Misaligned Beam Research Articles

Firmware and Software Implementation Status of the ICBLM and nBLM Systems for the ESS Facility

The European Spallation Source (ESS) is a neutron source powered by a 5 MW, 2 GeV superconducting proton linac which is currently being constructed in Lund. Due to high beam current, just a few microseconds of operation with a misaligned beam can melt the accelerating cavity wall. Therefore, a Beam Loss Monitor (BLM) system is necessary to protect hardware and personnel from the consequences of beam loss. The BLM’s main function is to inhibit beam production when excessively high beam losses are detected. Besides protection functionality, the system is anticipated to provide the means to monitor the beam losses during all modes of operation with the objective of preventing excessive machine activation. This paper concentrates on implementation for ionization chamber Beam Loss Monitor (ICBLM) and Neutron Sensitive Beam Loss Monitor (nBLM) systems. The required microsecond reaction time has been achieved due to the application of an FPGA. A single detector module was installed and successfully tested at LINAC4 at CERN, in the section where conditions are similar to the ones expected in the ESS linac. The summary of these tests is also included.

Frequency-dependent reflection of a misaligned beam by a Fabry–Pérot cavity

Gravitational-wave detectors, such as Virgo, LIGO, and KAGRA, are modified Michelson interferometers, with a system of coupled Fabry–Pérot cavities, to increase its sensitivity and bandwidth. In order to control the detector, several radio frequency sidebands, not resonant in the kilometric arm cavities but resonant in the central cavities of the interferometer, are added to the carrier frequency to extract longitudinal and alignment error signals. Misalignment of the laser in the Fabry–Pérot cavities causes sensitivity degradation through different mechanisms and results in non-superposition of carrier and sidebands. These relative misalignment between fields at different frequency contain clues to optimally align the interferometer, but the question of the direction of a reflected beam by a Fabry–Pérot cavity, as a function of the state of resonance of the incoming electromagnetic field, is neither straightforward nor intuitive. While numerical optical simulations used in the gravitational-wave detector community are able to answer the question, they do not give a qualitative and handy understanding of the observed phenomenon, useful for the commissioning and operation of the detectors. In this Letter, we present a model based on Gaussian beam first-order modal expansion to calculate analytically how misalignment on the input beam in a Fabry–Pérot cavity translates into misalignment of the reflected and circulating beams. We find a strong dependence not only on the beam resonance condition but also on the mirror geometry. Finally, we checked the consistency of our model by comparing its predictions with existing numerical simulators.

Moving object detection and tracking based on Doppler ultrasound

Fetal health monitoring during pregnancy has become a necessary procedure. Fetal heart rate (FHR) monitoring can determine fetal development or presence of heart disease and evaluate fetal well-being. The FHR measurement uses typically an acoustic probe-based Doppler ultrasound. Doppler ultrasound method transmits a continuous wave signal to the abdomen of a pregnant woman to receive a reflected signal from the fetal heart. Periodic displacement of the heart tissue produces the Doppler effect and the phase change of the reflected wave is proportional to the velocity of the fetal heart. The reflected signal is modulated into a phase signal and the received signal is demodulated to detect the heart rate. The current clinician system consists of a single probe and requires the probe to be manipulated to the optimal position to measure FHR. The system is highly dependent on trained diagnostic experts. The movement of the pregnant woman and the fetus leads to the misaligned acoustic beam which degrades the reliability of the measurement. This work presents a detection and tracking system using a Doppler signal to compensate for the target's movement. The system is implemented by integrating multi-channel probes interfaced to a Doppler signal converter with a 2-degree of freedom (DOF) motor device. This work describes the characteristics of two key components: Doppler signals of multi-channel probes according to the direction of the acoustic beam and the algorithm with a 2-DOF tracking system.

Diffraction Intensity Distribution of Pinhole for Misaligned Gaussian Beam Incidence

Diffraction Intensity Distribution of Pinhole for Misaligned Gaussian Beam Incidence

Direct Fabrication of Ultra-Sensitive Humidity Sensor Based on Hair-Like Laser-Induced Graphene Patterns.

Three-dimensional (3-D) porous graphitic structures have great potential for sensing applications due to their conductive carbon networks and large surface area. In this work, we present a method for facile fabrication of hair-like laser induced graphene (LIG) patterns using a laser scribing system equipped with a 355 nm pulsed laser. The polyimide (PI) film was positioned on a defocused plane and irradiated at a slow scanning speed using a misaligned laser beam. These patterns have the advantages of a large surface area and abundant oxidation groups. We have applied the hair-like LIG patterns to a humidity sensor. The humidity sensor showed good sensitivity characteristics and a large amount of electronic carriers can be stored.

Fatigue strength assessment of misaligned laser beam welded T-joints by effective stress method

This paper investigates the effect of the joint misalignment and the weld geometry on the fatigue strength of successive double-sided laser beam welded AA2024 and AA7075 T-joints using AA4047 filler wire. A finite element model was set up considering the misalignment and the weld geometry to obtain the notch stress. The effective notch stress was calculated by the fictitious radius concept and a microstructural length of 0.15 mm was estimated. Master design curves are proposed for different weld toe radii and microstructural length. The experimental results were better characterized by both a reference weld toe radius of 0 mm and a microstructural length of 0.15 mm. Furthermore, the geometric imperfections caused by variations on the weld toe generate fractures initiated by crack multinucleation.

Identifying the tilt angle and correcting the orbital angular momentum spectrum dispersion of misaligned light beam

The axis tilt of light beam in optical system would introduce the dispersion of orbital angular momentum (OAM) spectrum. To deal with it, a two-step method is proposed and demonstrated. First, the tilt angle of optical axis is identified with a deduced relation between the tilt angle and the variation of OAM topological charges with different reference axes, which is obtained with the help of a charge coupled device (CCD) camera. In our experiments, the precision of measured tilt angle is about 10−4rad with topological charges of −3~3. With the measured angle value, the additional phase delay due to axis tilt can be calculated so that the dispersion of OAM spectrum can be corrected with a simple formula while the optical axis is not aligned. The experimental results indicate that the original OAM spectrum has been successfully extracted for not only the pure state but also the superposed OAM states.

Impact of intermediate and high energy nuclear data on the neutronic safety parameters of MYRRHA accelerator driven system

Perturbation of external neutron source can cause significant local power changes transformed into undesired safety-related events in an accelerator driven system. Therefore for the accurate design of MYRRHA sub-critical core it is important to evaluate the uncertainty of power responses caused by the uncertainties in nuclear reaction models describing the particle transport from primary proton energy down to the evaluated nuclear data table range. The calculations with a set of models resulted in quite low uncertainty on the local power caused by significant perturbation of primary neutron yield from proton interactions with lead and bismuth isotopes. The considered accidental event of prescribed proton beam shape loss causes drastic increase in local power but does not practically change the total core thermal power making this effect difficult to detect. In the same time the results demonstrate a correlation between perturbed local power responses in normal operation and misaligned beam conditions indicating that generation of covariance data for proton and neutron induced neutron multiplicities for lead and bismuth isotopes is needed to obtain reliable uncertainties for local power responses.

SU-E-T-313: Dosimetric Deviation of Misaligned Beams for a 6 MV Photon Linear Accelerator Using Monte Carlo Simulations

Purpose: To quantify the dosimetric variations of misaligned beams for a linear accelerator by using Monte Carlo (MC) simulations. Method and Materials: Misaligned beams of a Varian 21EX Clinac were simulated to estimate the dosimetric effects. All the linac head components for a 6 MV photon beam were implemented in BEAMnrc/EGSnrc system. For incident electron beam parameters, 6 MeV with 0.1 cm full-width-half-max Gaussian beam was used. A phase space file was obtained below the jaw per each misalignment condition of the incident electron beam: (1) The incident electron beams were tilted by 0.5, 1.0 and 1.5 degrees on the x-axis from the central axis. (2) The center of the incident electron beam was off-axially moved toward +x-axis by 0.1, 0.2, and 0.3 cm away from the central axis. Lateral profiles for each misaligned beam condition were acquired at dmax = 1.5 cm and 10 cm depth in a rectangular water phantom. Beam flatness and symmetry were calculated by using the lateral profile data. Results: The lateral profiles were found to be skewed opposite to the angle of the incident beam for the tilted beams. For the displaced beams, similar skewed lateral profiles were obtained with small shifts of penumbra on the +x-axis. The variations of beam flatness were 3.89–11.18% and 4.12–42.57% for the tilted beam and the translated beam, respectively. The beam symmetry was separately found to be 2.95 −9.93% and 2.55–38.06% separately. It was found that the percent increase of the flatness and the symmetry values are approximated 2 to 3% per 0.5 degree tilt or per 1 mm displacement. Conclusion: This study quantified the dosimetric effects of misaligned beams using MC simulations. The results would be useful to understand the magnitude of the dosimetric deviations for the misaligned beams.

Atomic diffraction under oblique incidence: An analytical expression

The semiclassical perturbation method developed by Henkel et al. [J. Phys. II 4, 1955 (1994)] to model cold-atom diffraction by optical standing waves, is applied to the diffraction of fast atoms on crystal surfaces at grazing incidence (GIFAD or FAD). We first show that the interaction time and interaction length embedded in the obliquity factor is well suited to explain the transition from three-dimensional to two-dimensional (2D) diffraction. The situation of a slightly misaligned primary beam, corresponding to oblique incidence in the effective 2D system, is addressed pointing out discrepancies such as the absence of net deflection of the atomic beam. Guided by time-reversal considerations, we propose an arbitrarily symmetrized form significantly improving the agreement with experimental data recorded in oblique incidence.

Propagation properties of multi-Gaussian Schell-model beams through ABCD optical systems and in atmospheric turbulence

The propagation behavior of the multi-Gaussian Schell-model source through ABCD optical systems and its spreading in the turbulent atmosphere are investigated. We obtain the analytic expressions for the cross spectral density and spectral degree of coherence of such beams in misaligned ABCD optical systems and beam width in atmospheric turbulence. Numerical results show that when the beam is focused, it still has the flattened intensity profile around the optical axis. However, the degree of coherence of field in the focal plane assumes Gaussian-like profile. The turbulence inner scale and correlation width and index M of the source have some effects on beam spreading through atmospheric turbulence.

Formation of a large-area uniform ion beam using multipole magnets in the TIARA cyclotron

The formation of an ion beam with a uniform transverse intensity distribution based on a nonlinear focusing method has been studied at the azimuthally varying field cyclotron facility of Japan Atomic Energy Agency. The method enables us to perform uniform irradiation at a constant particle fluence rate over the whole area of a large sample. To obtain a large-area highly uniform beam on the target, we first smooth the intensity distribution of the beam from the cyclotron by multiple scattering and then focus the smoothed beam using octupole magnets. An efficient technique to enhance the scattering effect with a small energy loss is discussed. By collimating the tail of the smoothed beam before octupole focusing, a beam with a nearly uniform profile can be attained on the target. Moreover, we also show that a misaligned beam can be transformed into a uniform beam by adding sextupole focusing. The possibility of ultra-short-time and ultra-low-fluence irradiation has been demonstrated toward advanced applications in materials and biological science. The characteristics of the uniform beam produced by the nonlinear focusing method are discussed from the viewpoint of a uniform irradiation method as compared to other conventional methods.

Diffraction characteristic of a misaligned vortex beam through a phase-hologram grating

We study the analytical features of the output beam diffracted from a phase-hologram grating when the incident vortex beam is misaligned with respect to the grating. The analytical representation describing the diffracted beam of the 1st order is derived theoretically. Based on the representation, the central of gravity and the central intensity of the diffracted beam are investigated in the cases of the alignment, lateral displacement, angular tilt and simultaneous lateral misalignment and angular tilt, separately. It is shown that the diffracted beam is described through confluent hypergeometrical function. The misalignment of the incident vortex beam can give rise to the displacement of the beam center of gravity, which is independent of the misalignment direction and azimuth angle. The displacement is more obvious for the lager misalignment. In the case of angular tilt, the direction of beam center of gravity is nearly identical to the misalignment direction, whatever the topological charge of the incident beam and the fork number of the grating are. Besides, if the sum of the topological charge of the incident beam and the fork number of the grating is zero, with radial displacement and deflection angule increasing, the central intensity of the diffracted beam decreases gradually, otherwise, the central intensity is non-zero value any more. That means the misalignment between the phase-hologram grating and the incident vortex beam can influence the measurement of the topological charge of the vortex beam.

Application of orbital angular momentum to simultaneous determination of tilt and lateral displacement of a misaligned laser beam

We present an analysis of the combined effects of tilt and lateral displacement on the orbital angular momentum spectrum of a laser beam. Our theory explains the symmetries and properties of the spectrum under the influence of misalignments. We apply the theory to establish a reliable and efficient method for determining and subsequently eliminating tilt and lateral displacement. An improved technique for obtaining the orbital angular momentum spectrum employing Laguerre-Gaussian modes is proposed. Finally, a numerical experiment is carried out to verify the method.

Influences of misaligned optical beam carrying orbital angular momentum on the information transfer

An analysis of the orbital angular momentum of an optical beam misaligned with respect to a receiver during propagation in the information transfer is investigated. The analytical expressions of Laguerre-Gaussian beams laterally displaced， angularly deflected and simultaneously misaligned in both ways are derived, and the features of the spiral spectrum in these cases are studied. It is shown that both parallel displacement and angular tilt will lead to the spread of the spiral spectrum, which is independent of the misalign direction. The larger the misalign, the more serious the spectrum spread. However, with the increasing of the propagation distance, the degree of the spread for the laterally displacement is decreasing gradually, while that for the angular tilt is increasing. In the angular tilt case, the spread of the spiral spectrum always keeps symmetric during propagation, while in the case of lateral displacement, the spiral spectrum is asymmetric, and goes to a stable state. The spiral spectrum is also investigated for the case with simulataneous lateral misalignment and angular tilt, and we conclude that the effects of both misaligns on the spectrum are related with one another.

On the nature of optical nuclei in FR I radio-galaxies from ACS/HST imaging polarimetry

We obtained optical imaging polarimetry with the ACS/HRC aboard the HST of the 9 closest radio- galaxies in the 3C catalogue with an FR I morphology. The nuclear sources seen in direct HST images in these galaxies are found to be highly polarized with levels in the range � 2 - 11 % with a median value of 7 %. We discuss the different mechanisms that produce polarized emission and conclude that the only viable interpretation is a synchrotron origin for the optical nuclei. This idea is strengthened by the analogy with the polarization properties of BL Lac objects, providing also further support to the FR I/BL Lac unified model. This confirms previous suggestions that the dominant emission mechanism in low luminosity radio-loud AGN is related to non-thermal radiation produced by the base of their jets. In addition to the nuclear polarization (and to the large scale optical jets), polarization is detected co-spatially with the dusty circumnuclear disks, likely due to dichroic transmission; the polarization vectors are tangential to the disks as expected when the magnetic field responsible for the grains alignment is stretched by differential rotation. We explored the possibility to detect the polarimetric signature of a misaligned radiation beam in FR I, expected in our sources in the frame of the FR I/ BL Lac unification. We did not find this effect in any of the galaxies, but our the results are not conclusive on whether a misaligned beam is indeed present in FR I.

Analysis of orbital angular momentum of a misaligned optical beam

We report an analysis of the orbital angular momentum of an optical beam misaligned with respect to a reference axis. Both laterally displaced and angularly deflected Laguerre–Gaussian beams are represented in terms of the superposition of azimuthal harmonics with well-defined orbital angular momentum. Simultaneous parallel displacement and angular tilt cause the coupling between azimuthal harmonics and therefore change the projection of the orbital angular momentum on the reference axis. Rotation of beams around the reference axis was simulated by attributing corresponding rotational frequency shifts to the components.

Simulation of beam collector nonuniformities in gyro-amplifiers

The spiraling beams employed in gyro-amplifiers can create regions of enhanced local heating on the beam collector surface as a result of the turning-point behavior of the electron orbits. Although guiding center spread and the spread in Larmor radii will usually smooth out these irregularities, hot-spotting can reappear as a result of small transverse displacements or angular misalignments between the collector axis and the system magnetic axis, or from transverse magnetic fields in the electron gun. This paper presents a new, general procedure to efficiently simulate the spatial distribution of electron impacts in misaligned gyro-amplifier beam collectors, using adiabatic theory and analytic geometry. The procedure is used to investigate the distributions associated with various types of misalignments and the results are compared with experimental collector maps obtained from a high-average power, 94-GHz gyroklystron.

Coupling to an endfaced multi-clad optical fibre from a misaligned gaussian beam or a separated multi-clad fibre

The excitation of a multi-clad optical fibre by a misaligned gaussian beam is analysed using a least-squares boundary residual modal technique and a Born approximation. The modal technique is used to analyse the axial coupling via a gap between two aligned fibres. Comparison is made with a method based on a gaussian approximation for the fibre fields. Results of the modal technique confirm the accuracy of the Born approximation when corrected for Fresnel reflection. However, the modal technique is more precise than the other methods when serious mismatch exists and is efficient for treating the gap problem, where the Born approximation is not applicable.

Effect of the turbulent atmosphere on the autocovariance function for a speckle field generated by a laser beam with random pointing error

An analytic formulation for the speckle-intensity autocovariance function generated by a laser beam with random pointing error is developed. The analysis utilizes the extended Huygens–Fresnel principle and includes the effects of atmospheric turbulence on propagation of the misaligned laser beam to a diffuse target and on propagation of the speckle field back to the receiver. The main object of the development is to estimate the effects of deterministic and random pointing errors of the illuminating source on the statistics of the received intensity. In addition, methods for numerical evaluation of the autocovariance function are discussed, and some numerical results are shown. It is found that the autocovariance can be substantially increased above its vacuum value by the atmospheric turbulence.