Beam Imperfections Research Articles

Investigation of Lateral Stability of UHPC Beams Supported by Bearing Pads Considering the Nonlinear Lift-Off Effect

Ultrahigh-performance concrete (UHPC) is an advanced cement-based material with a compressive strength greater than 150 MPa. The application of UHPC on prestressed girders allows the design of very long spans with thin sections. However, the increased slenderness, low lateral flexural stiffness, flexible supports, and beam imperfections make lateral instability critical. Therefore, this paper presents an analytical prediction of precast beams’ behaviors considering the nonlinear contact interaction with the bearing pads. The nonlinear geometric theory for the slender element is coupled with the nonlinear contact mechanism developed between the beam and pad through the energy approach. The proposed solutions are compared with the equations available in the literature that consider the linear stiffness contact interaction. Changes in the limit load of up to 84% were a result of the lift-off effect (contact interaction). From the results, the UHPC is highly sensitive to lateral instability, giving a maximum load/dead weight ratio of up to 1.08 where the common technical recommendation is greater than four.

Simulation of X-ray Hartmann wavefront sensing with the Synchrotron Radiation Workshop.

X-ray wavefront measurement is an important beam diagnostic tool, especially for the diffraction-limited X-ray beam. These in situ diagnostics give a better understanding of beam imperfections, and they enable feedback for possible corrections and/or optical alignment improvements. Hartmann wavefront sensing is one of the promising techniques to perform in situ X-ray wavefront measurements. In this work, a simulation tool of the X-ray Hartmann Wavefront Sensor (HWS) is developed under the Synchrotron Radiation Workshop (SRW) framework. Using this new simulation capability, one can take advantage of the full SRW package to simulate Hartmann wavefront sensing with the beam traveling from the X-ray source to the sample through different X-ray optical components. This SRW HWS simulation tool can help to optimize the wavefront sensor parameters for a specific X-ray energy range. It can also simulate an in situ wavefront measurement experiment with a particular beamline optical layout and predict the expected results of the wavefront measurement under different beamline configurations.

Simple and robust method for determination of laser fluence thresholds for material modifications: an extension of Liu's approach to imperfect beams.

The so-called D-squared or Liu's method is an extensively applied approach to determine the irradiation fluence thresholds for laser-induced damage or modification of materials. However, one of the assumptions behind the method is the use of an ideal spatial Gaussian beam that can lead in practice to significant errors depending on beam imperfections. In this work, we rigorously calculate the bias corrections required when applying the same method to Airy-disk like profiles. Those profiles are readily produced from any beam by insertion of an aperture in the optical path. Thus, the correction method gives a robust solution for exact threshold determination without any added technical complications as for instance advanced control or metrology of the beam. Illustrated by two case-studies, the approach holds potential to solve the strong discrepancies existing between the laser-induced damage thresholds reported in the literature. It provides also an appropriate tool for new studies with the most extreme laser radiations.

Simple and robust method for determination of laser fluence thresholds for material modifications: an extension of Liu’s approach to imperfect beams

The so-called D-squared or Liu’s method is an extensively applied approach to determine the irradiation fluence thresholds for laser-induced damage or modification of materials. However, one of the assumptions behind the method is the use of an ideal Gaussian profile that can lead in practice to significant errors depending on beam imperfections. In this work, we rigorously calculate the bias corrections required when applying the same method to Airy-disk like profiles. Those profiles are readily produced from any beam by insertion of an aperture in the optical path. Thus, the correction method gives a robust solution for exact threshold determination without any added technical complications as for instance advanced control or metrology of the beam. Illustrated by two case-studies, the approach holds potential to solve the strong discrepancies existing between the laser-induced damage thresholds reported in the literature. It provides also an appropriate tool for new studies with the most extreme laser radiations.

Coherent extreme ultraviolet free-electron laser with echo-enabled harmonic generation

The echo-enabled harmonic generation (EEHG) scheme holds promising prospects for efficiently generating intense coherent radiation at very high harmonics of a conventional ultraviolet seed laser. We report the lasing of the EEHG free-electron laser (FEL) at an extreme ultraviolet (EUV) wavelength with a seeded FEL facility, the Shanghai soft x-ray FEL. For the first time, we have benchmarked the basic theory of EEHG by measuring the bunching factor distributions over one octave down to the EUV region. Our results demonstrated the key advantages of the EEHG FEL, i.e., generation of very high harmonics with a small laser-induced energy spread and insensitivity to beam imperfections, and marks a great step towards fully coherent x rays with the EEHG scheme.

Optical vortex trajectory as a merit function for spatial light modulator correction

Spatial light modulator (SLM) is a modern device extensively used for laser beam shaping. Over the years, its quality has been significantly improved. Nevertheless, there is still a need to correct its imperfections. In this paper we propose two criteria which allow to examine correction effectiveness of SLM. One is based on the Laguerre–Gauss transform and the second uses optical vortex dynamics which exposes a high sensitivity to any beam imperfections. It is based on the vortex internal scanning method, where the shape of vortex trajectory provides information about SLM correction quality. In this paper we test its sensitivity to astigmatism, which was introduced manually by SLM, after proper correction.

QuickPol: Fast calculation of effective beam matrices for CMB polarization

Current and planned observations of the cosmic microwave background (CMB) polarization anisotropies, with their ever increasing number of detectors, have reached a potential accuracy that requires a very demanding control of systematic effects. While some of these systematics can be reduced in the design of the instruments, others will have to be modeled and hopefully accounted for or corrected a posteriori. We propose QuickPol, a quick and accurate calculation of the full effective beam transfer function and of temperature to polarization leakage at the power spectra level, as induced by beam imperfections and mismatches between detector optical and electronic responses. All the observation details such as exact scanning strategy, imperfect polarization measurements, and flagged samples are accounted for. Our results are validated on Planck high frequency instrument (HFI) simulations. We show how the pipeline can be used to propagate instrumental uncertainties up to the final science products, and could be applied to experiments with rotating half-wave plates.

Bicep2. III. INSTRUMENTAL SYSTEMATICS

In a companion paper we have reported a $>5\sigma$ detection of degree scale $B $-mode polarization at 150 GHz by the BICEP2 experiment. Here we provide a detailed study of potential instrumental systematic contamination to that measurement. We focus extensively on spurious polarization that can potentially arise from beam imperfections. We present a heuristic classification of beam imperfections according to their symmetries and uniformities, and discuss how resulting contamination adds or cancels in maps that combine observations made at multiple orientations of the telescope about its boresight axis. We introduce a technique, which we call "deprojection", for filtering the leading order beam-induced contamination from time ordered data, and show that it removes power from BICEP2's $BB$ spectrum consistent with predictions using high signal-to-noise beam shape measurements. We detail the simulation pipeline that we use to directly simulate instrumental systematics and the calibration data used as input to that pipeline. Finally, we present the constraints on $BB$ contamination from individual sources of potential systematics. We find that systematics contribute $BB$ power that is a factor $\sim10\times$ below BICEP2's 3-year statistical uncertainty, and negligible compared to the observed $BB$ signal. The contribution to the best-fit tensor/scalar ratio is at a level equivalent to $r=(3-6)\times10^{-3}$.

FEL polarization control studies on Dalian coherent light source

The polarization switch of a free-electron laser (FEL) is of great importance to the user scientific community. In this paper, we investigate the generation of controllable polarization FEL from two well-known approaches for Dalian coherent light source, i.e., crossed planar undulator and elliptical permanent undulator. In order to perform a fair comparative study, a one-dimensional time-dependent FEL code has been developed, in which the imperfection effects of an elliptical permanent undulator are taken into account. Comprehensive simulation results indicate that the residual beam energy chirp and the intrinsic FEL gain may contribute to the degradation of the polarization performance for the crossed planar undulator. The elliptical permanent undulator is not very sensitive to the undulator errors and beam imperfections. Meanwhile, with proper configurations of the main planar undulators and additional elliptical permanent undulator section, circular polarized FEL with pulse energy exceeding 100 μJ could be achieved at Dalian coherent light source.

Illumination uniformity of capsules directly driven by a facility with thousands of laser beams

The uniformity of the illumination of a spherical capsule directly driven by laser beams has been assessed numerically. Different schemes characterized by ND = 12, 20, 32, 48 and 60 directions of irradiation associated to a single laser beam or a bundle of NB laser beams have been considered. The laser beams intensity profile is assumed super-Gaussian and the calculations take into account for beam imperfections as power imbalance and pointing errors. These facilities are characterized by a large number of laser beams Ntot = ND x NB . Assuming relatively small laser beams with an energy of a few hundred of J (200 J–500 J) it turn out that a few thousand of beamlets (1000–5000) are needed in order to provide a total energy of 0.5–1 MJ. A large parametric study has been performed showing that the optimum laser intensity profile, which minimizes the root-mean-square deviation of the capsule illumination, depends on the values of the power imbalance and pointing error. Moreover, when beams imperfections are taken into account it is found that the uniformity of the illumination is almost the same for all facilities and only depends on the total number of laser beams Ntot .

Six-dimensional weak-strong simulation of head-on beam-beam compensation in the Relativistic Heavy Ion Collider

To compensate the beam-beam tune spread and beam-beam resonance driving terms in the polarized proton operation in the Relativistic Heavy Ion Collider (RHIC), we will introduce a low energy DC electron beam into each ring to collide head-on with the opposing proton beam. The device to provide the electron beam is called an electron lens. In this article, using a 6D weak-strong beam-beam interaction simulation model, we will investigate the effects of head-on beam-beam compensation with electron lenses on the proton beam dynamics for the RHIC 250 GeV polarized proton operation. Frequency maps, dynamic apertures, and proton beam loss rates are calculated for this study. Key beam parameters involved in this scheme are varied to search for the optimum compensation condition. The sensitivities of head-on beam-beam compensation to beam imperfections and beam offsets are also studied.

Annular Cherenkov High Gradient Wakefield Accelerator: Beam-Breakup Analysis and Energy Transfer Efficiency

In this paper, we give a complete analytical solution for Cherenkov wakefields generated by an azimuthally asymmetric annular beam propagating in a coaxial two-channel dielectric structure. The transformer ratio of this type of structure is dramatically increased in comparison to a cylindrical wakefield accelerating structure. A particle-Green's function beam dynamics code (BBU-3000) to study beam breakup effects has been upgraded to incorporate annular drive beams and coaxial dielectric wakefield accelerating structures. Beam dynamics simulations of the annular drive beam with asymmetric charge distributions have been carried out to determine the sensitivity of this method to beam imperfections.

Stochastic homogenization of the laser intensity to improve the irradiation uniformity of capsules directly driven by thousands laser beams

Illumination uniformity of a spherical capsule directly driven by laser beams has been assessed numerically. Laser facilities characterized by N D = 12, 20, 24, 32, 48 and 60 directions of irradiation with associated a single laser beam or a bundle of N B laser beams have been considered. The laser beam intensity profile is assumed super-Gaussian and the calculations take into account beam imperfections as power imbalance and pointing errors. The optimum laser intensity profile, which minimizes the root-mean-square deviation of the capsule illumination, depends on the values of the beam imperfections. Assuming that the N B beams are statistically independents is found that they provide a stochastic homogenization of the laser intensity associated to the whole bundle, reducing the errors associated to the whole bundle by the factor \hbox{$1/N_{B}^{1{/}2}$} 1 / N B 1 / 2 , which in turn improves the illumination uniformity of the capsule. Moreover, it is found that the uniformity of the irradiation is almost the same for all facilities and only depends on the total number of laser beams N tot = N D × N B .

Irradiation uniformity of directly driven inertial confinement fusion targets in the context of the shock-ignition scheme

In direct drive inertial confinement fusion (ICF) the uniformity of the irradiation of the capsule still represents a crucial issue. The quality of the capsule irradiation in the context of the shock-ignition (SI) scheme has been assessed numerically. Schemes characterized by different directions of irradiation associated with a single laser beam or a bundle of laser beams have been considered. Beam imperfections as power imbalance and pointing errors have been taken into account and show that the focal spot that minimizes the root-mean-square deviation depends on these beam imperfections. We discuss the advantages provided by laser facilities accounting for a large number (up to a few thousand) of beamlets. Preliminarily results concerning the use of the Laser-Megajoule facility associated with a SI scheme will be discussed.

Illumination uniformity of a capsule directly driven by a laser facility with 32 or 48 directions of irradiation

The uniformity of the illumination of a spherical capsule directly driven by laser beams has been assessed numerically. Two schemes characterized by 32 and 48 directions of irradiation with associated a single laser beam or a bundle of laser beams characterized by a super-Gaussian intensity profile are considered. The calculations take into account beam imperfections as power imbalance and pointing errors. It is found that the optimum laser intensity profile, which minimizes the illumination uniformity, depends on the beams’ imperfections. A comparison between both schemes shows that the 48 beams configuration allows better illumination uniformity than the 32 beams’ configuration.

Buckling beam micromechanical memory with on-chip readout

We have used double clamped beams to implement a mechanical memory. Compressive stress is generated by resistive heating of the beams and beyond the buckling limit the bistable regime is accessed. Bits are written by applying lateral electrostatic forces. The state of the beam is read out by measuring the capacitance between beam and electrodes. Two ways to implement a mechanical memory are discussed: compensation of initial beam imperfections and snap through of the postbuckled beam. Although significant relaxation effects are observed, both methods prove reliable over thousands of write cycles.

Simulating systematic errors in X-ray absorption spectroscopy experiments: Sample and beam effects

The article presents an analytical model to simulate experimental imperfections in the realization of an X-ray absorption spectroscopy experiment, performed in transmission or fluorescence mode. Distinction is made between sources of systematic errors on a time-scale basis, to select the more appropriate model for their handling. For short time-scale, statistical models are the most suited. For large time-scale, the model is developed for sample and beam imperfections: mainly sample inhomogeneity, sample self-absorption, beam achromaticity. The ability of this model to reproduce the effects of these imperfections is exemplified, and the model is validated on real samples. Various potential application fields of the model are then presented.

Investigation of the resolution affecting parameters of a nuclear microprobe using a solenoid lens

The spatial resolution of a nuclear microprobe system may in practice be limited by a number of factors such as beam energy spread, system misalignment and imperfections of the final lens. A superconducting solenoid as final lens promises to have theoretically good ion optical properties. The experimental determination of relevant parameters of the solenoid lens installed at the Bochum heavy ion microprobe and of the incident ion beam is presented. With RBS and STIM the lateral dimension of the focal spot has been determined, while NRRA was used to measure the energy spread of the ion beam. The effect of alternating stray magnetic fields could be detected and was reduced. It was shown that mechanical vibrations present in the beam line setup have negligible influence on the focus quality. The shadow projection method allowed to investigate the effect of minor imperfections of the final lens. Introducing an electrostatic stigmator reduced the astigmatism of the beam spot significantly. A focus of 0.6 μm×0.7 μm could be established at a current of 100 pA.

‘‘Flicker’’ in small scale laser–plasma self-focusing

Small amplitude, short wavelength ion acoustic waves in laser-produced plasmas cause fluctuations in the trajectories of light rays that can lead to time-dependent, self-sustaining shifting of focal spots and a somewhat random redistribution of the light near the critical surface. This flickering is seen in simulations involving small scale beam inhomogeneities over a uniform background laser profile, which model the center of a realistic laser beam. The effect can cause significant intensity multiplication in long scale length high-Z plasmas with only modest beam imperfections.

Sciopticon array analysis of ORNL ion sources

The neutral beam sciopticon (pinhole camera) has been expanded to an array of pinhole apertures in order to simplify the process of distinguishing between beam imperfections caused by extraction nonuniformities and those caused by beamlet missteering. An analysis scheme is presented whereby the power transmission loss due to beamlet missteering may be estimated if the average beamlet divergence anlge is known. For a well-focused ion source, the sciopticon array can also give the approximate beam focal length and average beamlet divergence angle. Results from testing five ORNL ion sources indicate that the principal mechanism for differences in power transmission is beamlet missteering.