Wave frequency dependence of H- ion production and extraction in a transformer coupled plasma H- ion source at SNU

The effect of rf wave frequencies on the production of H(-) ion is investigated in a transformer coupled plasma H(-) ion source at Seoul National University. A Langmuir probe is installed to measure the plasma density and temperature, and these plasma parameters are correlated to the extracted H(-) beam currents at various frequencies. The Langmuir probe is also used to measure the density of H(-) ions at the ion source by generating photodetachment with an Nd:YAG laser. The extracted H(-) currents decrease to a minimum value until 13 MHz and then, increase as the driving frequency increases from 13 MHz while the relative H(-) population measured by photodetachment monotonically decreases as the driving rf frequency increases from 11 MHz to 15 MHz. A potential well formed at the extraction region at high frequencies of more than 13 MHz is considered responsible for the increased H(-) beam extraction even with a lower photodetachment signal. The variation in the driving rf frequency not only affects the density and temperature of the plasma but also modifies the plasma potential with the existence of a filtering magnetic field and consequently, influences the extracted H(-) current through the extraction as well as formation of H(-) ions.     

Laser measurement of H- ions in a field-effect-transistor based radio frequency ion source

Hydrogen negative ion density measurements are required to clarify the characteristics of negative ion production and ion source performance. Both of laser photodetachment and cavity ring down (CRD) measurements have been implemented to a field-effect-transistor based radio-frequency ion source. The density ratio of negative hydrogen ions to electrons was successfully measured by laser photodetachment and effect of magnetic filter field on negative ion density was confirmed. The calculated CRD signal showed that CRD mirrors with >99.990% reflectivity are required and loss of reflectivity due to cesium contamination should be minimized.     

Ion optical design of a collinear laser-negative ion beam apparatus

An apparatus for photodetachment studies on atomic and molecular negative ions of medium up to heavy mass (M ? 500) has been designed and constructed. Laser and ion beams are merged in the apparatus in a collinear geometry and atoms, neutral molecules and negative ions are detected in the forward direction. The ion optical design and the components used to optimize the mass resolution and the transmission through the extended field-free interaction region are described. A 90° sector field magnet with 50 cm bending radius in combination with two slits is used for mass dispersion providing a resolution of M∕ΔM?800 for molecular ions and M∕ΔM?400 for atomic ions. The difference in mass resolution for atomic and molecular ions is attributed to different energy distributions of the sputtered ions. With 1 mm slits, transmission from the source through the interaction region to the final ion detector was determined to be about 0.14%.     

Design of a cavity ring-down spectroscopy diagnostic for negative ion rf source SPIDER

The rf source test facility SPIDER will test and optimize the source of the 1 MV neutral beam injection systems for ITER. Cavity ring-down spectroscopy (CRDS) will measure the absolute line-of-sight integrated density of negative (H(-) and D(-)) ions, produced in the extraction region of the source. CRDS takes advantage of the photodetachment process: negative ions are converted to neutral hydrogen atoms by electron stripping through absorption of a photon from a laser. The design of this diagnostic is presented with the corresponding simulation of the expected performance. A prototype operated without plasma has provided CRDS reference signals, design validation, and results concerning the signal-to-noise ratio.     

The importance of current density ‘focusing’ in STM image resolution

In a previous paper we incorporated Tersoff and Hamann's model of an STM tip into Lang's transfer Hamiltonian result for the tunnelling current density and applied the resulting simple expression to Tersoff's six-plane-wave model of a monolayer of graphite. In this paper the results for a more realistic model of a graphite surface and a more complete selection of tip positions are presented. They support the previous conclusions that the normal component of the current density takes on both positive and negative values in a complex flow pattern and that its lateral falloff away from the (projected) tip position is much slower than expected from the extraordinary lateral resolution (～1 Å) evident in the best STM images of graphite. This shows that sharp ‘focusing’ of the current density directly under the tip is not a necessary condition for high lateral resolution.     

Development of a new photoelectron spectroscopy instrument combining an electrospray ion source and photoelectron imaging

A new apparatus has been constructed that combines electrospray ionization with a quadrupole mass filter, hexapole ion trap, and velocity-map imaging. The purpose is to record photoelectron images of isolated chromophore anions. To demonstrate the capability of our instrument we have recorded the photodetachment spectra of isolated deprotonated phenol and indole anions. To our knowledge, this is the first time that the photodetachment energy of the deprotonated indole anion has been recorded.     

基于DPIV的微气泡瞬态数密度与粒径谱测量研究

基于数字粒子图像技术及CCD成像的景深原理,提出了测算微气泡数密度和粒径谱的方法。推导出了测算数密度和粒径谱的公式,建立了算法流程。最后设计了实验系统,用该算法测量了由陶瓷管微气泡发生器产生的气泡数密度和粒径谱。研究发现,应用景深原理测量的气泡数密度规律与实际数据相符,水槽中气泡的数密度与调频器的频率正相关。基于内接圆标记的气泡粒径测算结果的变化规律与已有实验数据的规律一致,较之直接提取气泡图像像素计算半径更加准确。     

Beam purification by photodetachment (invited)

Ion beam purity is of crucial importance to many basic and applied studies in nuclear science. Selective photodetachment has been proposed to suppress unwanted species in negative ion beams while preserving the intensity of the species of interest. A highly efficient technique based on photodetachment in a gas-filled radio frequency quadrupole ion cooler has been demonstrated. In off-line experiments with stable ions, up to 10(4) times suppression of the isobar contaminants in a number of interesting radioactive negative ion beams has been demonstrated. For selected species, this technique promises new experimental possibilities in studies on exotic nuclei, accelerator mass spectrometry, and fundamental properties of negative atomic and molecular ions.     

Three-dimensional reconstructions from incomplete data: interpretability of density maps at "atomic" resolution

Three dimensional data collection in electron microscopy is normally limited to a range of tilt angles that is less than +/- 90 degrees. Thus, even under the best conditions, experimental values of the structure factors will be missing within a solid cone in reciprocal space. Previous work has already shown that the missing cone of data can produce serious artifacts in three-dimensional density maps at modest resolution, for example approximately 15A. We now report, however, that a missing cone as large as +/- 30 degrees appears to have no serious effect on the three-dimensional density map of a protein at 3.6 A resolution, and we attribute this result to the fact that the electron density features are quite well separated from one another at that resolution. The map calculated with a +/- 30 degree missing cone is, furthermore, no more sensitive to noise (error) than is the full (isotropic) Fourier map. This result does not seem to be unreasonable in view of the fact that less than 14% of the data (i.e., signal) is lost due to the missing cone. Our numerical simulations therefore indicate that there should be no difficulty in interpreting high-resolution Fourier maps that can be produced with data that fall within realistic estimates of achievable resolution, tilt angles, and experimental error.     

Two-dimensional spectral line emission reconstruction as a plasma diagnostic

A technique for reconstructing point-by-point light emission from a plasma using a series of one-dimensional, line-of-sight measurements is described. The method, an adaptation of tomographic x-ray scans, does not assume any symmetry in the plasma, unlike an Abel inversion. Results from the Tormac IV toroidal bicusp device are presented, in which plasma light was collected by a 16-channel polychromator with 0.4-A resolution and a 1-micros time response. Reconstructed plasma light emission with spatial, temporal, and wavelength resolution is then calculated. The profiles of reconstructed spectral lines (Stark broadened H(beta) and Doppler broadened He II 4686-A lines from the 90% H, 10% He plasma) are used to monitor local plasma density and temperature. A spatial resolution of 2 cm in a 14 x 24 cm vessel, which is determined by the number of angles that the plasma is viewed from, is realized. This technique is particularly useful when a high-temperature plasma is surrounded and obscured by a low-temperature, highly emissive plasma near the vessel walls.     

Measurements of electron density and temperature in the H-1 heliac plasma by helium line intensity ratios

Electron density and temperature distributions in the H-1 heliac plasma are measured using the helium line intensity ratio technique based on a collisional-radiative model. An inversion approach with minimum Fisher regularization is developed to reconstruct the ratios of the local emission radiances from detected line-integrated intensities. The electron density and temperature inferred from the He I 667.8/728.1 and He I 728.1/706.5 nm line ratios are in good agreement with those from other diagnostic techniques in the inner region of the plasma. The electron density and temperature values appear to be a little high in the outer region of the plasma. Some possible causes of the discrepancy in the outer region are discussed.     

Nonmarski differential-interference contrast microscopy in transillumination: its use on unstained or stained sections, smears, and wet mounts, or on fluorochromed sections and cell-culture monolayers

Unstained, lightly stained and conventionally stained microtome tissue sections of two different thicknesses (ca. 4 μm and 1 μm) and also unstained or stained wet mounts of cells were photographed under the microscope using bright-field, positive phase contrast, and Nomarski differential-interference contrast (DIC) in transillumination. The photomicrographs were critically compared. It was found that the density of various stains did not adversely affect the better resolution of the DIC image (as compared to the bright-field image); however, Optical sectioning' of darkly stained objects is not possible. Unstained or stained smears of blood or of epithelial cells of buccal mucosa were examined with DIC in transillumination, then after certain preparatory techniques, the same preparation was examined in the scanning electron microscope, and finally the same areas of the slide were viewed with DIC in epiillumination. Particular attention was given to structures (nuclei and cytoplasm) which appeared in positive or negative relief in the photomicrographs taken by the various techniques. It was concluded that the optically more dense nucleus which always appeared in positive relief by the various methods of examination, was in fact geometrically raised from the surrounding cytoplasm. Acridine orange (AO) stained cell-culture monolayers and H and E stained sections were examined under a fluorescence microscope with DIC optics. By comparing photographs which had been taken with DIC, epi-fluorescence or fluorescence in transillumination, and DIC-fluorescence, it was concluded that the DIC image, which had been superimposed on the fluorescence image, contributed a definite gain in information. Some common errors in the interpretation of the DIC image are discussed; methods of avoiding improper use of equipment are given. The conclusion is drawn that the DIC system is superior to positive and negative phase contrast for the examination of a variety of unstained or stained preparations. Therefore, this method can be used to advantage not only for the examination of unstained preparations, but also on some specimens which have been routinely stained or fluorochromed.     

窄光谱带宽X射线刻蚀多层膜光栅

结合X射线荧光分析和同步辐射单色器对窄光谱带宽多层膜的需求,开展了窄光谱带宽刻蚀多层膜光栅的理论和实验研究.用平均密度法从理论上阐明将多层膜刻蚀成不同刻蚀比的多层膜光栅后,光谱分辨率将得到提高.用磁控溅射方法制备了W/C多层膜,并用常规的光刻工艺对其进行刻蚀,得到了刻蚀后的多层膜光栅.掠入射X射线衍射测量表明,刻蚀后多层膜的衍射峰位置向小角方向移动,多层膜光栅没有改变剩余多层膜的结构,而且带宽减小,光谱分辨率得到提高,说明实验采用的工艺方法和工艺路线可以满足制作窄光谱带宽刻蚀多层膜光栅的要求,为今后进一步研究实用化元件打下了基础.     

Supersonic helium beam diagnostic for fluctuation measurements of electron temperature and density at the Tokamak TEXTOR

A supersonic helium beam diagnostic, based on the line-ratio technique for high resolution electron density and temperature measurements in the plasma edge (r/a > 0.9) was designed, built, and optimised at TEXTOR (Torus Experiment for Technology Oriented Research). The supersonic injection system, based on the Campargue skimmer-nozzle concept, was developed and optimised in order to provide both a high neutral helium beam density of n(0) = 1.5 × 10(18) m(-3) and a low beam divergence of ±1° simultaneously, achieving a poloidal resolution of Δ(poloidal) = 9 mm. The setup utilises a newly developed dead volume free piezo valve for operation in a high magnetic field environment of up to 2 T with a maximum repetition rate of 80 Hz. Gas injections are realised for a duration of 120 ms at a repetition rate of 2 Hz (duty cycle 1/3). In combination with a high sensitivity detection system, consisting of three 32 multi-channel photomultipliers (PMTs), measurements of edge electron temperature and density with a radial resolution of Δ(radial) = 2 mm and a maximum temporal resolution of Δt ? 2 μs (470 kHz) are possible for the first time. The diagnostic setup at TEXTOR is presented. The newly developed injection system and its theoretical bases are discussed. The applicability of the stationary collisional-radiative model as basis of the line-ratio technique is shown. Finally, an example of a fluctuation analysis demonstrating the unique high temporal and spatial resolution capabilities of this new diagnostic is presented.     

Thermal imaging diagnostics of powerful ion beams

Thermal imaging diagnostics of the total energy of a pulsed ion beam and energy-density distribution over the cross section is described. The diagnostics was tested on the TEMП-4M accelerator in the conditions of formation of two pulses: (i) the first plasma-forming pulse is negative (300–500 ns, 100–150 kV) and (ii) the second generated one is positive (150 ns, 250–300 kV). The beam composition includes carbon ions (85%) and protons, and the power density is 0.2–3.0 J/cm² (for various diodes). The diagnostics was applied in studies of the powerful ion beam, formed by an ion diode with self insulation (two-pulse mode) and external magnetic insulation in the single-pulse mode. The diagnostics was intended to measure the beam energy density in a range of 0.05–5.00 J/cm² in the absence of erosion and ablation processes on the target. When an infrared camera with a 140 × 160-pixel matrix is used, the spatial resolution is 0.9 mm. The measurement time does not exceed 0.1 s.     

Emission microscopy and related techniques: resolution in photoelectron microscopy, low energy electron microscopy and mirror electron microscopy.

A unified treatment of the resolution of three closely related techniques is presented: emission electron microscopy (particularly photoelectron microscopy, PEM), low energy electron microscopy (LEEM), and mirror electron microscopy (MEM). The resolution calculation is based on the intensity distribution in the image plane for an object of finite size rather than for a point source. The calculations take into account the spherical and chromatic aberrations of the accelerating field and of the objective lens. Intensity distributions for a range of energies in the electron beam are obtained by adding the single-energy distributions weighted according to the energy distribution function. The diffraction error is taken into account separately. A working resolution is calculated that includes the practical requirement for a finite exposure time, and hence a finite non-zero current in the image. The expressions for the aberration coefficients are the same in PEM and LEEM. The calculated aberrations in MEM are somewhat smaller than for PEM and LEEM. The resolution of PEM is calculated to be about 50 A, assuming conventional UV excitation sources, which provide current densities at the specimen of 5 x 10(-5) A/cm2 and emission energies ranging up to 0.5 eV. A resolution of about 70 A has been demonstrated experimentally. The emission current density at the specimen is higher in LEEM and MEM because an electron gun is used in place of a UV source. For a current density of 5 x 10(-4) A/cm2 and the same electron optical parameters as for PEM, the resolution is calculated to be 27 A for LEEM and 21 A for MEM.     

Edge and core Thomson scattering systems and their calibration on the ASDEX Upgrade tokamak

A new 10 channel Thomson scattering (TS) system was installed on the ASDEX Upgrade tokamak to measure radial profiles of electron density and temperature at the plasma edge with high radial resolution. Together with the already existing TS system, which is now used for the core plasma, electron density and temperature profiles extending from the edge to the core are now obtained in a single discharge. The TS systems are relatively calibrated by an optical parametric oscillator.     

A multi‐emitter fitting algorithm for potential live cell super‐resolution imaging over a wide range of molecular densities

Multi‐emitter fitting algorithms have been developed to improve the temporal resolution of single‐molecule switching nanoscopy, but the molecular density range they can analyse is narrow and the computation required is intensive, significantly limiting their practical application. Here, we propose a computationally fast method, wedged template matching (WTM), an algorithm that uses a template matching technique to localise molecules at any overlapping molecular density from sparse to ultrahigh density with subdiffraction resolution. WTM achieves the localization of overlapping molecules at densities up to 600 molecules μm^–2 with a high detection sensitivity and fast computational speed. WTM also shows localization precision comparable with that of DAOSTORM (an algorithm for high‐density super‐resolution microscopy), at densities up to 20 molecules μm^–2, and better than DAOSTORM at higher molecular densities. The application of WTM to a high‐density biological sample image demonstrated that it resolved protein dynamics from live cell images with subdiffraction resolution and a temporal resolution of several hundred milliseconds or less through a significant reduction in the number of camera images required for a high‐density reconstruction. WTM algorithm is a computationally fast, multi‐emitter fitting algorithm that can analyse over a wide range of molecular densities. The algorithm is available through the website. https://doi.org/10.17632/bf3z6xpn5j.1     

Layout and results from the initial operation of the high-resolution x-ray imaging crystal spectrometer on the Large Helical Device

First results of ion and electron temperature profile measurements from the x-ray imaging crystal spectrometer (XICS) diagnostic on the Large Helical Device (LHD) are presented. This diagnostic system has been operational since the beginning of the 2011 LHD experimental campaign and is the first application of the XICS diagnostic technique to helical plasma geometry. The XICS diagnostic provides measurements of ion and electron temperature profiles in LHD with a spatial resolution of 2 cm and a maximum time resolution of 5 ms (typically 20 ms). Ion temperature profiles from the XICS diagnostic are possible under conditions where charge exchange recombination spectroscopy (CXRS) is not possible (high density) or is perturbative to the plasma (low density or radio frequency heated plasmas). Measurements are made by using a spherically bent crystal to provide a spectrally resolved 1D image of the plasma from line integrated emission of helium-like Ar(16 +). The final hardware design and configuration are detailed along with the calibration procedures. Line-integrated ion and electron temperature measurements are presented, and the measurement accuracy is discussed. Finally central temperature measurements from the XICS system are compared to measurements from the Thomson scattering and CXRS systems, showing excellent agreement.     

Wire tomography in the H-1NF heliac for investigation of fine structure of magnetic islands

Electron beam wire tomography in the H-1NF heliac enables high resolution mapping of vacuum flux surfaces with minimal disruption of the plasma operations schedule. Recent experimental results have proven this technique to be a highly accurate and high resolution method for mapping vacuum magnetic islands. Islands of width as small as delta approximately 8 mm have been measured, providing estimates of the internal rotational transform of the island. Point-to-point comparison of the mapping results with computer tracing, in conjunction with an image warping technique, enables systematic exploration of magnetic islands and surfaces of interest. Recent development of a fast mapping technique significantly reduced the mapping time and made this technique suitable for mapping at higher magnetic fields. This article presents recent experimental results and associated techniques.