Depth Of He Research Articles

Helium diffusion and bubble evolution in single-phase tungsten-based W-Ta-Cr-V complex concentrated alloy

Tungsten-based complex concentrated alloy (CCA) W-Ta-Cr-V films with bcc single-phase have been investigated for its exceptional radiation tolerance. This work aims to investigate the formation of helium (He) bubble and He retention behavior in the CCA film under 100 eV He plasma at a fluence of 2.02 × 1025 He/m2. The results show that the size of He bubbles in the CCA film is smaller than that in the pure W film, and the depth distribution is shallower, which indicates that the evolution of He bubbles is inhibited. As for the underlying mechanism of resistance to the formation of He bubbles in CCAs, it is believed that the characteristic of low He mobility reduces the diffusion depth of He, and the sluggish diffusion of He inhibits the aggregation of He atoms or He clusters to a certain extent. In other words, the unique energy and microstructure characteristics also inhibit the diffusion of He in the CCA film, which affects the nucleation and growth of He bubbles. Furthermore, the release of He is mainly from He adsorption of the near surface, and the desorption peaks shift to lower temperatures in the CCA film within the desorption temperature of 1273 K.

Velocities of an Erupting Filament

Abstract Solar filaments exist as stable structures for extended periods of time before many of them form the core of a coronal mass ejection (CME). We examine the properties of an erupting filament on 2017 May 29–30 with high-resolution He i 10830 Å and Hα spectra from the Dunn Solar Telescope, full-disk Dopplergrams of He i 10830 Å from the Chromospheric Telescope, and EUV and coronograph data from SDO and STEREO. Pre-eruption line-of-sight velocities from an inversion of He i with the HAZEL code exhibit coherent patches of 5 Mm extent that indicate counter-streaming and/or buoyant behavior. During the eruption, individual, aligned threads appear in the He i velocity maps. The distribution of velocities evolves from Gaussian to strongly asymmetric. The maximal optical depth of He i 10830 Å decreased from τ = 1.75 to 0.25, the temperature increased by 13 kK, and the average speed and width of the filament increased from 0 to 25 km s−1 and 10 to 20 Mm, respectively. All data sources agree that the filament rose with an exponential acceleration reaching 7.4 m s−2 that increased to a final velocity of 430 km s−1 at 22:24 UT; a CME was associated with this filament eruption. The properties during the eruption favor a kink/torus instability, which requires the existence of a flux rope. We conclude that full-disk chromospheric Dopplergrams can be used to trace the initial phase of on-disk filament eruptions in real time, which might potentially be useful for modeling the source of any subsequent CMEs.

Helium retention depth profile in plasma-facing materials measured by glow-discharge optical emission spectroscopy

In order to quantitatively study the retention behaviors of helium (He) in the fusion relevant materials by Glow Discharge - Optical Emission Spectroscopy (GD-OES), tungsten (W) and copper (Cu) samples were implanted by He ions with different energies and fluences, and the He distributions in these samples were investigated by transmission electron microscopy (TEM) and simulated by the Stopping and Range of Ions in Matter (SRIM). After that these He implanted samples were used as calibration samples, and the electrical signal of GD-OES measurement was converted to He concentration depth profile. The results show that TEM investigations are consistent with SRIM simulation at the maximum concentration distribution depth of He. The experiment also found that He diffusion in the material is obvious, beyond the prediction range of SRIM simulation. After calibration, the He depth profiles in multi-energy implantation W, Cu and EUROFER97 samples measured by GD-OES agree with the SRIM simulation. GD-OES and thermal desorption spectroscopy (TDS) independent experiments proved that the total retained He amounts in materials are close to the total implanted He amounts, indicating that using He-implanted standard samples combined with SRIM simulation to obtain GD-OES depth profile is reliable.

The Evolution of the He ii-ionizing Background at Redshifts 2.3 < z < 3.8 Inferred from a Statistical Sample of 24 HST/COS He ii Lyα Absorption Spectra*

Abstract We present measurements of the large-scale (≈40 comoving Mpc) effective optical depth of He ii Lyα absorption, , at 2.54 < z < 3.86 toward 16 He ii-transparent quasars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope, to characterize the ionization state of helium in the intergalactic medium (IGM). We provide the first statistical sample of measurements in six signal-to-noise ratio ≳3 He ii sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of in 24 He ii sightlines. We confirm an increase of the median from ≃2 at z = 2.7 to at z > 3, and a scatter in that increases with redshift. The z > 3.5 He ii absorption is predominantly saturated, but isolated narrow (Δv < 650 km s−1) transmission spikes indicate patches of reionized helium. We compare our measurements to predictions for a range of UV background models applied to outputs of a large-volume (146 comoving Mpc)3 hydrodynamical simulation by forward-modeling our sample’s quality and size. At z > 2.74, the variance in significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median He ii photoionization rate of a fluctuating UV background from the median , finding a factor ≃5 decrease in between z ≃ 2.6 and z ≃ 3.1. At z ≃ 3.1, s−1 corresponds to a median He ii fraction of ≃2.5%, indicating that our data probe the tail end of He ii reionization.

Correlation between density and hydrogen content in vertically aligned carbon nanotube forests by ion beam analysis

Interactions of vertically aligned multiwall carbon nanotubes (CNTs) with high energy He+ beams were studied using elastic recoil detection analysis and ion beam channeling. The relationship between the elastic recoil of hydrogen, the depth of He–H interactions, and the number of carbon atoms per volume (denoted as effective density) was calculated. Ion channeling was observed in CNT forests shorter than 40 μm. It was found that the effective density and hydrogen content were inversely correlated with the CNT height. In compliance with channeling and density calculations, the authors propose that this effect is due to the weakening of Van-der-Waals forces in taller CNT forests. The methodology suggested in this work may be extended to assessing densities of thin, highly porous materials.

EARLY AND EXTENDED HELIUM REIONIZATION OVER MORE THAN 600 MILLION YEARS OF COSMIC TIME*

ABSTRACT We measure the effective optical depth of He ii Lyα absorption at in 17 UV-transmitting quasars observed with UV spectrographs on the Hubble Space Telescope. The median values increase gradually from 1.95 at to at , but with a strong sightline-to-sightline variance. Many comoving Mpc regions of the intergalactic medium (IGM) remain transmissive ( ), and the gradual trend with redshift appears consistent with density evolution of a fully reionized IGM. These modest optical depths imply average He ii fractions of and He ii ionizing photon mean free paths of comoving Mpc at , thus requiring that a substantial volume of the helium in the universe was already doubly ionized at early times; this stands in conflict with current models of He ii reionization driven by luminous quasars. Along 10 sightlines we measure the coeval H i Lyα effective optical depths, allowing us to study the density dependence of at . We establish that the dependence of on increasing is significantly shallower than expected from simple models of an IGM reionized in He ii. This requires higher He ii photoionization rates in overdense regions or underdense regions being not in photoionization equilibrium. Moreover, there are very large fluctuations in at all which greatly exceed the expectations from these simple models. These data present a distinct challenge to scenarios of He ii reionization—an IGM where He ii appears to be predominantly ionized at , and with a radiation field strength that may be correlated with the density field, but exhibits large fluctuations at all densities.

Influence of noble-gas ion irradiation on alumina barrier of magnetic tunnel junctions

The transport properties of Co∕Al2O3∕Co magnetic tunnel junctions with ion-irradiated tunneling barrier are reported. The irradiation by He+ and Ar+ with energies ranging from 15to105eV takes place in situ after oxidation of the 1.4-nm-thick Al layer. For both ion species the area resistance of the junctions increases strongly with ion energy, simultaneously the tunneling magnetoresistance is reduced. But the energy dependence of both properties is different for He+ and Ar+ irradiations. Additionally the bias voltage dependence of the tunneling magnetoresistance is deteriorated with increasing ion energy especially for Ar+ irradiation. These experimental results are discussed with respect to the energy-dependent penetration depth of He+ and Ar+ and their energy loss in the barrier.

The He I 10830 Å Line as an Indicator of Solar Activity

The behavior of the He I 10830 A infrared triplet parameters in active and quiet solar regions was traced from 1976 until 2000. We analyze the correlation between the central depth of the main He I line component and other solar activity indices: the Wolf number, the radiation flux at a frequency of 2800 MHz, the mean number of flares in sunspot groups, and the mean solar magnetic field. We show that the strong correlation between the He I 10830 A line depth and the phase of the 11-year solar cycle allows this depth to be effectively used as a new solar activity index both on long time scales (years) and on times scales of the order of a month or even days. The suggested new activity index is shown to have advantages over the universally accepted indices. The depth of the He I 10830 A line in quiet regions was found to increase from the phase of minimum solar activity to the phase of maximum by a factor of about 2. In active regions, this increase is less than 30%. The differences between the cyclic variations of the chromospheric He I 10830 A line radiation in active and quiet structures on the solar disk are indicative of the probable differences in the nature of cyclicity and its manifestations in magnetic fields of different spatial scales. The background magnetic fields appear to vary during the solar cycle more strongly than do the local fields associated with sunspots, faculae, and activity complexes. We suggest using regular observations in the He I 10830 A line to predict solar activity.

A calculation of the well depth for a pair of helium atoms

A new methods is proposed for the elimination of the basis superposition error in the calculation of small potentials. The method which involves CI is applied to the calculation of the well depth of He 2. It includes interatomic and intraatomic correlation effects simultaneously. A value for the well depth of 10.54 K at R = 3.03 au is obtained, in excellent agreement with current theoretical estimates.

The Ionisation Equilibrium for Heavy Elements

Calculations are being made of the distribution of the ions of heavy elements in planetary nebulae. Initial work has been concentrated on the central or He2+ zone of planetaries. The optical depths of ions of C, N, O, and Ne have been computed using ground state ionisation cross-sections and using approximations which should be substantially better than hydrogenic. A comparison has been made between the combined optical depth of the heavy elements and the optical depth of He+ in the far ultraviolet. The optical depths of the heavy elements in this spectral region may become significant, but a reasonable first approximation to the radiation field may be obtained by neglecting the absorption of all ions except He+. The distribution of the ions of the heavy elements has been calculated on this assumption.