Beam Impingement Research Articles

Formation of molecular hydrogen in diamond-like carbon films during deposition: Molecular dynamics simulations

The existence states of the H atoms in hydrogenated diamond-like carbon (a-C:H) films were simulated using a modified REBO II potential function-based large-scale atomic/molecular parallel simulator (LAMMPS) and neutral C and H atom beam impingement. The fraction of the H atoms in the incident source (H-flux fraction) in the source must be considered when investigating the effects of the incident energy combination on the H content and mass density of the films. In simulations a-C:H film deposition using a 15 % H-flux fraction, films with densities of 2.6–2.8 g/cm3 could be obtained, and their H content varied with the incident energy. However, the H contents of films simulated using a 70 % H-flux fraction could be kept in the range of 40–50 at.%. The H atoms in the films preferentially reacted with the C atoms to form CH bonds. Regardless of the incident energy combination used, the total sp3C fraction in the film increased with the H-flux fraction of the source. An incident energy of >36 eV for the H atoms was necessary to form H2 during the film deposition. Instead of being formed in the surface growth zone, H2 molecules were synthesized in the stabilized zone, where a high density of sp3C—Hn was favorable.

Scalable Microfabrication of Multi-Emitter Arrays in Silicon for a Compact Microfluidic Electrospray Propulsion System.

The recent proliferation of SmallSats and their use in increasingly demanding applications require the development of onboard electric propulsion compatible with the power, mass, and volume constraints of these spacecraft. Electrospray propulsion is a promising technology for SmallSats due to its unique high efficiency and scalability across the wide power range of these platforms, for example, from a few watts available in a CubeSat to a few hundred watts in a MiniSat. The implementation of electrospray propulsion requires the use of microfabrication techniques to create compact arrays of thousands of electrospray emitters. This article demonstrates the microfabrication of multi-emitter electrospray sources of a scalable size for electrospray propulsion. In particular, a microfabrication and assembly process is developed and demonstrated by fabricating sources with arrays of 1, 64, and 256 emitters. The electrospray sources are tested in a relevant environment for space propulsion (inside a vacuum chamber), exhibiting excellent propulsive performance (e.g., absence of beam impingement in the extractor electrode, absence of hysteresis in the beam current versus propellant flow rate characteristic, proper operation in the cone-jet electrospraying mode, etc.) and nearly coincident output per emitter. Several design elements contribute to this performance: the even distribution of the propellant among all emitters made possible by the implementation of a network of microfluidic channels in the backside of the emitter array; the small dead volume of the network of microfluidic channels; the accurate alignment between the emitters and extractor orifices; and the use of a pipe-flow configuration to drive the propellant through closed conduits, which protects the propellant.

A Thermo-Hydraulic Investigation of the Unprecedented TMRS Upper Neutron Spallation Target

Abstract The next-generation neutron spallation target station, the Target–Moderator-Reflector System (TMRS) Mk. IV, will be installed in 2021. This iteration features an unprecedented, water-cooled, third internal target aptly named the upper target. With the upper target designed completely by analysis, a complementary empirical investigation was undertaken to ascertain target conformance to those computational results which deemed the cooling efficacious. Three facets of the target were designated for verification: displacement under hydraulic load, critical fluid velocities, and the characteristic heat transfer coefficient (HTC). With the potential for flow maldistribution under excessive displacements, static pressure testing was performed. Discrepancies of an order of magnitude became evident between empirical and simulated displacements, 1.499 mm versus 0.203 mm, respectively. A closed-water flow loop reproducing the flow parameters intrinsic to the TMRS Mk. IV was constructed. Utilizing particle image velocimetry, global fluid dynamics were observed to be analogous to computer simulation. Furthermore, crucial velocities such as those at the point of beam impingement were met or exceeded, thus satisfying cooling requirements by a preponderance. A graphite susceptor mirroring nominal beam geometry was coupled to a solenoid coil to replicate a prodigious peak heat flux of 169 W/cm2 via induction heating. Matching peak heat flux within 3% engendered a HTC of 80% that of simulation. Consistent with analysis, the local HTC sufficiently mitigated nucleate/flow boiling. In summary, the analytically derived upper target design empirically demonstrated sufficient cooling despite quixotic beam conditions and unforeseen displacements.

Design, simulation and implementation of an efficient vacuum system and gas load calculations for a 6 MeV industrial LINAC

The design of vacuum system for a 6 MeV, commercial LINAC is presented. Unlike many commercial LINACs, the LINAC being developed has demountable electron gun, RF-waveguide and X-ray target assemblies. This design scheme is more efficient and economical. The LINAC has a total internal volume of 1.9 land surface area of ~3.2 × 103cm2, and is operated at Ultra High Vacuum. An iterative Simulation-Experimental approach is adopted to freeze a viable vacuum system for the LINAC. The vacuum simulations are performed in MOLFLOW+2.7.7, and the results are benchmarked experimentally. Due to compact geometry of the LINAC, reduced pumping speeds are experienced. The system was implemented using two 550 l/s commercially available turbo molecular pumps backed by rotary pumps, consuming ~2.4 kW. Particle dynamics simulations were performed using PARMELA 3. These simulations quantified the beam loss in the LINAC and predicted the sites of beam impingement. The system was experimentally verified for establishment of base pressure, activation of dispenser cathode, electron gun conditioning and the full power operation of LINAC at repetition rate of 50–110 Hz. Through all these regimes the vacuum system performed within permissible limits. The gas loads induced in the system were also evaluated using steady state and transient analysis. The pumping speed is optimized, and it is shown that base pressure of 1 × 10−9mbarcan be achieved at reduced pumping speed of ~100 l/s at the pumping ports. This reduced pumping speed also performed well for routine LINAC operation. Therefore, using benchmarked simulated results, an optimized and commercially economical vacuum system is proposed with reduced pumping speed of ~100 l/s consuming 6 times reduced power (0.4 kW) and ~40% reduced cost.

In Situ Local Measurement of Austenite Mechanical Stability and Transformation Behavior in Third-Generation Advanced High-Strength Steels

Austenite mechanical stability, i.e., retained austenite volume fraction (RAVF) variation with strain, and transformation behavior were investigated for two third-generation advanced high-strength steels (3GAHSS) under quasi-static uniaxial tension: a 1200 grade, two-phase medium Mn (10 wt pct) TRIP steel, and a 980 grade, three-phase TRIP steel produced with a quenching and partitioning heat treatment. The medium Mn (10 wt pct) TRIP steel deforms inhomogeneously via propagative instabilities (Luders and Portevin Le Châtelier-like bands), while the 980 grade TRIP steel deforms homogenously up to necking. The dramatically different deformation behaviors of these steels required the development of a new in situ experimental technique that couples volumetric synchrotron X-ray diffraction measurement of RAVF with surface strain measurement using stereo digital image correlation over the beam impingement area. Measurement results with the new technique are compared to those from a more conventional approach wherein strains are measured over the entire gage region, while RAVF measurement is the same as that in the new technique. A determination is made as to the appropriateness of the different measurement techniques in measuring the transformation behaviors for steels with homogeneous and inhomogeneous deformation behaviors. Extension of the new in situ technique to the measurement of austenite transformation under different deformation modes and to higher strain rates is discussed.

Effect of high power CO2 and Yb:YAG laser radiation on the characteristics of TIG arc in atmospherical pressure argon and helium

Abstract The effects of laser radiation on the characteristics of the DC tungsten inert gas (TIG) arc were investigated by applying a high power slab CO 2 laser and a Yb:YAG disc laser. Experiment results reveal that the arc voltage–current curve shifts downwards, the arc column expands, and the arc temperature rises while the high power CO 2 laser beam vertically interacts with the TIG arc in argon. With the increase of the laser power, the voltage–current curve of the arc shifts downwards more significantly, and the closer the laser beam impingement on the arc to the cathode, the more the decrease in arc voltage. Moreover, the arc column expansion and the arc temperature rise occur mainly in the region between the laser beam incident position and the anode. However, the arc characteristics hardly change in the cases of the CO 2 laser–helium arc and YAG laser–arc interactions. The reason is that the inverse Bremsstrahlung absorption coefficients are greatly different due to the different electron densities of the argon and helium arcs and the different wave lengths of CO 2 and YAG lasers.

Energy Absorption Numerical Analysis of Mine Impingement Thin-Walled Components with Different Configurations

For the effective prevention and control of mine bumps, or to a certain extent, to reduce the loss the losses caused by impact ground pressure accident,thin-walled components are proposed to be applied as a scaffold components in roadway supporting. Components impingement is reflected in the component crushed process ,in which the impacting energy absorption and crushing space provide certain amount of energy to free up space. The energy absorption characteristics of different configuration thin-walled components has been simulated with ABAQUS finite element, such as the conventional square,regular hexagon, round and origami square, regular hexagon etc,.and the results showed that: (1) The problem of conventional components is larger load fluctuation coefficient. Origami component is a kind of perfect impingement components ,as it can reduce peak loads and the load fluctuation coefficient effectively. (2) Under different ratios width-to-thickness, components can have different deformation modes, but the relationship of peak load and wall thickness is near linear and has nothing to do with the deformation modes. (3) The component can have a relatively lower crushing peak load and load fluctuation coefficient and relatively higher specific energy absorption,if given a suitable diamond concave angle. impingement components using combined with existing supports can make the existing supports to become top beam impingement support , mudsill impingement support ,two sides impingement support and impingement hydraumatic support and so on.

Cathodoluminescent stability of rare earth tantalate phosphors

Surface chemical changes were probed on the surface of YTaO4:Pr3+ (0.5mol%) and LaTaO4:Pr3+ (0.5mol%) phosphors that were prepared by solid state reaction at 1200°C, after they were exposed to a 900Ccm−2 electron dose. X-ray diffraction confirmed the successful formation of these phosphors. The structures of YTaO4:Pr3+ (0.5mol%) and LaTaO4:Pr3+ (0.5mol%) were found to be M′ monoclinic phases. The blue cathodoluminescence (CL) emission coming from the 3P0→3H4 transition of Pr3+ was found to be relatively more intense than the red emission from the 1D2→3H4 transition under electron beam irradiation of LaTaO4:Pr3+ (0.5mol%) and the red emission was dominant over the blue emission in YTaO4:Pr3+ (0.5mol%). This was attributed to the position of the intervalence charge transfer state as well as a possible cross-relaxation mechanism between Pr3+ ion pairs in the case of YTaO4:Pr3+ (0.5mol%). The CL and chemical changes of the surfaces of these phosphors were probed by a 2keV electron beam at a 1×10−6Torr O2 pressure using an Auger electron spectroscopy system in combination with a CL spectrometer. X-ray Photoelectron spectroscopy was used to analyze the chemical changes that took place on the surface as a result of the electron beam impingement, and it revealed the formation of new species on the surface of YTaO4 that were called YxOz, with 0<x<2, 0<z<3 and an increment contribution from Ta2O5 and Y2O3. No chemical changes were found for LaTaO4:Pr3+ during electron bombardment. The stability of the CL intensity of both phosphors opens the possibility of investigating the phosphors to be used as a possible field emission display phosphor.

Surface roughening of silicon, thermal silicon dioxide, and low-k dielectric coral films in argon plasma

The surface roughness evolutions of single crystal silicon, thermal silicon dioxide (SiO2), and low dielectric constant film coral in argon plasma have been measured by atomic force microscopy as a function of ion bombardment energy, ion impingement angle, and etching time in an inductively coupled plasma beam chamber, in which the plasma chemistry, ion energy, ion flux, and ion incident angle can be adjusted independently. The sputtering yield (or etching rate) scales linearly with the square root of ion energy at normal impingement angle; additionally, the angular dependence of the etching yield of all films in argon plasma followed the typical sputtering yield curve, with a maximum around 60°–70° off-normal angle. All films stayed smooth after etching at normal angle but typically became rougher at grazing angles. In particular, at grazing angles the rms roughness level of all films increased if more material was removed; additionally, the striation structure formed at grazing angles can be either parallel or transverse to the beam impingement direction, which depends on the off-normal angle. More interestingly, the sputtering caused roughness evolution at different off-normal angles can be qualitatively explained by the corresponding angular dependent etching yield curve. In addition, the roughening at grazing angles is a strong function of the type of surface; specifically, coral suffers greater roughening compared to thermal silicon dioxide.

Materials modification using intense ion beams

Pulsed intense ion beams have been developed for applications including surface modification and alloying, and thin-film and nanopowder synthesis. Rapid thermal processing with ions is quite promising for large-scale commercial use, due to the high specific ion energy deposition (joules per cubic centimeter) without reflection, and to the relative efficiency and low cost of the pulsed power ion-beam drivers compared to other high-kinetic energy alternatives. We discuss in this paper the basis for the use of ions in materials processing and the methods of beam formation and impingement on material to be treated, and give examples of recent and ongoing work in materials processing.

Probability of a Laser Illuminating a Space Object

Laser beam impingement probability is needed to quantify the risk of accidental or unwanted illumination of a spaceobject.Atheoreticalmethodofcombininglaserbeampointingerrorandspaceobjectposition errorisused to calculatebeam impingementprobability.Themethodaccountsforposition,velocity,anderrorcovariancematrices of laser emitter and space object. Laser slew rate, pointing uncertainty, and beam divergence half-cone angle, as well asspace object sizeand shape, arealso included in the formulation. In performing thecalculation, parameters are transformed to a laser beam coordinated frame that is centered on the nominal beam axis. Instantaneous and time-dependent impingement probabilities are reduced to one-dimensional contour integrals. The cumulative impingement probability is expressed as a simple integral. A software tool was created to implement the method. Numerical results for example data are presented. Nomenclature A = unit vector normal to the laser beam having the largest pointing uncertainty a = standard deviation of the symmetric probability density,m B§ = y-axis intercept parameter in the diagonal frame, m b = standard deviation along the y axis of the diagonal frame, m C = satellite and laser combined error covariance matrix

THE LOW PRESSURE GAS EFFECTS ON THE POTENCY OF AN ELECTRON BEAM ON CERAMIC FABRIC MATERIALS FOR SPACE WELDING

This investigation was undertaken to evaluate if molten metal or electron beam impingement could damage or burn through the fabric of the astronauts extravehicular mobility unit (EMU) during electron beam welding exercises performed in space. An 8 kV electron beam with a current in the neighborhood of 100 mA from the Ukrainian space welding “Universal Hand Tool” burned holes in Nextel AF-62 ceramic cloth designed to withstand temperatures up to 1427°C. The burnthrough time was on the order of 8 s at standoff distances between UHT and cloth ranging from 6 to 24 in . At both closer (2 in) and farther (48 in) standoff distances, the potency of the beam against the cloth declined and the burnthrough time went up significantly. Prior to the test it had been expected that the beam would lay down a static charge on the cloth and be deflected without damaging the cloth. The burnthrough is thought to be an effect of partial transmission of beam power by a stream of positive ions generated by the high voltage electron beam from contaminant gas in the “vacuum” chamber. A rough quantitative theoretical computation appears to substantiate this possibility.

Polycarbonate surface modified by argon cluster ion beams

X-ray induced photoelectron spectroscopy and atomic force microscopy were used to gain detailed information on surface topography, composition and bonding among atoms in a near-surface region of untreated and modified polycarbonate surfaces by argon cluster and argon monomer ion irradiation. As a consequence of the cluster ion beam impingement on the polycarbonates, their surfaces become rough with polymer structure partially damaged. Surprisingly, the surface irradiated by argon monomer ions was found less rough but the polymer structure in a surface region completely lost its integrity and an amorphous (hydrogenated) carbon layer was formed.

Designing the performance of a thick-film laser power detector by means of a heat-transfer analysis using finite-element method

Besides the usual application of thick-film technology to produce circuits of high reliability, this technology is also attractive for the fabrication of sensors. A laser power detector has been realized utilizing the thick-film technique. A special detector configuration provides an exact laser power measurement independent of the position of beam impingement on the detector area and independent of the intensity distribution of the laser beam. The detector, which actually operates as a heat-flow meter, consists of a thermopile circularly applied on an AlN substrate. The geometrical proportions of the detector have been accommodated to an NdYAG laser with a given specification by means of thermal heat-transfer analysis using the finite-element method (FEM). Based on the FEM simulation of steady state and transient temperature distributions on the detector area induced by laser beam impingement, the performance and the operating range of the detector have been specified.

Utilizing of hydrocarbon contamination for prevention of the surface charge-up at electron-beam assisted chemical etching of a diamond chip

Electron beam assisted chemical etching (EBACE) with oxygen gas is not applicable for direct fine patterning of diamond devices, because the diamond is an electrical insulator and electron beam impingement of the diamond causes the surface charge-up. It is possible to form conductive layer of hydrocarbon on the diamond surface by electron beam irradiation in the atmosphere of diffusion pump oil vapors. In this paper, a scanning electron microscope (SEM) combined with oxygen gas introduction system was used for EBACE of the diamond. It was found by in-situ SEM observation that rectangular patterns with several μm 2 area and sub-μm depth were formed on the diamond chip.

Novel features of photoluminescence spectra from acceptor-doped GaAs: formation of acceptor—acceptor pair emissions and optical compensation effect

Abstract Low temperature photoluminescence measurements were carried out for acceptor-doped GaAs. Impurities were introduced by doping during molecular beam epitaxy (MBE) or liquid phase epitaxy (LPE) growth or by ion beam impingement into substrates using mass-separated ions. In the case of carbon incorporation, doping of mass-separated C + ions using a low energy ion beam was carried out during MBE growth of GaAs. Results revealed that just below the band edge emissions a large number of novel strong emissions are produced. These emissions were found to be totally missing in specimens prepared by conventional methods. Dual incorporation of acceptor and donor impurities indicated that these emissions are easily quenched by an extremely small amount of donor atoms, which is a principal reason for the absence of these emissions in previous samples. Most of these novel emissions were also obtained in InP, implying that these observations are ubiquitously established in the whole range of III–V compound semiconductors. It was concluded that for the comprehensive understanding of impurity properties in semiconductors, mass-separated impurity introduction into ultra pure substrates is essential to avoid this strong optical (and corresponding electrical) compensation effect induced by oppositely charged impurities. Taking this conclusion into consideration, we emphasize that the optical properties of doped semiconductors should be totally re-examined in a more detailed and systematic manner.

Laser Machining for Secondary Finishing Applications

A laser-based technique for finishing axisymmetric parts is presented which allows the efficient finishing of polymers and ceramics without tool wear, tool breakage, or cutting forces. In this process, a laser beam impinges tangentially onto the surface of a cylindrical workpiece. A flexible machine tool can be developed to grind parts of differing geometries and materials by changing process parameters instead of setups or machines, as well as integrate primary machining and secondary finishing in one machine tool. The precision of laser finishing can be enhanced by using oblique beam impingement angles. Initial results show that Ra values less than 1 μm can be achieved on PMMA workpieces with a fixed beam. This paper presents the elements of the laser machine tool and preliminary results on parametric dependencies for laser finishing of polymer workpieces.

Theoretical Model of Laser Grooving for Composite Materials

The influence of directionally-dependent thermal properties on laser machining effectiveness is analyzed theoretically and experimentally. A laser grooving model was developed which incorporated the effects of directionally-dependent thermal conductivity. Laser grooving experiments on graphite/vinylester composites showed differences in groove depth and profile depending on the laser beam direction relative to fiber orientation. The groove depth predictions from the composite grooving model showed an improved correlation with experimental data compared to predictions from the model with homogeneous material properties. The predictions of groove profiles and centerlines from the composites model are also presented for laser beam impingement parallel and perpendicular to fiber orientation.

Application of integrated thick-film thermocouples for a laser power detector

Abstract A laser power detector has been developed, where the sensor elements have been built up utilizing standard thick-film technique. The operating principle is based on a heat-flow measurement. Heat flow may be determined by means of temperature differences applying surface temperature sensors. Thick-film thermopiles arranged in a special configuration act as heat-flow meters. Optimizing the geometrical proportions of the detector configuration provides an accurate laser power measurement independent of the position of beam impingement on the detector area and independent of the intensity distribution of the laser beam.

Application of rf superconductors to linacs for high-brightness proton beams

A series of superconducting structures for the acceleration of high-current ion beams is being developed, and two prototype Nb cavities are under construction. These cavities operate in a frequency-velocity range which, for superconducting structures, has been little explored: frequencies of 0.4 GHz to more than 1 GHz, and velocities of 0.1 c to 0.5 c. Issues discussed include: need for strong beam loading (∽10 4), need for strong focusing elements located close to the cavities, minimization of beam impingement and beam instabilities.