Minimum Beam Divergence Research Articles

Optical beam diffraction tensor in birefringent crystals

We demonstrate that anisotropy of Fresnel diffraction in a birefringent medium can be quantitatively characterized by a planar tensor. Eigenvectors of the tensor correspond to directions of minimum and maximum beam divergence. Zero eigenvalues exist for the slow eigenmode near the optic axis of biaxial crystals and correspond to autocollimated beam propagation along the cone of external conical refraction. Applications of the beam diffraction tensor are demonstrated for analysis of noncritical phase matching in acousto-optics and nonlinear three-wave mixing.

Optimizing the detection, ablation, and ion extraction efficiency of a single-particle laser ablation mass spectrometer for application in environments with low aerosol particle concentrations

Abstract. The aim of this study is to show how a newly developed aerodynamic lens system (ALS), a delayed ion extraction (DIE), and better electric shielding improve the efficiency of the Aircraft-based Laser ABlation Aerosol MAss spectrometer (ALABAMA). These improvements are applicable to single-particle laser ablation mass spectrometers in general. To characterize the modifications, extensive size-resolved measurements with spherical polystyrene latex particles (PSL; 150–6000 nm) and cubic sodium chloride particles (NaCl; 400–1700 nm) were performed. Measurements at a fixed ALS position show an improved detectable particle size range of the new ALS compared to the previously used Liu-type ALS, especially for supermicron particles. At a lens pressure of 2.4 hPa, the new ALS achieves a PSL particle size range from 230 to 3240 nm with 50 % detection efficiency and between 350 and 2000 nm with 95 % detection efficiency. The particle beam divergence was determined by measuring the detection efficiency at variable ALS positions along the laser cross sections and found to be minimal for PSL at about 800 nm. Compared to measurements by single-particle mass spectrometry (SPMS) instruments using Liu-type ALSs, the minimum particle beam divergence is shifted towards larger particle sizes. However, there are no disadvantages compared to the Liu-type lenses for particle sizes down to 200 nm. Improvements achieved by using the DIE and an additional electric shielding could be evaluated by size-resolved measurements of the hit rate, which is the ratio of laser pulses yielding a detectable amount of ions to the total number of emitted laser pulses. In particular, the hit rate for multiply charged particles smaller than 500 nm is significantly improved by preventing an undesired deflection of these particles in the ion extraction field. Moreover, it was found that by using the DIE the ion yield of the ablation, ionization, and ion extraction process could be increased, resulting in up to 7 times higher signal intensities of the cation spectra. The enhanced ion yield results in a larger effective width of the ablation laser beam, which in turn leads to a hit rate of almost 100 % for PSL particles in the size range from 350 to 2000 nm. Regarding cubic NaCl particles the modifications of the ALABAMA result in an up to 2 times increased detection efficiency and an up to 5 times increased hit rate. The need for such instrument modifications arises in particular for measurements of particles that are present in low number concentrations such as ice-nucleating particles (INPs) in general, but also aerosol particles at high altitudes or in pristine environments. Especially for these low particle number concentrations, improved efficiencies help to overcome the statistical limitations of single-particle mass spectrometer measurements. As an example, laboratory INP measurements carried out in this study show that the application of the DIE alone increases the number of INP mass spectra per time unit by a factor of 2 to 3 for the sampled substances. Overall, the combination of instrument modifications presented here resulted in an increased measurement efficiency of the ALABAMA for different particle types and particles shape as well as for highly charged particles.

Комбинированные окна для газовых лазеров высокой мощности

Based on the previously developed mathematical model of the behavior of the multi-kilowatt laser window with an unstable cavity, the case of a two-component output window is considered. The two-component window consists of a transparent polycrystalline diamond ring and a central opaque area separated by a plastic vacuum gasket. The central opaque area is equipped with a cryoaccumulator to reduce heat load. Numerical calculations of thermomechanical processes are performed for such windows used in high-power CO2 lasers. Mathematical model used for the calculations consists of three parts - thermophysical, mechanical and optical. The advantages of using a two-component design with a cryoaccumulator under the conditions of a gas laser operating in the multi-kilowatt power range are demonstrated. The dependences of the maximum output power, temperature distribution and mechanical stresses versus the thickness of the window are obtained. The optimal conditions providing maximum radiation strength and minimum beam divergence are considered.

Thermal analysis of beamline heat load components due to increased power delivery from EAST neutral beam injector

According to EAST’s experimental plan, the long-pulse and high-power operation of neutral beam injector is the requirement for EAST. In this paper, the operation range of beam perveance is adjusted from 1.9–2.5 μP to 2.5–3.3 μP through changing the voltage ratio of gradient grid/plasma grid from 0.84 to 0.78. The thermal analysis of beamline heat load components shows that with the decrease in the voltage ratio of gradient grid/plasma grid from 0.84 to 0.78, the maximum neutral beam power deposition on the calorimeter increases from 825 to 950 kW when accelerating voltage is 50 kV. The results show that the minimum beam divergence increases from 1.25° to 1.32° and the maximum temperature rise of bending magnet collimator increases from 10 to 27 °C. The results in this paper are helpful to find a balance between increase in power delivery capability and prolongation of beam extraction duration and lay a basis for achieving plasma heating according to the physical experimental requirement of EAST.

Evaluation of beam divergence of a negative hydrogen ion beam using Doppler shift spectroscopy diagnostics

The Doppler Shift Spectroscopy (DSS) diagnostic is in the conceptual stage to estimate beam divergence, stripping losses, and beam uniformity of the 100 keV hydrogen Diagnostics Neutral Beam of International Thermonuclear Experimental Reactor. This DSS diagnostic is used to measure the above-mentioned parameters with an error of less than 10%. To aid the design calculations and to establish a methodology for estimation of the beam divergence, DSS measurements were carried out on the existing prototype ion source RF Operated Beam Source in India for Negative ion Research. Emissions of the fast-excited neutrals that are generated from the extracted negative ions were collected in the target tank, and the line broadening of these emissions were used for estimating beam divergence. The observed broadening is a convolution of broadenings due to beam divergence, collection optics, voltage ripple, beam focusing, and instrumental broadening. Hence, for estimating the beam divergence from the observed line broadening, a systematic line profile analysis was performed. To minimize the error in the divergence measurements, a study on error propagation in the beam divergence measurements was carried out and the error was estimated. The measurements of beam divergence were done at a constant RF power of 50 kW and a source pressure of 0.6 Pa by varying the extraction voltage from 4 kV to10 kV and the acceleration voltage from 10 kV to 15 kV. These measurements were then compared with the calorimetric divergence, and the results seemed to agree within 10%. A minimum beam divergence of ∼3° was obtained when the source was operated at an extraction voltage of ∼5 kV and at a ∼10 kV acceleration voltage, i.e., at a total applied voltage of 15 kV. This is in agreement with the values reported in experiments carried out on similar sources elsewhere.

First neutral beam injection experiments on KSTAR tokamak

The first neutral beam (NB) injection system of the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak was partially completed in 2010 with only 1∕3 of its full design capability, and NB heating experiments were carried out during the 2010 KSTAR operation campaign. The ion source is composed of a JAEA bucket plasma generator and a KAERI large multi-aperture accelerator assembly, which is designed to deliver a 1.5 MW, NB power of deuterium at 95 keV. Before the beam injection experiments, discharge, and beam extraction characteristics of the ion source were investigated. The ion source has good beam optics in a broad range of beam perveance. The optimum perveance is 1.1-1.3 μP, and the minimum beam divergence angle measured by the Doppler shift spectroscopy is 0.8°. The ion species ratio is D(+):D(2)(+):D(3)(+) = 75:20:5 at beam current density of 85 mA/cm(2). The arc efficiency is more than 1.0 A∕kW. In the 2010 KSTAR campaign, a deuterium NB power of 0.7-1.5 MW was successfully injected into the KSTAR plasma with a beam energy of 70-90 keV. L-H transitions were observed within a wide range of beam powers relative to a threshold value. The edge pedestal formation in the T(i) and T(e) profiles was verified through CES and electron cyclotron emission diagnostics. In every deuterium NB injection, a burst of D-D neutrons was recorded, and increases in the ion temperature and plasma stored energy were found.

Multimode laser beam analyzer instrument using electrically programmable optics

Presented is a novel design of a multimode laser beam analyzer using a digital micromirror device (DMD) and an electronically controlled variable focus lens (ECVFL) that serve as the digital and analog agile optics, respectively. The proposed analyzer is a broadband laser characterization instrument that uses the agile optics to smartly direct light to the required point photodetectors to enable beam measurements of minimum beam waist size, minimum waist location, divergence, and the beam propagation parameter M(2). Experimental results successfully demonstrate these measurements for a 500 mW multimode test laser beam with a wavelength of 532 nm. The minimum beam waist, divergence, and M(2) experimental results for the test laser are found to be 257.61 μm, 2.103 mrad, 1.600 and 326.67 μm, 2.682 mrad, 2.587 for the vertical and horizontal directions, respectively. These measurements are compared to a traditional scan method and the results of the beam waist are found to be within error tolerance of the demonstrated instrument.

Aberration coefficients of multi-element cylindrical electrostatic lens systems for charged particle beam applications

In order to analyze the imaging properties of an electrostatic lens system, it is necessary to know how various sources of aberration combine to increase the size of final image or spot. In this paper, we investigated the spherical and chromatic aberration coefficients of multi-element electrostatic lens systems as a function of the lens voltages and magnification, using the electron ray tracing simulation programs S imion and L ensys. These programs can be used to obtain electron optical aberration integrals which involve the axial potential distribution and its derivative, and two independent trajectories and their derivatives for the determination of the third- or fifth-order aberration coefficients of multi-element lenses. Optical simulation of the intensity distribution has quantitatively shown that the aberration in the crossover image causes an electron beam blur and a positioning error on the focus spot. If a high positive voltage with respect to the first element's potential is applied to the lens elements, the aberrations as well as the minimum beam divergence can be reduced. The reason, obtained from numerical simulation, is that a positive voltage increases the electron velocity, shortening the electron drift time across the region with aberrant field.

Design of a miniature picosecond low-energy electron gun for time-resolved scattering experiments

We present the design and performance tests of a miniaturized pulsed low-energy electron gun. Electrons photoemitted from a gold cathode are accelerated over a distance of 75 μm and then collimated by a microchannel plate. According to calculations, this novel concept will allow the time spread of the electron pulses to be kept below 5 ps for kinetic energies as low as 100 eV. The achievement of a minimum angular beam divergence (≈1°) along with an energy resolution of 1.1 eV has to be paid for by low signal intensities. We demonstrate the performance of the gun and the high electron-beam coherence by presenting low-energy-electron diffraction images taken from a submonolayer of lead adsorbed on the germanium (111) surface. We anticipate that this electron gun will open up new possibilities for following structural changes on solid surfaces in real time.

Acceleration experiments for an intense H− ion beam

Intense H− beams have been extracted from a large multicusp plasma source operated with cesium seeding. The H− beams were accelerated up to 100 keV by a single-stage or a two-stage electrode system. Spatial profiles of the beams are measured calorimetrically and the beam divergence angle is obtained from half of the e-folding width. A minimum beam divergence angle of 5 mrads is achieved at a H− current density of 30 mA/cm2 with a beam energy of 100 keV. The ratio of acceleration current to H− current increases abruptly when a H− current saturates in the space charge limited region. This enhancement is mainly due to secondary electrons caused by the intersection of H− beams with an extraction grid. When the operating gas pressure decreases, the ratio of the acceleration current to the H− current decreases. This is related to a stripping loss of H− ions in the electrodes. A beam divergence angle reaches a minimum when a ratio of Vacc to Vext is set at an optimum value of 1.6 in the single-stage acceleration. This ratio is almost the same as that in the double-stage acceleration, where the optimum ratio of Eaccl/Eext is 1.5. In the optimum Eaccl/Eext ratio the divergence angle is not affected by Vacc2. The divergence angle can be reduced by changing Vacc2 even if the ratio of Eaccl/Eext is not optimized. The beam steering effect by permanent magnets buried in an extraction grid is observed in nine beamlets experiments. A simple calculation of a single particle trajectory gives a good approximation of the beam deflection angle.

High-brightness femtosecond laser using titanium–sapphire technology and amplification in dyes

We modify a conventional picosecond dye-amplifier system to achieve maximum brightness for 100-fs pulses derived from a titanium-sapphire oscillator. We obtain near transform-limited 4-GW pulses by using efficient chirped-pulse amplification and compression based on a prism-pair stretcher and a glass-block compressor. Good beam quality and minimum beam divergence are obtained with a Bethune cell final amplifier. The system is shown to be capable of generating beam brightness in excess of 2 × 10(7) W cm(-2) sr(-1), within a factor of 2 of that expected for a diffraction-limited beam.

A compact rf-driven, dc ion source

A helium atom beam has been extracted from a rf plasma source with beam energies ranging from 22 to 35 kV. At optimum conditions, a beam of approximately 15 mA equivalent current of 30-keV neutral atoms displays a minimum angular beam divergence of 0.3°. The beam system can be operated continuously in a chopped mode with a 50% duty factor by square-wave modulation of the extraction voltage. Frequency of the modulation can be varied up to 500 Hz. The beam system is also capable of dc operation, having been run in this mode for periods of time exceeding 2 h.

Modified perveance law for neutral-beam ion sources

The perveance law for ion sources is modified based on experimental data gained during the conditioning phase of three multiaperture ion sources for neutral-beam injection into TEXTOR. The ion sources are modified JET PINIs with a three-grid accel-decel extraction system designed for 55 kV, 88 A, 4.8 MW, operated in hydrogen. Results show that best agreement of the perveance law with the experimental data is achieved when the measured ion species mixture is used and, additionally, when the total width of the first acceleration gap, usually determined by the geometrical distance d1 between grid 1 and grid 2 and thickness D1 of grid 1, is enlarged by a third distance term r2, taking the effect of distortion of the electrical field lines at the decel grid into account. At perveance matched conditions, i.e., minimum beam divergence, the optimum value is found to be r′2 =0.8r2, where r2 is the aperture radius of the decel grid. The species fractions were measured by optical and magnetic spectrometry.

A multiaperture ion source with adjustable optics to provide well-collimated, high-current-density, low- to medium-energy ion beams

The design, operation, and characterization of a new, adjustable-grid, multiaperture ion source are described. The capability for in situ relative lateral motion of the grids allows beam steering and alignment during source operation. The maximum beam deflection angle is ∼10° at a beam acceleration voltage Vb=1000 V. The grid separation distance dg is also adjustable in situ, allowing independent control over beam current and voltage while maintaining a minimum beam divergence. Decreases in beam divergence angle α, by as much as a factor of 3 over a comparable fixed-grid source, were obtained. Beam accelerating voltages Vb ranging from 200 to 1000 V were investigated for dg values between 1 and 4 mm with a grid hole diameter of 2 mm. Minimum α values ranged from 4° to 7°. The total current density at minimum α for dg=1 mm and Vb=1000 V was 2.2 mA cm−2. The dependence of the measured beam divergence on source operating conditions and grid geometry is explained using a simple ion-optics model.

Analysis of beam divergence in InGaAsP/InP semiconductor laser diodes

AbstractThe beam divergence angles for the fundamental TE mode both in the directions parallel and perpendicular to the junction plane of InGaAsP/InP semiconductor laser diodes emitting at 1.3 and 1.55 μm have been investigated. The analysis is based on the effective‐index method and the results can be used to design laser diodes (LDs) with minimum beam divergence which will improve the coupling efficiency between LDs and single‐mode fibers.

Beam Expansion and Focusing Effects on Evaporative Laser Cutting

In recent years there has been a considerable increase in the use of high-power lasers as a tool for material processing, such as welding, trimming, scribing, cutting, etc.. Material processing is achieved by evaporating material with a focused Gaussian laser beam as a heat source. A Gaussian laser beam at TWM{sub 00} mode is focused by a lens through which the laser beam passes and behind which it converges to a minimum beam waist around the focal point of the lens. With this concentrated energy at the focal point of the lens it is possible to heat, melt, and vaporize any known material. Investigations in the ares of laser material processing have treated cases with and without phase change, and a variety of irradiation or source conditions have been studied both theoretically and experimentally. Thermal models may be divided into two main categories: (1) models with detailed treatment of thermal conduction and (2) models where details of phase transition vaporization are considered. In the present work, evaporative cutting of a semi-infinite material with a moving, focused continuous-wave laser is considered. The effect of focusing parameters, such as position of lens focal point above or below the surface, minimum beam radius,more » and beam divergence rate are investigated.« less

Production of Fast Neutral Lithium Beam for High Temperature Plasma Probing

A high-current lithium ion beam of a few mAcm-2 with an energy of 12 keV and a minimum beam divergence of about 1.2° has been produced by using an ion source of the discharge type. Characteristics of the lithium ion beam regarding the intensity and divergence were experimentally investigated in detail. A lithium neutral beam has been produced using a sodium neutralizer. The neutralization efficiency of the lithium ion beam in the energy range of 2–15 keV was about 80% with a neutralizer at a vapor pressure of 1.7×10-3 Torr. A mass analysis was performed when a mixed discharge of lithium and argon was used. The ratio of R (argon-ion current/lithium-ion current) became less than 0.1 at the oven temperature of the ion source to about 700°C. The applicability of a fast neutral lithium beam to plasma probing is discussed.

A preliminary study of a retroreflective mirror resonator

The characteristics of the laser radiation emitted from a long pulse, electron beam sustained CO 2 laser employing a retroreflective mirror resonator have been studied. The minimum beam divergence is found to be determined by the retroreflective array imperfections. The effect of tilting both the output window and the array is studied and examination of the temporal behaviour of the laser output to determine both mode structure and the effect of the Laser Induced Medium Perturbation (LIMP) is reported for the first time.

Ion-extraction performance of the circular magnetic-multipole bucket ion source applied for the neutral beam injection system of the Heliotron E machine

The ion-beam extraction performance of the 30-kV–35-A circular magnetic-multipole bucket ion source applied for the neutral beam injection system of the Heliotron E fusion machine has been investigated with special reference to ion-beam optics. By using the numerical simulation code for the cylindrical ion-extraction system, the computational beam-divergence angles have been compared with the experimental data. The experiments show that the applied three-electrode beam optics are satisfactorily stable against source-plasma variation and that the minimum beam-divergence angle is less than 1.15 deg. The obtained optimal perveance of the ion source is nearly 6.3×10−6 (A/V3/2), and the experimental optics behavior is in reasonable agreement with the numerical prediction. On the basis of the experiments, the effect of the edge angle of the first beam-forming, high-potential electrode is also discussed. The experiment suggests that the obtuse angle (∼73°) for that edge is preferable to the acute angle (∼30°).

Performance characteristics of a circular multicusp neutral beam source

The performance of the Heliotron-E neutral beam system, composed of intense cylindrical magnetic multipole bucket sources and beam line facilities, has been investigated. The investigation of the influence of the filament shape on the arc discharge stability shows that a wide-hairpin shaped filament results in more stable arc discharge without increasing the arc voltage than does a narrow-hairpin filament. An electrolytic polishing process has been tested for surface treatment of the Molybdenum grid and found to be very effective in reducing the number of conditioning shots. Beam extraction experiments show that a single bucket source can deliver the total beam current of 35 A at the accelerating voltage of 30 kV with the corresponding power transmission efficiency of more than 85% at the focal point of the grid curvature, the minimum beam divergence is less than 1.15° and the proton yield of the source is above 85%. Measurements of the equivalent gas line density in the neutralizer cell suggest the existence of the gas flow process different from a standard vacuum model.