Beam Uniformity Research Articles

Physical design of an Atmospheric Neutron Irradiation Spectrometer at China Spallation Neutron Source

An Atmospheric Neutron Irradiation Spectrometer (ANIS) with a neutron spectrum resembling that of atmospheric neutrons was physically designed at the China Spallation Neutron Source (CSNS). The general layout and the main components of ANIS are presented and discussed. The beam specifications including the atmospheric neutron spectrum, neutron flux, beam spot and beam uniformity are obtained via Monte Carlo simulations. Charged particles and photons were also analyzed. Simulation results indicate that ANIS is a world-class neutron irradiation facility based on proton accelerator, with very distinctive neutron beam quality and wide range adjustment capability. The first neutron beam will be supplied in the end of 2021. It will supply a powerful platform for the accelerated Single Event Effect (SEE) test that is used in aviation and terrestrial electronics, in compliance with the requirements of the associated international standards.

Manipulation of laser-accelerated proton beam spatial distribution by laser machined microstructure targets

Experimental study of laser proton acceleration was carried out using laser machined line targets and cross targets with tens of micrometers scale. We have found that both the shape and material of the microstructure targets have significant influences on the distribution of the proton beam. For the aluminum line target, the proton beam spot expands in a direction perpendicular to the boundary of the target; while for the plastic line target, it expands parallel to the boundary of the target. Detailed PIC simulations of the aluminum target have been carried out, which show that due to the lateral transport of hot electrons and the sheath fields accumulated at the edge of the microstructure target, the divergence angle of the accelerated proton beam is changed accordingly, thereby modulating the spatial distribution: the elliptical beam spot can be obtained from the line target, and the quasi-square beam spot can be obtained from the cross target. Simulations of the plastic target indicate that the difference in the electron transport properties for the two types of targets may be the reason for the completely different beam spot shapes. This work shows that the microstructure targets can be a potential method to manipulate the spatial distribution and uniformity of the proton beam.

A performance analysis of a new secondary standard dosimetry laboratory irradiation system

This study reviews the performance analysis of a commercial 137Cs irradiation system that has recently been installed in the Secondary Standard Dosimetry Laboratory (SSDL) at the Soreq Nuclear Research Center (SNRC).The SSDL is responsible for calibrating calibration facilities and for calibrating most protection level radiation monitoring instruments used in medical, nuclear, industrial and academic settings in Israel. Until recently, these monitoring instruments have been calibrated by a 40-year-old irradiation system.At the end of 2018, a new Hopewell Designs Inc. ® irradiation system was installed at our SSDL. This system employs the most advanced radiation safety standards, along with 21st century source automation methods. In this work, we review the system’s performance. Our analysis shows excellent results in terms of repeatability, robustness, linearity, and beam uniformity. Additionally, the system accuracy was tested by participating in an international IAEA postal audit, and here too, excellent results were obtained.

Effect of a non-uniform annular electron beam on the microwave characteristics in a coaxial structure Ka-band RKA output cavity

The influence of electron beam uniformity on the microwave characteristics of coaxial structure Ka-band RKA output cavity is investigated with particle-in-cell simulations. The electron beam non-uniformity is simulated in four different electron emission models: (1) continuous non-emission area, (2) spaced emission, (3) enhanced emission and (4) current density variation emission. The simulation results with the first emission model shows that the output power of Ka-band RKA decreases as the non-emission area increases, while the continuous non-emission area has little effect on the frequency and pulse width of Ka-band microwave; The simulation results with the second emission model shows that the output power of Ka-band RKA is related to the distribution of Z-component of electric field and excitation electric field strength in the beam–wave interaction gap. The more uniform the distribution of Z-component of electric field is, the greater the output power is. The stronger the excitation electric field is, the greater the output power is; The simulation results with the third emission model shows that the intense current density in local area will cause the reduction of output power. The higher the current density of the enhanced emission area is, the smaller the output power is; The simulation results with the fourth emission model shows that the fluctuation of current density during the steady-state phase will cause the fluctuation of microwave frequency and output power. The fluctuation amplitude of frequency and output power are positively related to the fluctuation amplitude of the current density. The larger the fluctuation amplitude of current density is, the greater the fluctuation amplitude of frequency and output power are. The faster the current density changes, the greater the fluctuation amplitude of frequency and output power are.

Study on the alignment error of complex modulation for focusing flattop beam shaping

The complex modulation is applied to indirectly generate a focusing flattop beam, which can modulate the amplitude and phase in the optical pupil combining with the phase retrieval theory. The simulation results show that this algorithm has good uniformity. However, the alignment errors of various components in the actual optical system have an impact on the beam uniformity, especially after the implementation of SLM (Spatial Light Modulator) error calibration, the beam uniformity still hardly meets the requirements. In this paper, the error source of the complex modulation beam shaping system is theoretically analyzed, while the Mixed Region Amplitude Freedom (MRAF) method is selected as a comparative reference in this research. It is found that the defocus aberration in the SLM illumination is the main aberration which has a special grid-facula, and the error source is proved through the actual experiment. Finally, then the correction is carried out, the complex modulation has better uniformity than MRAF.

Properties of the 90° cylindrical energy analyzer with internal deceleration of the beam in real conditions of operation

To measure plasma potential in toroidal magnetically confined fusion plasma, the Heavy Ion Beam Diagnostic (HIBD) employs of the electrostatic energy analyzer. The 90° cylindrical energy analyzer (CEA) developed for the HIBD on the ISTTOK tokamak due to compactness and multislit operation capability, is characterized by large angular aberrations, if operated in conventional mode. A new operation regime was proposed based on numerical investigations, which demonstrated the efficiency of the 90°CEA operation in reducing angular aberrations via the deceleration of the ion beam inside analyzer, resulting also on an improved energy resolution. In this paper, the properties of the 90 ° CEA featuring with guard rings (GR) to shield the analyzer from the surrounding structures are investigated in the internal deceleration mode (ID-mode) for “centric” and “paracentric” injection geometries. The numerical simulations are conducted using SIMION code. Special attention is paid to uniformity of the beam image on detector plate, which is of importance in ion beam energy measurements by split-plate detector technique. The results confirm coupling of analyzer angle aberration properties with beam deceleration inside analyzer. For both “centric” and “paracentric” entrances the existence of the first-order focus in ID-mode is obtained. However, the first order focus ID-mode of 90°CEA with GR is characterized by heterogeneity of the image for the initially uniform beam, determined by modification of electric field inside analyzer. It is revealed that the energy dispersion of the initially monoenergetic ions due to chromatic-like aberration introduced by analyzer plays a contributive role in the beam image non-uniformity. From the beam energy measurements point of view, it is shown that the energy variation due to this chromatic aberration is of the second order of the analyzer intrinsic resolution. The results show that “centric” entrance in ID-mode is preferable as the image non-uniformity is less pronounced and can be effectively restored by manipulation of entrance slit dimensions and potentials on GR. It is concluded that besides detail preparatory numerical optimization, the beam image shape and uniformity control is essential, which can be realized with the dedicated multiple cell detection.

Beam shaping with high energy utilization and uniformity using gradient orthogonal gratings.

A flattop beam is useful in ultrafast laser processing. A laser beam shaping method for high energy utilization and uniformity is presented using a complex hologram displayed on a spatial light modulator. The hologram consists of a geometric mask, an external blazed grating, and internal gradient orthogonal gratings. The gradient orthogonal gratings can change the incident light energy distribution and obtain flattop beams with high energy utilization. Experimental results show that the presented method can obtain an arbitrary geometric shape with a steep edge and high uniformity. Meanwhile, the bigger the geometric mask size, the higher the energy utilization will be, and it is up to 78.70%.

Neutron Beam Characterization at Neutron Radiography (NRAD) Reactor East Beam Following Reactor Modifications

The Neutron Radiography Reactor at Idaho National Laboratory (INL) has two beamlines extending radially outward from the east and north faces of the reactor core. The control rod withdrawal procedure has recently been altered, potentially changing power distribution of the reactor and thus the properties of the neutron beams, calling for characterization of the neutron beams. The characterization of the East Radiography Station involved experiments used to measure the following characteristics: Neutron flux, neutron beam uniformity, cadmium ratio, image quality, and the neutron energy spectrum. The ERS is a Category-I neutron radiography facility signifying it has the highest possible rank a radiography station can achieve. The thermal equivalent neutron flux was measured using gold foil activation and determined to be 9.61 × 106 ± 2.47 × 105 n/cm2-s with a relatively uniform profile across the image plane. The cadmium ratio measurement was performed using bare and cadmium-covered gold foils and measured to be 2.05 ± 2.9%, indicating large epithermal and fast neutron content in the beam. The neutron energy spectrum was measured using foil activation coupled with unfolding algorithms provided by the software package Unfolding with MAXED and GRAVEL (UMG). The Monte-Carlo N-Particle (MCNP6) transport code was used to assist with the unfolding process. UMG, MCNP6, and measured foil activities were used to determine a neutron energy spectrum which was implemented into the MCNP6 model of the east neutron beam to contribute to future studies.

Tractable Resonant Circuit With Two Nonuniform Beams for a High-Power 0.22-THz Extended Interaction Oscillator

In vacuum electronics, the influence of the uniformity of electron beams on the performance of two- or multi-beam devices is a key scientific problem in the development of compact, high power millimeter-wave and terahertz (THz) sources. In this letter, we focus on this problem in extended interaction oscillators (EIOs) and propose a tractable extended resonant circuit that has no requirement for the uniformity of beams. The circuit design builds up a TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> mode mechanism to support two nonuniform beams, based on maintaining the same electromagnetic characteristics of conventional resonant circuits used in single-beam EIOs. To verify this idea, we designed and simulated a 0.22-THz EIO with the proposed circuit. As the current of one beam is I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and that of another beam is increased from 0 to 4I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , the EIO has shown the behavior of linearly increasing the output power and then becoming saturated through particle-in-cell (PIC) simulations. The ideal case is the currents of the two beams are the same at the same voltage to drive the EIO. This case can reduce the high current density required for start oscillation in single-beam EIOs and obtain high power. Simulation results show that the power of 0.22-THz wave over 1.32 kW is obtained using double beams at 22.2 kV and each current of 0.25 A.

The influence of various vascular access ports on MV photon beam uniformity examined on the PMMA phantom

The presence of a high-density material object such as the vascular access port made of titanium, can affect the homogeneity of dose distribution in underlying tissues. This influence depends on numerous factors but in the first place on the composition material of such an object and its geometry. In this work an influence of the various titanium-made vascular ports, placed in the 6 MV photon field, have been analyzed. The vascular ports of various sizes were placed on the top of thepolymethyl-methacrylate (PMMA) phantom which is then scanned on a computed tomography (CT) simulator to generate the digitized 3-dimensional images for the purpose of treatment planning. The treatment plans were prepared in matRad treatment planning toolkit. The beam profiles and the percentage depth doses have been analyzed. The observed maximum dose values, for ports A, B, C and D, relatively to the maximum dose value in PMMA phantom alone, were 102.25%, 100.62%, 101.78% and 102.48%, respectively. The titanium edges of the ports reduce the dose below them in amount of 8.52%, 8.64%, 10.01% and 10.04% observed for ports A, B, C and D, respectively, in comparison to the central axis dose value obtained in PMMA phantom for the port-free case. The established changes in PDD curves and beam profiles depend on the vascular access port dimensions, reservoir volumes as well as of the amount of titanium content.

First Tests of a New Facility for Device-Level, Board-Level and System-Level Neutron Irradiation of Microelectronics

The very limited availability of fast neutron facilities, particularly in Europe, for testing of microelectronics has motivated the construction of ChipIr, a new beamline at the ISIS neutron and muon source in the UK. ChipIr has been designed for Single Event Effect testing at the device-level, board-level and system-level which requires a beam of uniform intensity over a selectable area in the order of hundreds of cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Measurements of the beam uniformity are presented in this paper. A memory chip of interest for space applications, based on SRAMs and developed by ESA for monitoring of radiation fields, has been used for a comparative characterization of the beam. Consistent results have been found, giving confidence on the intensity and shape of the fast neutron spectrum of ChipIr.

Bench tests of a helium ion source for the neutral particle diagnostic system of the ITER tokamak.

Bench tests of a 15 keV helium ion source, which has been developed for the neutral particle diagnostic system of the International Thermonuclear Experimental Reactor (ITER), are described. Being part of the diagnostic system, the ion source will be used to monitor the intactness of carbon stripping foils as well as to check the detection and dispersion systems of the main diagnostic instruments-neutral particle analyzers (NPAs). The ion source produces a wide 5-cm diameter (FWHM) ion beam at a distance of about 50 cm; the ion beam uniformity at a 2-cm area corresponding to the size of the stripping foil is not worse than 10%. The beam current over the area of the stripping foil can be adjusted in the range of 0.1 pA-1 pA. After initial heating, the temporal stability of the ion beam is better than 10%. Pulse height measurements of registered signals show that 15 keV He+ ions can be reliably registered by the NPA detector system. The obtained results allow us to conclude that the developed ion source can provide a reliable check of the NPA system during the diagnostic performance on the ITER tokamak.

Stealth dicing of sapphire sheets with low surface roughness, zero kerf width, debris/crack-free and zero taper using a femtosecond Bessel beam

Previous approaches for laser beam cutting of sapphire often lead to chipping, debris, large kerf widths, tapering and high surface roughness or nonuniform surfaces. Laser beam stealth dicing can remove kerf width and tapering, reduce defects. However, sidewall uniformity is poor as part of the material is laser cut and part is broken by an external mechanical force. In previous approaches of Bessel beam full depth cutting of sapphire, uniformity can be improved. However, the sidewall surface roughness is poor. There has been lack of an ideal solution to achieving minimum defects and low surface roughness. Here we show a much improved sapphire cutting method with a femtosecond Bessel beam achieving a 200 nm Ra cut surface roughness, which is almost an order of magnitude improvement over the previous Bessel beam cutting approaches without losing the uniformity. By using a diameter reduced Gaussian beam passing through a 20° physical angle axicon lens, a highly uniform non-diffraction Bessel beam is generated. Under circular polarization, the effects of Bessel beam scanning speed on flexural strength and the sidewall surface roughness are analyzed for cutting sapphire sheets of 0.38 mm, 1 mm, and 1.5 mm in thickness. Zero taper, zero kerf width, free of debris/chipping sapphire cutting with both straight and curved lines are demonstrated. The fundamental mechanisms involved are discussed. The uniformity of Bessel beam and appropriate separation of pulses have been identified as the key factors for achieving the low surface roughness.

Design and implementation of a linear array laser emitting optical system based on diffractive principles

This study investigated the design and implementation of a linear array laser emitting optical system based on diffractive optical elements. The system offered relatively good energy efficiency and beam uniformity, as well as a simpler structure than conventional line laser illumination systems. Based on the diffractive model, a hybrid local-global search algorithm was proposed to solve for the structural parameters of the diffractive element. This method is simple to describe and flexible upon implementation. It also offers high efficiency. Using this method, a diffractive optical element was designed to generate 13 1.06 μm laser beams. The theoretical diffractive efficiency approached 80 % and the inter-beam non-uniformity was less than 20 %. The diffractive element was fabricated via photolithography and a corresponding optical system was built to verify its performance. Experimental measurements showed that the total diffractive efficiency of the 13 beams was 70.83 % and their non-uniformity was 11.96 %. These results approach those calculated theoretically, and thus verify the correctness and effectiveness of the system.

Influence of annular electron beam uniformities on the microwave characteristics in an S-band relativistic klystron amplifier

The non-uniform annular electron beam models are established based on the annular cathode explosive emission luminescence images. The influence of electron beam uniformities on the microwave characteristics of an annular structure S-band relativistic klystron amplifier output cavity is investigated with particle-in-cell simulations. The electron beam non-uniformities are simulated using four different electron emission models: (1) continuous area without emission, (2) spaced emission, (3) enhanced emission, and (4) current density variation emission. The simulation results with the first emission model show that the output power decreases as the continuous area without emission increases, while the continuous area without emission has little effect on the frequency and pulse width. The simulation results with the second emission model show that the output power of the spaced emission is related to the distribution of non-emission areas. The more evenly the areas without emission are distributed on the annular cathode, the greater the output power is. The simulation results with the third emission model show that the higher the current density in the local area is, the smaller the output power is. The simulation results with the fourth emission model show that the fluctuation of current density causes the fluctuation of frequency and output power. The larger the change area of the current density is, the greater the fluctuation amplitudes of the output power and frequency are. The larger the fluctuation amplitude of current density is, the greater the fluctuation amplitudes of the output power and frequency are.

Evaluation of lensed fibers used in photodynamic therapy (PDT).

The combination of diode laser and lensed optical fiber is a commonly used technique in photodynamic therapy (PDT). The design of the lensed fiber can affect the beam quality, therefore, directly or indirectly effect treatment outcomes. In this study, several commercially available microlens and convex lens fibers used in PDT were evaluated. A total of five types of lensed fibers were evaluated. The beam uniformity, expansion, and transmittance properties of lensed fibers to laser beams of 532 nm and 630 nm were analyzed. In addition, the spectral transmissions of lensed fibers were also examined using a broad band light source (400-1000 nm). Using microlens alone could significantly improve the light beam uniformity. The combination of lens and macrobendings could further improve the light beam uniformity. The beam expansion effect was determined by the numerical aperture (NA) of lens. The microlens fibers of higher NA showed better beam expansion effect than the convex lens of lower NA. The overall transmittance of tested lensed fibers ranged between 70 % and 77 %. Tested fibers showed good transmission property to the tested PDT wavelengths. Generally speaking, utilizing macrobending could improve the transmission quality of lens based frontal fiber-optic light distributors in PDT applications.

Spatial autocorrelation study for laser beam quality estimation

High brightness electron beam is required by several applications in the accelerator physics field. In order to have a high brightness beam, that means a high current and a low emittance beam, it is important to study, among other things, the beams non uniformity due to the non perfect transverse laser beam uniformity. Regarding the transverse analysis of the beam, statistical tools as mean and standard deviation are usually used. In this work we will show how the autocorrelation function of a photocathode laser can be used for monitoring the spatial distribution of the beam non-uniformity, strictly connected with the high electron beam emittance: we will apply our analysis on the SPARC_LAB data.

Subterahertz refractive flat-top beam shaping via 3D printed aspheric lens combination

The viability of 3D printed aspheric lenses for the purpose of frequency-scalable subterahertz Gaussian to flat-top beam shaping is evaluated. A cylindrical one-dimensional Fresnel–Kirchhoff diffraction equation was implemented in Matlab and used to design a pair of aspheric lenses with customized vertex radius of curvature and conic constant. The lenses were printed at maximum possible resolution in acrylonitrile butadiene styrene ( n = 1.6 ) and tested with a 102 GHz continuous-wave subterahertz source. The aspheric lens combination produced a flat-top profile from a low-quality incident Gaussian beam at the minimal cost of 3D printing substrate. The flat-top profile exhibited small ( &lt; 14 % root-mean-square deviation over a flat region) intensity fluctuations and is expected to prove useful in future terahertz applications that require a high degree of beam uniformity.

On the vertical uniformity of an ITER-like large beam

Abstract A beam uniformity higher than 90% is a crucial requirement for the ITER neutral beam injection (NBI) system, together with very low beam core divergence ( The ELISE test facility, equipped with an ITER-like multi-aperture 3-grid system, produces large ITER-like beams, with half the size in vertical direction. Due to the distance of the beam diagnostics from the grids, the overlapping of the beamlets is large, and information on divergence and uniformity can be retrieved in terms of beamlet groups or beam segments. In general, good symmetry in accelerated current density between beam segments is achieved. The vertical beam uniformity is determined by caesium distribution and by plasma parameters at the plasma grid, which are inhomogeneous due to the interplay of filter field and bias potential. In this work, the role of filter field and bias potential on the vertical beam uniformity (in intensity and divergence) between the two beam segments is identified: the filter field mainly affects the extracted current density and its distribution within and between the segments; the bias potential can be used to tune the beam divergence, differently between the beam segments.

Optical System Design for Gaussian Beam Uniformization with Long Focal Depth Based on Aberration Effect of Aspheric Mirror

Near infrared Gaussian laser beams are crucial in many research studies,such as the interaction between intense laser and materials,laser cleaning,and laser combustion diagnostics.However,the non-uniformity of energy distribution of Gaussian beams hinders the further development of these fields.To improve cleaning efficiency and measuring accuracy,beam energy is typically expected to distribute uniformly within a larger working distance in practical applications.Nevertheless,the current beam shaping methods cannot satisfy requirements of long focal depth and high laser power tolerance simultaneously.Therefore,a novel optical system of Gaussian beam uniformization with long focal depth is proposed and designed based on the aberration effect of aspheric surface lens,which comprises an aspheric beam uniformization system and a spherical long focal collimation system.All lenses are fabricated with fused quartz and coated with antireflective film on the surface,which can achieve a 99.9%transmission efficiency in the optical system.The working wavelength of the system is 1064 nm,the working distance is 1000 mm,the total length of the system is 135.2 mm,and the laser power is higher than 300 W.The design results show that the effective focal depth of the shaped flat-topped Gaussian beam is±100 mm,the uniformity of the beam is better than 95%,and the convergence angle is 17.52 mrad,which satisfy the requirements of the abovementioned application fields.Compared with other laser beam uniformization systems,the beam shaping system designed in this study possesses a simple structure and affords easy processing,low cost,long focal length,high laser power tolerance,and good beam uniformization.