools, and technological equipment, as well as in modern micro- and nanoelectronics for applying conductive and dielectric layers of various thicknesses. The study presents a method of thermionic coating that combines in one technological cycle the processes of surface cleaning by a stream of low-energy ions and plasma electron beam evaporation of a substance under conditions of thermionic plasma formation. Aim. The research aims to determine the possibility of using the presented methodology to create and process functional coatings without radiation damage to the substrate surface. Methodology. The experimental studies were carried out in a plasma electron beam system with a primary plasma source based on an arc discharge with a filament cathode. The samples were diagnosed using a scanning electron microscope. Results. The ion current density from the primary plasma was investigated to determine the efficiency of the surface cleaning mechanism by ion bombardment. The dependences of the ionic current density on the initial parameters of the experiment were determined. The results of the treatment of the (TiZr/TiSi) N sample surface with a plasma ion stream are presented. The obtained scanning electron microscope images showed that such treatment leads to the cleaning of the surface layer from various contaminants without damage and creates a substrate for further thermionic deposition of a coating with a high degree of adhesion. The mechanism of thermionic deposition of a titanium monolayer on a stainless-steel substrate previously cleaned by ion bombardment was studied. The revealed order of magnitude higher values of the ionic current to the substrate during the formation of thermionic plasma indicate a significant increase in the rate of the deposition process and contribute to the formation of a high-quality ion-plasma coating. Conclusions. The scanning electron microscope images of the sample surface showed that the titanium coating during thermionic deposition was uniformly distributed over the substrate surface, without any droplet phase, which indicates the suitability of this technique for applying functional coatings without radiation damage to the substrate surface

BOMBARDMENT OF THE SURFACE BY A LOW-ENERGY ION FLOW ACCELERATED IN THE NEAR-SURFACE LAYER OF THE SPACE CHARGE

The paper presents a method for the formation of low-energy ion flow for creation and treatment of functional coatings without radiation damage. Plasma source based in arc discharge with filament cathode creates the primary plasma in the volume of vacuum chamber where the treated sample is placed. The formation of low-energy ion flow takes place in the layer of space charge near the samples surface. The behavior of the ion current density depending on the potential drop in the layer has been studied for various experimental parameters and conditions. The sample surface treatment by low-energy ion flow has been also evaluated.

Vanadium pentoxide thin films deposited by the thermionic vacuum arc plasma

The Vanadium pentoxide (V2O5) thin films were deposited on glass substrates by a Thermionic vacuum arc (TVA) plasma system. The structural, elemental, surface morphological, and optical properties of the deposited thin films were investigated in detail using X-ray diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet-visible (UV–VIS) spectroscopy, field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). In this study, the V2O5 thin films deposited on the glass substrates were produced at three different arc powers at fixed cathode filament current and deposition time, and all characteristics of the deposited thin films were analyzed separately for these three arc powers. The thicknesses of the V2O5 thin films deposited at arc powers of 80, 280, and 640 W were measured as 92, 98, and 204 nm, respectively. The main objective of the research is to show that V2O5 thin films can be deposited by the TVA with the features specified by the technology, which cannot be arranged easily in other methods. While all the deposited thin films have an amorphous structure and roughness as small as 0.4 – 1.8 nm, the arc power did not cause any different bonding in the atomic structure. The transmittance of each film, in the spectral range of 300 to 900 nm, was measured and the optical energy band gap (Eg) was calculated for three films deposited at various arc powers and found to be 3.84, 3.42, and 3.20 eV for the direct allowed transitions respectively.

Revisiting “Non-Thermal” Batch Microwave Oven Inactivation of Microorganisms

Over the last few decades there has been active discussion concerning the mechanisms involved in “non-thermal” microwave-assisted inactivation of microorganisms. This work presents a novel non-invasive acoustic measurement of a domestic microwave oven cavity-magnetron operating at fo = 2.45 ± 0.05 GHz (λo ~ 12.2 cm) that is modulated in the time-domain (0 to 2 minutes). The measurements reveal the cavity-magnetron cathode filament cold-start warm-up period and the pulse width modulation periods (time-on time-off and base-time period, where time-on minus base-time = duty cycle). The waveform information is used to reconstruct historical microwave “non-thermal” homogeneous microorganism inactivation experiments: where tap-water is used to mimic the microorganism suspension; and ice, crushed ice, and ice slurry mixture are used as the cooling media. The experiments are described using text, diagrams, and photographs. Four key experimental parameters are indentified that influence the suspension time-dependent temperature profile. First, where the selected process time > the time-base, the cavity-magnetron continuous wave rated power should be used for each second of microwave illumination. Second, external crushed ice and ice slurry baths induce different cooling profiles due to difference in their heat absorption rates. In addition external baths may shield the suspension resulting in a retarding of the time-dependent heating profile. Third, internal cooling systems dictate that the suspension is directly exposed to microwave illumination due to the absence of surrounding ice volume. Fourth, four separated water dummy-loads isolate and control thermal heat transfer (conduction) to and from the suspension, thereby diverting a portion of the microwave power away from the suspension. Energy phase-space projections were used to compare the “non-thermal” energy densities of 0.03 to 0.1 kJ·m-1 at 800 W with reported thermal microwave-assisted microorganism inactivation energy densities of 0.5 to 5 kJ·m-1 at 1050 ± 50 W. Estimations of the “non-thermal” microwave-assisted root mean square of the electric field strength are found to be in the range of 22 to 41.2 V·m-1 for 800 W.

The Features of Intense Electron Flow Impact on Metal Hydride Electrode

The features of generation of a shielding plasma layer by a Zr50V50 metal hydride surface which prevents the sample from melting have been studied. The sample was interacting with an electron beam formed directly by the metal hydride. The electron beam was emitted from primary plasma generated by an additional discharge with a filament cathode and accelerated in the space charge layer at the front of the shielding plasma, which is formed on hydrogen desorbed from metal hydride or on the sample material in case of the depletion of stored hydrogen. Three different stages of the formation of shielding plasma layer have been identified depending on the ratio between the current to the metal hydride IMH and the current of the primary plasma source Id. When IMH/Id < 1 the classical conditions for charged particles transfer are realized. At IMH/Id > 1 the classical conditions for the transfer of charged particles are violated and double layer appears at the front of the shielding plasma, which ensures the efficient energy transfer from external electrical field to the energy of bipolar motion of charged particles.

100 s negative ion accelerations for the JT-60SA negative-ion-based neutral beam injector

In the negative-ion-based neutral beam injector (N-NBI) of JT-60SA, a hydrogen negative ion beam with energy of 500 keV and a current density of 154 A m−2 for 118 s has been successfully demonstrated. This achievement exceeds the requirements (500 keV, 130 A m−2, 100 s) for the first time. To maintain stable negative ion production for a long pulse, the optimal temperatures of the chamber wall and plasma grid were analytically examined and were experimentally demonstrated. It was confirmed that the temperature during the beam pulse should be <50 °C for the chamber wall and >200 °C for the plasma grid. Damage to the filament cathode due to an abnormal discharge, so-called arcing, has been mitigated by developing a fast cut-off system of the arc current for around 100 μs after the arcing. To maintain sufficient voltage holding capability and to reduce the grid heat load due to the beam in the accelerator, techniques developed for the beam acceleration have been applied to this test. As a result of the integration of these techniques, a stable beam over 100 s has been demonstrated successfully. This is the first achievement over a 100 s stable beam with intensity of >75 MW m−2, which is the required practical level in the N-NBI. These results contribute to the coming NBI system for ITER and DEMO.

HEATER OF THE HOLLOW CATHODE WITH LaB6 FOR OPERATION WITH HALL THRUSTERS

The article presents the results of design, development and laboratory tests at Space Electric Thruster Systems (Dnipro, Ukraine) of a hollow filament cathode based on lanthanum hexaboride (LaB6). The cathode is designed for joint operation with Hall thrusters of various input powers. The emitter material LaB6 is very promising, because has a high emissivity and low ionization potential. However, to realize all the advantages of considerate material, it is necessary to provide a sufficiently high operating temperature (1400 ° C). Therefore, when developing the filament cathode, special attention was focused on the development and research of one of the key elements of the filament cathode - the heater. Thermal calculations of the hollow cathode were carried out when it was heated with a heater, as well as when the cathode was operating in auto mode. The results of thermal calculations made it possible to identify the shortcomings of the cathode structure and parameters and determine the ways to eliminate them. Laboratory tests of a cathode with a different number of heater thermal shields have been carried out. The results obtained made it possible to provide the required operating temperature of the cathode emitter with a minimum number of heat shields. Laboratory tests of the cathode were carried out both in diode mode (to the external anode) and in conjunction with the ST-25 Hall thruster developed by the company. In the course of laboratory tests of the cathode, the power supplies for the discharge, the keeper, and the heater, which are components of the power processing unit of the Hall thruster, were used. In the frame of experimental studies, a cyclogram of the cathode starting process was obtained, which is necessary to form the Hall thruster starting sequence. The tests carried out confirmed the correctness of the selected technical solutions for the choice of the cathode and its heater. The developed cathode can be used both in low-power (up to 200 W) Hall thrusters and in thrusters with increased input power (up to 5 kW).

Radial control of plasma uniformity and electron temperature by external plate biasing in a back diffused partially magnetized plasma

The possibility of achieving uniform and controllable plasma distribution remain a fundamental challenge in low pressure plasma discharges, which has numerous applications in plasma technologies. An external object when introduced in a magnetized plasma can significantly alter the spatial plasma distribution. In this paper, the effect of external plate biasing on both axial as well as radial characteristics of an expanding, partially magnetized plasma column, created by hot cathode filament inside a linear plasma device is presented. It is found that by applying a positive potential to a conducting external electrode, placed at a remote location away from the primary discharge region; the back diffused plasma tends to become uniform. The plate biasing also results in an overall increase in electron temperature of the expanding plasma; however, it does not appear to alter the radial plasma characteristics inside the source and the biasing region. A possible mechanism behind observing this effect has been provided.

Investigation of Sulfur Melter Heating Coil as an Industrial Product: A Study Case on Technical Design and Structural Inspection

The rapid development of technology in the industrial center encourages the need for skilled and expert human resources. The industry always innovates in developing the quality of products to achieve the desired targets. One of them is a heater to heat a fluid, which one them is heat exchanger type. The most important part of the heater is the cathode heater coil or filament which is used to heat the cathode in a vacuum tube or cathode ray tube. Fabrication of coil heater products is interesting to study, which covers design, manufacture, and machining activities. This article discusses the fabrication process of the sulfur melter heating coil, which is carried out in one of manufacture industries. The research method is conducted by field survey to observe the design and fabrication process. Overall production stages are divided into the design stage which is to obtain technical illustration of the product, material selection stage which is to calculate and determine product material, manufacturing stage which is to determine the machining process of each product part, assembly stage which is to determine the type of jointing in each product part, and finally, the structural inspection stage which is to check the standard of fabricated products.

A prototype Multi-X-ray-source array (MXA) for digital breast tomosynthesis

The design and testing of a prototype Multi-X-ray-source Array (MXA) for digital breast tomosynthesis is reported. The MXA is comprised of an array of tungsten filament cathodes with focus cup grid-controlled modulation and a common rotating anode housed in a single vacuum envelope. Prototypes consisting of arrays of three-source elements and eleven-source-elements were fabricated and evaluated. The prototype sources demonstrated focal spot sizes of 0.3 mm at 45 kV with 50 mA. Measured x-ray spectra were consistent with the molybdenum anode employed, and the tube output (air kerma) was between 0.6 mGy/100 mAs at 20 kV and 17 mGy/100 mAs at 45 kV with a distance of 100 cm. HVL measurements ranged from 0.5 mm Al at 30 kV to 0.8 mm Al at 45 kV, and x-ray pulse widths were varied from 20 ms to 110 ms at operating frequencies ultimately to be limited by source turn-on/off times of ∼1 ms. Initial results of reconstructed tomographic data are presented.

POWER SUPPLY SYSTEM FOR THE HELIUM IONS LINAC’S INJECTOR

This paper presents compact power supply system for the helium ion linac’s injector. The system consists of high-voltage modulator, power supplies of cathode filament, solenoid and arc modulator. It is designed on modern semi-conductor base. Using of the designed power supply system allowed to decrease the power consumption and to get rid of spurious capacitances and pickups. As a result of improving the continuous adjustment modulator and high-voltage pulse stabilization scheme, the accelerated beam current has increased twice.

Development of the MiXI Thruster with the ARCH Discharge

The Miniature Xenon Ion (MiXI) thruster with the Axial Ring-Cusp Hybrid ‘MiXI(ARCH)’ discharge was developed and operated with beam extraction at 1 kV. The thruster achieved 59% cathode-free total discharge efficiency at 23.7 mA xenon beam current with filament cathodes and low temperature operation, corresponding to a discharge loss of 226 W/A and propellant utilization of 72%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ∼320 °C due to the thermal isolation of the thruster head. Reducing the discharge chamber aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57% but required slow ramping of beam voltage and was limited to stable operation to above 0.5 sccm discharge propellant flow. In contrast to the 3-ring cusp configuration, MiXI(3-Ring), the performance is generally higher but is not able to achieve lower thrust levels and requires more complex start-up for stable operation. An analytical single-cell model was developed and applied to investigate internal processes of the MiXI(ARCH) discharge. The model emulated the effect of increasing flow on performance, indicating that the dominant loss mechanism is plasma electron current to the anode, in contrast to the 3-Ring geometry, which is dominated by primary electron losses. This model also matched trends reported in previous works of strongly increasing electron temperature and primary density with propellant utilization. Through this effort, the MiXI thruster’s highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.

Mechanical Stability of Filaments for ITER Diagnostic Pressure Gauges Relating to Creep and Fatigue

Diagnostic pressure gauges (DPGs) shall provide measurements of the neutral gas pressure in various locations of the International Thermonuclear Experimental Reactor (ITER) vacuum vessel. This parameter is essential for the basic control of ITER operation as well as for input to physics models of the plasma boundary. The hot cathode (filament) is the component of the DPG sensor that is exposed to most demanding loads. In order to reach the required electron emission from the filament, it has to be heated to high temperatures by a direct current. As a result, thermal stresses appear, and in the presence of a magnetic field, additional mechanical stresses caused by Lorentz forces arises. On the one hand, due to a large number of pulses foreseen in ITER, this load will be cyclic, which may result in filament failure caused by fatigue. On the other hand, the longest pulses in ITER are expected to be in the order of 30 min, and thus, filaments could fail due to creep. In order to verify that the filament of the DPG sensor can withstand fatigue and creep dedicated experiments have been conducted. Results of these tests are discussed in this article.

Signatures of ring currents in a magnetic mirror plasma experiment

Spatial profiles of electron density and temperature obtained from Langmuir probe data in a magnetic mirror plasma experiment using permanent rare-earth magnet stacks show clear signatures of azimuthal or ring currents generated by grad B and curvature drifts. The plasma-generating hot cathode filament is placed within the mirror so that the primary electrons generated with energies ≃75 eV are confined by the magnetic mirror effect resulting in a combination of a rapid bounce motion with a slower azimuthal drift whose direction is determined by the orientation of the magnet stacks. A spatial scan using a movable Langmuir probe system shows two peaks of unequal amplitude in the hot electron density profile at locations along the probe path corresponding to ϕ = 90° and ϕ = 270° where the filament is located at ϕ = 0. The position of the stronger peak is consistent with the shorter path in the electron drift direction for the choice of orientation of the magnets. Reversing the magnetic orientation exchanges the locations of the strong and weak hot electron density peaks. The dependence of the ratio of the two peak amplitudes on gas pressure p is consistent with exponential attenuation of the hot electron density along the drift orbit with a mean free path λmfp ∝ 1/p.

Testing an indirectly heated cathode ion source: Temperature and thermionic emission

Here we compare the cathode surface temperatures and thermionic emissions of a novel thermal barrier cathode against original equipment manufacturing (OEM). The new indirectly heated cathode ion source assembly employs a cathode cup unit and filament arrangement, where the filament has a flat face that is separated from the cup-shaped cathode body. The filament is placed in a space that is surrounded by a thermal barrier to reduce thermal losses. The thermal barrier is formed by a plurality of concentric foils that are closely spaced. We observed an improvement on the cathode surface temperature and thermionic emission relative to OEM. The addition of the thermal barrier in combination with the large cathode surface area increased the thermionic emission.