Development of laser ion source for heavy ion applications

We have been developing a high-performance laser ion source (LIS) for practical applications since 2009. Ideally, the LIS should generate a carbon beam with a peak current of 20 mA and a pulse duration of over 1 μs. We selected a Nd:YAG laser with a Gaussian-coupled resonator as the laser source based on our experience of generating high-charge-state ion beams. This laser can produce fundamental pulses with a power of 650 mJ and durations of about 6 ns. The graphite target used is 10 cm high and 10 cm in diameter, as it can be irradiated with up to 10⁵ laser shots. The maximum extraction voltage was designed to be 50 kV. We have already finished designing the LIS and we commenced fabrication. We intend to measure the source performance by performing plasma and beam tests up to the end of March 2011.     

Effect of distances between lens and sample surface on laser-induced breakdown spectroscopy with spatial confinement

Spatial confinement can significantly enhance the spectral intensity of laser-induced plasma in air.It is attributed to the compression of plasma plume by the reflected shockwave.In addition,optical emission spectroscopy of laser-induced plasma can also be affected by the distance between lens and sample surface.In order to obtain the optimized spectral intensity,the distance must be considered.In this work,spatially confined laser-induced silicon plasma by using a Nd:YAG nanosecond laser at different distances between lens and sample surface was investigated.The laser energies were 12 mJ,16 mJ,20 mJ,and 24 mJ.All experiments were carried out in an atmospheric environment.The results indicated that the intensity of Si (I) 390.55 nm line firstly rose and then dropped with the increase of lens-to-sample distance.Moreover,the spectral peak intensity with spatial confinement was higher than that without spatial confinement.The enhancement ratio was approximately 2 when laser energy was 24 mJ.     

Generation of double pulses at the Shanghai soft X-ray free electron laser facility

In this article, we present the promise of a new method generating double electron pulses in picosecondscale pulse length and tunable interpulse spacing at several picoseconds. This has witnessed an impressive potential of application in pump–probe techniques, two-color X-ray free electron laser, high-gradient witness bunch acceleration in a plasma, etc. Three-dimensional simulations are carried out to analyze the dynamic of the electron beam in a linear accelerator. Comparisons are made between the new method and existing ways.     

Generation of double pulses at the Shanghai soft X-ray free electron laser facility

In this article,we present the promise of a new method generating double electron pulses in picosecondscale pulse length and tunable interpulse spacing at several picoseconds.This has witnessed an impressive potential of application in pump-probe techniques,two-color X-ray free electron laser,high-gradient witness bunch acceleration in a plasma,etc.Three-dimensional simulations are carried out to analyze the dynamic of the electron beam in a linear accelerator.Comparisons are made between the new method and existing ways.     

紫外超短激光驱动铜薄膜靶产生质子的实验研究

在中国原子能科学研究院的放电泵浦的紫外KrF超短脉冲激光放大装置上,开展了紫外超短脉冲激光与铜薄膜靶相互作用加速产生质子束的实验研究。紫外超短脉冲激光输出能量为30 mJ、波长为248 nm、脉冲宽度为500 fs,采用离轴抛物面镜聚焦获得激光聚焦功率密度为1.2×10¹⁷ W/cm²。激光以45°入射5 μm厚的铜薄膜靶,质子最大能量超过300 keV。紫外超短脉冲激光的高对比度和高吸收效率是紫外激光加速的优点。     

High energy X-ray computed tomography for industrial applications

A high energy X-ray computed tomography (CT) system with an electron linear accelerator was developed to image cross-sections of large-scale and high-density materials. An electron linear accelerator is used as the X-ray source. The maximum X-ray energy is 12 MeV, and the average energy is around 4 MeV. The intensity of the X-ray fan beam passing through the test object is measured by a 15-channel detector array. CWO (CdWO₄) scintillators and photodiodes are used as the X-ray detectors. The crosstalk noise due to scattering of X-ray photons by adjacent detectors is reduced to less than 1.6% by installing tungsten shields between the scintillators. Extra channels are used to compensate for base line shift of the circuits. These techniques allowed attainment of a dynamic range of more than 85 dB and a noise level comparable to the signal amplitude of X-rays transmitted in a 420-mm thick iron block. A spatial resolution of 0.8 mm was confirmed with an iron test piece 200 mm in diameter     

Plasma-laser characterization by electrostatic mass quadrupole analyzer

A study of laser ablation of different metals (aluminium, zinc, tantalum and lead), in vacuum, by using 3 ns Nd:YAG laser radiation, at 532 nm wavelength, is reported. Laser pulse, at intensities of the order of 10⁹ W/cm², produces high non-isotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, associated to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species within the plume as a function of the incident laser energy. Neutrals show typical Boltzmann distributions while ions show Coulomb-Boltzmann-shifted distributions. The plasma characterization is rationalized in terms of kinetic energies of ejected particles, ion, electron and neutral temperatures, ion charge states, and plasma density. A special regard is given to the parameters which regulate the plasma temperature: the boiling point, the electron density and the ionization potentials of the ablated elements. The ion acceleration processes occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions are presented and discussed.     

Control and acquisition for MAST Thomson scattering diagnostics

The MAST (mega-amp spherical tokamak) Thomson scattering (TS) diagnostics have been radically upgraded and expanded. Eight 30 Hz 1.6 J Nd:YAG lasers have been combined to produce a sampling rate of 240 Hz. The scattered signals are acquired by two spectrometer systems: core and edge. The core system has been built anew: collection optics, polychromators, digitizers, and control computers. It allows measurement of electron temperature and density at 130 spatial points with ∼10 mm resolution across the plasma.The Nd:YAG scattered light signals are registered in 650 channels as polychromator outputs; each channel is registered on two ADCs: at 1 GHz rate in a short interval around each laser pulse and at 100 kHz for background data. The fast ADCs are combined in 26 data acquisition units. Each unit is assembled in a 6 U PXI chassis with embedded controller and six 4-channel 1 GHz ADC cards. Some chassis contain a 96-channel slow ADC card with Ethernet control.The Ruby TS has been rebuilt with a new spectrometer and CCD camera to provide higher spatial resolution - 512 points; the laser has been modified to add double pulse capability.A new control and acquisition system has been developed; it has modular design allowing flexibility and seamless expansion. The system supports event-triggered and real-time operation (will be added in a later stage).A smart trigger device has been developed for TS timing and synchronisation. It provides complex pulse sequences for laser firing with resynchronisation on a number of digital and analogue inputs including plasma events. This device also triggers TS acquisition.The system is integrated by a TS master process running on the dedicated computer; it is represented as a standard MAST data acquisition unit. The Ruby TS is also implemented as a standard MAST unit linked with the Nd:YAG TS by MAST system services.     

Influence of sample temperature on the expansion dynamics of laser-induced germanium plasma

In this paper, we investigated the influence of sample temperature on the expansion dynamics and the optical emission spectroscopy of laser-induced plasma, and Ge was selected as the test sample. The target was heated from room temperature(22 °C) to 300 °C, and excited in atmospheric environment by using a Q-Switched Nd:YAG pulse laser with the wavelength of 1064 nm. To study the plasma expansion dynamics, we observed the plasma plume at different laser energies(5.0, 7.4 and 9.4 mJ)and different sample temperatures by using time-resolved image. We found that the heated target temperature could accelerate the expansion of plasma plume. Moreover, we also measured the effect of target temperature on the optical emission spectroscopy and signal-to-noise ratio.     

Uranium measurements using laser-induced breakdown spectroscopy in lithium chloride-potassium chloride salt of pyroprocessing

In pyroprocessing,uranium(U) is recovered from molten LiCl-KCl salt,and,for safeguard purposes,it is important to analyze the U and Plutonium(Pu) concentrations in a timely manner.In the present work,salt samples containing U were fabricated.The laser used in the present work was an Nd:YAG laser with a wavelength of 532 nm,a laser energy on the sample of11.5 mJ,and a pulse repetition rate of 10 Hz.The plasma emission light was measured with an Echelle spectrometer.A total of 100 points on the sample surface were measured as the laser incident position was changed.The U and potassium(K) peaks in the spectrum were identified.Univariate and multivariate analyzes were conducted to determine the accuracy and limit of detection(LOD) of the laser-induced breakdown spectroscopy.     

Photoneutron interrogation of low-enriched uranium induced by bremsstrahlung from a 4 MeV linac

Revealing smuggled nuclear material by passive γ-detection is hindered, because the weak radiation can easily be shielded. Neutrons, as penetrate shielding, represent a detection potential, by inducing fission in the nuclear material. A 4 MeV linear accelerator was used as a pulsed neutron source for active interrogation of U-bearing material. Produced in heavy water by bremsstrahlung, neutrons subsequently induced fissions in UO₂ samples. Delayed fission neutrons were detected in a neutron collar built up by ³He counters in a polyamide container. The counters were gated to be detached from high voltage during the electron pulse. Irradiation-measurement cycles were carried out with a 25 Hz pulse repetition rate as optimum setting. The time analyser start-up was externally triggered and synchronised by the electron beam pulse. The response of the system was studied as a function of the intensity of the electron current, the amount of heavy water, U enrichment, and total U content. Sensitivity limit was achieved as 0.5 g ²³⁵U and/or 30 g ²³⁸U in a 20 s measurement time (500 cycles) with the amount of heavy water of 100 g and a mean electron current of 2 μA. Because of the long decay time of the prompt (interrogating and fission) neutron pulse, about a half of the time interval (40 ms) between pulses is only available for counting delayed neutrons.     

Nanosecond laser ablation of Thoria fuel pellets for microstructural study

A two-dimensional numerical model has been developed simulating the process of laser based surface etching of Thoria targets via pulsed laser ablation enabling their surface preparation for subsequent metallographic investigation. The heat conduction equation solved by an explicit finite difference method provides simulated data on the temperature distribution at the surface and within the target, melt depth and evaporation rate from the target as a function of time, during and after the laser pulse. Calculations have been performed for laser and target parameters corresponding to experimental conditions matching our reported experimental observations on pulsed laser etching of Thoria pellets via laser ablation. The calculated maximum surface temperature reached by the laser treated Thoria target exceeds the estimated value of thermodynamic critical temperature of Thoria. Thus, our results on simulation of pulsed laser ablation for an average laser flux of 10 J/cm² delivered by a 10 ns Nd:YAG laser pulse corresponding to a peak laser intensity of 3.87 × 10⁹ W/cm² suggest, that explosive boiling could probably be an additional material-removal mechanism other than normal boiling and evaporation when surface etching Thoria with such intense laser radiation. Since explosive boiling is usually accompanied by intense material ejection, this mechanism of material-removal should be avoided to ensure minimum induced target damage associated with the technique of laser based etching. Our calculations thus help us to make a proper choice of laser parameters facilitating subsequent metallographic investigation of laser etched Thoria fuel pellets, at the same time, minimizing unwanted associated thermal effects such as target damage through crater formation, as has been experimentally observed.     

The role of radiolysis products in in situ luminescence of Li2O

A new phenomenon of an “excess luminescence” (EL) in Li₂O observed at 4.5–2.5 eV under light ion (H⁺, He⁺) irradiation during the rise of temperature (>573 K) was studied. The essence of the EL is in the rapid pulse increase of the luminescence intensity. It is proposed that this phenomenon is based on the thermo-dissociation of colloidal Li into Li lattice ions, F⁺ and F⁰ centers, and oxygen vacancies. Formed oxygen vacancies capture electrons during the irradiation and form excited F-centers, whose relaxation gives the EL. This phenomenon was reproduced using X-ray irradiation and a sample containing colloidal Li introduced by irradiation with electron accelerator to an absorbed dose of 105 MGy.     

Laser induced surface alloy formation and diffusion of antimony in aluminium

We propose laser quenching of a metal, covered with a suitable layer of a solute species as a method for forming metastable surface alloys similar to those obtained by splat cooling and ion implantation techniques. The proposed technique is tested for Sb layers (1000–4000 Å thick) evaporated on Al and irradiated with a single Nd: glass laser pulse (7 ns fwhm) of energy density upto 10.2 J cm⁻². Rutherford backscattering of 1.8 MeV He⁺ ions was used to determine depth profiles of Sb in Al. Considerable loss and diffusion of Sb in Al is observed. Loss of Sb is studied systematically for various energy densities. X-ray diffraction was used to identify phases formed by laser quenching. The diffused antimony is found to form a metastable alloy similar to that formed by ion implantation. Surface topological changes produced by laser treatment are studied using a scanning electron microscope. A comparative study of laser treated, implanted Sb layer (30 keV, 1.3 × 10¹⁷ Sb⁺ ions cm⁻²) in Al and an evaporated Sb layer of comparable thickness is also presented.     

Si nanocrystals formation by a new ion implantation device

Metallic and non-metallic ion beams can be used to modify the properties of wafer surfaces if accelerated at moderate energies. We developed a new “implantation machine” able to generate ions and to accelerate them up to 80 kV. The ion generation is achieved by a laser-plasma source which creates plasma in expansion. The device consists of a KrF excimer laser and a generating vacuum chamber made of stainless steel. The laser energy was 45 mJ/pulse with a power density of 2.25 × 10⁸ W/cm². The target was kept to positive voltage to accelerate the produced ions. The ion dose was estimated by a fast polarised Faraday cup. This machine was utilised to try synthesizing silicon nanocrystals in SiO₂ matrix. Preliminary results of Si nanocrystals formation and the glancing-angle X-ray diffraction analyses are reported.     

Laser induced plasma plume imaging and surface morphology of silicon

Shot-to-shot variation in the characteristics of laser produced plasma plume and surface profile of N-type silicon (1 1 1) are investigated. In order to produce plasma, a Q-switched Nd: YAG laser (1064 nm, 10 mJ, 9-14 ns) is tightly focused on silicon target in air at room temperature. Target was exposed in such a way that number of laser shots was increased from point to point in ascending order starting from single shot at first point. Target was moved 2 mm after each exposure. In order to investigate shot-to-shot variation in the time integrated emission intensity regions within the plasma plume, a computer controlled CCD based image capture system was employed. Various intensity regimes were found depending strongly on the number of incident laser pulses. Plasma plume length was also found to vary with the number of pulses. The topographic analysis of the irradiated Si was performed by Scanning Electron Microscope (SEM) which shows the primary mechanisms like thermal or non-thermal ablation depend on the number of shots. Surface morphological changes were also studied in terms of ripple formation, ejection, debris and re-deposition of material caused by laser beam at sample surface. The micrographs show ripples spacing versus wavelength dependence rule [Λ ≈ λ/(1 − sin θ)]. Intensity variations with number of shots are correlated with the surface morphology of the irradiated sample.     

激光同步辐射源特性的线性康普顿散射分析

激光同步辐射源（Laser Synchrotron Source,简称LSS）是利用强激光与相对论电子束散射,产生准单色、能量可调的X射线脉冲的新型X射线源。本文介绍了LSS的线性康普顿散射理论;分析了LSS的主要特性,包括X射线通量、脉冲时间结构、能谱等。     

Optical design for divertor Thomson scattering system for JT-60SA

Optical design for divertor Thomson scattering system in JT-60SA has been conducted. The measurement system will use a Nd:YAG laser at 1064 nm, and scattered photons are collected by a collection optical system. The collection optics consists of primary mirror, secondary mirror, relay optics, and fiber collection optics. The laser transmission mirror and collection optics were designed to be installed in a slender lower port of JT-60SA. The assessment of the measurement errors in temperature was conducted for the designed collection optical system. Because of spatial limitation, the solid angle from the measurement points would be small especially for the measurement points in high field side, and consequently, the temperature errors in the high field side would be considerably large. The effects of several improvements on the error are discussed. Moreover, an assessment for the in-vessel laser transmission metallic mirrors is conducted for the present design.     

Effects of Nd:YAG laser irradiation on structural and magnetic properties of Li0.5Fe2.5O4

In the present paper we report our results on the effect of Nd:YAG laser irradiation on the structural and magnetic properties of Li_0.5Fe_2.5O₄ spinel ferrite prepared by solid-state reaction technique. X-ray diffraction analysis was carried out to confirm the formation of the single phase cubic spinel structure. The lattice constant calculated from X-ray diffraction data (XRD) goes on increasing with non irradiated phase to exceeding higher doses of irradiation. The distribution of the substituted ions over the different lattice sites is determined from XRD and infrared spectra. The damage structure and morphological investigations were carried out by using scanning electron microscopy and transmission electron microscopy techniques. It has been observed from our data on magnetic properties that laser irradiation severely affects the magnetization. From the magnetization measurements it has been observed that the saturation magnetization decreases with increase in the laser dose rate. The observed reduction in the saturation magnetization after irradiation is understood on the basis of the partial formation of paramagnetic centers and rearrangement of cations in the lattice.     

Cooling as a means of obtaining a bright and achromatic proton microbeam

An electron cooling ring for a 50 MeV proton beam is proposed for the University of Manitoba Accelerator Centre, With suitable extraction this system would produce 0.25 nA of monochromatic dc beam with a pulse length of 0.1 s deliverable within 1 μm².