Low Beam Current Density Research Articles

Analysis of geological glasses by electron probe microanalysis under low beam current density conditions

An electron probe microanalysis method was developed to mitigate alkali ion migration effects under low beam current density conditions with a time-saving mode to collect alkali-ion X-ray signals. The method is suitable for estimating H2O content in H2O-rich geological glasses as a substitute technique for SIMS and FTIR.

Quantitative Mapping of the Charge Density in a Monolayer of MoS2 at Atomic Resolution by Off-Axis Electron Holography.

The electric potential, electric field, and charge density of a monolayer of MoS2 have been quantitatively measured at atomic-scale resolution. This has been performed by off-axis electron holography using a double aberration-corrected transmission electron microscope operated at 80 kV and a low electron beam current density. Using this low dose rate and acceleration voltage, the specimen damage is limited during imaging. In order to improve the sensitivity of the measurement, a series of holograms have been acquired. Instabilities of the microscope such as the drifts of the specimen, biprism, and optical aberrations during the acquisition have been corrected by data processing. Phase images of the MoS2 monolayer have been acquired with a sensitivity of 2π/698 rad associated with a spatial resolution of 2.4 Å. The improvement in the signal-to-noise ratio allows the charge density to be directly calculated from the phase images using Poisson's equation. Density functional theory simulations of the potential and charge density of this MoS2 monolayer were performed for comparison to the experiment. The experimental measurements and simulations are consistent with each other, and notably, the charge density in a sulfur monovacancy (VS) site is shown.

A multiple gap plasma cathode electron gun and its electron beam analysis in self and trigger breakdown modes.

In the present paper, a pseudospark discharge based multiple gap plasma cathode electron gun is reported which has been operated separately in self and trigger breakdown modes using two different gases, namely, argon and hydrogen. The beam current and beam energy have been analyzed using a concentric ring diagnostic arrangement. Two distinct electron beams are clearly seen with hollow cathode and conductive phases. The hollow cathode phase has been observed for ∼50 ns where the obtained electron beam is having low beam current density and high energy. While in conductive phase it is high current density and low energy electron beam. It is inferred that in the hollow cathode phase the beam energy is more for the self breakdown case whereas the current density is more for the trigger breakdown case. The tailor made operation of the hollow cathode phase electron beam can play an important role in microwave generation. Up to 30% variation in the electron beam energy has been achieved keeping the same gas and by varying the breakdown mode operations. Also, up to 32% variation in the beam current density has been achieved for the trigger breakdown mode at optimized trigger position by varying the gas type.

Microbeam methods for the analysis of glass in fine-grained tephra deposits: a SMART perspective on current and future trends

Abstract Correlation of tephra deposits frequently relies on the analysis of glass shards separated from their host. Small shards from distal deposits (marine-, ice- or lake-cores, peat bogs) are difficult to analyse. Here current methods for glass shard analysis from (particularly) marine tephra deposits are reviewed. These methods apply equally to other distal deposits, where linking repositories of climatic information is central to many research programmes. Electron probe microanalysis is used widely to determine the major element compositions of volcanic glass. However, electron beam irradiation causes permanent damage to glass, especially hydrated or silica- and alkali-rich compositions. Recent developments have shown that reliable data is obtained with beam diameters >3 µm on basaltic or moderately hydrated rhyolitic glass, provided low electron beam current densities are used. For robust correlation of tephra deposits from different environments, it is recommended that analytical data are normalized to an anhydrous basis. Glass trace element analysis is commonly performed by laser ablation inductively coupled plasma mass spectrometry, which determines approx. 30 elements at <1 ppm from ablation craters 10 µm diameter, and although element fractionation occurs, it can be corrected. Recent developments in both methods should facilitate analysis of smaller, more distal material, expanding the geographical range over which reliable tephra correlations can be achieved.

Non-self-maintained discharge supported by a proton beam for studying stretched dust structures

The main parameters of non-self-maintained discharge supported by a proton beam in inert gas are obtained. The discharge is used to study dust structures in nuclear excited plasma. Its main features include low gas pressure (∼133 Pa) and low proton beam current density (∼10−6 A/cm2). It is shown that a trap for negatively charged dust particles is formed in the discharge near-collector area at a sufficiently large negative voltage on the collector (−100 V), which is caused by a large negative potential jump in the Langmuir electrode layer whose width increases with distance from the center of the discharge. The possibility of generating stretched dust structures in non-self-maintained discharge is considered.

Development of a plasma generator for a long pulse ion source for neutral beam injectors

A plasma generator for a long pulse H(+)/D(+) ion source has been developed. The plasma generator was designed to produce 65 A H(+)/D(+) beams at an energy of 120 keV from an ion extraction area of 12 cm in width and 45 cm in length. Configuration of the plasma generator is a multi-cusp bucket type with SmCo permanent magnets. Dimension of a plasma chamber is 25 cm in width, 59 cm in length, and 32.5 cm in depth. The plasma generator was designed and fabricated at Japan Atomic Energy Agency. Source plasma generation and beam extraction tests for hydrogen coupling with an accelerator of the KSTAR ion source have been performed at the KSTAR neutral beam test stand under the agreement of Japan-Korea collaborative experiment. Spatial uniformity of the source plasma at the extraction region was measured using Langmuir probes and ±7% of the deviation from an averaged ion saturation current density was obtained. A long pulse test of the plasma generation up to 200 s with an arc discharge power of 70 kW has been successfully demonstrated. The arc discharge power satisfies the requirement of the beam production for the KSTAR NBI. A 70 keV, 41 A, 5 s hydrogen ion beam has been extracted with a high arc efficiency of 0.9 -1.1 A/kW at a beam extraction experiment. A deuteron yield of 77% was measured even at a low beam current density of 73 mA/cm(2).

Isotope study of far IR absorption of bistable centers in hydrogen-implanted silicon

Far IR study of high-purity crystalline silicon implanted by protons and deuterons at low ion beam current density and room temperature was performed. Isotope analysis shows that a set of new absorption bands at 200–1500 cm−1 are connected to hydrogen-related centers including bistable centers. It is found that the behavior of the bands correlates with that of high-order bands observed in the near infrared absorption spectra of neutron-irradiated silicon. The bands are tentatively associated with nanostructured hydrogen-related defects.

Characterization of p+-n Junctions and a Quasi-One-Dimensional Structure Fabricated by Low-Energy Focused Ion Beam

The electrical properties of implanted p+ layers, shallow p+-n junctions, and a quasi-one-dimensional structure fabricated without masks by 20-keV Ga+ focused ion beam (FIB) on crystalline Si substrates were investigated in details. The distribution profile from SIMS showed discrepancies from LSS theory, which is considered to be the results of low beam energy and high beam current density. From Hall effect measurements, the Hall drift mobility was 131 cm2 / V-sec, which is 62% of the projected value due to remained defects after annealing. The I - V characteristics of p+-n junctions illustrated four distinct regions in the forward bias region and rising saturation current in the reverse bias region. Higher than expected leakage current is considered to be the results of residue damages after annealing. For the first time, the resistance of a quasi-one-dimensional structure was investigated. The resistance is 1 x 1 ,which is 647 times higher than the projected value. The cause of this phenomenon is considered to be the combined effects of interfacial roughness, the residue damages, and the shape of the structure.

The impact of electron beam damage on the detection of indium-rich localisation centres in InGaN quantum wells using transmission electron microscopy

High-resolution transmission electron microscope (HRTEM) lattice fringe images and lattice parameter maps are used to reveal the rapid modification of InGaN quantum wells by the electron beam in a TEM. Images acquired within seconds of first irradiating a region of quantum well do not exhibit the strong nanometre-scale strain contrast which has been reported to signify the presence of very indium-rich regions in InGaN quantum wells. However, after a very short period of irradiation by a relatively low electron beam current density, images of the specimen could be interpreted as indicating the presence of these indium “clusters”. The beam damage is shown to occur for the InGaN quantum wells grown in our lab as well as those in a commercial blue light emitting diode (LED) and in TEM specimens prepared only using mechanical polishing. Possible mechanisms for the beam damage are discussed and we make suggestions as to what may cause exciton localisation in quantum wells that do not contain gross composition fluctuations.

High-accuracy aerial image measurement for electron beam projection lithography

A direct means of measuring the image blur of electron beam projection lithography (EPL) tools is described. We developed an aerial image sensor using a Si membrane knife-edge and a transmitted electron detection technique. The aerial image sensor is designed to increase signal amplitude and signal contrast in order to yield a large signal to noise ratio even under a low beam current density condition. The image blur can be quantified accurate to a few nanometers because the measurement error due to the sensor is extremely small. The aerial image sensor was installed in Nikon's electron beam projection experimental column and was evaluated. The measured image blur, defined as the distance between the 12% and 88% points of the beam edge profile, under the optimum condition was 13 nm, and the measurement repeatability was 3 nm (3 sigma). The application of this technique to a system calibration is demonstrated. Focus and astigmatism were measured and the optimum settings of focus coils and stigmators were determined with excellent repeatability. The potential for this technique to provide an automated self-calibration system on EPL tools is clearly shown.

Effects of Beam Current Density on Ion Beam Synthesis OF SiC

ABSTRACTThin SiC layers were synthesized by high dose C implantation into silicon using a metal vapor vacuum arc ion source at various conditions. Characterization of the ion beam synthesized SiC layers was performed using various techniques including x-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) absorption, and Raman spectroscopy. The XPS results showed that for samples with over-stoichiometric implant doses, if the implant beam current density was not high enough, even after prolonged thermal annealing at high temperatures, the as-implanted gaussian-like carbon depth profile remained unchanged. However, if the implant beam current density was sufficiently high, there was significant carbon redistribution during annealing, so that a thicker stoichiometric SiC layer can be formed after annealing. The XPS and Raman results also showed that there were carbon clusters formed in the as-implanted layers for the low beam current density implanted samples, while the formation of such carbon clusters was minimal in the high beam current density as-implanted samples. The effect of beam current density on the fraction of different bonding states of the implanted carbon atoms was studied.

Distributed generator with extended interaction on field emitter arrays

A possibility for designing a distributed microwave generator on field emitter arrays has been studied which involves a waveguide interaction system as a circular distributed resonator. This generator was shown to be able of operating in the millimeter wavelength band with low voltages and beam current densities and to provide an efficiency to 20%. The electrical tuning of the generator in frequency can achieve 30% for one of its versions.

Improving the mechanical properties of steels using low energy, high temperature nitrogen ion implantation

The use of nitrogen ion implantation to increase the surface hardness of structural steels is well documented. Traditionally this involves the use of high energy nitrogen ion beams (approximately 100 keV), with a relatively low beam current density because high energy beams are necessary to produce the required penetration into the material to achieve a significant depth of hardened material. Hardening needs to occur in a region whose size is comparable with the scale of the deformation associated with the tribocontact. 100 keV nitrogen ions typically penetrate into steels only about 0.1 μm and the range of possible tribological applications is thus restricted by this shallow treatment depth. In plasma nitriding processes the nitrogen ions approach the substrate with much lower energies but the ion currents are sufficiently high to cause considerable substrate heating. In this study an ion beam process has been used which more closely approximates plasma nitriding conditions. This involves the use of comparatively low energy nitrogen ion beams (1–2 keV), with higher beam current densities and, where necessary, additional heating to increase the sample temperature to approximately 400 °C at the surface. Secondary ion mass spectrometry has been used to assess the extent of nitrogen diffusion into a range of steels. The nitrogen penetration depth (and profile shape) depends both on the ion beam parameters and the structure and composition of the steel. Additional experiments conducted with a supplementary supply of nitrogen gas in the form of a vacuum chamber backfill demonstrate that increasing the nitrogen supply can increase nitrogen take-up for some materials. For a 30 min implantation at 263 μA cm −2 the penetration depth was a substantial fraction of a micrometre and is greater than the profile produced by high energy implantation in some cases. The hardness of the implanted steels can be correlated with the extent of nitrogen take-up, the amount of diffusion during implantation and softening induced by annealing of the steel substrates. Considerable hardening is achieved for some steels (e.g. M2 high speed steel) whereas for other steels the improvements are less obvious (e.g. 304 stainless). These observations are discussed in the light of the nitridability of the steels.

Simulations of magnetically insulated multistage ion diodes

An analytic theory for magnetically insulated multistage acceleration of high intensity ion beams has been presented [J. Appl. Phys. 67, 6705 (1990)]. This theory predicts an operating behavior that is strongly dependent on the electron density profile. A numerical investigation, using both two-dimensional (2-D) and three-dimensional (3-D) particle-in-cell codes, of multistage diode operating behavior is presented in this paper. The 2-D results are consistent with the analytic results based on a very thin electron sheath. In contrast, the 3-D simulations are consistent with the analytic theory based on a thick electron sheath. The different results are due to the growth of electromagnetic instabilities in the 3-D simulations, which generate fluctuations that broaden the electron sheath. The 2-D simulations did not properly model these instabilities because they propagate in the direction that was ignored. In addition to these results, the 3-D code was used to study the generation of ion divergence due to the instability induced fluctuations. These simulations show a positive correlation between the ion current density (normalized for space-charge effects) and the growth of transverse ion velocities during acceleration. It is found that, at low beam current densities, ion divergence can be reduced significantly by postacceleration.

Controlled Formation of Buried Layers of Carbon

ABSTRACTPartially cured and cured PF-resin samples were prepared at 150°C, 170°C and at 200°C in an inert environment and then bombarded by MeV ion beams using protons, alphas and nitrogen. Using low ion beam current density, 100 to 500 pA/mm2 for the nitrogen ions, 10 to 20 nA/mm2 for the alpha ions, and 50 to 500 nA/mm2 for protons, we have produced buried carbon layers without breakdown i.e., crack formation. The thicknesses of the carbon layers produced were of the order of a few tens of nanometers at a depth of a few nanometers to several micrometers depending on the energy, the type of bombarding ions and the curing temperature of the precursor. The electrical resistivity of these layers was measured in situ and was reduced from 109 Ω-cm to 10 Ω-cm. The lowest resistivity, 10 Ω-cm, was measured in the alpha bombarded, 150°C heat-treated resin. The carbonized volumes were analyzed by Raman microprobe spectroscopy which showed that the strongest graphitic (G-line) and distorted (D-line) Raman signals observed were from the nitrogen and alpha irradiated samples.

Mechanism of production and the current density effects of doubly charged ion emission from Cd+-bombarded Nb and V targets

Abstract Secondary emission of doubly-charged postive ions from 3–15 keV Cd+ ion-bombarded polycrystalline Nb and V samples at target current densities (Jp ) in the range 0–1.4 mA cm−2 has been studied in detail. For each target the ratio of doubly-charged to singly-charged ion currents (Is ++ /Is ++)has been found to be 0.01 at low beam current densities and does not vary appreciably with increase in the primary ion current (Ip ), there is a rapid increase in the value of Is ++ as compared to I+ s such that at Jp ∼ 15μm UA cm−2, the ratio Is ++/Is + increase by 10 to 300 times depending upon the target and bombarding conditions. The value of Jp at which maximum enhancement of Is ++ is observed often corresponds to the saturation region in the current density variation curve, observed in case of singly-charged ion emission. This region possibly corresponds to the formation of an intermetallic compounds layer of a definite stochiometric composition due to the action of reactive Cd+ ions at the metallic tar...

Operation of repetitively pulsed virtual cathode oscillators on the TEMPO pulser

The design and performance characteristics of two virtual cathode oscillators operated at a 1-Hz repetition rate for a 10-shot burst using the TEMPO pulser are presented. The 2.4-GHz hardware generated a 300 MW per pulse (radiated) in the TM/sub 02/ mode with a 1.3% total energy conversion efficiency and with a 10.5% frequency bandwidth. A 2:1 scale-tip of this hardware was used to achieve an 840-MHz operation, but it only radiated 70 MW per pulse in the TM/sub 01/ mode with a 7.3% bandwidth, since hardware constraints prevented the TM/sub 02/ component from being radiated. The relatively low beam current density of the TEMPO VCOs yielded a low diode gap closure rate that should make them suitable for long-pulse operation. In addition, the low beam current density minimized damage to the thin anode screen.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multiwave lasing in an electron-beam-pumped Ar–Xe mixture

Some results are given of theoretical and experimental investigations of lasing in an Ar–Xe mixture pumped by an electron beam. A kinetic model of an XeI laser was developed to take into account the level structure of the 6p and 5d states of xenon. The calculations were in satisfactory agreement with the experimental data in a wide range of pump pulse durations and beam densities. The calculated high efficiency (~ 2%) at low beam current densities and an operating pressure of ~ 1 atm resulted from a reduction in the rate of quenching of the upper active level by the electrons. High-power lasing at λ = 2.03 μm was obtained at high pump levels of an Ar–Xe–He mixture. The addition of helium led to an increase in the laser efficiency.

Radiation damage in PIXE analysis of museum paper-like objects

Paper discoloration and its tensile strength degradation caused by 2.1 MeV external proton beam bombardment under low beam current density ( < 10 nA/cm 2) were measured as a function of the number of incident protons. The target temperature reached during bombardment was measured using an infrared thermometer. Under low beam current density, paper heating is negligible and the paper deterioration depends only on the total beam fluence. Some of the experimental results are presented and discussed for several types of paper, including one used for traditional Chinese paintings.

Microstructure and mechanical properties of high dose nitrogen-implanted iron, chromium and titanium sheets

A study has been made of the mechanical properties of high dose nitrogen-implanted iron, chromium and titanium sheets. The ion implantation of N + ions was performed with doses ranging from 7.5 × 10 16 to 1 × 10 18 ions cm −2 at an energy of 100 kev. The substrate temperature during implantation was kept at about 20°C using a low beam current density and water cooling. The depth profile of implanted nitrogen was estimated by Auger electron spectroscopy (AES) combined with Ar + sputtering. The surface structure produced by implantation was identified by X-ray diffraction (XRD). The near-surface hardness and friction coefficient were measured by a Knoop hardness tester and a Bowden-Leben-type friction-testing machine, respectively. AES measurements revealed that nitrogen formed a Gaussian distribution for a low dose and error functional or trapezoid-like distributions for the highest dose, and the nitrogen concentration did not exceed a certain stoichiometric ratio. The XRD patterns showed that nitrides peculiar to substrates such as Fe 2N, CrN and TiN were formed at room-temperature implantation. The hardness of iron had a maximum at a dose of 1 × 10 17 ions cm −2, and the increase in hardness of chromium and titanium corresponded to the increase in nitrogen dose. The friction coefficients of iron and chromium decreased stably at the highest dose, but that of titanium scattered widely as the dose increased. These mechanical properties are discussed with respect to the surface composition and structure.