Hollow Anode Research Articles

Voltage-current characteristics of an anodic arc producing carbon nanotubes

Voltage-current (V-I) characteristics of the carbon nanotube producing anodic arc are measured for different gap sizes (up to 13 mm), anode compositions, and background He pressures (in the range of about 300–500 Torr). Arc is supported by the erosion of 6.35 mm hollow anode packed with composition of Ni, Y, and carbon powders. Experiments were carried out for the arc currents in the range of about 10–100 A. It is shown that V-I characteristics have a V-type shape with minimum point corresponded to arc current of about 25–30 A and arc voltage of 20–25 V at the gap of 2.5 mm. V-I characteristic shifts 15–25 V to higher arc voltages with increasing gap from 2.5 to 13 mm, while the position of voltage minimum shifts by about 15 A to higher arc currents. The increase in the metallic catalyst fraction in the anode composition (from C:Ni:Y=56:4:1 to 36:4:1) results in the slight decrease in the arc voltage (about a few volts) and slight shift of the minimum position to higher arc currents (5–10 A). Anode erosion rate increases from below 5 mg/s to about 44 mg/s with arc current increase from 40 A to about 100 A.

Neutron yield and pressure evolution during a dense plasma focus device shot series

Adsorption and desorption of the filling gas, by the electrodes, the insulator and chamber materials of plasma focus devices, have been suggested as probable causes for the fluctuations in their neutron yield. This work describes analysis of data, aimed at looking for evidence to support this hypothesis. Before starting each series of discharges, a vacuum around 10−6 Torr is achieved. The filling gas, pure deuterium, is maintained under static conditions. A sudden fall of the initial pressure, around 5%, is systematically observed after the first shot in each series, before creeping back at an almost constant rate, in successive shots. On the other hand, for the first shot with fresh filling gas, the neutron yield is always low and systematically increases for the second one. The pressure evolution for the following shots shows no correlation with the neutron yield fluctuations. Thus, except for the first two shots, we find no evidence to support the hypothesis that the neutron yield fluctuations are related to an adsorption–desorption process. It is also observed that a tendency exists for the last shots of each series to yield a larger number of neutrons but with a larger dispersion. This study has been done with both solid and hollow anodes, showing qualitatively similar results in both cases.

Non-disturbing measurements of hollow-anode plasma parameters

Thomson scattering and visible spectroscopy were applied to study the parameters of plasma formed in a hollow-anode discharge (about 1 kA) prior and during electron beam extraction (approximately 1 kA; 300 kV). The discharge was ignited and sustained by ferroelectric plasma sources incorporated in the hollow anode. It was found that during the hollow-anode operation the density and energy of the ferroelectric surface plasma electrons are about 1015 cm−3 and not more than 5 eV, respectively. The density and temperature of the bulk hollow anode plasma electrons were found to be about 6×1013 cm−3 and about 10 eV, respectively. During an acceleration pulse the surface plasma electron density and energy increased to approximately 6×1016 cm−3 and not more than 20 eV respectively, while the bulk plasma parameters remain unchanged. Also, it was shown that the plasma formation on the ferroelectric surface is accompanied by the generation of microparticles having velocities up to 3×105 cm s−1.

Development of soft plasma ionization (SPI) source for analysis of organic compounds

We present a newly designed soft plasma ionization (SPI) source developed for mass spectrometric study of organic compounds in this study. The SPI cell having a relatively small size consists of a hollow anode and a hollow mesh cathode. The voltage–current characteristic depending on the pressure was investigated, indicating that it has similar characteristics to conventional hollow cathode glow discharges. To investigate the emission characteristics of the SPI source, some molecular band emission spectra (N 2, N 2 + and OH +) were measured by using argon and helium discharge gases. The SPI source was installed to a commercially used quadrupole mass analyzer for analyzing organic compounds. To demonstrate the SPI source, the mass spectra of some organic compounds (methylene chloride, toluene, benzene, cyclohexane and chloroform) were measured. The organic compounds were ionized with good stability in the plasma, and the fragmentation depended on the applied current. When helium and argon gases were used as the discharge gas, the helium plasma was more suitable for SPI-MS rather than argon because the argon plasma not only suffers from spectral interference but also has lower sensitivity.

Effect of Anode Designs on Ion Emission Characteristics of a Plasma Focus Device

A comparative study on the ion emission characteristics such as flux and energy, and their variation in angular positions and operating gas pressures has been carried out in a nitrogen-filling plasma focus device. Three different designs of cylindrical anode (central electrode) having hollow, solid and hemispherical tip have been tested for this study. The ion emission characteristics were investigated by employing three Faraday cups at various angular positions. The ion flux depends on the operating gas pressure irrespective of the anode designs and the maximum ion flux is found to be in the pressure range 0.3 to 0.5 Torr for all the anode designs. The hemispherical anode yields highest ion flux while the hollow anode emits lowest ion flux. The angular variation of ion flux is seen to be anisotropic irrespective of the anode designs with an ion dip at 0° (axis of the device) and maximal at 5° angular positions. The anisotropic character of ion emission is less in the case of the hemispherical anode than the hollow anode. The ion energy, measured by the time of flight method, shows its dependence on the anode designs. The maximum ion energy is found to be around 830 keV at an angular position 5° in the case of the hemispherical anode design. The most probable ions are found to be with energy less than 100 keV irrespective of the anode designs and the angular positions. This study indicates that the plasma focus device could be optimized to a great extent for optimal ions yield by using an appropriate anode design.

Broad beam gas ion source with hollow cathode discharge and four-grid accelerator system

A broad beam gas ion source based on low-pressure hollow cathode glow discharge is described. An axial magnetic filed produced by AlNiCo permanent magnets enhances the glow discharge in the ion source as a result of the magnetizing electrons between the hollow cathode and rod anode. The gas plasma is produced by magnetron hollow cathode glow discharge in the hollow cathode and a collimated broad ion beam is extracted by a four-grid accelerator system. A weak magnetic field of several millitesla is enough to ignite the magnetron glow discharge at pressure lower than 0.1Pa, thereby enabling stable and continuous high-current discharge to form the homogeneous plasma. A four-grid accelerator, which separates the extraction and acceleration of the ion beam, is used in this design to generate the high-energy ion beam from 10keV to 60keV at a working pressure of 10−4Torr. Although a higher gas pressure is necessary to maintain the low-pressure glow discharge when compared to hot filament discharge, the hollow cathode ion source is operational with reactive gases such as oxygen in the high-voltage continuous mode. A laterally uniform ion beam can be achieved by using the four-grid accelerator system. The effects of the rod anode length on the characteristics of the plasma discharge as well as ion beam extraction from the ion source are discussed.

Thomson scattering diagnostics of the plasma generated in a hollow anode with a ferroelectric plasma source

Thomson scattering of a laser beam was applied to study the plasma parameters inside a hollow anode having a ferroelectric plasma source incorporated in it. This method allowed avoiding difficulties related to spectroscopical measurements in the case of unknown electron energy distribution. It was found that the electron density and energy of the ferroelectric plasma are ∼1015cm−3 and ⩽5eV, respectively, and the density of the hollow anode bulk plasma is ∼6×1013cm−3. Applying an accelerating pulse for electron extraction from the bulk plasma leads to an increase in the electron density and energy of the ferroelectric plasma up to 6×1016cm−3 and ⩽20eV, respectively.

Passive and Active Plasma Emission Sources for High-current Electron Beam Generation

In this review we present main results of experimental research of passive and active plasma sources for high-current electron beam generation obtained during the last few years in our Plasma & Pulsed Power Laboratory. We will describe passive plasma sources (ceramic-metal, velvet and carbon fiber cathodes) based on flashover plasma and active plasma sources based on a ferroelectric plasma source (FPS) as well as on an FPS-assisted hollow anode (HA) plasma source. The main data concerning the plasma parameters (plasma density and temperature, plasma uniformity and plasma potential) and the main features of these plasma sources (plasma formation, life time, vacuum compatibility) will be discussed. Also, data concerning electron diode operation and parameters of the generated electron beam while using these plasma sources will be presented.

Effects of electron-focusing electric field upon enhanced glow discharge plasma ion implantation

The effects of the electron-focused electric field on enhanced glow discharge plasma ion implantation (EGDPII) are investigated and an enhancement mechanism is proposed. By using a small pointed anode and large area tabular cathode, an electric field that can focus the electrons is set up. The electrons concentrate on the pointed hollow anode and the secondary electrons play an important role in producing and sustaining the plasma.

Reactive sputter-deposition of AlN films by dense plasma focus

A low energy (1.45kJ) dense plasma focus device is used to deposit thin films of aluminum nitride (AlN) at room temperature on silicon substrates. For deposition of films, a conventional hollow copper anode is replaced with a solid aluminum anode and nitrogen is used as fill gas. The films are deposited using a multiple number of focus shots by placing the substrate in front of the anode. The deposited films are characterized using x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, and a microhardness test. The XRD analysis of the films shows that the deposited films show strong c-axis alignment. The Raman spectra of the films indicate that the deposited films are under compressive stress and crystalline quality decreases with increasing number of focus shots. The microhardness results point toward the uniform deposition of hard AlN layers on silicon substrates.

Novel Trigger Mechanism High-Power Switch: The Electrostatic Plasma Injection Switch

High-power gas switches like the Pseudospark have been developed for decades, and past research has focused on the trigger mechanism of Pseudospark. This paper presents a new type of switch called the electrostatic plasma injection switch (EPIS), which uses transmission-line reflections of incoming trigger pulse to generate voltage gradients on a vertical multisection trigger structure that extends from a hollow cathode into a symmetrical hollow anode. Low-current preionization glow discharge at the bottom of the cathode region provides seed electrons for fast turn on. The trigger voltage injects plasma into the trigger structure and then accelerates plasma vertically into the gap between anode and cathode. Similar to Pseudospark, a superdense-glow-discharge plasma is responsible for the high current. The trigger electron density is measured by optical spectroscopy to be 3times1015/cm3 inside the hollow cathode and 8times1014/cm3 at the anode-cathode gap. plasma injection speed is 1-4times107 cm/s between 0.1and 0.4-torr pressure and is proportional to pressure and trigger voltage. Delay time decreases when the pressure increases. The EPIS has been operated at 16.6-kV hold-off voltage and 4.96 kA with 95 mtorr of hydrogen. This switch operates in the left-hand branch of the Paschen curve, as do Thyratron and Pseudospark, but it should provide better trigger reliability and higher current capability. Hold-off voltage is limited by local field emission. This device is operational, and waveforms of anode voltage, anode current, trigger voltage, and plasma emissions are recorded. Future applications are discussed

High-current large-area uniform electron beam generation by a grid-controlled hollow anode with multiple-ferroelectric-plasma-source ignition

We report results on the generation of a large-cross-section (about 170 cm2) high-current (about 1000 A) uniform electron beam by a hollow anode (HA) plasma source at a pressure of approximately 8 × 10−5 Torr, in a diode with an accelerating pulse of 300 kV and approximately 300 ns duration. The HA discharge was sustained for about 10 μs by seven Ba–Ti-based ferroelectric plasma sources. The resistive decoupling of each plasma source produces a uniform plasma density distribution at the HA output grid at a discharge current of not more than 1000 A. It was found that the HA plasma is characterized by a density of about 1012 cm−3, an electron temperature of approximately 8 eV and a group of fast electrons with an energy of about 50 eV. It was shown that an increase in the HA output grid potential allows the plasma prefilling of the accelerating gap to be reduced significantly.

Potential distribution in an ion sheath of non-Maxwellian plasma

A model is suggested that shows that in a plasma with a group of fast (non-Maxwellian) electrons, a nonmonotonic distribution of the potential can be formed inside the ion sheath with the potential maximum larger than the biased electrode potential. It was shown that the excess of the maximum potential above the biased electrode potential value depends on the ratio between the density of non-Maxwellian electrons and the density of the plasma bulk electrons. The results obtained in experiments with a hollow anode plasma source, which is characterized by the existence of a fast electron group, qualitatively agree with the model predictions.

Nano-structured Fe thin film deposition using plasma focus device

This paper reports the deposition of nano-structured Fe thin films using 3.3 kJ Mather-type plasma focus. The conventional hollow copper anode was replaced by anode fitted with solid Fe top and the deposition was done using different numbers of deposition shots at two different angular positions. Scanning Electron Microscopy shows that the size of nano-phase agglomerate is smaller when the sample is deposited using either lesser number of deposition shots or at higher angular position with respect to anode axis. X-ray Diffraction shows that crystal structure characteristics change with increase in number of deposition shots. Measurements of magnetic properties using Vibrating Sample Magnetometer identify intermediate magnetization and coercivity in Fe thin films deposited at smaller angular position with respect to anode axis. It is concluded that the morphological, structural and magnetic characteristics of Fe thin films deposited using plasma focus device depend not only on the number of focus deposition shots but also on the angular position of the sample.

Striations in dielectric barrier discharge systems with a hollow anode

The dynamics of the charging of an elongated hollow anode covered with a dielectric layer by an electron current is studied via three-dimensional particle-in-cell/Monte Carlo kinetic simulations. It is shown that this process is accompanied by the successive formation of striations in the plasma created by the electron impact ionization of the background gas. A number of specially designed numerical experiments are performed to further explore the nature of this phenomenon. An experimental setup where this type of striations should be observed is suggested.

アノードレイヤ型ホールスラスタ内部のシース構造と放電電流振動の数値解析

To investigate a role of a hollow anode for discharge stabilization in anode-layer hall thrusters, plasma dynamics inside a thruster was computed using the fully kinetic 2D3V Particle-in-Cell (PIC) and Direct Simulation Monte Carlo (DSMC) methods. The developed code successfully reproduced the measured discharge current waveform and threshold magnetic flux density for the oscillation onset. At the low magnetic flux density corresponding to the stable discharge case, ionization in the hollow anode was found vigorous and an ion sheath was created on the anode surface. This sheath contributed to the discharge stabilization. However, the amount of ionization in the anode decreased with the magnetic flux density, and the sheath structure changed to an electron sheath at the threshold magnetic flux density.

Repetitively pulsed CO2 laser driven by an electron accelerator with a gas-discharge plasma cathode

A repetitively pulsed electron-beam-controlled CO2 laser driven by an electron accelerator with a plasma electron emitter based on low-pressure glow discharge with hollow anode and cathode is presented. The application of the proposed emitter makes it possible to sharply increase the current and to control the electron-beam duration and, hence, the energy and time characteristics of the laser. It is demonstrated that the radiation pulse duration ranges from 300 to 1200 μs, whereas the radiation energy amounts to 200 J at an efficiency of 18%. In the course of lasing, the focal spot is not affected by the heterogeneous heating of the active medium. A significant heterogeneity in the gas flow upon an incomplete renewal of the gas in the zone filled with radiation leads to an increase in the focal spot owing to the wavefront distortions.

Experimental studies of anode sheath phenomena in a Hall thruster discharge

Both electron-repelling and electron-attracting anode sheaths in a Hall thruster were characterized by measuring the plasma potential with biased and emissive probes [L. Dorf, Y. Raitses, V. Semenov, and N. J. Fisch, Appl. Phys. Lett. 84, 1070 (2004)]. In the present work, two-dimensional structures of the plasma potential, electron temperature, and plasma density in the near-anode region of a Hall thruster with clean and dielectrically coated anodes are identified. Possible mechanisms of anode sheath formation in a Hall thruster are analyzed. The path for current closure to the anode appears to be the determining factor in the anode sheath formation process. The main conclusion of this work is that the anode sheath formation in Hall thrusters differs essentially from that in the other gas discharge devices, such as a glow discharge or a hollow anode, because the Hall thruster utilizes long electron residence times to ionize rather than high neutral pressures.

Low-pressure hollow-anode plasma sources

The operation of hollow anodes (HAs) with either multi-arc or magnetron-like or ferroelectric plasma source (FPS) ignition is described. These HA sources produce in the vicinity of the HA output grid a plasma with a density n ≈ 5 × 1012 cm−3 and this plasma acquires a positive potential of several tens of volts. HAs with an incorporated FPS have demonstrated the best operation characteristics and smallest dimensions. Also, the FPS allows us to achieve an HA discharge with a current amplitude I d ≤ 1.2 kA at a gas pressure of about 10−5 Torr. These HA modifications were used as a cathode in a diode powered by a 200 kV, 400 ns pulse. The parameters of the electron beam with an amplitude of 1.5 kA or less are presented. The accelerating pulse causes an increase in the HA plasma potential of up to 6 kV. A model that explains the electron emission from the positively charged plasma is proposed.

Plasma Cathode for a Broad-Beam Electron Accelerator

We propose a grid-stabilized plasma cathode based on a slit-contracted low-pressure glow discharge with hollow anode. The area of the plasma cathode is one order of magnitude higher than that in systems where electrons are extracted immediately from plasma in the cathode cavity. Conditions for the discharge initiation, the current switching to the hollow anode, and the obtaining of uniform emission from the plasma cathode are determined. At an accelerating voltage of 160 kV, an electron beam with a 1000 × 180 mm cross section, a total current of several amperes, and a current pulse duration of up to 10−3 s was obtained. The plasma cathode operates under technical vacuum conditions (air, 0.1 Pa) and ensures high stability and reproducibility of the beam current pulses.