Low-energy Neutral Beam Research Articles

Calibrating beam fluxes of a low-energy neutral atom beam facility.

Scientific detection and imaging instruments for low-energetic neutral atoms (ENA) onboard spacecraft require thorough pre-flight laboratory calibration against a well-characterized neutral atom beam source. To achieve this requirement, a dedicated test facility is available at the University of Bern, which is equipped with a powerful plasma ion source and an ion beam neutralization stage. Using surface neutralization, low-energy neutral atom beams of any desired gas species can be produced in the energy range from 3keV down as low as 10eV. As the efficiency of the neutralization stage is species and energy dependent, the neutralizer itself needs to be calibrated against an independent reference. We report on the calibration and characterization of this neutral atom beam source using our recently developed Absolute Beam Monitor (ABM) as a primary calibration standard. The ABM measures the absolute ENA flux independent of neutral species in the energy range from 10eV to 3keV. We obtain calibration factors of a few 100cm-2s-1 pA-1, depending on species at beam energies above about 100eV, and a power-law decrease for energies below 100eV. Furthermore, the energy loss of neutralized ions in the surface neutralizer is estimated from time-of-flight measurements using the ABM. The relative energy loss increases with ENA energy from low levels near zero up to 20%-35% at 3keV, depending on atomic species. Having calibrated our neutral beam source allows for accurate calibration of ENA space instruments.

Mitigation of disruption on IR-T1 tokamak by means of low-energy neutral beam injection to control runaway electron generation

In a tokamak, the poloidal magnetic field provided by the toroidal plasma current forms an essential part of the magnetic field confining the plasma. However, instabilities of magnetohydrodynamic equilibrium can lead to an uncontrolled sudden loss of plasma current and energy, which is called a disruption. Disruptions are of significant concern to future devices due to the large amount of energy released during the rapid quenching of the plasma. One important consequence of disruption is the generation of significant current carried in multi-MeV runaway electrons that are eventually lost into plasma components. They can damage the tokamak walls and its structure if they are not controlled. Disruption control by neutral beam injection has been performed on IR-T1 to study the effect on runaway electron generated by plasma disruptions. Noble gases are used for injection, pure Hydrogen, Helium and Argon. The use of these non-reactive gases for disruption control ensures they fast removed from the vessel after the termination of a tokamak discharge. A piezo-valve is used for injection which has the precision of 1 ms. The effect of runaway electron generation control during disruption is studied using a comparison between reference disruptive discharge and a discharge into which different impurity species are injected. The data collected can then be used to optimize the performance of these energetic electrons control generated in disruption.

Low-temperature InGaAs oxidation using oxygen neutral beam

The oxidation of InGaAs and the mechanisms underlying this process were investigated using a low-energy oxygen neutral beam. The thickness of the oxide layer and the components of In, O, Ga, and As were examined by cross-sectional transmission electron microscopy (TEM) and X-ray photoemission spectroscopy (XPS). Extending the oxidation duration together with the use of a high indium concentration generated a thicker InGaAs oxide layer because of the difference in chemical bond strength. The oxidation of InGaAs, which was successful even at room temperature, resulted in a high-quality interface because of the highly reactive neutral beam and its extremely low activation energy.

Constructing a Heterostructural LiNi0.4Mn1.6O4-δ Material from Concentration-Gradient Framework to Significantly Improve Its Cycling Performance.

A heterostructural LiNi0.35Mn1.65O4-δ-LiNi0.5Mn1.5O4 was achieved via constructing the concentration-gradient material with an average composition of LiNi0.4Mn1.6O4-δ. The as-obtained samples were characterized by cross-sectional scanning electron microscopy and energy dispersive X-ray spectrometry, and it was found that the core material LiNi0.35Mn1.65O4-δ is encapsulated completely by a concentration-gradient shell with the outmost layer of LiNi0.5Mn1.5O4. Demonstrated by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy analysis with a low-energy Ar neutral beam etching method, the structurally stable P4332 phase on the surface coordinates the high conductive Fd3̅m phase in the bulk without any detectable interface. At room temperature, the heterostructural samples deliver the initial discharge capacity of 145.1 mAhg-1 at 1 C, close to the theoretical capacity, and the capacity retention after 300 cycles was up to 96.5% of the first discharge capacity. Moreover, it shows excellent rate capability with discharge capacity of 144.3 mAhg-1 at 10 C and significantly improved cycling stability with capacity retention of 85.51% over 50 cycles between 3.0 and 4.95 V at 55 °C. The as-obtained material with coordination of two crystallographic structure domains is a promising cathode to develop high energy, high power, long lifespan, and low cost lithium-ion batteries.

Low energy, high power hydrogen neutral beam for plasma heating.

A high power, relatively low energy neutral beam injector was developed to upgrade of the neutral beam system of the gas dynamic trap device and C2-U experiment. The ion source of the injector produces a proton beam with the particle energy of 15 keV, current of up to 175 A, and pulse duration of a few milliseconds. The plasma emitter of the ion source is produced by superimposing highly ionized plasma jets from an array of four arc-discharge plasma generators. A multipole magnetic field produced with permanent magnets at the periphery of the plasma box is used to increase the efficiency and improve the uniformity of the plasma emitter. Multi-slit grids with 48% transparency are fabricated from bronze plates, which are spherically shaped to provide geometrical beam focusing. The focal length of the Ion Optical System (IOS) is 3.5 m and the initial beam diameter is 34 cm. The IOS geometry and grid potentials were optimized numerically to ensure accurate beam formation. The measured angular divergences of the beam are ±0.01 rad parallel to the slits and ±0.03 rad in the transverse direction.

Si etching with reactive neutral beams of very low energy

A Si etching process has been investigated with reactive neutral beams (NBs) extracted using a low acceleration voltage of less than 100 V from CF4 and Ar mixed plasmas. The etched Si profile shows that the etching process is predominantly anisotropic. The reactive NB has a constant Si etching rate in the acceleration voltage range from 20 V to 80 V. It is considered that low-energy NBs can trigger Si etching because F radicals adsorb onto the Si surface and weaken Si–Si bonds. The etching rate per unit beam flux is 33 times higher than that with Ar NB. These results show that the low-energy reactive NB is useful for damage-free high speed Si etching.

Mechanical quality factor enhancement in a silicon micromechanical resonator by low-damage process using neutral beam etching technology

The fabrication and evaluation of silicon micromechanical resonators using neutral beam etching (NBE) technology is presented. An etching technique based on a low energy neutral beam of Cl2/F2/O2 is introduced for making nano-trench patterns on 5 µm-thick silicon. The NBE technology has been investigated to form a highly-anisotropic etching shape. A 5 μm-deep trench pattern having smooth side walls with a gap width of 230 nm is achieved by using NBE. Additionally, a fabrication method for silicon resonators using NBE technology is proposed. The resonant frequency of the fabricated devices with a length of 500 μm, width of 440 μm and thickness of 5 μm is 9.66 MHz, and the average quality factor (Q) value is around 78 000. The devices fabricated by both deep reactive ion etching (DRIE) and NBE are evaluated and compared. The devices fabricated by NBE show that the motional resistances are reduced by almost 11 times from 645 kΩ to 59 kΩ and their output signals (insertion loss) are increased by approximately 15 dB in comparison with those fabricated by DRIE. Especially, devices fabricated by NBE provide the higher Q factors (average Q factor value of around 78 000) than those (average Q factor value of around 61 000) fabricated by DRIE in the same resonator parameters and measurement conditions.

Extension of operation regimes and investigation of three-dimensional currentless plasmas in the Large Helical Device

The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-Ti plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.

Development of a low-energy and high-current pulsed neutral beam injector with a washer-gun plasma source for high-beta plasma experiments

We have developed a novel and economical neutral-beam injection system by employing a washer-gun plasma source. It provides a low-cost and maintenance-free ion beam, thus eliminating the need for the filaments and water-cooling systems employed conventionally. In our primary experiments, the washer gun produced a source plasma with an electron temperature of approximately 5 eV and an electron density of 5 × 10(17) m(-3), i.e., conditions suitable for ion-beam extraction. The dependence of the extracted beam current on the acceleration voltage is consistent with space-charge current limitation, because the observed current density is almost proportional to the 3/2 power of the acceleration voltage below approximately 8 kV. By optimizing plasma formation, we successfully achieved beam extraction of up to 40 A at 15 kV and a pulse length in excess of 0.25 ms. Its low-voltage and high-current pulsed-beam properties enable us to apply this high-power neutral beam injection into a high-beta compact torus plasma characterized by a low magnetic field.

Development of Neutral Beam Source Using Electron Beam Excited Plasma

A low-energy neutral beam (NB) source, which consists of an electron-beam-excited plasma (EBEP) source and two carbon electrodes, has been developed for damageless etching of ultra-large-scale integrated (ULSI) devices. It has been confirmed that the Ar ion beam energy was controlled by the acceleration voltage and the beam profile had good uniformity over the diameter of 80 mm. Dry etching of a Si wafer at the floating potential has been carried out by Ar NB. Si sputtering yield by an Ar NB clearly depends on the acceleration voltage. This result shows that the NB has been generated through the charge exchange reaction from the ion beam in the process chamber.

Ion beam modification of biological materials in nanoscale

Ion interaction with biological objects in nanoscale is a novel research area stemming from applications of low-energy ion beams in biotechnology and biomedicine. Although the ion beam applications in biotechnology and biomedicine have achieved great successes, many mechanisms remain unclear and many new applications are to be explored. We have carried out some research on exploring the mechanisms and new applications besides attaining ion beam induction of mutation breeding and gene transformation. In the studies on the mechanisms, we focused our investigations on the direct interaction in nanoscale between ions and biological living materials. Our research topics have included the low-energy ion range in DNA, low-energy ion or neutral beam bombardment effect on DNA topological form change and mutation, low-energy ion or neutral beam bombardment effect on the cell envelope and gene transformation, and molecular dynamics simulation of ultra-low-energy ion irradiation of DNA. In the exploration of new applications, we have started experiments on ion irradiation or bombardment, in the nanoscaled depth or area, of human cells for biomedical research. This paper introduces our experiments and reports interesting results.

The properties of low energy neutral particles in a neutral beam source: A molecular dynamics study

Application of a hyperthermal neutral beam source is one of the candidate methods of reducing plasma induced damage problems. The neutral beam is generated by vertical collisions between energetic ions and a reflector composed of metal. However, it is difficult to measure the neutral angle and energy distribution experimentally. We simulate the hyperthermal neutral beam (HNB) generation using a molecular dynamics algorithm. In order to obtain a low energy neutral beam, ions with various energies are vertically projected onto the reflector surface. A rough surface structure that has been experimentally measured is used for a realistic simulation. The energy distributions are obtained and the ratio of energy of reflected neutral particles agrees with experimental data.

Controllable modification of self-assembled monolayer surface by using N2 neutral beam process

A novel technique of low-energy N2 neutral beam (NB) (<10 eV) irradiation was investigated as a means of controlling the surface nitridation of self-assembled monolayers (SAMs). This low-energy process without UV photons was able to nitride the surfaces of terphenyl SAMs. Furthermore, by varying the on/off period for irradiation, a pulse-time modulated NB process could be used to control the atomic ratio of nitrogen in terphenyl molecules. Time-of-flight secondary ion mass spectra revealed that one H in the terphenyl group was substituted by a NH2 group because of irradiation by the N2 beam. The data suggested that the low-energy NB technique caused less damage to modified surfaces than the conventional plasma process did. Therefore, we believe that it is a potential damage-free process for modifying the surfaces of soft organic materials and films.

Cold-cathode Penning discharge-based ionizer for detection of hyperthermal neutral beams

Plasmas produced in a cold-cathode Penning discharge have been studied for possible use as an active ionizing medium in commercial quadrupole mass/energy analyzers for detection of low-energy neutral beams. Two distinct Penning discharge modes have been examined: (1) high-pressure (HP) mode and (2) high magnetic field (HMF) mode. It is shown that the ionization efficiency in the HP mode is independent of the length of ionization region; however, somewhat high working pressures (p>10(-4) Torr) and large discharge currents limit the practical use of this mode. This is not the case in the HMF mode, which appears at lower pressures, with an effective ionization region length of the order of electron cyclotron radius. The design and operation of a compact (5x4x4 cm(3)), low-maintenance ionizer based on a Penning cell with permanent magnets is described. The ability to ionize 40 eV neutral-argon beams with subsequent detection in a Hiden EQP energy-resolved mass spectrometer is shown. The ionization efficiency of the ionizer was found to be as high as 10(-3). Unlike conventional electron impact ionizers, the Penning discharge configuration allows to eliminate the thermal background component in the detected signal. The ionizer has potential application for the detection of hyperthermal neutral beams of various species.

Coulomb-staircase observed in silicon-nanodisk structures fabricated by low-energy chlorine neutral beams

A silicon-nanodisk structure, a nanometer-scale silicon disk on extremely thin SiO2 film, was fabricated by etching a 3.5–4-nm-thick polycrystalline silicon (poly-Si) thin film/1.4–3-nm-thick underlying oxide layer/Si substrate structure with a 7-nm-diameter ferritin iron-core mask and Cl neutral beam etching (NBE). The degree of etching was precisely controlled by detecting its depth using x-ray photoelectron spectroscopy. Cross-sectional scanning transmission electron microscopy (STEM) with elemental mapping by electron energy-loss spectroscopy (EELS) revealed that the underlying oxide layer remained while the Si layer was accurately etched. The STEM-EELS observation also revealed that there was an Si layer about 1–2-nm thick even in the nanodisk, while the nanodisk’s surface region was covered by native oxide. Removing the surface oxide layer prior to the NBE process could decrease the nanodisk diameter. Irradiation by Cl NB of the underlying 1.4-nm-thick SiO2 film increased the thickness of the SiO2 film and drastically decreased the SiO2 leakage current. This worked as self-aligned isolation in the space between the nanodisks when measuring their current-voltage (I−V). This may also be useful for fabricating future quantum-effect devices using nanodisks. Coulomb staircases could be observed by measuring the I−V of nanodisks even at room temperature. These results indicated that the nanodisks fabricated in this research have a precise quantum-effect structure and they attained single-electron properties. This research has great potential for the development of practical and robust fabrication processes for future quantum-effect devices.

Fabrication and Power-Management Demonstration of Four-Terminal FinFETs

A high-aspect-ratio and damage-free vertical ultrathin channel (UTC) for a vertical-type double-gate (DG) MOSFET has been fabricated by using low-energy neutral-beam (NBE) etching. Also, dynamically power-controllable CMOS technology has been investigated using separated-gate four-terminal (4T) FinFETs. We demonstrate that the power consumption of the CMOS inverter can dynamically be controlled using the 4T-FinFET.

Measurement of Relative Flux of Fractional-Energy Emissions Using Beam Emission Diagnostic

Fraction of a neutral beam particle with the different energy has been investigated in the Large Helical Device. We adopt a low-energy (40keV-3MW) neutral beam injection system in order to improve the ion temperature. A visible-spectrometer coupled intensified charge coupled device detector has been installed in order to measure a hydrogen beam emission spectrum inside of a target plasma. The spectral resolution and the reciprocal dispersion are 0.21nm and 1.4nm/mm, respectively. We typically use the 40ms sampling times with the 20ms exposure times. We have clearly observed the beam emission spectra from a hydrogen neutral beam with the energy of full, half and 1/3. We have also estimated the fraction of the full-energy beam component from the beam emission intensity. Total injection beam power is improved by increasing of the fraction of the full-energy beam component.

Characterization of neutral beam source based on pulsed inductively coupled discharge: Time evolution of ion fluxes entering neutralizer

Low-energy neutral beam sources are very promising candidates for realization of next generation ultralarge-scale integrated devices. The use of pulsed inductively coupled plasma and surface (wall) neutralizer appears to be an efficient way of producing high-flux low-energy neutral beams. Measurement of the time evolution of ion fluxes entering the neutralizer plays an essential role in understanding and control of these neutral beam systems. Here the authors present a simple method for measuring the temporal dynamics of ion fluxes in neutral beam source described elsewhere [S. Samukawa et al., J. Vac. Sci. Technol. A 20, 1566 (2002)]. The method is based on the use of a low aspect ratio orifice in the center of neutralizer, magnetic filter, and Faraday cup. At some conditions, it allows (1) to measure the magnitudes of positive and negative wall ion fluxes in pulsed plasmas with an extremely high temporal resolution (better than 1μs) and (2) to examine the difference in surface neutralization between positive and negative ions. The measurements show that neutralization of hyperthermal ions is mainly controlled by geometry of plasma sheath adjacent to the surface neutralizer; however, negative ions are neutralized more easily than positive ones. The experimental results for SF6 (ion-ion) and Ar plasmas in combination with dc/rf bias are reported.

Effects of neutral-beam excited states on charge-exchange emission cross sections

Charge-exchange recombination spectroscopy can be used as a diagnostic for local densities of fully stripped impurities. For example, impurity profiles are valuable for transport studies. A major impediment to analysis is the availability of cross sections which relate the observed spectroscopic emission to the impurity densities. In this work, this impediment is removed for boron ions. Emission cross sections for B+4 states excited by charge exchange between beams of neutrals and fully stripped B+5 impurities in high density plasmas are computed for beams with multiple energy components. We show that even the low-energy neutral beam components make significant contributions to the emission for high-density plasmas which are commonly generated in the laboratory. Excited states as high as n=3 are important. This implies that a detailed description of the beam components including density, excited state population, and penetration is required for prediction of intensities of spectral lines excited by diagnostic neutral beams in high-density plasmas. Plasma experiments performed on the Alcator C-Mod tokamak will be described which clearly show the efficacy of our analysis method. This technique can be extended to species other than boron.

Low-damage atomic layer modification of self-assembled monolayer using neutral beam process

Damage-free surface nitridation of terphenyl methanethiol self-assembled monolayers (TP1-SAMs) was investigated using a novel low-energy N2 neutral beam (NB) irradiation technique. When a conventional inductively coupled plasma process was used, x-ray photoelectron spectra confirmed that the TP1-SAM was quickly broken because ions or ultraviolet (UV) photons enhance the surface decomposition and molecular desorption. Conversely, with the N2 NB radiation process, which is free of ions and UV photons, there was little difference in the atomic ratios of x-ray photoelectron spectra before and after NB irradiation. These results suggest that low-damage surface modification is possible through the authors’ NB technique.