Energy Helium Ions Research Articles

Ионно-лучевое формирование серебросодержащей поверхности пористого кремния

The results of a study of the formation of a silver-containing porous layer on the surface of single-crystal silicon KEF-4,5 by ion-beam alloying are presented. A computer simulation of the process of ion doping of silicon with KEF-4,5, high-energy helium and silver ions was carried out using the TRIM/SRIM software package in order to determine the dose and energy neighborhoods of helium and silver ions necessary and sufficient in the experiment. The relief of the porous surface, the density of quasi-pores on the surface of implanted silicon, the elemental composition of the obtained porous composite silver-containing layers, taking into account the effect of intermediate heat treatment on it, are studied. The ion-beam formation of a silver-containing coating ensures the creation of a developed surface relief at the nanoscale with high reproducibility and controllability inherent in the ion doping method, which contributes to an increase in the yield of usable solar cell products. The technological modes of high reproducibility and controllability inherent in the ion doping method are determined based on the experimental data obtained for studying the relief of a porous surface and the density of quasi-pores on the surface of implanted silicon from the energy and dose of helium ions. Based on the study of the elemental composition of the quasi-porous composite silver-containing layer obtained by implantation of silver ions, the recommended parameters of implantation of silver ions have been established.

In situ work function measurements of W, WO3 nanostructured surfaces

Surface nanostructuring enables the fabrication of materials with highly desirable properties. Nanostructured tungsten surfaces have potential applications in solar water splitting. Exposing a polished tungsten surface to helium plasma induces various surface morphological changes. Depending on the helium ion energy, temperature, and fluence, helium clusters, helium bubbles and foam-like nanostructures develop on the tungsten surface. In this study, tungsten foam-like nanostructures were formed and/or oxidised, and then examined using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) without breaking the vacuum. The chemical state of nanostructured W or WO3 was not modified in comparison to the pristine one. However, measuring the line width of the emitted electrons from the onset of the secondary electrons up to the Fermi edge and subtracting value from the incident photon energy, the work function acquired in situ by UPS for a nanostructured W surface increased by 0.9 eV in comparison to the pristine one. Helium ions effectively eliminated field emission sites via sputtering/implantation and thereby increased the work function. No change in work function was measured for WO3-pristine and its fuzz: the oxidation hindered the effect of helium. In contrast to the W-fuzz sample, no helium bubbles were identified in WO3-fuzz, as helium diffused out during oxidation.

Relative stopping power resolution in time-of-flight proton CT

Objective. Proton computed tomography (CT) is similar to x-ray CT but relies on protons rather than photons to form an image. In its most common operation mode, the measured quantity is the amount of energy that a proton has lost while traversing the imaged object from which a relative stopping power map can be obtained via tomographic reconstruction. To this end, a calorimeter which measures the energy deposited by protons downstream of the scanned object has been studied or implemented as energy detector in several proton CT prototypes. An alternative method is to measure the proton’s residual velocity and thus its kinetic energy via the time of flight (TOF) between at least two sensor planes. In this work, we study the RSP resolution, seen as image noise, which can be expected from TOF proton CT systems. Approach. We rely on physics models on the one hand and statistical models of the relevant uncertainties on the other to derive closed form expressions for the noise in projection images. The TOF measurement error scales with the distance between the TOF sensor planes and is reported as velocity error in ps/m. We use variance reconstruction to obtain noise maps of a water cylinder phantom given the scanner characteristics and additionally reconstruct noise maps for a calorimeter-based proton CT system as reference. We use Monte Carlo simulations to verify our model and to estimate the noise due to multiple Coulomb scattering inside the object. We also provide a comparison of TOF helium and proton CT. Main results. We find that TOF proton CT with 30 ps m−1 velocity error reaches similar image noise as a calorimeter-based proton CT system with 1% energy error (1 sigma error). A TOF proton CT system with a 50 ps m−1 velocity error produces slightly less noise than a 2% calorimeter system. Noise in a reconstructed TOF proton CT image is spatially inhomogeneous with a marked increase towards the object periphery. Our modelled noise was consistent with Monte Carlo simulated images. TOF helium CT offers lower RSP noise at equal fluence, but is less advantageous at equal imaging dose. Significance. This systematic study of image noise in TOF proton CT can serve as a guide for future developments of this alternative solution for estimating the residual energy of protons and helium ions after the scanned object.

Evaluation of water equivalent ratio (WER) values for polyethylene, polymethyl methacrylate, polystyrene, lead, tungsten and aluminum at helium ion energies ranging from 25-250 MeV/u through Monte Carlo simulation

Evaluation of water equivalent ratio (WER) values for polyethylene, polymethyl methacrylate, polystyrene, lead, tungsten and aluminum at helium ion energies ranging from 25-250 MeV/u through Monte Carlo simulation

Evaluation of water equivalent ratio (WER) values for polyethylene, polymethyl methacrylate, polystyrene, lead, tungsten and aluminum at helium ion energies ranging from 25-250 MeV/u through Monte Carlo simulation

Evaluation of water equivalent ratio (WER) values for polyethylene, polymethyl methacrylate, polystyrene, lead, tungsten and aluminum at helium ion energies ranging from 25-250 MeV/u through Monte Carlo simulation

Effect of temperature and incident ion energy on nanostructure formation on silicon exposed to helium plasma

AbstractHelium plasma can be used to deliver low‐energy (<100 eV) helium ions to stimulate the growth of nanostructures on silicon surfaces. This can produce a wide range of surface features including nanoscale roughening, nanowires and porous structures. In this study, nanostructure sizes varied from ∼10 to over 100 nm in diameter. The effect of these structures on surface reflectivity for photovoltaic and photocatalytic applications is also investigated. Broadband suppression of photoreflectivity is achieved across the 300–1,200 nm wavelength range studied for silicon exposed to helium plasma at 600°C, with an average reflectivity of 3.2% and 2.9% for incident helium ion energies of 42 and 62 eV, respectively.

Enhanced fuzzy tungsten growth in the presence of tungsten deposition

Using a magnetron sputtering device operating in helium, fibre-form ‘fuzz’ has been grown on tungsten samples in the presence of a significant auxiliary source of depositing tungsten. In this system, fuzzy tungsten was grown over a range of helium ion fluences, , sample temperatures and helium ion energies, but with operator control over the tungsten atom-to-helium ion arrival rate ratio at the sample (from 0.003 to 0.009). In the presence of tungsten deposition, it appears that the fuzz growth has two distinct stages: at low to intermediate helium ion fluence the fuzzy layer thickness follows the expected diffusive law augmented by approximately the ‘effective’ thin film thickness of deposited tungsten; at high fluences the fuzz thickness increases very steeply with . These observations are explained through the increase in the porosity of the fuzzy layer as it reaches thicknesses larger than ∼1 m. It was observed that during the second phase of fuzz growth the thickness was highly dependent on both the sample temperature and the tungsten atom-to-helium ion arrival rate ratio. For the same helium ion exposure, an increase in the sample temperature from 1050 to 1150 K lead to a six-fold increase in the fuzzy layer thickness, whilst increasing the tungsten atom-to-helium ion arrival rate ratio over the full range produced a two-fold increase in the thickness. Microscopy and electron diffraction studies of the grown structures show clearly helium bubbles within polycrystalline tendrils.

Generation of Ultrahigh-Velocity Collisionless Electrostatic Shocks Using an Ultra-Intense Laser Pulse Interacting with Foil-Gas Target**Supported by the Science Challenge Project under Grant Nos TZ2018005 and TZ2016005, and the National Key Research and Development Program of China under Grant No 2016YFA0401100.

Ultra high-velocity collisionless shocks are generated using an ultra-intense laser interacting with foil-gas target, which consists of copper foil and helium gas. The energy of helium ions accelerated by shock and the proton probing image of the shock electrostatic field show that the shock velocity is 0.02c, where c is the light speed. The numerical and theory studies indicate that the collisionless shock velocity exceeding 0.1c can be generated by a laser pulse with picosecond duration and an intensity of 1020 W/cm2. This system may be relevant to the study of mildly relativistic velocity collisionless shocks in astrophysics.

Surface and structural analyses of helium ion irradiated beryllium

We present the findings of pulsed helium ion irradiation on beryllium that is emitted from a tabletop pulsed ion source. The source delivers a broad range of helium ion energy having most probable energy of 60 keV with a flux of ~1025 m−2s−1. The FESEM micrographs of irradiated samples depict the formation of helium bubbles having tri-modal size distribution. The appearance of larger size bubbles is supposed to be resulted due to Ostwald ripening phenomenon. The calculated rate of irradiation swelling for the beryllium sample due to the development of helium bubbles is found to ~0.24%. The three dimensional AFM micrographs suggest the formation of the hill like structures. An increase in the roughness value of irradiated samples is observed when the helium ion pulse number increases (up to 5 shots) and finally the roughness value decreases for 10 shots helium ion irradiation sample. The XRD patterns reveal the polycrystalline nature of the beryllium samples. In the case of irradiated samples, two feeble planes of beryllium oxide are speculated and most intense peak (002) confirms the formation of compressive stress due to helium ion irradiation.

BCA simulations of ion irradiation of tungsten fuzz

A model based on the Binary Collision Approximation (BCA) approach is developed to study the penetrative and sputtering properties of tungsten “fuzz” under low-energy ion bombardment. In the model, tungsten fuzz structures are created with building blocks and then are processed to generate finite element meshes, which are used as input in the BCA code IM3D. Fuzz structures of different densities and textures are created by changing the parameters of the building blocks. Helium and argon ions with different energies are applied to bombard the fuzz structures. Simulation results confirm that low-energy helium ions can penetrate several hundred nanometers into tungsten fuzz just by going through the open channels or by bouncing between the fuzz tendrils. The penetrative properties depend on the fuzz density, the orientation and length of the building blocks, and do not depend on the energy of helium ions. These results provide an adequate explanation on how helium atoms transport through thick fuzz layers and offer new insights to fuzz growth mechanisms. Sputtering properties of W fuzz are also studied. Results show that the sputtering yield of W fuzz is significantly lower than flat tungsten surfaces, which is in good consistence with experimental findings.

A coupled rate theory-Monte Carlo model of helium bubble evolution in plasma-facing micro-engineered tungsten

A multiscale model of helium bubble evolution in plasma-facing materials is developed. The model links different stages of helium bubble evolution: deposition, nucleation, growth, motion, and coalescence. Helium deposition is simulated with the SRIM Monte Carlo program to give spatial information on helium and displacement damage distributions near the surface. This deposition profile is then introduced into a space-dependent rate theory of bubble nucleation and growth to describe the early stages of the distribution and size of helium bubbles. The coarsening stage of bubble evolution as a result of whole bubble motion, interaction, and coalescence is modeled by a new Object Kinetic Monte Carlo (OKMC) model, for which initial conditions are taken from the mean-field rate theory calculations. The model is compared to experimental data on low-energy helium plasma interaction with micro-engineered tungsten (W), and on high-energy helium ion deposition in flat W samples. Novel features of the multiscale model include: (1) space-dependent rate theory; (2) OKMC model of bubble motion in stress and temperature fields; and (3) application of the model to micro-engineered materials, and comparison with experiments on the same time-scale. At low helium ion energy, it is found that the mechanism of trap mutation is essential in achieving good agreement with experimental measurements. On the other hand, good agreement with experiments at high incident ion energy and temperature showed the importance of bubble coalescence and coarsening as main mechanisms. The results of the model are compared with experiments on flat W surfaces irradiated at high ion energy (30 keV), and with micro-engineered W, where the surface is coated with high-density micro-pillars at low ion energy (around 100 eV). The predicted average bubble radius and density are in qualitative agreement with experimental results.

Characterization of high flux magnetized helium plasma in SCU-PSI linear device

A high-flux linear plasma device in Sichuan University plasma-surface interaction (SCU-PSI) based on a cascaded arc source has been established to simulate the interactions between helium and hydrogen plasma with the plasma-facing components in fusion reactors. In this paper, the helium plasma has been characterized by a double-pin Langmuir probe. The results show that the stable helium plasma beam with a diameter of 26 mm was constrained very well at a magnetic field strength of 0.3 T. The core density and ion flux of helium plasma have a strong dependence on the applied current, magnetic field strength and gas flow rate. It could reach an electron density of 1.2 × 1019 m−3 and helium ion flux of 3.2 × 1022 m−2 s−1, with a gas flow rate of 4 standard liter per minute, magnetic field strength of 0.2 T and input power of 11 kW. With the addition of −80 V applied to the target to increase the helium ion energy and the exposure time of 2 h, the flat top temperature reached about 530 °C. The different sizes of nanostructured fuzz on irradiated tungsten and molybdenum samples surfaces under the bombardment of helium ions were observed by scanning electron microscopy. These results measured in the SCU-PSI linear device provide a reference for International Thermonuclear Experimental Reactor related PSI research.

Isolated nano-tendril bundles on tungsten surfaces exposed to radiofrequency helium plasma

The DIONISOS experiment is used to study the impact of RF helium (He) plasma on the surface morphology of tungsten (W) at a frequency of 13.56MHz. Helium ion energy distributions with a span of 70–75eV, while still below the sputtering threshold result in nano-tendril bundles (NTBs) and free-standing W whiskers on surfaces at 1020K. The NTBs are distributed intragranularly with coverage of less than 10% while reaching up to 30µm normal to the surface for He ion fluence of 7.6 ×1025m−2 and flux density of 1022m−2s−1. Analysis of the NTB interior and sub-surface structure is provided through focused ion beam cross section.

Broad ion energy distributions in helicon wave-coupled helium plasma

Helium ion energy distributions were measured in helicon wave-coupled plasmas of the dynamics of ion implantation and sputtering of surface experiment using a retarding field energy analyzer. The shape of the energy distribution is a double-peak, characteristic of radiofrequency plasma potential modulation. The broad distribution is located within a radius of 0.8 cm, while the quartz tube of the plasma source has an inner radius of 2.2 cm. The ion energy distribution rapidly changes from a double-peak to a single peak in the radius range of 0.7–0.9 cm. The average ion energy is approximately uniform across the plasma column including the double-peak and single peak regions. The widths of the broad distribution, ΔE, in the wave-coupled mode are large compared to the time-averaged ion energy, ⟨E⟩. On the axis (r = 0), ΔE/⟨E⟩ ≲ 3.4, and at a radius near the edge of the plasma column (r = 2.2 cm), ΔE/⟨E⟩ ∼ 1.2. The discharge parameter space is scanned to investigate the effects of the magnetic field, input power, and chamber fill pressure on the wave-coupled mode that exhibits the sharp radial variation in the ion energy distribution.

Impact of helium ion energy modulation on tungsten surface morphology and nano-tendril growth

Time-modulated helium (He) ion energy (e.g. VBias = −50 + 25·sin(2πfRF · t), fRF = 13.56 MHz) is demonstrated to strongly affect the development of tungsten (W) surface morphology that results from He plasma irradiation in the DIONISOS linear plasma experiment. Nano-tendril bundles (NTBs), which appear as isolated ‘islands’ of nano-tendrils, can rapidly grow on an otherwise smooth W surface. This is in contrast to previously seen full-surface coverage of nano-tendril growth known as ‘fuzz’. When tall NTBs form, less than 15% of the surface contains nano-tendrils. The NTB surface coverage changes with growth conditions and the total volume of nano-tendrils in the NTBs is observed to be up to a factor of 16 larger than when fuzz is grown. This indicates that long-range W surface transport underlies nano-tendril formation.Surface temperature 870–1220 K, the DC bias potential −30 to −70 V, and the ion flux density 4.4 × 1021–1.1 × 1022 He · m−2 · s−1 are varied in the experiments. NTBs form at similar conditions as fuzz with the critical difference being the RF modulation of the ion energy bombarding the W, another indication of the importance of W surface transport. Mass loss measurements indicate net erosion with a yield of 1–8 × 10−4 W/He when NTBs form; erosion that is not attributable to chemical or physical sputtering by He or impurities in the plasma. The erosion is correlated to the NTB growth, based on post-exposure inspection by electron microscopy indicating that NTBs are prone to loss from the surface. NTB growth is compared to the empirical growth-erosion model of fuzz, showing NTBs grow up to a factor of 100 times taller than the expected fuzz layer depth under DC bias conditions. Insights into nano-tendril growth provided by this new growth regime are discussed. Strategies to mitigate W fuzz growth may inadvertently result in rapid localized nano-tendril bundle growth with a higher probability of dust production.

Low energy helium ion irradiation induced nanostructure formation on tungsten surface

We report on the low energy helium ion irradiation induced surface morphology changes on tungsten (W) surfaces under extreme conditions. Surface morphology changes on W surfaces were monitored as a function of helium ion energy (140–300 eV), fluence (2.3 × 1024–1.6 × 1025 ions m−2), and flux (2.0 × 1020–5.5 × 1020 ion m−2 s−1). All the experiments were performed at 900° C. Our study shows significant effect of all the three ion irradiation parameters (ion flux, fluence, and energy) on the surface morphology. However, the effect of ion flux is more pronounced. Variation of helium ion fluence allows to capture the very early stages of fuzz growth. The observed fuzz growth and morphology changes were understood in the realm of various possible phenomena. The study has relevance and important impact in the current and future nuclear fusion applications.

Helium ions acceleration by ultraintense laser interactions with foil-gas target

Laser-driven helium ion source with multi-MeV energy has an important application in the field of fusion reactor material irradiation damage. At present, the generating of high energy helium ions by relativistic ultraintense laser interacting with helium gas jet is the main scheme of laser-driven helium ion source. However, so far, this scheme has been hard to generate the helium ion beam with the characteristics, i.e., it is forward and quasi-monoenergetic and has multi-MeV in energy and high yield. These characteristics of helium ion beam are important for studying the material irradiation damage. In this paper, we propose a new scheme in which an ultraintense laser interacting with foil-gas complex target is used to generate helium ions. With this method, we perform an experiment on XingGuang III laser facility which has three laser beams with different laser durations (nanosecond, picosecond and femtosecond). In our experiment, we use a picosecond laser beam. The wavelength of this laser beam is 1054 nm and its duration is 0.8 ps. We use an off-axis parabola mirror to focus the 100 J energy of this laser beam onto a focal spot of 25 m far away. The laser intensity reaches 51018 W/cm2. The foil-gas target is composed of a copper foil with 7 m in thickness and a helium gas nozzle which is behind the copper foil. The helium gas nozzle can generate a helium gas jet with a full ionization electron density of 51019/cm3. We use the Thomson Parabola Spectrometer to record the helium ion signals and the Electron Magnetic Spectrometer to diagnose the hot electron temperature. In the experiment, the laser pulse interacts with the front surface of the copper foil and generates lots of hot electrons. These hot electrons result in the expansion of the rear surface of the copper foil. The expanding plasma accelerates the helium ions behind the copper foil. The experimental results show that the obtained helium ions are forward and quasi-monoenergetic (the peak energy is 2.7 MeV), and the total energy of the helium ions whose energies are all higher than 0.5 MeV is about 1.1 J/sr, and correspondingly the yield of helium ions is about 1013/sr. The helium ion spectrum and hot electron temperature given by particle in cell (PIC) simulation with using the experimental parameters are consistent with the experimental results. In addition, the PIC simulations also show that helium ions are accelerated by target normal sheath acceleration and collisionless shock acceleration-like mechanisms, and the maximum helium ion energy is proportional to the hot electron temperature.

Experimental investigation on the effect of surface electric field in the growth of tungsten nano-tendril morphology due to low energy helium irradiation

The mechanisms responsible for and controlling the growth of tungsten nano-tendrils (or “fuzz”) under low-energy helium plasma exposure remain unclear. For the first time in nano-tendril experiments, the plasma sheath-produced electric field and the helium (He) ion energy have been decoupled, showing that the sheath electric field has little impact on nano-tendril growth, eliminating a possible cause for tendril growth. The well-established necessary growth conditions for W fuzz were maintained with He ion flux density ΓHe > 1021 He m−2 s−1, surface temperature Ts = 1273 K, He ion energy EHe = 64 eV, and He ion fluence ΦHe > 1024 He m−2. A grid is situated between the tungsten sample and plasma, with the grid and sample potentials independently controlled in order to control the electric field at the surface of the sample while maintaining the same incident He ion energy to the surface. A calculation of the potential profile in the drift space between the grid and sample was used to account for space charge and calculate the electric field at the surface of the sample. Tungsten fuzz formed at all electric fields tested, even near zero electric field. Also, the depth of the resulting W fuzz layer was unaltered by the electric field when compared to the calculated depth determined from an empirical growth model. The conclusion is that the sheath electric field is not necessary to cause the changes in surface morphology.

Observation of a helium ion energy threshold for retention in tungsten exposed to hydrogen/helium mixture plasma

Helium retention is measured in tungsten samples exposed to mixed H/He plasma in the Magnum-PSI linear plasma device. It is observed that there is very little He retention below helium ion impact energies of eV, indicating the existence of a potential barrier which must be overcome for implantation to occur. The helium retention in samples exposed to plasma at temperatures >1000 K is strongly correlated with nano-bubble formation measured using grazing incidence small-angle x-ray scattering. The diameters of nano-bubbles were not found to increase with increasing helium concentration, indicating that additional helium must be accommodated by increasing the bubble concentration or an increase in bubble pressure. For some samples pre-irradiation with heavy ions of 2.0 MeV energy is investigated to simulate the effects of neutron damage. It is observed that nano-bubble sizes are comparable between samples pre-irradiated with heavy-ions, and those without heavy-ion pre-irradiation.

Simulation of alpha decay of actinides in iron phosphate glasses by ion irradiation

A surrogate approach of ion beam irradiation is employed to simulate alpha decay of actinides in iron phosphate nuclear waste glasses. Bismuth and helium ions of different energies have been selected for simulating glass matrix modification owing to radiolysis and ballistic damage due to recoil atoms. Structural modification and change in coordination number of network former were probed by employing Reflectance Fourier-Transform Infrared (FT-IR), and Raman spectroscopies as a consequence of ion irradiation. Depolymerisation is observed in glass sample irradiated at intermediate energy of 2MeV. Helium blisters of micron size are seen in glass sample irradiated at low helium ion energy of 30keV.