Beam Fluctuations Research Articles

Validation and reproducibility of in vivo dosimetry for pencil beam scanned FLASH proton treatment in mice

PurposeTo investigate quality assurance (QA) techniques for in vivo dosimetry and establish its routine uses for proton FLASH small animal experiments with a saturated monitor chamber. Methods and Materials227 mice were irradiated at FLASH or conventional (CONV) dose rates with a 250 MeV FLASH-capable proton beamline using pencil beam scanning to characterize the proton FLASH effect on abdominal irradiation and examining various endpoints. A 2D strip ionization chamber array (SICA) detector was positioned upstream of collimation and used for in vivo dose monitoring during irradiation. Before each irradiation series, SICA signal was correlated with the isocenter dose at each delivered dose rate. Dose, dose rate, and 2D dose distribution for each mouse were monitored with the SICA detector. ResultsCalibration curves between the upstream SICA detector signal and the delivered dose at isocenter had good linearity with minimal R2 values of 0.991 (FLASH) and 0.985 (CONV), and slopes were consistent for each modality. After reassigning mice, standard deviations were less than 1.85 % (FLASH) and 0.83 % (CONV) for all dose levels, with no individual subject dose falling outside a ± 3.6 % range of the designated dose. FLASH fields had a field-averaged dose rate of 79.0 ± 0.8 Gy/s and mean local average dose rate of 160.6 ± 3.0 Gy/s. In vivo dosimetry allowed for the accurate detection of variation between the delivered and the planned dose. ConclusionIn vivo dosimetry benefits FLASH experiments through enabling real-time dose and dose rate monitoring allowing mouse cohort regrouping when beam fluctuation causes delivered dose to vary from planned dose.

Innovative Cesiation Deriving Incredible 145 mA Beam from J-PARC Cesiated RF-Driven H− Ion Source

In NIBS2022, the stable 8 hours operation of the J-PARC cesiated RF-driven H− ion source (IS) in a test-stand with a 69.9 keV 120 mA beam and a beam duty factor of 4 % (1 ms x 40 Hz) was reported. However, the Cesiation condition was produced after many times and rather large amount of Cs and H2Os injections. The fluctuation of the H− ion beam intensity (IH −) in a pulse and the transverse emittances were rather larger than those for the previous 65 keV 110 mA operation. The plasma electrode temperature (TPE) of 254 °C much higher than that not only for the usual J-PARC IS (about 70 °C) but also for the standard cesiated H− ion sources (180 ∼ 200 °C) suggested the existence of novel H2O mediated cesiation surviving against the high TPE. In this paper, the J-PARC IS improvements based upon a hypothesis of the best cesiation constituted of sub monolayer H2O (chemically bound with Mo) mediated cesiation and Cs half monolayer on remaining surface are presented. The innovative cesiation derived a 76.5 keV 145 mA beam stably with the small beam fluctuation and transverse emittances suitable for Radio Frequency Quadrupole LINACs of high energy LINACs. The measured results of the 145 mA / 83 mA beam, extraction electrode current and RF waveforms, parameter trends of an 8 hours 145 mA operation and the transverse emittances are presented. The optimum TPE was increased from 70 °C to 230 °C. Furthermore, the beam intensity for the J-PARC IS operation energy of 52.5 keV was increased from 72 mA to 83 mA, which was consistent with the 1.5 power perveance law on the beam energy compared with 145 mA for 76.5 keV.

Taxonomy of hybridly polarized Stokes vortex beams.

Structured beams carrying topological defects, namely phase and Stokes singularities, have gained extensive interest in numerous areas of optics. The non-separable spin and orbital angular momentum states of hybridly polarized Stokes singular beams provide additional freedom for manipulating optical fields. However, the characterization of hybridly polarized Stokes vortex beams remains challenging owing to the degeneracy associated with the complex polarization structures of these beams. In addition, experimental noise factors such as relative phase, amplitude, and polarization difference together with beam fluctuations add to the perplexity in the identification process. Here, we present a generalized diffraction-based Stokes polarimetry approach assisted with deep learning for efficient identification of Stokes singular beams. A total of 15 classes of beams are considered based on the type of Stokes singularity and their associated mode indices. The resultant total and polarization component intensities of Stokes singular beams after diffraction through a triangular aperture are exploited by the deep neural network to recognize these beams. Our approach presents a classification accuracy of 98.67% for 15 types of Stokes singular beams that comprise several degenerate cases. The present study illustrates the potential of diffraction of the Stokes singular beam with polarization transformation, modeling of experimental noise factors, and a deep learning framework for characterizing hybridly polarized beams.

Effect due to Cs injection upon current oscillation of the beam extracted from the J-PARC negative hydrogen ion source

A high-speed emittance measurement system installed at the J-PARC RF negative hydrogen (H-) ion source test stand can construct emittance diagrams of the beam of a specific frequency component. The driving 2 MHz RF power produces small amplitude oscillating components at 2 MHz and at higher harmonic frequencies in the total H- ion beam detected by a Faraday installed downstream of the emittance measurement system. Magnitudes of oscillating component at 2 MHz and 4 MHz frequencies become appreciable as the slits of the emittance measurement system separate the H- ion beam into the position/angle resolved phase space elements. The reconstructed emittance diagrams of H- ion beams for the operation condition with continuous supply of Cs realizing large H- ion current to total extraction current ratios indicate that 2 MHz and 4 MHz frequency oscillating beam components propagate surrounding the center DC beam. Both 2 MHz and 4 MHz oscillating beam components comprise two to three beam particle groups in the phase space. The DC component of the H- ion beam exhibit a single peak in the angular distribution. Meanwhile, oscillating components often take the minima where the DC H- ion beam takes the maximum in the angular distribution diagram. Thus, the regions that produce large amplitude oscillations of the H- ion beam are considered located at the circumference of the beam extraction aperture. One possible explanation for observing larger amplitude beam oscillation can be the enhanced H- ion surface production at the plasma facing the wall of the beam extraction aperture. The operation without the Cs delivery to the source increased electron current coextracted with H- ions and changed the structure of the beam emittance diagram. In the direction perpendicular to the magnetic fields created by a magnetic filter and electron suppression magnets, the peak of the 4 MHz oscillating beam components appeared at the center of the DC beam. This observation suggests the extraction of H- ions formed in the central region by the volume production process near the internal RF antenna. Mechanisms that can excite and diminish the beam fluctuation amplitudes of the extracted H- ion beam are discussed.

2D-smoothing of laser beam fluctuations in optical compressor

We propose a modification of the four diffraction gratings Treacy compressor (TC)—a double-smoothing grating compressor (DSGC)—which enables smoothing of spatial fluctuations of a laser beam in two directions. Smoothing along the groves is due to oblique incidence on the gratings or tilted grooves. Smoothing in the direction normal to the grooves is achieved due to the use of nonidentical pairs of gratings. It is shown that the far-field fluence and the focal beam intensity after the DSGC are like those after the TC. Smoothing is a consequence of spatial harmonics lagging behind or overtaking the main pulse in proportion to the transverse wave vector. Analytical expressions are obtained for the spectrum of fluence fluctuations and fluence rms at the DSGC output. The efficiency of suppressing small-scale self-focusing in transmissive optical elements after the DSGC, for example, in the case of post-pulse compression is assessed.

Implication of the double-gating mode in a hybrid photon counting detector for measurements of transient heat conduction in GaAs/AlAs superlattice structures.

Understanding and control of thermal transport in solids at the nanoscale are crucial in engineering and enhance the properties of a new generation of optoelectronic, thermoelectric and photonic devices. In this regard, semiconductor superlattice structures provide a unique platform to study phenomena associated with phonon propagations in solids such as heat conduction. Transient X-ray diffraction can directly probe atomic motions and therefore is among the rare techniques sensitive to phonon dynamics in condensed matter. Here, optically induced transient heat conduction in GaAs/AlAs superlattice structures is studied using the EIGER2 detector. Benchmark experiments have been performed at the Austrian SAXS beamline at Elettra-Sincrotrone Trieste operated in the hybrid filling mode. This work demonstrates that drifts of experimental conditions, such as synchrotron beam fluctuations, become less essential when utilizing the EIGER2 double-gating mode which results in a faster acquisition of high-quality data and facilitates data analysis and data interpretation.

Stability improvement of multi-beam picosecond–petawatt laser system for ultrahigh peak-power applications

Pointing fluctuations of beams reduce the possibility of incoherent or coherent addition for ultrahigh peak power in a multi-beam picosecond–petawatt laser system. Pointing fluctuations on the target were observed on Shenguang Upgrade Petawatt (SGII-UP-Petawatt) beam using a high-speed and high-resolution active pointing stabilization control system. The maximum frequency of the pointing fluctuations was less than 50 Hz, and the amplitude was approximately 2.8 µrad (RMS). An online test of pointing fluctuations with active stabilization control demonstrated that pointing fluctuations could be reduced to 0.63 µrad (RMS), approximately one-quarter of that without active stabilization control. The benefits of reduced pointing fluctuation were estimated using a multi-beamlet petawatt laser system; the results demonstrated that peak power could be increased by 51.7% when active stabilization control was used in an eight-beamlet picosecond–petawatt laser system.

Laser power stabilization via radiation pressure

Extremely stable laser sources are essential for high-precision experiments, such as interferometric gravitational wave detectors. Marina Trad Nery explains how to use radiation pressure to measure and stabilize power fluctuations in a laser beam.

Nonthermal and selective crystal bridging of ZnO grains by irradiation with electron beam as nonequilibrium reaction field.

Ceramic particles, such as titanium oxide and indium tin oxide, are expected to be used as electric or catalytic materials for various applications. In this work, we progressed to employ the irradiation with an electron beam as the nonequilibrium reaction field for ceramic composition, and we successfully obtained the basic technology for a ceramic thin-film fabrication using a field emission (FE) electron beam with low energy resolution having a half width under 100 meV that had a homogeneous planar electron emission as the nonequilibrium reaction field. In particular, ZnO particles synthesized by electron beam irradiation show selective crystal bridging along the c-axis during FE electron beam irradiation, which is important for synthesizing poly-ZnO crystals without a heating process, because the energy fluctuations of FE electron beams are small and affect the directionality of ZnO crystal growth along the c-axis. This accomplishment may make a significant contribution to the analysis of the formation mechanism of ZnO particles with a uniform morphology and crystal structure by the FE electron beam during the crystallization. Moreover, we will be able to provide basic elements for next-generation nanodevices with highly functional properties by controlling each terminal crystal interface of metals, ceramics, and semiconductors with this technique.

Superatomic Signature and Reactivity of Silver Clusters with Oxygen: Double Magic Ag 17 – with Geometric and Electronic Shell Closure

Understanding the stability and reactivity of silver clusters toward oxygen provides insights to design new materials of coinage metals with atomic precision. Herein, we report a systematic study o...

Artifacts and quantitative biases in neutron tomography introduced by systematic and random errors

Systematic and random errors are often introduced in white-beam neutron Computed Tomography (CT) due to the nature of the neutron source, neutron-matter interactions or limitations in hardware. These errors can cause bias in measured linear attenuation coefficients (LACs) and artifacts in the reconstructed images, reducing image quality and impeding quantitative analysis. Three sources of error in neutron tomography were investigated and quantified that are particularly pertinent to spallation source facilities. These include detector to sample stage misalignment, beam fluctuations and undersampling due to missing CT projections. Quantitative analyses of the grayscale biases and misalignment artifacts were performed on the reconstructed data using ImageJ. Furthermore, the efficacy of classical iterative reconstruction algorithms for the suppression of missing projection artifacts was explored. Several image qualifying metrics were used to compare Filtered Back Projection (FBP) and iterative algorithms quantitatively. We show that beam intensity fluctuations if left uncorrected provide a minimal bias even when exaggerated across a stack of projections. In addition, detector-sample stage misalignment causes a geometrical misalignment blur when not corrected for and severe ring artifacts despite image rotation correction. We suggest that iterative algorithms are more favourable in suppressing missing projection neutron artifacts, however they inaccurately reproduce the LACs for missing-wedge reconstructions compared to FBP.

Multi-photon enhancement of the Schwinger pair production mechanism under strong FEL radiation

In this work we study the production of electron-positron pairs from vacuum in presence of a field resulting from the collision of two counter-propagating FEL beams that undergo Gaussian amplitude fluctuations. Our work aims to the extension of previous works based on the standing wave hypothesis, by including the inherent stochastic amplitude fluctuations of the individual FEL beams. As shown, depending on the order of the process, a large non-linear enhancement in the number of created pairs can be expected over a big intensity window in the multi-photon regime. Vacuum pair creation in view of future plans on the production of ultra-strong and high energy radiation in FEL facilities is also discussed.

The Analysis of Intensity Correlation in Laser Transceiving Systems for Formation of a Cryptographic Key

The potentials for and limitations to the use of intensity fluctuations of laser beams propagating through a turbulent atmosphere for generating random data when keying in confidential optical communication systems are analyzed. The technique is based on the reciprocity theorem for optical fields. Light propagation in a system of two transceivers directed at each other, the signals from which are distorted by an atmospheric channel, is numerically simulated. An experimental setup is created; the generation of random correlated signals in this system is experimentally studied. A need for low-pass filtration of signals received is experimentally ascertained. The efficiency of this filtration is estimated. The dependences of the correlation coefficient on the geometrical parameters of the system and turbulent conditions along the propagation path are derived from the numerical simulation in a wide range of distances, aperture radii, and turbulence strength values. Theoretical results are shown to be in a good agreement with the results of laboratory experiments.

Experimental investigations of seeding mechanisms of TMI in rod fiber amplifier using spatially and temporally resolved imaging.

In this work we investigate transverse mode instability (TMI) in the presence of pump intensity noise and a controlled perturbation of the input coupling for a rod-type fiber amplifier using spatially and temporally resolved imaging (ST). We show that inherent pump intensity noise from the power supply can define significant peaks in the resulting TMI spectrum. ST measurements show that the TMI in the transition region consists of different orientations of LP11. This finding indicates that the simple picture of TMI being seeded by the combination of a static initial fraction of LP11 and pump or signal intensity noise is not valid for our measurements. Furthermore we present seeding of TMI by perturbing the input coupling dynamically. ST measurements of the resulting TMI as a function of perturbation frequency provides quantitative information regarding the frequency response of the non-linear coupling coefficient. Finally, ST measurements of the resulting TMI as a function of signal power shows that the TMI experiences an exponential gain long before visible beam fluctuations appear.

Oscillations of the variable cross beams under seismic and technogenic influences (part II)

The forced transverse oscillations of the variable cross section compressed beams under seismic and technogenic impacts are considered. The source of oscillations is the kinematic perturbations of the beam ends in the form of a harmonic or random vector process. The mathematical model of oscillations is presented as a boundary value problem from the basic partial differential equation of the fourth order hyperbolic type in spatial coordinate and the second order in time, which is supplemented by the boundary conditions. The beam fluctuations are interpreted as the spatiotemporal random fields inhomogeneous in space and stationery in time. In the deterministic case, the amplitudes of the forced oscillations are determined, the influence of the kinematic disturbances’ frequency and the beam oscillations’ shift of their phases is studied. In the stochastic problem, the standard deviations for the deflections are calculated, the influence of the characteristic frequency and correlation of the components of the vector perturbation process on the deflections’ output function is studied. The problem is solved by the methods of the variables’ separation, finite differences and the theory of random processes. The examples are considered, the conclusions are drawn from the calculations results in the Matlab computing complex environment.

Quantum-enhanced stimulated emission detection for label-free microscopy

Nonlinear optical microscopy techniques have emerged as a set of successful tools for biological imaging. Stimulated emission microscopy belongs to a small subset of pump–probe techniques, which can image non-fluorescent samples without requiring fluorescent labeling. However, its sensitivity has been shown to be ultimately limited by the quantum fluctuations in the probe beam. We propose and experimentally implement sub-shot-noise limited stimulated emission microscopy by preparing the probe pulse in an intensity-squeezed state. This technique paves the way for imaging delicate biological samples that have no detectable fluorescence with sensitivity beyond standard quantum fluctuations.

Phase fluctuations in vortex beams

Phase fluctuations of vortex beams are examined using the random phase screen setup. By taking Gaussian, Bessel Gaussian and Laguerre Gaussian vortex beams, with the main focus being on Gaussian vortex beam, we investigate the phase fluctuations in a region around on-axis. These phase fluctuations are quantified in terms of phase variance and phase structure function. Variations of phase fluctuations against some of the source parameters, propagation distance and receiver plane radial position are plotted. Our analysis shows that the studied vortex beams have more phase fluctuations on and around the on-axis region in comparison to Gaussian beam. This is reflected into the phase variance of the vortex beams starting approximately aroundπfollowed by a fall. Similar trend applies to the phase structure function which exhibits a rapid increase just past the on-axis point. These observations change however at longer propagation distances where strong turbulence characteristics prevail. Here we observe saturation regimes, whereby all phase variances and structure functions settle around π and 2π respectively.

Misalignment measurement with dual-frequency moiré fringe in nanoimprint lithography.

We explore an easy-to-implement moiré-based measurement scheme for the mask-wafer misalignment in nanoimprint lithography. By introducing the beat signal of moiré fringes, the measurement range increase by dozens or even hundreds of times, while the measurement accuracy doesn't get affected and still kept in nanoscale. Moreover, the alignment signal, collected throughout the whole imprint process, is independent of the wafer-mask gap and beam fluctuation, which makes it very suitable for the misalignment measurement in NIL. The experiment shows that sub-10 nm alignment could be obtained within a measurement range of 500µm, which is expected to be improved after the parameter optimization.

Experimental demonstration and analysis of underwater wireless optical communication link: Design, BCH coded receiver diversity over the turbid and turbulent seawater channels

AbstractIn this article, we demonstrate an experimental underwater wireless optical communication (UWOC) system in the presence of air bubbles and weak turbulence for varying turbidity levels of the aquatic optical medium. The major limiting factors of the UWOC system are: absorption, scattering, and beam fluctuations; these effects can be mitigated by employing transmitter/receiver diversity schemes and channel codes. In this proposed system, we have employed receiver diversity (selection combining (SC), majority logic combining (MLC), and equal gain combining (EGC)) techniques augmented with Bose‐Chaudhuri‐Hocquenghem (BCH) codes to improve the performance of on‐off keying modulated UWOC system. The bit error rate (BER) expressions are derived for the proposed system and results are validated using analytic and experimental means. The results show that the proposed system, that is, the receiver employing SC, MLC, and EGC receiver combining techniques augmented with the BCH code provides a transmit power gain of 4, 6, and 8 dB respectively, when compared with the uncoded single‐input single‐output system, at a BER of 10−5.

2 kW high-efficiency Raman fiber amplifier based on passive fiber with dynamic analysis on beam cleanup and fluctuation.

In this paper, we study the power scaling in high power continuous-wave Raman fiber amplifier employing graded-index passive fiber. The maximum output power reaches 2.087 kW at 1130 nm with an optical conversion efficiency of 90.1% (the output signal power versus the depleted pump power). To the best of our knowledge, this is the highest power in the fields of Raman fiber lasers based merely on Stokes radiation. The beam quality parameter M2 improves from 15 to 8.9 during the power boosting process, then beam spot distortion appears at high power level. This is the first observation and analysis on erratic dynamic properties of the transverse modes in high power Raman fiber amplifier.