Target Radius Research Articles

Definition and Application of Proton Source Efficiency in Accelerator-Driven Systems

In order to study the beam power amplification of an accelerator-driven system (ADS), a new parameter, the proton source efficiency ψ* is introduced. ψ* represents the average importance of the external proton source, relative to the average importance of the eigenmode production, and is closely related to the neutron source efficiency φ*, which is frequently used in the ADS field. φ* is commonly used in the physics of subcritical systems driven by any external source (spallation source, (d,d), (d,t), 252Cf spontaneous fissions, etc.). On the contrary, ψ* has been defined in this paper exclusively for ADS studies where the system is driven by a spallation source. The main advantage with using ψ* instead of φ* for ADS is that the way of defining the external source is unique and that it is proportional to the core power divided by the proton beam power, independent of the neutron source distribution.Numerical simulations have been performed with the Monte Carlo code MCNPX in order to study ψ* as a function of different design parameters. It was found that, in order to maximize ψ* and therefore minimize the proton current needs, a target radius as small as possible should be chosen. For target radii smaller than ~30 cm, lead-bismuth is a better choice of coolant material than sodium, regarding the proton source efficiency, while for larger target radii the two materials are equally good. The optimal axial proton beam impact was found to be located ~20 cm above the core center. Varying the proton energy, ψ*/Ep was found to have a maximum for proton energies between 1200 and 1400 MeV. Increasing the americium content in the fuel decreases ψ* considerably, in particular when the target radius is large.

Influence of grid and target radius and ion–neutral collisions on grid-enhanced plasma source ion-implantation process

Grid-enhanced plasma source ion implantation (GEPSII) is a newly proposed technique for inner surface modification of materials with cylindrical geometry. In this paper, a collisional fluid model is used to investigate the ion sheath dynamics between the grid electrode and the inner surface of a cylindrical bore during the GEPSII process. Assuming the initial ion density along the radial direction is not uniform but determined by diffusion mechanisms, the effects of grid electrode radius, target radius and ion–neutral collisions on the ion dose and impact energy are investigated by solving fluid equations for ions coupled with Boltzmann assumption for electrons and Poisson's equation. The results show that small gap distance between grid electrode and target is favourable to increase the ion dose and impact energy on the target. In addition, ion–neutral collisions can reduce both the ion dose and impact energy.

Validation of Confocal Microscopy Through Focusing for Corneal Sublayer Pachymetry

To evaluate the accuracy and precision of confocal microscopy through focusing (CMTF) for corneal sublayer pachymetry. A tandem scanning confocal microscope equipped with a nonapplanating contact objective was used to perform CMTF. The accuracy of CMTF measurements was evaluated using nine custom-made calibration contact lenses (PMMA) with varying thickness (300-600 microm) and radius of curvature (7.0-9.0 mm). The influence of immersion fluid stabilization and the consequence of prolonged corneal examination were assessed by performing CMTF in rabbits. Additionally, factors related to the instrumental setup and to sedation of experimental animals were examined. For all calibration contact lenses, the thickness measured by CMTF was within +/-1.0 microm of the certified value. Varying the target thickness or radius of curvature had no consistent impact on the high accuracy of CMTF. When performing CMTF in vivo, z-axis motion was readily identified by sampling and comparing both in- and out-scans. Apart from involuntary eye movements, z-axis motion was due to initial thinning of the immersion fluid with stabilization obtained after approximately 1.5 minutes. Continued confocal examination led to slight but significant swelling of both the stroma (0.5 microm/min) and epithelium (0.1 microm/min). CMTF is an accurate and precise technique for corneal sublayer pachymetry with concurrent display of cellular morphology. The precision of CMTF can be improved by allowing time for methylcellulose stabilization and by performing repeated two-way (in and out) scans to account for z-axis motion.

A novel target-field method for finite-length magnetic resonance shim coils: II. Tesseral shims

This paper presents a new approach for the design of finite-length shim and gradient coils of the type that are used in magnetic resonance imagingapplications. A cylindrical target region is located arbitrarily within the coil, and the exact geometry of the coil is treated without approximation. Target fields of any desired type may be specified, and may involve zonal and tesseral harmonics. The method presented here then designs the coil windings needed to produce the specified target field. In this new approach, there is complete freedom in the choice of target field, with no requirement to restrict the target field to any one spherical harmonic. The method is therefore able to design coils in which the region of interest is located asymmetrically with respect to the coil length. In addition, to control the accuracy of the fields produced by the shim coil, the design methodology presented here is also extended to show how the desired target field can be matched at two or more different co-axial target radii. The method is illustrated for the design of shim coil that involves high-order tesseral fields located asymmetrically in a finite-length coil.

Fragmentation rates of small satellites in the outer solar system

The narrow rings of Uranus and Neptune exist in a system of observed and hypothesized small moons. Catastrophic fragmentation of these moons by comet impact has been proposed as the mode of origin of those rings, and earlier efforts to model the process showed that small moons are destroyed by impact on short timescales, leading to rapid collisional erosion of any primordial satellite system (Colwell and Esposito, 1992). We reexamine the question of impact fragmentation of small satellites in the light of new observational data on the population of Kuiper Belt and Centaur objects that produce the impacting flux and new theoretical and computational studies of catastrophic fragmentation. We find that the impacting flux used by Colwell and Esposito (1992) is consistent with the new observations of Kuiper Belt objects and calculations of their transport into the solar system. However, new fragmentation criteria from modeling of the asteroid belt and hydrocode simulations lengthen the model collisional lifetimes of satellite systems. The observed distribution of rings, dust bands, and moons at Uranus and Neptune suggest a catastrophic disruption model with a relatively weak dependence on target radius.

90° Bremsstrahlung Source Term Produced in Thick Targets by 50 MeV to 10 GeV Electrons

The 90° bremsstrahlung source terms produced in thick targets were studied using the EGS4 and FLUKA Monte Carlo codes. Calculations were performed for cylindrical targets of aluminum, iron, copper and lead. In the calculations, the electron beam energies varied from 50 MeV to 10 GeV, and the target radii varied from 0.5 to 3 Moliere units. The results were compared with the SLAC SHIELD 11 code.

Experimental study of the surface waves on a dielectric cylinder via terahertz impulse radar ranging

Employing an ultrafast optoelectronic terahertz impulse radar range with subpicosecond resolution, we have characterized the electric-field time-domain response from an impulsively excited dielectric cylinder. The bandwidth of the measurement extends from 200 GHz to 1.4 THz and late time response is observed at times exceeding that to traverse 40 target radii at c. A physical optics (PO) model is employed to identify the different mechanisms of scattering for the temporally isolated signals. Through analysis of the first and second surface-wave signals it is determined that the surface wave has a propagation velocity of 0.91c and an effective index of refraction of n=1.10+0.073i. The first measurement of the coupling efficiency of this surface wave through the cylinder via an interior chord at the critical angle is performed along with the determination of the /spl pi//2 phase shift associated with the single axis caustic of this interior chord in the PO model.

Compression of Plasma to Megabar Range using Imploding Liner

Axial current discharges were used to implode spherical aluminum shells with thickness tapered to match the external magnetic pressure. These implosions were used to compress hydrogen plasmas with an initial pressure $g100\mathrm{bar}$ and an initial temperature $g1\mathrm{eV}$. The implosion and compressed plasma effect on a central target were observed radiographically. The target's radius vs time indicates that the plasma pressure reaches $\ensuremath{\sim}1\mathrm{Mbar}$ prior to liner-target contact.

Regularities of the dynamics of spherical explosions in homogeneous gases

It is found that the calculated recoil momentum for sufficiently large target radii is negative irrespective of the variations in the equation of state, whereas the experimentally measured value is positive regardless of the target dimensions. An explanation for the difference revealed is proposed.

Design and modeling of precision solid liner experiments on Pegasus

Pulsed power driven solid liners may be used for a variety of physics experiments involving materials at high stresses. These include shock formation and propagation, material strain-rate effects, material melt, instability growth, and ejecta from shocked surfaces. We describe the design and performance of a cylindrical solid liner that can attain velocities in the several mm/μs regime, and that can be used to drive high-stress experiments. An approximate theoretical analysis of solid liner implosions is used to establish the basic parameters (mass, materials, and initial radius) of the driver. We then present one-dimensional and two-dimensional simulations of magnetically driven, liner implosions which include resistive heating and elastic–plastic behavior. The two-dimensional models are used to study the effects of electrode glide planes on the liner’s performance, to examine sources of perturbations of the liner, and to assess possible effects of instability growth during the implosion. Finally, simulations are compared with experimental data to show that the solid liner performed as predicted computationally. Experimental data indicate that the liner imploded from an initial radius of 2.4 cm to a target radius of 1.5 cm, and that it was concentric and cylindrical to better than the experimental resolution (60 μm) at the target. The results demonstrate that a precision solid liner can be produced for high-stress, pulsed power applications experiments.

Dependences of the parameters of a recombination x-ray laser, based on the 3d5/2—2p3/2 transition in the CVI ion, on the pump pulse duration and on the target parameters

The SND-VEGA suite of programs was used to model numerically an x-ray laser with recombination pumping and based on the 3 — 2 (λ=18.2 nm) transition in the CVI ion. In the calculations it was assumed that a plasma is formed by irradiation of a cylindrical target with a number of neodymium laser harmonics of different pulse durations and of the maximum intensity 6 × 1013 W cm-2. The modelling yielded the temporal and spatial dependences of the local and average gain coefficients on the target radius and on the variable parameters of the Nd laser pulses. The parameters corresponding, in the investigated range, to the maximum gain coefficient were determined.

動的空洞発達を考慮した平板衝突解析における変形の３次元性と空洞体積率の影響

Spall damage analyses are conducted in the framework of continuum damage mechanics with special emphasis on the 3-dimensionality and the effect of void volume fraction on the spall process. The evolution equation of a scalar damage variable taking account of the nucleation and growth of spherical voids, and the elastic-viscoplastic constitutive equations extended to damaged materials are incorporated into the commercial hydrocode MANJUSRI-3D. The axisymmetric two-dimensional analyses of the plate-impact tests on OFHC copper plates are performed to examine the effect of the ratio of the target radius to its thickness and that of the volumetric strain related to the void development on the relation between volumetric strain and hydrostatic pressure.

Inverse problem in planar magnetron sputtering

Mathematical consideration is given to the optimal source distribution which produces a film thickness profile having the least deviation from the flat in axisymmetric planar magnetron sputtering. When the definition domain of the solution is expanded to the Schwartz distribution space, the optimal solution is found to be represented by a finite number of Dirac’s δ-functions, that is, discretized erosion rings of zero width. Numerical analysis yields the bifurcation diagram of the optimal solution. With increasing target radius for a fixed target-substrate distance, the number of δ-functions increases infinitely. A newly bifurcated δ-function first appears at the center of the target remains there for a time, and then begins to move outwards.

Kinetics of muonic hydrogen drift in axial symmetric geometry

The kinetics of muonic atoms of hydrogen isotopes in an axially symmetric trap is studied. The problem of the determination of the initial kinetic energy distribution of µp and µd atoms from time-of-flight spectra is discussed. The effects of the scattering of muonic atoms from gas and of the stopping distribution are evaluated. When the collision length is much larger than the target radius, the moments of the kinetic energy distribution are shown to be determined by the time-of-flight spectrum in a model-independent way.

The inherent background in acoustic wave scattering by a submerged target.

A new concept for the background in resonance scattering of acoustic waves by a submerged target is proposed. The idea is that the remainder after removing only the resonances from the target response must be used as the background. The complex wave numbers whose imaginary parts are the attenuation coefficients of elastic bulk waves in target are introduced to damp out the resonances. From the assumption that the structural damping coefficient, the product of the attenuation coefficients of elastic bulk waves, and the outer radius of target, is a large constant compared with the radiation damping coefficients of resonances, a simple mathematical expression is derived for elastic cylindrical shells and named the inherent background. It is shown that the inherent background leads to the impenetrable (i.e., rigid or soft) background in some limiting cases and also describes exactly the background of elastic shell over entire frequencies regardless of shell thickness.

Radar Astrometry of Asteroids, Comets and Planetary Satellites

Groundbased radar has considerable astrometric potential for asteroids, comets and natural satellites that enter the detectability windows of available telescopes. For very closely approaching asteroids and comets, measurements of the distribution of echo power in time delay (range) and Doppler frequency (radial velocity) can achieve a fractional precision between 10−5 and 10−9, and consequently are invaluable for refining orbits and prediction ephemerides. Even for mainbelt asteroids and the more readily detectable planetary satellites whose orbits are very well known, radar can collapse range uncertainties from a few target radii to a few percent of a target radius, with direct implications for the navigation of spacecraft on flyby or rendezvous trajectories.

Experimental and Theoretical Cross Sections for Escherichia coli Mutants B, B/r, and B s-1 after Heavy-Ion Irradiation

Data for the inactivation of three Escherichia coli mutants by energetic heavy ions are fitted by the track theory of a one-hit detector in an extended target mode. The respective E0's are 46, 36.5, and 12.6 Gy for E. coli B, B/r, and Bs-1 and a0, the assumed target radius, is 0.5 microns for all three. The parameter E0, the D37 with gamma rays, is measured directly, while a0 is fitted to the data. It is significant that neither a point target model nor calculations with a0 = 0.2 and 1.0 micron fit the data. Our fitted target radius, a0, approximates the size of the bacterium. These results raise questions as to why the E. coli mutants are one-hit detectors, and concerning the differences in the E0's in relation to a mechanistic interpretation of cell killing.

Symmetry of illumination and implosion of hotraum targets for heavy ion inertial fusion

The symmetry of implosion of the hotraum target that has been proposed by J. Meyer-ter-Vehn and the author is analysed in the case when the target is driven by a nonspherical ensemble of heavy ion beams. The ion beams, each with a final focus radius rbf ⩾ R (R is the target radius), are all assumed to irradiate the target at the same angle α0 and symmetrically with respect to the equatorial plane. Separate one dimensional (1-D) simulations for the pole and equator target sectors indicate that the l = 2 mode in the asymmetry of fuel implosion vanishes for α0 ≈ 36°. The l = 4 mode can also be eliminated by adjusting the ion current profile across each beam. It is conjectured that the higher asymmetry modes with l ⩾ 6 will be smoothed out by radiative symmetrization to ≲ 1%. A 1-D average (over the target latitude) energy gain of G = 57 is calculated

Energy gain of spherical shell targets in inertial confinement fusion

An analytical model for the energy gain of a spherical shell target is developed. Realistic ignition profiles are calculated in a consistent way, taking into account the implosion phase. The restrictions on the hot spot size posed by the symmetry requirements are included in the model, and scaling relations for the fuel and the implosion parameters are obtained. The target gain is found to be strongly dependent on the ratio between the hot spot size and the initial target radius, whereas the preheating effects by radiation or hot electrons are seen to be less deleterious in this model than in previous models The presence of a relatively heavy layer around the fuel, which acts as a pusher during the implosion phase, has multiple effects on the energy gain

Measurements of spatial and temporal sheath evolution for spherical and cylindrical geometries in plasma source ion implantation

A comparison of experimental measurements and numerical calculations of temporal and spatial sheath evolution is presented. Spherical targets of copper and stainless steel (radius=2 cm) and a cylindrical target (radius=0.95 cm, height=18 cm) were immersed in an argon plasma with plasma densities of 2×108–8×109 cm−3 and biased negatively (20–50 kV). A Langmuir probe was used to detect the propagating sheath edge. Experimental measurements of sheath edge position were in good agreement with those determined by numerical calculations.