Ion Beam Current Density Research Articles

Electron heating mode in magnetic field diffusion region of microwave discharge ion thruster

In magnetic field design principle of microwave discharge ion thruster, it is universally received that enlarging the magnetic mirror region can confine more electrons to acquire better energy utilization rate, while reducing the magnetic field diffusion region can prevent electrons from losing at wall to reduce the discharge loss. However, recently the integrated simulation proposes a hypothesis that electrons can also be heated in the magnetic field diffusion region when the Child-Langmuir sheath is considered as a constraint condition for electrons. Therefore, herein a magnetic field structure for the magnet array microwave discharge ion thruster is designed to verify the hypothesis, in which the magnetic field diffusion region is located near the screen grid. Then, an integrated simulation is conducted for studying the initial discharge and ion beam extraction stages of the thruster. The simulation results show that in the magnetic field diffusion region, the electron temperature is 4–8 eV when the grid system voltage is not applied, while the electron temperature is 4–12 eV when the the grid system voltage is applied. And the plasma density in the latter case has one order of magnitude higher than that in the former case. It means that electrons are obviously heated in the magnetic field diffusion region when they are confined among the Child-Langmuir sheath, the plasma sheath at antenna surface, and magnetic mirror. This electron heating mode produces more high-energy electrons outside the magnetic mirror region to generate plasma in front of the grid system, which can significantly increase the plasma density and ion beam current density. The result shows that under the conditions of 0.3 sccm (1 sccm = 1 mL/min) xenon gas flow, 1 W input microwave power, 300 V screen grid voltage and –50 V acceleration grid voltage, the ion beam current and its density are 0.47 mA and 0.60 mA/cm<sup>2</sup> for the magnet array microwave discharge ion thruster, while the ion beam current and its density are 1.2 mA and 0.38 mA/cm<sup>2</sup> for the 2-cm microwave discharge ion thruster. The ion beam current density increases by 57.9%. Through the integrated simulation, a new electron heating mode in the magnetic field diffusion region is proved theoretically, which provides a theoretical basis for the magnetic field structure optimization of microwave discharge ion thruster.

Solid-State Dewetting of Thin Silver Films into Spherical Nanoparticles under High-Current Pulsed Ion Beam Irradiation.

Noble metal nanoparticles (NPs) are important in many applications, including light trapping of photovoltaic cells, photoelectrochemical applications, etc. The present study reports the formation of silver NPs from the as-deposited silver coatings on fused silica substrates by solid-state dewetting induced by high-current intense pulsed ion beam (IPIB) irradiation. We described the effects of IPIB irradiation with different ion beam current densities and numbers of pulses on NP morphology and compared the results with conventional rapid thermal annealing (RTA). IPIB irradiation enables superfast heating (higher than 109 K/s) and cooling, providing a superfast annealing solid-state dewetting mechanism. Our results demonstrate that the sphericity of silver NPs is enhanced after IPIB irradiation relative to RTA-annealed silver NPs. Our results suggest further possibilities of shape and sphericity control of silver NPs with very fast heating/cooling annealing rates.

Theoretical basis and performance research on advanced non-contacting ion beam manufacturing process for optical materials

Ultra-smooth surface is very important for the performance of high energy optical system in the defence field. Low quality optical elements will cause energy loss and affect the performance of the optical system. Ultra-precision manufacturing technology of optical components is the key technology to obtain high quality optical components. As an advanced ultra-precision and no-contact surface processing technology, Ion Beam Manufacturing (IBM) was studied. Based on the analysis of the relationship between the ion beam current density distribution parameters obtained by faraday scan and the removal function, more effective removal function calculation method was found, the removal function model for IBF fused silica was established. The Molecular Dynamics(MD)simulation to study interaction between incident ion and optical substrate. Ion particle motion behaviour were obtained under different incident energies. The effect of incidence angle on surface roughness were carried out, lower surface roughness by smaller angle incident, ultra-precision surface of optical components were obtained by IBF experiments.

Study of the characteristics of the system: Ion source-ion decelerator.

The features of the method for the formation of dense low-energy ion beams in the system consisting of ion source-ion decelerator are considered. According to this method, a wide ion beam is formed using an ion source with an ion energy of 500eV and higher, and then the ions are decelerated just before landing on the substrate surface by an electric field created in the ion decelerator. In the considered ion decelerator, the electric field that slows down the ions is created in a gas discharge in E × B fields due to the Hall effect. The decelerated electrode imitating the substrate is located in the field of action of a magnetic field with an induction of 0.45T. The ion decelerator is characterized as a Hall magnetohydrodynamic converter of the ion beam kinetic energy into electrical energy. The analysis of plasma processes in the converter has been carried out on the basis of the model of the ion-vacuum regime of a discharge in E × B fields for estimating the expected properties of the converter and their verification based on experimental results. In experiments, an ion source of the Kaufman type formed an argon ion beam with a density of 0.5-2 mA/cm2 and ion energy of 300-1500eV was used. The voltage-current characteristics of the converter are given. The ratio of electron and ion components of the current in the circuit of the decelerated electrode was determined when the beam space charge neutralizer in the form of a heated filament was switched on and off. Using a Langmuir probe and a thermionic probe, the distributions of the ion beam current density and the potential of the plasma surrounding the ion beam in the beam transport space has been studied. A significant influence of the potential of the decelerator electrode on the plasma potential in the region of ion beam transport is shown. The features of the discharge in a specific design of the ion decelerator for a deceleration degree of ions to an energy of 50-75eV are revealed.

Study of the propagation of an intense ion beam to the target

The results of studying the intense ion beam propagation with peak energy up to 330 keV are presented. The ion beam was injected into the metal tube made from stainless steel in the form of a hollow cone. It was found that the ion beam focused due to neutralizing the space charge and reducing the divergence of ions. Neutralization of the beam space charge was investigated by measuring its electric potential. The average energy density of a beam increased up to 2.9 J/cm2 over the beam cross-section in the focal plane. The ion beam current density in the tube rose by a factor of 3 in comparison with geometric focusing. The analysis of the surface structure of the ion beam spot obtained on the stainless steel AISI 304 plate was done.

Status of a point-like neutron generator development

In this paper we report on a high current density ion beam profile diagnostics with a slit-based system as a reliable method, capable of high thermal load applications. The task arose in frames of a point-like neutron source development for neutron radiography. In previous research, it was suggested to construct such a system as a D-D neutron generator based on the high current gasdynamic ion source, which utilises the plasma of electron cyclotron resonance discharge sustained by powerful millimeter wave gyrotron radiation. This device is able to produce focused D+ beams with a characteristic diameter of 1 mm, total current above 100 mA, and current density at a level of several A/cm2. Study of such intense beams profile to obtain the best focusing efficiency and minimize neutron producing area appeared to be a challenging task. The paper also demonstrates the possibility of fast neutron imaging with a point-like powerful neutron generator (neutron yield on the level of 1010 s−1).

Negative ion beam extraction in volume mode on the RF negative ion source at ASIPP

• A test equipment of RF negative ion source was modified according to the commissioning results of negative ions extraction. • The negative ion beam current density without Cs seeding was increased to 25 A/m 2 at 50 kW RF power. • The extracted H- current tended to be saturated at high RF power, but the co-extracted electron current kept increasing. • The e/H- ratio is monotonously decreased with the rising bias voltage. Hefei utility negative ions test equipment with RF source (HUNTER) has been developed at the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP), in order to study the RF negative ion source for neutral beam injection application. This paper reports on several recent modifications made to HUNTER, including magnetic confining field, magnetic filter field, extraction area, pumping system, RF generator, high voltage power supply. And then, the performance of negative hydrogen ion beam extraction in volume mode without Cs seeding was reviewed and compared to the former commissioning results. The current of extracted negative ions and co-extracted electrons were recorded by the Hall current sensors installed on the electric wires connecting to the extraction grid, and ground grid and calorimeter. A similar extraction experiment from argon plasma verified the effective deflection and absorption of co-extracted electrons on the extraction grid, where the leaking electrons were less than 2%. The characteristic of negative ion beam extraction in volume mode was analyzed in different RF power, extraction voltage, bias voltage. The extracted negative ion density of 25 A/m 2 was achieved and the corresponding ratio of electron and negative ion current I e /I H- was 9.1, with the RF power of 50 kW and the bias voltage of 29 V. The extracted H − current saturated with RF power, while the co-extracted electron current increased. The current experimental results indicated the HUNTER source was operated on a right trace and is ready for the surface mode operation with Cs seeding.

Nanostructures on Sapphire Surfaces Induced by Metal Impurity Assisted Ion Beam

The metal impurity assisted ion beam technology has shown its uniqueness and effectiveness in the formation and precise control of nanostructures on the surface of materials. Hence, the investigation in this area is vital. The morphology evolution of self-organized nanostructures induced by Fe co-deposition assisted Ar+ ion beam sputtering at a different distance from the impurity target was investigated on sapphire, using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). We also investigated the role of metal impurities on sapphire ripple formation. Experiments were carried out at an oblique angle of incidence 65° with constant ion beam current density 487 μA/cm2 and the erosion duration of 60 min at room temperature (20 °C). The introduction of Fe impurity increased the longitudinal height and roughness of the surface nanostructures. Moreover, the amounts of Fe deposited on the surface decreased with increasing distance, and the morphology of the smooth sapphire surface demonstrated a strong distance dependence. Differences in surface morphology were attributed to changes in metal impurity concentration. With an increase of impurity target distance, island-like structures gradually evolved into continuous ripples. At the same time, the orderliness of nanostructures was enhanced, the longitudinal height gradually decreased, while the spatial frequency was unchanged. In addition, there were very few metal impurities on the etched sample. During the ion beam sputtering process, island-like structures promoted the growth of ripples but destroyed their orderliness.

The impact of focused ion beam induced damage on scanning spreading resistance microscopy\xa0measurements

Scanning Spreading Resistance Microscopy is a well-established technique for obtaining quantitative two- and three-dimensional carrier profiles in semiconductor devices with sub-nm spatial resolution. However, for sub-100 nm devices, the use of focused ion beam becomes inevitable for exposing the region of interest on a sample cross section. In this work, we investigate the impact of the focused ion beam milling on spreading resistance analysis and we show that the electrical effect of the focused ion beam extends far beyond the amorphous region and depends on the dopant concentration, ion beam energy, impact angle, and current density. For example, for dopant concentrations between 1.0 × 1020 and 1.5 × 1016 cm−3 we observe dopant deactivation at least between 23 and 175 nm for a glancing 30 keV ion beam. Further, we show that dopant deactivation is caused by defect diffusion during milling and is not directly impacted by the presence of Gallium in the sample. Later, we also discuss potential ways to mitigate these effects.

A study of an external-magnetic insulated ion beam diode with the No. 25 transformer oil anode

An experimental study of a semi-cylindrical external magnetically insulated diode using a polyethylene film and the No. 25 transformer oil acting as anode discharging materials is carried out. The luminescence images of anode plasma and the ion beam current density are acquired using an intensity charge-coupled detector (ICCD) camera and a calibrated Faraday, respectively. The experimental results show that the ion beam current density of the No. 25 transformer oil is higher than that of the polyethylene film. The maximum ion beam current densities at the focus point of the No. 25 transformer oil and the polyethylene film are 325 A/cm2 and 310 A/cm2, respectively. Meanwhile, the lifetime of the No. 25 transformer oil is significantly longer than that of the conventional polyethylene film. The lifetime of the No. 25 transformer oil is about 5000 discharges, while that of the conventional polyethylene film is about 400 discharges. In addition, the ion beam current density of the No. 25 transformer oil is more stable than that of the polyethylene film, and the anode plasma of the No. 25 transformer oil is more uniform than that of the polyethylene film.

Sputtering of Silicon Surface during Low-Energy High-Dose Implantation with Silver Ions

We report on the results of first practical observations of sputtering of the Si surface during the implantation with Ag+ ions with an energy of 30 keV depending on irradiation dose D in the interval from 2.5 × 1016 to 1.5 × 1017 ion/cm2 for a fixed value of ion beam current density J = 8 μA/cm2, as well as for variation of J = 2, 5, 8, 15, and 20 μA/cm2 at constant D = 1.5 × 1017 ion/cm2. In the former case, the thickness of the porous Si (PSi) layer being sputtered increases monotonically to 50 nm at the maximum value of D; in this case, the effective sputtering ratio of the implanted Ag : PSi layer is 1.6. We have also established that the thickness of the sputtered layer increases with current density J.

The plume diagnostics of 30 cm ion thruster with an advanced plasma diagnostics system

In order to achieve a better understanding of plume characteristics of LIPS-300 ion thruster, the beam current density, ion energy and electron number density of LIPS-300 ion thruster plume are studied with an Advanced Plasma Diagnostics System (APDS) which allows for simultaneous in situ measurements of various properties characterizing ion thruster, such as plasma density, plasma potential, plasma temperature and ion beam current densities, ion energy distribution and so on. The results show that the beam current density distribution has a double ‘wing’ shape. The high energy ions were found in small scan angle, while low energy ions were found in greater scan angle. Electron number density has a similar shape with the beam current density distribution.

A study of self-magnetic insulated ion beam diode with liquid anode.

An experimental study of the semi-cylindrical self-magnetic insulated diode with No. 25 transformer oil and methyl silicone oil filled into stainless steel metal foam as anode discharging materials is carried out for the first time. The results show that the lifetime of this type of ion beam diode is about 3000 discharges, significantly higher than that of the conventional solid-film ion beam diode, which is about 400 discharges. Meanwhile, the ion beam current density of transformer oil and methyl silicone oil is higher than the polyethylene film, and the maximum ion beam current density of No. 25 transformer oil and methyl silicone oil is 330 A/cm2 and 250 A/cm2, respectively. In addition, the ion beam current density of No. 25 transformer oil and methyl silicone oil is more stable than that of the polyethylene film.

Temperature gradients in targets at low energy high-intensity ion implantation

This paper is devoted to the study of the gradient temperature field dynamics and distribution in the metal targets irradiated with high-intensity beams of gas ions. The investigations concerned ion implantation modes with the ion beam current density from several tens of mA/cm2 up to A/cm2 were presented. The ion beam power was varied due to the change of ion energy in the range from 0.6 to several keV and the pulse duty factor in the range of 0.2–0.8. The integral temperature of the target was measured with an electrically isolated thermocouple. To measure the dynamic change in the local temperature on the irradiated surface of target a high-temperature pulsed pyrometer KLEIBER 740-LO was used. Analysis of the experimental data obtained with the use of electrically isolated thermocouple, pulsed pyrometer, and numerical simulation revealed the presence of significant gradient temperature fields both over the surface and along the depth of targets irradiated by high-intensity ion beams.

Temperature gradients in targets with high-intensity implantation and their influence on the characteristics of ion-modified layers

This paper is devoted to the study of the gradient temperature field dynamics and distribution in the metal targets irradiated with high-intensity beams of gas and metal ions. The investigations concerned ion implantation modes with the ion beam current density from several tens of mA·cm−2 up to A·cm−2 were investigated. The ion beam power was additionally varied due to the change of ion energy in the range from 0.6 to several keV and the pulse duty factor in the range of 0.2–0.8. The integral temperature of the target was measured with an electrically isolated thermocouple. To measure the dynamic change in the local temperature on the irradiated target a high-temperature pulse pyrometer KLEIBER 740-LO was used. The problem of temperature evolution and metal sample melting under the exposure of a high-intensity repetitively pulsed ion beam was solved numerically using the heat conduction equation written in cylindrical coordinates. Analysis of the experimental data obtained with the use of electrically isolated thermocouple, pulse pyrometer, and numerical simulation revealed the presence of significant gradient temperature fields both over the surface and along the depth of targets irradiated by high-intensity ion beams.

Effect of structure on the secondary electron emission of tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) films are appealing for electronic device due to its unique electronic and optical properties. In this work, ta-C films were prepared by filtered cathodic vacuum arc (FCVA) method which is developed by adopting a focusing-deflection system for adjusting the ion beam current density and increasing the deposition field size. The surface morphology of ta-C films prepared by this developed FCVA was presented by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results reveal that the roughness of these ta-C films is only ∼0.4 nm and bias voltages almost unaffected to the surface morphology of the films. Moreover, secondary electron emission properties of ta-C films were investigated with various bias voltages. Compared with bare stainless steel substrate, the secondary electron yield (SEY) of ta-C films decreases significantly. Furthermore, the films deposited at the bias voltages in the range of 110–155 V possess the highest maximum secondary electron emission coefficient (δmax) compared to others, which matches well with the variation of sp3content in the films from Raman spectra analyses. These results indicate that the SEY can be tailored by the sp3/sp2 ratio of ta-C films.

Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

Using an electrostatic double probe, profiles of the ion beam current density, electron density, and electron temperature were measured in the background plasma that appears by injecting a hydrogen ion beam into low-pressure hydrogen gas in a propagation chamber. The hydrogen ion beam is extracted using concave-shaped electrodes at extremely low energies (Eib = 60–120 eV). Focusing of the beam occurs when Eib exceeds a certain threshold. One probe electrode P3 is far from the beam source and is set in the shadow of another electrode in the beam trajectory. The ion saturation current in P3 is then estimated without considering the beam contribution. The measured electron densities are much larger than those of the ion beam density, the electron temperatures are very low (<1 eV), and the ion beam current densities exhibit fairly good agreement with those measured by Faraday cups. The profiles of the space potential are also estimated by measuring the potential of P3 with respect to ground with a voltage divider having an extremely high resistance. The space potentials obtained are quite low at <10 eV with focusing and ∼23 eV without focusing. The data with and without focusing are compared and conditions for focusing are examined. Focusing achieved through additional electron beam injection in the ground electrode is also examined. The results obtained indicate that a large electron source is required to balance the ion charges. Secondary electron emissions and/or small electron beam injection are effective sources.

Computational design of a high-efficiency accelerator grid for a miniature ion thruster by full-aperture ion optics simulations

Full-aperture ion optics simulations have been conducted for the inhomogeneous plasma source of a miniature ion propulsion system (MIPS) to design a high-efficiency accelerator grid that provides high degree of the neutral confinement and absence of direct ion impingement. The designed accelerator grid has flat upstream and smoothly curved downstream surfaces with straight holes for easy low-cost manufacture. The diameter of the accelerator aperture was changed from the nominal value of 0.40 mm to 0.25 mm, which decreased neutral leakage and increased the propellant utilization efficiency from 31 to 50%. The direct impingement of ions caused by decreasing the accelerator aperture diameter was compensated by reducing the thickness of the accelerator grid while taking into account the inhomogeneous ion beam current density profile of the MIPS. An off-design performance simulation was conducted to validate the proposed grid design; the obtained results showed that the ion beam could be accelerated smoothly even during throttling the beam current between 75 and 150%. A grid wear simulation was also performed to compare the changes in the propellant utilization efficiency between the nominal and high-efficiency grids caused by erosion. It was found that the propellant utilization efficiency of the high-efficiency grid was greater than that of the nominal grid within the first 5,000 h of operation and that its lifetime exceeded 10,000 h of the accumulated operation time. By using the proposed high-efficiency accelerator grid and MIPS plasma source, the propellant utilization efficiency was increased, while the accelerator impingement current became negligible.

Microscopic Examination of the Silicon Surface Subjected to High-Dose Silver Implantation

Low-energy (E = 30 keV) Ag+ ions have been implanted into single-crystalline Si wafers (c-Si) with an implantation dose varying from 1.25 × 1015 to 1.5 × 1017 ions cm–2 and an ion beam current density varying from 2 to 15 μA/cm2. The surface morphology of implanted wafers has been examined using scanning electron microscopy, transmission electron microscopy, and atomic force microscopy, and their structure has been studied by means of reflection high-energy electron diffraction and elemental microanalysis. It has been shown that for minimal irradiation doses used in experiments, the surface layer of c-Si experiences amorphization. It has been found that when the implantation dose is in excess of the threshold value (~3.1 × 1015 ions cm–2), Ag nanoparticles uniformly distributed over the Si surface arise in the irradiated Si layer. At a dose exceeding 1017 ions cm–2, a porous Si structure is observed. In this case, the Ag nanoparticle size distribution becomes bimodal with coarse particles localized at the walls of Si pores.

Микроскопия поверхности кремния, имплантированного ионами серебра высокими дозами

AbstractLow-energy ( E = 30 keV) Ag^+ ions have been implanted into single-crystalline Si wafers (c-Si) with an implantation dose varying from 1.25 × 10^15 to 1.5 × 10^17 ions cm^–2 and an ion beam current density varying from 2 to 15 μA/cm^2. The surface morphology of implanted wafers has been examined using scanning electron microscopy, transmission electron microscopy, and atomic force microscopy, and their structure has been studied by means of reflection high-energy electron diffraction and elemental microanalysis. It has been shown that for minimal irradiation doses used in experiments, the surface layer of c-Si experiences amorphization. It has been found that when the implantation dose is in excess of the threshold value (~3.1 × 10^15 ions cm^–2), Ag nanoparticles uniformly distributed over the Si surface arise in the irradiated Si layer. At a dose exceeding 10^17 ions cm^–2, a porous Si structure is observed. In this case, the Ag nanoparticle size distribution becomes bimodal with coarse particles localized at the walls of Si pores.