Virtual Beam Research Articles

SHEEBA: A spatial high energy electron beam analyzer

Electron bunches with large energy and angle spread are not easy to be analyzed with conventional spectrometers. In this article, a device for the detection of high energy electrons is presented. This detector, based on the traces left by electrons on a stack of dosimetric films, together with an original numerical algorithm for traces deconvolution, is able to characterize both angularly and spectrally (up to some mega-electron-volts) a broad-spectrum electron bunch. A numerical test was successfully performed with a virtual electron beam, which was in turn reconstructed using a Montecarlo code (based on the CERN library GEANT4). Due to its simplicity and small size, the spatial high energy electron beam analyzer (SHEEBA) detector is particularly suitable to be used in laser plasma acceleration experiments.

Bistatic coherent laser radar signal-to-noise ratio.

We investigate the signal-to-noise ratio (SNR) for a bistatic coherent laser radar (CLR) system. With a bistatic configuration, the spatial resolution is determined by the overlap of the transmit beam and the virtual backpropagated local oscillator beam. This eliminates the trade-off between range resolution and the bandwidth of the transmitted pulse inherent in monostatic systems. The presented analysis is completely general in that the expressions can be applied to both monostatic and bistatic CLR systems. The heterodyne SNR is computed under the assumption of untruncated Gaussian optics and untruncated Gaussian beam profiles. The analysis also includes the effects of refractive turbulence. The results show that, for maximum SNR, small transmit and local oscillator beam profiles (e-1 intensity radius) are desired.

“Phase Centers” of Far Infrared Multi-Moded Horn Antennas

Multi-moded horn antennas can be used as high efficiency feeds for bolometers when diffraction limited resolution is not required. For example, such horns are proposed for the PLANCK Surveyor, a satellite telescope due to be launched in 2007 to make definite measurements of the Cosmic Microwave Background. In a previous paper we described an accurate approach involving electromagnetic modelling using a rigorous mode matching technique to obtain both the horn aperture fields and the corresponding far field radiation patterns. In this paper we extend this description to determine the “phase center” of such horns when used on large reflecting telescopes. The “phase center” is ill defined as the individual spatially coherent fields making up the far field pattern all appear to come from different phase centers. The best average phase center location is therefore redefined in terms of the virtual beam waist position behind the horn aperture at which the focus of the telescope should be located in order to optimise angular resolution and on-axis gain for the beam on the sky. A number of alternative techniques to locating the phase center are discussed in detail in thes paper.

Measurement of in-service vibration of interior mirrors using virtual laser technique

The Spanish company IDIADA AT, Santa Oliva (Tarragona), has developed a novel measurement technique that uses a virtual laser technique to quantify in-service vibration levels of interior and exterior mirrors and their impact on the visual quality perceived by the driver. This technique allows the calculation of the trajectory of a virtual beam of light reflected by the mirror onto the driver’s eyes in operational conditions.

Continuous-wave bistatic laser Doppler wind sensor

A coherent laser radar has been built by use of a master-oscillator power-amplifier arrangement in which the master oscillator is an external-cavity semiconductor laser and the power amplifier is an erbium-doped fiber amplifier with approximately 1-W output at a wavelength of 1.55 microm. The beams are routed within single-mode optical fiber, allowing modular construction of the optical layout with standard components. The system employs separate transmit and receive optics (a bistatic configuration) and has sufficient sensitivity for reliable Doppler wind-speed detection in moderate scattering conditions at short range (to as much as approximately 200 m). The bistatic arrangement leads to a well-defined probe volume formed by the intersection of the transmitted laser beam with the virtual backpropagated local-oscillator beam. This could be advantageous for applications in which the precise localization of wind speed is required (e.g., wind tunnel studies) or in which smoke, low cloud, or solid objects can lead to spurious signals. The confinement of the probe volume also leads to a reduction in the signal power. A theoretical study has been carried out on the reduction in wind signal strength compared with the monostatic arrangement, and the results are compared with experimental observation.

Production of isoscalar pion pairs in the [formula omitted] reaction near threshold

The production near threshold of isoscalar pion pairs in the pd→ 3 He(ππ) 0 reaction is estimated in a two-step model which successfully describes the production of η, ω and η′ mesons. A virtual pion beam, generated through an NN→ dπ reaction on one of the nucleons in the deuteron, produces a second pion via a πN→ ππN reaction on the other nucleon. Using the same scale factor as for heavy meson production, the model reproduces the total π 0 π 0 production rate determined at an excess energy of 37 MeV. There are some indications in the data for a suppression of events with low ππ masses, as in the π − p→ π 0 π 0 n reaction, and this is confirmed within the model. The model suggests that a significant fraction of the charged pion production in the pd→ 3 Heπ +π − reaction at Q=70 MeV might be associated with isoscalar pion pairs, though this does not explain the strong dependence observed on the π + π − relative momentum angle.

Generation of periodic accelerating structures in plasma by colliding laser pulses.

A mechanism for generating large (>1 GeV/m) accelerating wakes in a plasma is proposed. Two slightly detuned counterpropagating laser beams, an ultrashort timing pulse and a long pump, exchange photons and deposit the recoil momentum in plasma electrons. This produces a localized region of electron current, which acts as a virtual electron beam, inducing intense plasma wakes with phase velocity equal to the group velocity of the short pulse. Modulating the pumping beam generates periodic accelerating structures in the plasma ("plasma linac") which can be used for particle acceleration unlimited by the dephasing between the particles and the wake. An important difference between this type of plasma accelerator and the conventional wakefield accelerators is that this type can be achieved with laser intensities I<<10(18) W/cm(2).

Transcranial Doppler sonography guided by magnetic resonance angiography for improved monitoring of intracranial arteries.

Transcranial Doppler (TCD) sonography combines the advantages of real-time hemodynamic information, cost-effectiveness, and bedside application. However, measurements can be difficult to reproduce because the spatial resolution and the determination of insonation angles are limited. The purpose of this study was to use the high anatomic resolution of three-dimensional (3D) magnetic resonance angiography (MRA) images for the stereotactic guidance of TCD in order to improve the accuracy and reproducibility of TCD examinations. The MRA examinations were performed on a 1.5 T scanner using a 3D flow compensated gradient-echo sequence. A noninvasive stereotactic mask was used for image registration. The MRA data were then transferred to a personal computer. An infrared tracking system registered the position of the head and the ultrasound probe during TCD. This enabled the authors to superimpose a virtual ultrasound beam onto the MRA projections of the intracranial arteries displayed on the monitor of the personal computer. This allows the examiner to easily identify the insonated intracranial artery and displays the insonation angle. In volunteer examinations (n = 10), the accuracy and reproducibility for the localization of specific vessel segments was 2.48 mm for the middle cerebral artery and 2.81 mm for all insonated intracranial arteries (middle cerebral artery, anterior cerebral artery, internal carotid artery, and posterior cerebral artery). Without navigation the reproducibility of vessel segment insonation dropped to 4.7 mm for the middle cerebral artery and to 4.84 mm for all vessels. The authors conclude that 3D MRA, acquired as an initial procedure in patients with intracranial vascular disorders, can be used to provide stereotactic guidance for repeated TCD examinations. This facilitates the reproducible insonation of specific vessel segments.

Buckling and postbuckling behavior of composite cross-ply laminates with multiple delaminations

In this study, buckling and postbuckling analysis was conducted for the composite cross-ply laminates with multiple delaminations under compressive loading. The geometries of multiple delaminations are a one-dimensional through-the-width type and a two-dimensional embedded circular type. In a nonlinear finite element formulation, the updated Lagrangian description and the modified arc-length method were adopted. For a finite element modeling of composite laminates, the eight-node degenerated shell element was used. To avoid overlapping between delaminated areas, the contact node pair was defined by virtual beam element. Numerical results showed that multiple delaminations lower buckling loads and load carrying capacities in the postbuckling region. The buckling load varies with the projected area of the delaminations. However, the geometry of multiple delaminations has an influence on the postbuckling behavior. The load carrying capacity differed with the distribution type due to the local reduction of bending stiffness and the contact behavior of the sublaminates.

Modeling of Composite Laminates with Multiple Delaminations under Compressive Loading

Multiple delaminations are more realistic damage types in the laminated composite structures. In this study, buckling and postbuckling analysis was conducted for the composite laminates with multiple delaminations under compressive loading. In a nonlinear finite element formulation, the updated Lagrangian description and modified arc-length method were adopted. For a finite element modeling of composite laminates, the eight-node degenerated shell element was used. To avoid the overlapping between delaminated layers, which is a physically inadmissible buckling mode, the contact node pair was defined by use of virtual beam element. It employed constraints at the overlapped region in the load incremental procedure. Using this modeling of multiple delaminations, accurate results were obtained for the buckling and postbuckling behavior of composite laminates. Numerical results showed that multiple delaminations lower buckling loads and load carrying capacities in the postbuckling region.

Effects of jet azimuthal angular distributions on dijet production cross sections in DIS

A Monte Carlo study of the azimuthal angular distribution around the virtual boson-proton beam axis for dijet events in DIS at HERA is presented. In the presence of typical acceptance cuts on the jets in the laboratory frame, the azimuthal distribution is dominated by kinematic effects rather than the typical $\cos\phi$ and $\cos 2\phi$ dependence predicted by the QCD matrix elements. This implies that the $\phi$ dependent part of the QCD matrix elements contributes even to the dijet production cross section. Neglecting this $\phi$ dependence leads to an error of about 5-8\% in the production cross section for typical acceptance cuts in the laboratory frame. We also present first NLO results on the $\phi$-decorrelation of the jets through NLO effects.

Evaluation of Virtual Source Beam Configurations for Rapid Tomographic Reconstruction of Gas and Vapor Concentrations in Workplaces

Beam path average data from an Open Path Fourier Transform Infrared (OP-FTIR) spectrometer can be used to reconstruct two-dimensional concentration maps of the gas and vapor contaminants in workplaces and the environment using computed tomographic (CT) techniques. However, a practical limitation arises because in the past, multiple-source and detector units were required to produce a sufficient number of intersecting beam paths in order to reconstruct concentration maps. Such a system can be applied to actual field monitoring situations only with great expense and difficulty. A single monostatic OP-FTIR system capable of rapid beam movement can eliminate this deficiency. Instead of many source and detector units, a virtual source arrangement has been proposed using a number of flat mirrors and retroreflectors to obtain intersecting folded beam paths. Three virtual source beam configurations generated for a single-beam steerable FTIR system were tested using 54 flat mirrors and four retroreflectors or 54 flat mirrors and 56 retroreflectors mounted along the perimeter walls of a typical 24- x 21-ft test room. The virtual source CT configurations were numerically evaluated using concentration maps created from tracer gas concentration distributions measured experimentally in a test chamber. Synthetic beam path integral data were calculated from the test maps and beam configurations. Computer simulations of different beam configurations were used to determine the effects of beam geometry. The effects of noise and peak-reducing artifacts were evaluated. The performance of the tomographic reconstruction strategy was tested as a function of concentration and concentration gradients.

Growth models for virtual molecular beam epitaxy

Virtual molecular beam epitaxy involves growth simulations on computers where various atomistic kinetic processes such as deposition, diffusion, etc. can be numerically controlled to mimic real growth under precisely defined conditions. This article provides a brief non-mathematical overview of the status of molecular beam epitaxial growth simulations with the emphasis on a minimal stochastic Monte Carlo growth model which simulates deposition as a spatially random atomistic kinetic process and surface diffusion at the growth front as an atomistic Arrhenius activated hopping process. Concepts of generic scale invariance and kinetic roughening, as these apply to molecular beam epitaxy, are discussed qualitatively. A very brief discussion of dynamic scaling and continuum equations pertaining to molecular beam epitaxy is provided emphasizing the physical motivation underlying these ideas.

The Effectiveness of Predictive Display with Virtual Beam in Force-Controlled Space Telerobotic System.

During earth-to-space teleoperation, time delay seriously reduces the operator's ability to perform given tasks. Time delay effects appear in both the visual feedback loop and the force feedback loop. Using visual feedback with time delay, the operator is forced to adopt the strategy of "move and wait." On the other hand it is well known that time delay in the force feedback loop causes instabilities. To overcome these problems, we propose a predictive display technique based on the concepts of optimal approach velocity and virtual beam. Experimental results show that a peg-in-hole task, which includes a contact task, is performed very effectively by our method even with long time delay. The effectiveness of our method has been verified from various viewpoints. As a result, we show that our predictive display of the end-effector for noncontact mode is helpful not only for noncontact tasks but also for contact tasks.

Threshold production of heavy mesons in the [formula omitted] reaction

The threshold production rates for isoscalar mesons X 0 formed through the pd → 3 HeX 0 reaction are estimated in a two-step model. A virtual pion beam, produced in an NN → dπ reaction on one of the nucleons in the target deuteron, is converted into an X 0-meson on the second nucleon via a πN → X 0 N process. Though too low in magnitude by an overall factor of about 2.4, the model reproduces well the cross section variation deduced from X 0 = η, ω, η′, and Φ production data.

宇宙ロボットの遠隔操作力制御システム

For the teleoperation of force controlled space robots in orbit by a human operator on the ground, communication time delay becomes a difficult problem. In this paper, to solve this problem, a new teleoperation control scheme is proposed. In this scheme, the robot has two control modes: velocity and force control modes, either of which is selected automatically according to contact conditions between the robot and its environments. The coefficient that converts the force command into the velocity command is decided based on the analysis of the optimal approach velocity. Since the both control modes are implemented by an on-board computer in orbit, no force reflection to the human operator is needed even in the force control mode. Moreover, to aid the operator's 3 D visual perception, it is proposed to use a virtual beam which is generated by a ground computer using a geometrical model. With this aid, operation in the velocity control mode is improved.

Real-pion states formed by a virtual-pion beam:: New family of nuclear excited states

Deeply bound pionic states are discussed from various points of view; highly excited nuclear states as a cluster family of pionic bound states, Σ atom/Σ hypernuclei, halo-like density distributions, virtual pion beam to produce pionic states, etc.

On the Masur Paradox∗

It was shown by Barnett, Shield, and Prager in the sixties that in optimal elastic beam design for a deflection constraint, the continuously variable stiffness is proportional to the square root of the product of the real and virtual beam moments. In the case of a deflection constraint at a single point, the virtual moments equilibrate a unit load at that point. As Masur pointed out in the early seventies, the above optimality condition implies that the real and virtual beam moments must everywhere have the same sign. Tn view of the fact that the corresponding real and adjoint displacement fields must be kinematically admissible, Masur justifiably concluded that a solution satisfying all the above requirements may not exist for some problems. An explanation of this apparent paradox, involving an unusual type of singularity in the optimal solution, is given in this paper. It is also shown that mathematical programming (MP) methods do not yield a close approximation of the exact solution for the class of problems considered, while iterative optimality criteria (OC) methods fully confirm the above findings.

Vehicle charging and associated HF wave phenomena during electron beam emissions

We present data obtained during tests of the Beam‐Emitting Rocket Test (BERT) payload in the Johnson Space Center's Chamber A. During the tests an electron gun on the main payload emitted beams of 0.5, 1, and 2 keV, which stepped through currents of 0.1–20 mA. Simultaneously, an instrumented nose cone, separate from the main payload, measured electric field fluctuations from DC to 5.5 MHz. Abrupt changes in HF wave activity and vehicle potential occurred during 0.5‐ and 1.0‐keV gun operations. Whenever beam current exceeded a threshold level, wave emissions above the chamber electron gyrofrequency, ƒce, were dramatically enhanced and emissions below ƒce were disrupted. At the same time the potential on the electrically isolated BERT payload, which steadily increased with beam current, suddenly ceased to increase and often decreased. The threshold current, Ic, is proportional to the three‐halves power of beam energy, ε, as in beam plasma discharge. However, other facts indicate we are observing a different phenomenon. We propose that the Ic ∝ ε3/2 relation arises from the space charge limitation of current flow from the payload to the chamber wall. The sudden change in the payload potential/beam current relationship results when the beam is partially reflected toward BERT by a virtual cathode which forms at the threshold current. The appearance of HF waves can be attributed to electron density enhancement in this virtual cathode or beam stagnation region. A simple one‐dimensional, analytical model is presented to explain the Ic ∝ ε3/2 proportionality and the generation of HF waves.

Scatter component and its effect on virtual source and electron beam quality

Scattered electrons from the accelerator components and applicator walls and their effect on the virtual source off and on the central axis have been studied for electron beams from a 20 MeV linear accelerator. Measurements were done with and without one of four commercially available fixed applicators of 5 × 5, 10 × 10, 15 × 15, and 20 × 20 cm 2. The effects of various settings of primary collimator jaws were studied. This approach allows for a first order discrimination of scatter from the various components in the system. The results of this study clearly indicate that the scatter contribution is significant. The effect of the scatter distribution decreases with increased beam energy, and significantly increases with the distance from the central axis. It is a major contributor near the edges of the field. In addition, scattered electrons affect the depth dose characteristics of these beams. The most significant changes in beam quality are observed in the surface to depth of maximum region while the extrapolated range is not significantly affected.