Intensity Of He Research Articles

Characterization of cascaded arc He plasma in a compact linear plasma device using voltammetry and optical emission spectroscopy

Cascaded arc plasma has been widely applied in linear plasma devices (LPDs) to produce high flux plasma for the study of plasma-material interaction. In this work, cascaded arc He plasma produced in an LPD with a compact arrangement is investigated by voltammetry and optical emission spectroscopy (OES). The results show that the cathode potential increases with the discharge current while it firstly decreases and then increases as increasing the gas flow rate. A local reverse electric field is observed at low gas flow rates between two cascaded plates (i.e. floating electrodes) near the cathode. The OES’ results reveal that as the gas flow rate increases, the intensity of He I lines increases and the electron excitation temperature (T exc ) decreases. As increasing the discharge current, the intensity of He lines exhibits various trends at different gas flow rates, showing a monotonic decline at 1.94 slm and a first increase followed by a reduction at 3.52 slm. The T exc increases with the discharge current. These findings could preliminarily shed light on the properties of cascaded arc of He plasma in the compact LPD and aid in the optimization of the device to generate the high-flux divertor-relevant plasma.

Optics of a multiple color gas puff imaging diagnostic on the HL-2A tokamak

The optics of a new Multiple Color Gas Puff Imaging (MC-GPI) diagnostic on the HL-2A tokamak is presented. MC-GPI utilizes the helium I (He I) line ratio technique to infer plasma density ne and electron temperature Te in edge plasma simultaneously. A compact four-channel beam splitter and dichroic mirrors enable the selection of the intensities of He I lines at specific wavelengths, i.e., λ=587.6 nm, λ=667.8 nm, λ=706.5 nm and λ=728.1 nm. The transmittance of these four specific He I lines after passing through the optical fiber bundle and the beam splitter is 34%, 33%, 33% and 30%, respectively. The temporal resolution of MC-GPI is 100μs and the spatial resolution is 2.5 mm × 2.5 mm. This diagnostic has been experimentally tested on an optical table as well as on a linear plasma device to verify its capability for ne and Te measurements. A newly designed light splitter with double-layer layout is under developing to absorb the stray light, aiming at improve the performance of the diagnostic.

No correlation of the Lyman continuum escape fraction with spectral hardness

The properties that govern the production and escape of hydrogen-ionizing photons (Lyman continuum, LyC; with energies > 13.6 eV) in star-forming galaxies are still poorly understood, but they are key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relation between the hardness of ionizing radiation and the LyC leakage in a large sample of low-z star-forming galaxies from the recent Hubble Space Telescope Low-z Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep X-shooter observations, we investigate the hardness of the ionizing spectra (QHe+/QH) between 54.4 eV (He+) and 13.6 eV (H) from the optical recombination lines He II 4686 Å and Hβ 4861 Å for galaxies with LyC escape fractions spanning a wide range, fesc(LyC) ≃ 0−90%. We find that the observed intensity of He II/Hβ is primarily driven by variations in the metallicity, but is not correlated with LyC leakage. Both very strong (<fesc(LyC)> ≃ 0.5) and nonleakers (<fesc(LyC)> ≃ 0) present similar observed intensities of He II and Hβ at comparable metallicity, between ≃0.01 and ≃0.02 for 12 + log(O/H)> 8.0 and < 8.0, respectively. Our results demonstrate that QHe+/QH does not correlate with fesc(LyC), which implies that strong LyC emitters do not show harder ionizing spectra than nonleakers at similar metallicity.

Observation of Contrary Thermo-responsive Trend for Single Crystal and Powder Samples in Mechano-, Thermo- and Solvato-responsive Luminescent Cubane [Ag4I4L4] Cluster

A new silver(I) iodide cluster [Ag4I4(TMP)4] (TMP = tris(3-methylphenyl)-phosphine) 1 shows triply stimuli-responsive luminescent chromism, namely, mechano-, thermo- and solvent-responsive chromism, which is isostructural to our previously reported [Cu4I4(TMP)4] 2 but shows quite different luminescence in response to the external stimuli. Especially, during the mechanical grinding, the relative intensities of HE and LE of 1 varied with a concomitant hypsochromic shift, and when the temperature was decreased from 300 to 5 K, unprecedented contrary thermo-responsive trend for single crystal and powered samples (blueshift of single crystals and redshift of powdered samples) was observed. These distinct characters of 1 should be due to the different molecular packing modes, metallic interactions and the unique character of Ag(I) ion.

Interferometric study of the effect of laser intensity and polarization on the cold-drawing of virgin polypropylene fibres

With the aid of the Mach–Zehnder interferometer, the drawability of polypropylene fibres (PP) was optically studied. The effect of varying the intensity of He–Ne laser on PP opto-mechanical properties was investigated. The state of polarization of the used laser was found to influence the optical and mechanical properties of PP fibres, such as the refractive index, elongation at break, work of rupture and the stress–strain curves. As a key finding, it is found that the PP fibres break at different draw ratios when the state of polarization is changed from 0° to 90°.

Formation of the helium extreme-UV resonance lines

Context: While classical models successfully reproduce intensities of many transition region lines, they predict helium EUV line intensities roughly an order of magnitude lower than the observed value. Aims: To determine the relevant formation mechanism(s) of the helium EUV resonance lines, capable of explaining the high intensities under quiet sun conditions. Methods: We synthesise and study the emergent spectra from a 3D radiation-magnetohydrodynamics simulation model. The effects of coronal illumination and non-equilibrium ionisation of hydrogen and helium are included self-consistently in the numerical simulation. Results: Radiative transfer calculations result in helium EUV line intensities that are an order of magnitude larger than the intensities calculated under the classical assumptions. The enhanced intensity of He I 584 is primarily caused by He II recombination cascades. The enhanced intensity of He II 304 and He II 256 is caused primarily by non-equilibrium helium ionisation. Conclusion: The analysis shows that the long standing problem of the high helium EUV line intensities disappears when taking into account optically thick radiative transfer and non-equilibrium ionisation effects.

Temporal and spatial profiles of emission intensities in atmospheric pressure helium plasma jet driven by microsecond pulse: Experiment and simulation

A needle-circular electrode structure helium plasma jet driven by microsecond pulsed power is studied. Spatially resolved emission results show that the emission intensity of He(33S1) line decreases monotonically along the axial direction, while those of N2(C3Πu), N2+(B2∑+u), and O(3p5P) reach their maxima at 3 cm, 2.6 cm, and 1.4 cm, respectively. The plasma plume of the four species shows different characteristics: The N2 emission plume travels at a fast speed along the entire plasma jet; the N2+ emission plume is composed of a bright head and relatively weak tail and travels a shorter distance than the N2 emission plume; the He emission plume travels at a slower speed for only a very short distance; propagation of the O emission plume is not observed. Results of calculation of radiation fluxes emitted by positive streamers propagating along helium plasma jets are presented. It is shown, in agreement with the results of the present experiment and with other available experimental data, that the intensities of radiation of N2(C3Πu) molecules and He(33S1) atoms vary with time (along the plasma jet) quite differently. The factors resulting in this difference are discussed.

The measurement of electron number density in helium micro hollow gas discharge using asymmetric He I lines

The electron number density Ne in helium micro hollow gas discharge (MHGD) is measured by means of optical emission spectroscopy (OES) techniques. The structure of MHGD is a gold–alumina–gold sandwich with 250 μm alumina thickness and 100 μm diameter hole. The electron temperature Te and gas temperature Tg in the discharge is determined using the relative intensity of He I lines and R branch lines in the frame of BP technique, respectively. The simple procedure based on spectral line broadening theory was developed in MATLAB to generate synthetic neutral line asymmetric profiles. The synthetic profiles were compared with an experimental He I 447.1 nm and He I 492.2 nm line to obtain Ne from the centre of a micro hollow gas discharge (MHGD) source in helium. The Ne results were compared with Ne values obtained from the forbidden-to-allowed (F/A) intensity ratio technique. The comparison confirmed higher Ne determined using a F/A ratio due to large uncertainty of the method. Applying the fitting formula for a He I 492.2 nm line derived from computer simulation (CS) gives the same Ne values as the one determined using the MATLAB procedure in this study. The dependence of Ne on gas pressure and electric current is investigated as well.

Spectroscopic investigation of the spatiotemporal dynamics of an electric field in plasma of a beam-type high-voltage discharge in helium

In the present work, a method of determining the spatiotemporal characteristics of the electric field strength in an accelerating gap and a plasma flare of a beam-type high-voltage pulsed discharge in He at moderate pressure is presented. The method is based on spectroscopic data on the Stark splitting of π-components of He 2P–4Q transitions; Q = D(4921.93 Å), F(4920.35 Å) in the near-cathode region of the discharge and on the spatiotemporal characteristics of intensities of He(2s1S–3p1P0) spectral lines with λ = 5015 Å and He+(3d–4f) spectral lines with λ = 4685 Å measured experimentally and calculated from the data of statistical simulation of the electron kinetics by the Monte Carlo method. The shape and strength of the electric field of the space charge in the interelectrode gap and the drift region of the discharge are estimated. It is demonstrated that the electric field created by the current pulse of uncompensated space charge in the near-anode region changes the discharge regime and causes degradation of the electron beam in the plasma flare. It is established that the flux of secondary electrons from the drift region to the anode has significant effect on the radiation intensity distribution in this region of the discharge.

Determination of Ag submonolayer structures on Pt(111) from simulation and data of He scattering experiments

We present a method that adapts the parameters describing the surface dynamics of an epitaxial system to a set of experimental intensities of He scattering at thermal energies. The method is applied to the problem of Ag growth on a Pt(111) surface at temperatures between 250 and 600 K.

2D Measurement of Edge Plasma Dynamics by Using High‐Speed Camera Based on HeI Line Intensity Ratio Method

AbstractIn order to investigate dynamic plasma behaviors in scrape‐off layer (SOL) regions, such as ELMs and plasma blobs, we have developed a 4‐channel high‐speed camera equipped with four interference filters to obtain 2D successive images of spectral intensity of He I emission lines at 667.8 nm, 706.5 nm and 728.1 nm in addition to HeII at 656.0 nm. By using calculated results based on a Collisional Radiative (CR) model for He, the intensity ratios of the He I lines (728.1/706.5 and 667.8/706.5), provides 2D distributions of the electron temperature and density. The 2D profiles of electron density and temperature reconstructed from 2D images of He I emission lines taken in a small tokamak, HYBTOK‐II were compared with those measured with the triple probe in order to evaluate the accuracy of this diagnostics. Electron density and temperature measured with the 4‐channel high‐speed camera are in good agreement with those obtained with the triple probe (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spatially resolved spectra of excited particles in homogeneous dielectric barrier discharge in helium at atmospheric pressure

In this paper, the optical emission spectra of He (3s(3)S-->2p(3)P, 706.5 nm), OH (A(2)Sigma-->X(2)Pi, 0-0), N(2) (C(3)Pi(u)-->B(3)Pi(g), 0-0, 337.1 nm), and N(2)(+) (B(2)Sigma(u)(+)-->X(2)Sigma(g)(+), 0-0, 391.4 nm) emitted from plate-plate electrodes configuration homogeneous dielectric barrier discharge in helium are recorded at atmospheric pressure. The spatially resolved measurements are performed and the asymmetry discharge mode of homogeneous dielectric barrier discharge is observed with electrical and optical measurements when the micro-impurity (N(2), H(2)O) exists. The effects of applied voltage and driving frequency on the spatial distributions of the emission intensity of He (3s(3)S-->2p(3)P) are also investigated. It is found that the spatial distributions of the emission intensities of He (3s(3)S-->2p(3)P), OH (A(2)Sigma-->X(2)Pi, 0-0), N(2) (C(3)Pi(u)-->B(3)Pi(g), 0-0), and N(2)(+) (B(2)Sigma(u)(+)-->X(2)Sigma(g)(+), 0-0) exhibit two peaks at 1 and 6 mm from the upper electrode respectively in the 7 mm gas gap and the emission intensities at 1 and 6 mm are about two to three times higher than those at 3-4 mm; when micro-impurity (N(2), H(2)O) exists, the asymmetry discharge mode of homogeneous dielectric barrier discharge can be observed, the emission intensities at 6 mm are about 1.4 times higher than those at 1 mm. Also, the main involved physicochemical processes are discussed.

THE EFFECT OF HOT CORONAL ELECTRONS ON EXTREME-ULTRAVIOLET SPECTRAL LINES OF He II EMITTED BY SOLAR TRANSITION REGION PLASMAS

The cause of the lower intensities of extreme-ultraviolet (EUV) He II lines emitted by coronal hole (CH) plasmas compared with quiet Sun (QS) plasmas has been the subject of many studies dating back over half a century. In this paper, we study the effect of small amounts of 'hot' electrons at coronal temperatures (T{sub e} = 1.4 x 10{sup 6} K) on the intensities of EUV He II lines, as well as on the intensities of EUV lines of C III, C IV, O III, and O IV emitted by 1.5 x 10{sup 4} K-1.5 x 10{sup 5} K (4.2 <= log T{sub e} <= 5.2) plasmas in the QS. We show that although the influence of a fraction as small as 10{sup -4}-10{sup -3} of hot electrons on the intensities of the C and O lines is noticeable, the effect on the intensities of the He lines is much larger, to the extent that it could explain the excess brightness of He II lines emitted by QS regions relative to CH plasmas.

Near infrared spectral and polarization imaging observation of coronal emission lines during the 2008 total solar eclipse

During the 2008 total solar eclipse, the coronal emission lines were observed by using optical fibre spectrometric and polarization imaging system in near infrared waveband. The profiles of the coronal emission lines including Fe XIII 10747 Å, 10798 Å and He I 10830 Å were obtained with dispersion of 0.5 Å /pix. The intensity of Fe XIII 10747 Å remained unchanged in the two different coronal regions while the intensity of He I 10830 Å varied considerably in the two coronal locations no matter whether the prominence appeared or not. The coronal polarization images were observed at Fe XI 7892 Å with a bandpass of 30 Å in a series of exposure times.

Large-scaled line plasma production by evanescent microwave

Long line-shaped microwave plasma has been generated, in helium and argon gas pressure, over a pressure range of 1–50 torr. To this end, waves of 2.45 GHz are fed into a narrow flat rectangular waveguide, through a tapered waveguide. The power of microwave is coupled to the discharge gas through a linear-slot antenna. The discharge is realized in a rectangular Pyrex tube, placed-in along the slot groove position. In this configuration the microwave is cut off, but the evanescent wave with a long wavelength is propagated and employed to produce the plasma. Measurements of the field distribution in the waveguide revealed that the wavelength of the excited field increases as waveguide width decreased to a width close to the cut-off width of 61.3 mm. The relative intensities of He I lines show very high uniformity along the whole line plasma length, and helium excitation temperatures were in the range of 3000–5000 K.

ELECTRON TEMPERATURES OF PLANETARY NEBULAE DETERMINED FROM THE He I DISCONTINUITIES

We have used the He I discontinuities at 3421A to determine the electron temperatures, designated Te(He I), for a sample of five Galactic planetary nebulae (PNe). We compared Te(He I) with the electron temperatures derived from the hydrogen Balmer jump at 3646A, designated Te(H I), and found that Te(He I) are generally lower than Te(H I). There are two possible interpretations, a) the presence of substantial He+2 zone, or b) the presence of hydrogen-deficient cold clumps within diffuse nebulae. A series of photoionization models were constructed to test the two scenarios. We found that the observed Te(He I)/Te(H I) discrepancies are beyond the predictions of chemically homogeneous models. Our modelling shows that the presence of a small amount of hydrogen-deficient inclusions seems to be able to reproduce the observed intensities of He I discontinuities. We stress the value of He I discontinuities in investigating nebular physical conditions. Albeit with some observational and technical limitations, He I discontinuities should be considered in future modelling work.

Photoluminescence Degradation and Passivation Mechanisms of Si Nanoclusters in Silicon Oxide Matrix

The Si nanoclusters embedded in a silicon oxide matrix were prepared using a laser-assisted chemical vapor deposition (LACVD) system. A degradation of photoluminescence (PL) by irradiating the sample with helium–cadmium (He–Cd) laser was observed. The dependence of PL degradation on long-term irradiation of He–Cd laser was investigated. We found that the He–Cd laser-induced breakage of Si–H-related bonds resulted in the formation of Si dangling bonds such as D centers and Pb centers, which are known to decrease PL intensity. The PL intensity of He–Cd laser-irradiated samples can be increased to that of as-deposited samples after exposing the samples to a H2 ambient at 400°C for 5 min. Post annealing in H2 could also help increase PL intensity by passivating the defect centers in as-deposited samples.

Non‐LTE Radiative Transfer in Model Prominences. I. Integrated Intensities of HeiTriplet Lines

In this work we use new results of radiative transfer calculations out of local thermodynamical equilibrium to study the triplet lines emitted by neutral helium in solar quiescent prominences. We compare two types of prominence atmospheres: isothermal and isobaric models versus nonisothermal and nonisobaric ones. We can thus investigate the effect of the presence of a prominence-to-corona transition region (PCTR) on the emergent intensities in detail. It is found that the presence of the PCTR affects the emitted intensities of the triplet lines, even though they are formed in the central parts of the prominence. We show that the inclusion of a transition region reduces the impact of collisional excitation at high temperatures in comparison with the isothermal and isobaric case. A simple study of helium energy level populations shows how statistical equilibrium is changed when a transition region is present. This points to the necessity of including an interface between the prominence body and the corona to predict all emergent intensities, whatever the region of formation of the radiation. We have found a correlation between most of the He I triplet line ratios and the altitude of the model prominence. Comparisons of our predicted intensity ratios with observations yield generally good agreement. Remaining discrepancies may be resolved by extrapolating our predicted results to higher altitudes.

The White-Light Limb Flare of 16 August 1989 and its Chromospheric Counterpart

After carefully comparing the white-light (5600+/-800 Angstrom) and the slit-jaw Halpha images (0.5 Angstrom passband) of the 2N/X20 white-light flare of 16 August 1989, we found that the Halpha counterpart identification of the bright kernels in continuum by Hiei, Nakagomi, and Takuma (1992) was incorrect. Now we come to the conclusion that none of the two white-light kernels has a corresponding bright Halpha area. Moreover, the loop shapes in white-light are also different from those in Halpha. Halpha loops rose more rapidly than white-light loops. However, their height-time variations on the whole are similar. This indicates that the continuum and chromospheric emissions of the flare presumably come from different plasmas, but may be modulated by some mutual factors, such as large-scale magnetic fields. Analysis of the He I 10 830 Angstrom spectra taken simultaneously with the slit-jaw Halpha images shows that the line-center intensity of He I 10 830 Angstrom doesn't have a good correlation with the intensity of nearby continuum, which supports the above conclusions. In addition, the electron density at the white-light loop top estimated from the continuum around 5600 Angstrom and 10 830 Angstrom is as high as 10(12)-10(13) cm(-3).

A structural analysis of Bi/Si(1 0 0) 2 × n surfaces by ICISS

We have investigated the structure of Bi-adsorbed Si(100) 2×n surfaces with various Bi coverages by low-energy electron diffraction (LEED) and impact-collision ion scattering spectroscopy (ICISS). Intensities of He+ scattered from Bi and Si atoms were measured as a function of ion incidence angle. Bi–Bi interatomic distances for the toplayer including Bi-dimers were estimated by the shadow-cone technique. Bi-signal peaks appeared even at considerably large angles of incidence, which are attributed to collisions with Bi atoms in sub-surface layers. Bi adsorption is not completed at a mono-layer level, but continues to form multiple layers, fitted in the Si lattice, in the Frank-van der Merwe growth mode.