Vacuum Ultraviolet Spectrometer Research Articles

Nanograting-based compact vacuum ultraviolet spectrometer and beam profiler for in situ characterization of high-order harmonic generation light sources

A compact, versatile device for vacuum ultraviolet (VUV) beam characterization is presented. It combines the functionalities of a VUV spectrometer and a VUV beam profiler in one unit and is entirely supported by a standard DN200 CF flange. The spectrometer employs a silicon nitride transmission nanograting in combination with a microchannel plate-based imaging detector. This enables the simultaneous recording of wavelengths ranging from 10 to 80 nm with a resolution of 0.25-0.13 nm. Spatial beam profiles with diameters up to 10 mm are imaged with 0.1 mm resolution. The setup is equipped with an in-vacuum translation stage that allows for in situ switching between the spectrometer and beam profiler modes and for moving the setup out of the beam. The simple, robust design of the device is well suited for nonintrusive routine characterization of emerging laboratory- and accelerator-based VUV light sources. Operation of the device is demonstrated by characterizing the output of a femtosecond high-order harmonic generation light source.

Space-resolved vacuum ultraviolet spectrometer system for edge impurity and temperature profile measurement in HL-2A

A 1 m normal incidence vacuum ultraviolet (VUV) spectrometer has been developed for spatial distribution measurement of edge impurity line emission in the wavelength range of 300-3200 A on HL-2A tokamak. A vertical profile of the impurity line emission is measured with a space-resolved slit placed between an entrance slit and a grating of the spectrometer. Two concave 1200 grooves/mm gratings blazed at 800 and 1500 A are set on a rotatable holder in the spectrometer, which gives wavelength dispersion of 0.12 mm/A. A back-illuminated charge-coupled device is used as a detector with an image size of 6.7 x 26.6 mm(2) (26 x 26 microm(2)/pixel). An excellent spatial resolution of 2 mm is obtained with good spectral resolution of 0.15 A when the space-resolved slit of 50 microm in width is used. The space-resolved spectra thus provide three radial profiles of emission line intensity, ion temperature, and poloidal rotation. The electron temperature can be measured by the intensity ratio, e.g., CIII 2s(2)-2s3p (386 A)/2s(2)-2s2p (977 A). The sensitivity of the spectrometer is calibrated in situ by using the VUV bremsstrahlung continuum radiation emitted from the tokamak plasma. A good performance of the spectrometer system for the edge impurity and temperature profile measurements is presented with results on Ohmic and H-mode discharges.

Self-Reversal in Hydrogen Lyman-α Line Profile

A detailed spectral profile of the Lyman-α line of neutral hydrogen, i.e., the transition from n = 2 to the ground state, where n is the principal quantum number, was measured with a vacuum ultraviolet spectrometer for the plasma in the Large Helical Device. Self-reversal was observed in the spectral profile when the plasma density was increased with repetitive injection of hydrogen pellets. A one-dimensional radiation transport model was used for creating the Lyman-α spectral profile, for which an emission and absorption medium with a slab geometry and constant plasma parameters were assumed. The population density of the n = 2 level generally has a peaked spatial profile even with constant ground state density because of the reabsorption effect which is essential for the emergence of self-reversal in the spectral profile. We used the n = 2 level population distribution in the medium by Molisch et al. [Radiation Trapping in Atomic Vapours, Oxford University, Oxford, 1998] and evaluated the Lyman-α spectral profile as a function of the optical thickness. The observed line profile was found to be well fitted, for example, with a ground state density of 5.2 × 1018 m−3 and a medium thickness of 10 cm when a Lorentzian profile having a full width at half maximum of 0.0018 nm is adopted for the emission and absorption coefficients.

Calibration of a high harmonic spectrometer by laser induced plasma emission

We present a method that allows for a convenient switching between high harmonic generation (HHG) and accurate calibration of the vacuum ultraviolet (VUV) spectrometer used to analyze the harmonic spectrum. The accurate calibration of HHG spectra is becoming increasingly important for the determination of electronic structures. The wavelength of the laser harmonics themselves depend on the details of the harmonic geometry and phase matching, making them unsuitable for calibration purposes. In our calibration mode, the target resides directly at the focus of the laser, thereby enhancing plasma emission and suppressing harmonic generation. In HHG mode, the source medium resides in front or after the focus, showing enhanced HHG and no plasma emission lines. We analyze the plasma emission and use it for a direct calibration of our HHG spectra.

Preliminary Thermal and Hydraulic Analysis on the ITER Upper Diagnostic Port Plug During Normal Operation and Baking

A preliminary thermo-hydraulic analysis was performed on the ITER diagnostic upper port plug. Relevant thermal and hydraulic parameters, such as coolant pressure drop, maximum structure temperature and bake-out time, were calculated for normal operation and baking. The upper port plug considered is based on the preliminary generic structure design of Princeton Plasma Physics Laboratory and the Blanket Shield Module (BSM) developed in Europe. The diagnostic shield modules are modeled so that the Korean diagnostic procurement package, which includes Vacuum Ultra-Violet (VUV) spectrometer and neutron activation system, can be integrated. The analysis provides design inputs to optimize flow in the cooling channels of the plug. The conjugated heat transfer analysis for the port plug confirms that it is important to secure accurate nuclear heat and accurate electro-magnetic (EM) force for the design of the joining flange between the BSM and the main body. Thermal analysis shows that it will take ten hours for the port plug to reach the bake-out temperature (240°C), if the window plate is heated additionally from the rear side.

Absolute vacuum ultraviolet flux in inductively coupled plasmas and chemical modifications of 193 nm photoresist

Vacuum ultraviolet (VUV) photons in plasma processing systems are known to alter surface chemistry and may damage gate dielectrics and photoresist. We characterize absolute VUV fluxes to surfaces exposed in an inductively coupled argon plasma, 1–50 mTorr, 25–400 W, using a calibrated VUV spectrometer. We also demonstrate an alternative method to estimate VUV fluence in an inductively coupled plasma (ICP) reactor using a chemical dosimeter-type monitor. We illustrate the technique with argon ICP and xenon lamp exposure experiments, comparing direct VUV measurements with measured chemical changes in 193 nm photoresist-covered Si wafers following VUV exposure.

First Measurement of VUV Spectroscopy Using 1 m Normal Incidence Spectrometer on HL-2A

A 1 m vacuum ultraviolet (VUV) spectrometer with temporal and spatial resolution was developed for impurity study of HL-2A tokamak. The instrument is equipped with two concave gratings blazed at 80 nm and 150 nm, respectively, and a windowless back-illuminated charge coupled device (CCD) detector of 256 × 1024 pixels. The total wavelength coverage of spectrometer is 30 ∼ 320 nm with a spectral resolution of 0.015 nm at a width of entrance slit of 10 μm. A portion of this range is observed during a plasma discharge with a spectral range of 20 nm. The minimum integration time of the detector system is about 6.7 ms for each frame in a full binning mode. Using a space-resolved slit located between the entrance slit and the grating a radial profile on the vertical direction with a range of 400 mm can be obtained. The primary results were successfully obtained with high signal-to-noise ratio and good spectral resolution, which demonstrated the instrument functions very well.

Vacuum ultraviolet optical properties of a micro-pulling-down-method grown Nd^3+:(La_09,Ba_01)F_29

Nd3+:(La1−x,Bax)F3−x(x=0.1) is efficiently grown by the micro-pulling-down method. Characterization of its optical properties reveals that it is transparent in the vacuum ultraviolet region with an absorption edge at around 180 nm. It has fluorescence centered at 175 nm with a bandwidth of 12 nm and a lifetime of 6.1 ns as measured by a vacuum ultraviolet streak camera and spectrometer combination. It is proposed to be suitable as a vacuum ultraviolet scintillator and a potential laser material.

Multichannel detector for ion temperature determination in vacuum ultraviolet spectrum

A vacuum ultraviolet spectrometer equipped with a charge coupled device and an open multichannel plate has been used to analyze the temperature of carbon and oxygen ions in the NOVA-UNICAMP tokamak. The detection system was optimized and aligned to minimize the instrumental broadening. Also, higher order diffractions of the emissions were analyzed, resulting in lower experimental errors. The ion temperature was monitored during the tokamak discharge, presenting values between 30 and 70 eV.

Impurity Transport in Alcator C-Mod Plasmas

Trace nonrecycling impurities (scandium and CaF2) have been injected into Alcator C-Mod plasmas in order to determine impurity transport coefficient profiles in a number of operating regimes. Recycling Ar has also been injected to characterize steady-state impurity density profiles. Subsequent impurity emission has been observed with spatially scanning X-ray and vacuum ultraviolet spectrometer systems, in addition to very high spatial resolution X-ray and bolometer arrays viewing the plasma edge. Measured time-resolved brightness profiles of helium-, lithium-, and beryllium-like transitions have been compared with those calculated from a transport code that includes impurity diffusion and convection, in conjunction with an atomic physics package for individual line emission. Similar modeling has been performed for the edge observations, which are unresolved in energy. The line time histories and the profile shapes put large constraints on the impurity diffusion coefficient and convection velocity profiles. In L-mode plasmas, impurity confinement times are short (~20 ms), with diffusivities in the range of 0.5 m2/s, anomalously large compared to neoclassical values. During Enhanced Dα (EDA) H-modes, the impurity confinement times are longer than in L-mode plasmas, and the modeling suggests that there exists inward convection (≤50 m/s) near the plasma edge, with greatly reduced diffusion (of order 0.1 m2/s), also in the region of the edge transport barrier. These edge values of the transport coefficients during EDA H-mode are qualitatively similar to the neoclassical values. In edge localized mode-free H-mode discharges, impurity accumulation occurs, dominated by large inward impurity convection in the pedestal region. A scaling of the impurity confinement time with H-factor reveals a very strong exponential dependence. In internal transport barrier discharges, there is significant impurity accumulation inside of the barrier foot, typically at r/a = 0.5. Steady-state impurity density profiles in L-mode plasmas have a large up-down asymmetry near the last closed flux surface. The impurity density enhancement, in the direction opposite to the ion B × ΔB drift, is consistent with modeling of neoclassical parallel impurity transport.

Overview of HIT-SI Diagnostic Systems

The Helicity Injected Torus with Steady Inductive current drive (HIT-SI) device is a spheromak which uses two inductively driven helicity injectors to provide constant helicity injection. The HIT-SI diagnostic set includes 96 3-D magnetic surface probes, 20 toroidal flux loops, an internal array of three stems of 3-D magnetic probes, a far infrared interferometer, a SPRED diagnostic, two tangentially viewing vacuum ultraviolet (VUV) spectrometers, a bolometer, a Zeff diagnostic, H-alpha detectors, a time integrated CCD camera, and Ion Doppler Spectroscopy (IDS). In addition, a multi point Thomson Scattering system, a microwave interferometer/polarimeter, injector voltage probes, an edge Langmuir probe array, and soft X-ray camera are under development.

Radial profile measurement of impurity line emissions using space-resolved 3m vacuum ultraviolet spectrometer in LHD

A space-resolved 3m vacuum ultraviolet (vuv) spectrometer has been developed to measure a radial (vertical) profile of impurity line emissions in the Large Helical Device (LHD). The system consists of a 3m normal incidence spectrometer equipped with a space-resolved slit and a back-illuminated charge-coupled device (CCD) detector, view-angle adjustable mirrors, and a toroidal slit. A full vertical profile of the vuv line radiated from the LHD plasma is observed with a convex cylindrical mirror by extending the view angle. The vertical profiles of intrinsic impurities of carbon (C III–C V) and externally injected impurities of neon (Ne VIII), aluminum (Al XI), and argon (Ar VIII) are successfully obtained with a time interval of 125ms in the VUV range of 500–3100Å. The ion temperature profiles are also measured from the C IV line emitted in the ergodic layer of LHD.

Investigating the chemical composition of mixed organic–inorganic particles by “soft” vacuum ultraviolet photoionization: The reaction of ozone with anthracene on sodium chloride particles

The ozone reaction with anthracene coated sodium chloride particles is measured by single photon ionization of the resulting volatilized gas phase molecules. To investigate in detail the organic fraction of particles, a vacuum ultraviolet aerosol mass spectrometer (VUV-AMS) has been developed at the Chemical Dynamics Beamline at the Advanced Light Source. This instrument combines a thermal vaporization particle source with tunable synchrotron radiation. Since VUV is a “soft” ionization method, the roles of both the vaporization and ionization steps on the fragmentation patterns of C 30H 62 hydrocarbons are observed. Conditions can be found to obtain mass spectra without significant fragmentation of the molecules. This technique is used to characterize the products of the heterogeneous reaction of gas phase ozone with particle phase anthracene on size-selected sodium chloride particles, conducted in a flow tube experiment. The recorded fragment-free mass spectra show several new products from mono- to penta-oxygenated anthracene. The kinetics of the product formation and particle size measurements are simultaneously recorded, revealing that an increase of the particle size accompanies the formation of the products.

Modified branching ratio method for absolute intensity calibration in VUV spectroscopy

A new modified branching ratio (MBR) method for the absolute intensity calibration of a vacuum ultraviolet (VUV) spectrometer is presented. The spectrometer is equipped with a multichannel detector, consisting of an open microchannel plate coupled to a charge-coupled device (CCD), or with a single channel photomultiplier. This technique extends the number of calibration points available from those provided by the branching ratio (BR) calibration technique. The MBR method is a variation of the conventional BR method, where we relax the condition that the two spectral emissions, in the visible and VUV spectra, come from the same excited level, to include transitions from different sublevels of the same energy level. However, a critical study of the statistical equilibrium of sublevels from the same ion energy level was necessary. As a result, we have more than doubled the number of calibration points for our spectrometer used in tokamak plasma diagnostics. The appropriate identification of new spectral line pairs for absolute calibration here presented opens the path for future works in other devices with similar plasma conditions or impurities content.

Vacuum ultraviolet photon detector with improved resolution for inverse photoemission spectroscopy

We have significantly improved the energy resolution of a vacuum ultraviolet isochromat spectrometer for inverse photoemission spectroscopy. The detector is based on a Geiger–Müller counting tube with acetone as filling gas and a CaF2 entrance window working as an energy selective optical bandpass at a mean energy of 9.84eV. Under this operating conditions the detector achieves an optical energy resolution of 320meV (FWHM). By adding a gas chamber with two CaF2 windows at the entrance of the counting tube we are able to use absorption lines of oxygen and krypton to further enhance the optical resolution of the detector system. In the case of krypton the mean energy shifts to a somewhat lower value of 9.72eV. The energy resolution improves to about 115meV (FWHM), while the integrated sensitivity decreases to approximately 30%. Due to the ability to easily switch between high sensitivity and high resolution this new counting tube differs from other resolution-enhancing suggestions. Measurements on the prominent (n=1) image potential state on the Cu(111) surface indicate that the photon detector is no longer the major resolution limiting component in the spectrometer.

Radiated Power and Impurity Concentrations in the EXTRAP-T2R Reversed-Field Pinch

A numerical and experimental study of the impurity concentration and radiation in the EXTRAP-T2R device is reported. The experimental setup consists of an 8-chord bolometer system providing the plasma radiated power and a vacuum-ultraviolet spectrometer providing information on the plasma impurity content. The plasma emissivity profile as measured by the bolometric system is peaked in the plasma centre. A one dimensional Onion Skin Collisional-Radiative model (OSCR) has been developed to compute the density and radiation distributions of the main impurities. The observed centrally peaked emissivity profile can be reproduced by OSCR simulations only if finite particle confinement time and charge-exchange processes between plasma impurities and neutral hydrogen are taken into account. The neutral hydrogen density profile is computed with a recycling code. Simulations show that recycling on metal first wall such as in EXTRAP-T2R (stainless steel vacuum vessel and molybdenum limiters) is compatible with a rather high neutral hydrogen density in the plasma centre. Assuming an impurity concentration of 10% for oxygen and 3% for carbon compared with the electron density, the OSCR calculation including lines and continuum emission reproduces about 60% of the total radiated power with a similarly centrally peaked emissivity profile. The centrally peaked emissivity profile is due to low ionisation stages and strongly radiating species in the plasma core, mainly O4+ (Be-like) and C3+ Li-like.

Spectroscopic Study of Impurity Behavior in Heliotron-J Plasmas

Behavior of intrinsic and injected impurities has been investigated in Heliotron-J plasmas by spectroscopic methods. Intrinsic impurities are identified with the vacuum ultraviolet grazing incidence spectrometer in neutral beam injection (NBI)-heated plasmas. Na-like Ni XVIII and Mg-like Ni XVIII are observed only in NBI heating phase. Helium gas is injected into electron cyclotron heating plasmas. In the density scan experiments, He II line intensities, which are normalized by the electron density, increase with decreasing electron density. For intrinsic impurities, similar dependence of line intensities on the electron density is observed. The normalized line intensity indicates the particle number of ions penetrated into the core plasma. In addition, the edge electron density is in proportion to the core electron density. These results may reflect the screening effect due to electron collisional ionization at the edge plasma. In the carbon limiter insertion, the CH radical band spectrum is observed. The carbon limiter head is formed in the hemisphere. The spatial distribution of the band emission is asymmetrical to the main axis of the limiter head. A good agreement is obtained between the spatial distribution of emissions of the band spectrum and the camera image with bandpass filter.

Electron impact dissociative excitation of O2: 1. Kinetic energy distributions of fast oxygen atoms

The kinetic energy distribution of oxygen atoms resulting from electron impact dissociation of O2 has been measured. A high‐resolution vacuum ultraviolet spectrometer was employed for the measurement of the OI(115.2 nm, 130.4 nm, 135.6 nm) fine‐structure emission line profiles at 35 eV and 100 eV electron impact energies. The deconvolved line profiles of atomic oxygen reveal the existence of a broad line profile (∼30–40 mÅ full width at half maximum) produced from a combination of dissociative excitation and dissociative ionization excitation processes. The atomic oxygen kinetic energy distributions at 100 eV electron impact energy spans the energy range from 1 to 10 eV, with a peak value near 1 eV. The excitation functions of OI(115.2 nm, 130.4 nm, 135.6 nm) were measured in the vicinity of threshold, from 14 to 50 eV. The PACS classification is 34.80.Gs (electron scattering‐molecular dissociation) and 33.50Dq (molecular spectra‐fluorescence).

Impurity transport studies by means of tracer-encapsulated solid pellet injection in neutral beam heated plasmas on LHD

The quantitative properties of impurity transport in large helical device (LHD) plasmas heated by neutral beam injection have been investigated by means of tracer-encapsulated solid pellet (TESPEL) injection. In the case of a titanium (Ti) tracer, the behaviour of the emission lines from the highly ionized Ti impurity, Ti Kα(EHe−like∼4.7 keV) and Ti XIX (λ = 16.959 nm), has been observed clearly by a soft x-ray pulse height analyzer and a vacuum ultraviolet spectrometer, respectively. A fairly longer decay time of the Ti Kα emission lines is obtained above the value of a line-averaged electron density, 3.0×1019 m−3. The dependence of the behaviour of the Ti tracer impurity on the line-averaged electron density below the value of that, 3.5×1019 m−3 is in qualitative agreement with the characteristics obtained from the observation of the behaviour of an intrinsic metallic impurity in neutral beam heated plasmas on LHD. In order to estimate the properties of the Ti impurity transport quantitatively, the one-dimensional impurity transport code, MIST has been used. As a result of the transport analysis with the MIST code, even an small inward convection should be necessary to account for the experimental results with the value of the line-averaged electron density, 3.5×1019 m−3. In order to examine the experimentally obtained transport coefficients, neoclassical analysis with respect to the radial impurity flux has been performed. The inferred rise of the inward convection cannot be explained solely by neoclassical impurity transport. Therefore, in order to account for the inward convection, the effect of a radial electric field and/or some other effect must be taken into account additionally.

Development of optical coatings for 157-nm lithography. I. Coating materials.

In a basic study to identify low-loss optics for applications in F2 lithography, five potential coating materials (AlF3, Na3AlF6, MgF2, LaF8, and GdF3) and three deposition methods (thermal evaporation by a resistance heater and by electron beam and ion-beam sputtering) were investigated in the vacuum ultraviolet (VUV) region. Samples were supplied as single-layer coatings on CaF2 substrates by four Japanese coating suppliers. Refractive indices (n) and extinction coefficients (k) of these coatings at 157 nm were evaluated; the transmittance and the reflectance were measured by a VUV spectrometer and were compared. As a result, resistance heating thermal evaporation is seen to be the optimal method for achieving low-loss antireflection coatings. The relation among optical constants, microstructures, and stoichiometry is discussed.