Electron-impact collision strengths for inner-shell excitation of doubly excited levels from singly excited levels in He-like ions

A simple Coulomb-Born-Exchange method has been used to calculate electron-impact colision strengths for inner-shell excitation of the doubly excited levels with n = 2 in He-like ions from the singly excited levels. The results are of particular interest for plasma diagnostic purposes. The calculations were made for nine impact electron energies in threshold units ? = 1.0, 1.2, 1.5, 1.9, 2.5, 4.0, 6.0, 10.0 and 15.0 for 19 He-like ions with nuclear charge number Z in the range 6 ≤ Z ≤ 74. Fits are given of the scaled collision strength $\text{Z}{}^2\text{Ω}$ to simple functions of ? that are readily integrated over a Maxwellian distribution to obtain collision rates. Configuration-interaction and intermediate-coupling effects have been included in the calculations. Calculated values for transition energies and radiative line strengths are also given.     

Relativistic energy contributions to the N = 2 triplet levels in helium-like ions

The 1s2s³S?1s2p³P_J (J = 0, 1, 2) transitions in helium-like ions can be used to test QED, but only if the other contributions to the transition energy, and especially the relativistic contributions are known to high accuracy.This work tests the relativistic calculations by comparing the theoretical ${}^3\text{P}{}_{\mathrm{J}}$ splittings (which are nearly ndependent of QED) with each other and with experiment. An estimate of the largest uncalculated relativistic term, of relative order α⁴Z⁴, is also made.It is found that the calculations of Hata and Grant are not very accurate, and that the small discrepancy between Drake and experiment vanishes if the α⁴Z⁴ relativistic term is included.     

Calculation of the behaviour of neutral hydrogen in the wall region of high temperature plasmas including wall interaction

In the wall regime of high temperature discharges (e.g. tokamak experiments) neutral hydrogen plays a dominating role. The neutrals influence the local particle density and energy losses, and contribute to wall interactions by desorption of adsorbed molecular and atomic species, sputtering and back-scattering. This investigation of the behaviour of the neutral component in the wall layer starts from the one-dimensional time dependent Boltzmann-equation for neutral hydrogen and assumes the data of the background plasma to be known. Charge exchange and ionisation processes are taken into account as well as interaction with the wall and an influx of atoms. Depending on density and temperature profiles of the background plasmas, the distribution function of the neutrals and their fluxes on to the wall are calculated for steady conditions. By solving the time dependent Boltzmann equation the relaxation time for the influx of cold neutral atoms into a tokamak plasma is also derived.     

Multi-layer structure formation of relativistic electron beams in plasmas

A two-dimensional electromagnetic particle-in-cell simulation model is proposed to study the density evolution and collective stopping of electron beams in background plasmas. We show here the formation of the multi-layer structure of the relativistic electron beam in the plasma due to the different betatron frequency from the beam front to the beam tail. Meanwhile, the nonuniformity of the longitudinal wakefield is the essential reason for the multi-layer structure formation in beam phase space. The influences of beam parameters (beam radius and transverse density profile) on the formation of the multi-layer structure and collective stopping in background plasmas are also considered.     

Scintillation light produced by low-energy beams of highly-charged ions

Measurements have been performed of scintillation light intensities emitted from various inorganic scintillators irradiated with low-energy beams of highly-charged ions from an electron beam ion source (EBIS) and an electron cyclotron resonance ion source (ECRIS). Beams of xenon ions Xe^q+ with various charge states between q = 2 and q = 18 have been used at energies between 5 and 17.5 keV per charge generated by the ECRIS. The intensity of the beam was typically varied between 1 and 100 nA. Beams of highly charged residual gas ions have been produced by the EBIS at 4.5 keV per charge and with low intensities down to 100 pA. The scintillator materials used are flat screens of P46 YAG and P43 phosphor. In all cases, scintillation light emitted from the screen surface was detected by a CCD camera. The scintillation light intensity has been found to depend linearly on the kinetic ion energy per time deposited into the scintillator, while up to q = 18 no significant contribution from the ions’ potential energy was found. We discuss the results on the background of a possible use as beam diagnostics, e.g. for the new HITRAP facility at GSI, Germany.     

Intermediate-coupling collision strengths for transitions between doubly excited levels produced by electron impact on highly charged He-like ions

Coulomb-Born-Exchange data for intermediate-coupling scaled collision strengths $\text{Z}{}^2\text{Ω}$ for fine-structure transitions of type 2l_a2l_b^2S_a+1L_aja?2l_a2l′_b^2S′_a+1L′_aj′a or 2l_a2l_b(j_aj_b)_Ja?2l_a2l′_b(j′_aj′_b)_J′a between the doubly excited levels in He-like ions with 6 ≤ Z ≤ 74 are fitted by simple functions of the impact-electron energy ? in threshold units. To obtain collision rates these functions can be readily integrated over a Maxwellian energy-distribution function for the electrons. Fits to the data for transitions between energy terms are also included. In addition results are given for the transition energies and electric-dipole radiative line strengths. The basic collision-strength data cover impact-electron energies $\text{≤}\text{3.25Z}{}^2\text{Ry}$ or 44.2Z² eV.     

Hydrogenation of zirconium film by implantation of hydrogen ions

In order to understand the dnve-in target in a D-D type neutron generator,it is essential to study the mechanism of the interaction between hydrogen ion beams and the hydrogenabsorbing metal film.The present research concerns the nucleation of hydride within zirconium film implanted with hydrogen ions.Doses of 30 keV hydrogen ions ranging from 4.30×10~(17) to1.43×10~(18) ions cm~(-2) were loaded into the zirconium film through the ion beam implantation technique.Features of the surface morphology and transformation of phase structures were investigated with scanning electron microscopy,atomic force microscopy and x-ray diffraction.Confirmation of the formation of 5 phase zirconium hydride in the implanted samples was first made by x-ray diffraction,and the different stages in the gradual nucleation and growth of zirconium hydride were then observed by atomic force microscope and scanning electron microscopy.     

Sputtering of silicon carbide coatings by low-energy hydrogen ions

Various types of silicon carbide coatings made by reactive ion-plating have been bombarded with a 3.0 keV H⁺₃ ion beam at temperatures around 500°C. The sputtering yield in stoichiometric samples (i.e. Si : C = 1 : 1) at 500°C was $\text{1.15 × 10}{}^{\mathrm{?2}}$ atoms/H⁺. As the stoichiometry deviates from this point, the sputtering yield has larger values. The temperature dependence of the sputtering yield in stoichiometric samples was negligible below 600°C. No surface topography changes occurred in stoichiometric samples even at a high fluence of $\text{2 × 10}{}^{20}\text{H}{}^{\mathrm{+}}\text{/cm}{}^2$, while severe erosion took place in non-stoichiometric samples. By Auger electron spectroscopy (AES), carbon exists on the surface in the form of carbide in stoichiometric SiC before and after bombardment, while it exists in the form of graphite in carbon rich samples, which suggests that the bound state of carbon in the form of carbide should correspond to the low sputtering yield in stoichiometric SiC coatings. The surface stoichiometry changes due to hydrogen bombardment were observed by AES, where the carbon population increases in stoichiometric SiC, while it decreases in carbon rich samples, which was supported as well by the results from electron probe X-ray microanalysis.     

Numerical simulations of the wavefront distortion of inter-spacecraft laser beams caused by solar wind and magnetospheric plasmas

Plasma turbulence may lead to additional wavefront distortion of inter-spacecraft laser beams during the operation of spaceborne gravitational wave (GW) observatories, e.g. TianQin. By making use of the Space Weather Modelling Framework (SWMF) model and realistic orbit data for the TianQin constellation, the characteristic parameters of the plasma turbulence present at the TianQin orbit are obtained. As a first step, this work is based on the assumptions that the cold plasma approximation is valid and that the effects of the electromagnetic field induced by charge separation within the Debye length on the laser’s wavefront can be ignored. An atmospheric turbulence–laser interaction model is then applied to analyze the effects of the plasma turbulence on the inter-spacecraft laser’s wavefront. The preliminary results show that the wavefront distortion caused by the plasma turbulence is 10^–9 rad, which is significantly less than the designated error budget, i.e. 10^–6 rad, and thus will not affect the laser interferometry.     

Energy distributions of neutral atoms sputtered by very low energy heavy ions

Polycrystalline Cu was sputtered by normally incident, very low energy Ar⁺ ions (E₀ = 40–1000 eV). The kinetic energy (E) distributions of the neutral Cu atoms sputtered normally from the Cu surface were measured, using secondary neutral mass spectrometry. For values of E₀ above approximately 600 eV, the observed energy distributions agreed closely with the Thompson-Sigmund theory. For values of E₀ less than about 600 eV the distributions fell off faster than predicted by the Thompson-Sigmund theory, and the peak value of the distribution shifted to somewhat lower energies. Both these effects were exaggerated as E₀ was further lowered. The average kinetic energy of the sputtered neutral Cu atoms increased with increasing E₀. The rate of this increase was less at higher values of E₀.     

An improved method for the study of neutral species produced in processing plasmas

Neutral, electronegative particles, including radicals, produced in a processing plasma may be studied by using a mass spectrometer to examine the negative ions formed from them in the source of the mass spectrometer. The technique has advantages over the more usual technique of examining positive ion formation from sampled neutral species. Experimental data for a carbon dioxide plasma are shown to illustrate the method.     

Transition probabilities and branching ratios for electric-dipole transitions between arbitrary levels of hydrogen-like atoms

Transition probabilities and branching ratios in hydrogen-like atoms due to electric-dipole transitions are tabulated for the initial principal and angular-momentum quantum numbers n′l′, and final principal and angular-momentum quantum numbers nl. The electric-dipole matrices are evaluated in parabolic coordinates, and by a transformation their values in spherical coordinates are found. In Table I transition probabilities for transitions n′l′ → n and n′ → n given.Branching ratios are tabulated in Table II for transitions n′l′ → n and n′ → n, while in Table III these ratios are tabulated for transitions n′l′ → nl and n′ → nl.The lifetime of an excited state nl can be expressed as (μZ⁴)^?1c_nln³l², where μ and Z are the reduced mass and nuclear charge of the hydrogen-like atom, and c_l is a slowly varying coefficient of n and l, approaching a constant as n,l → ∞. Values of c_nl are given in Table IV.     

Temporal and spatial study of differently charged ions emitted by ns-laser-produced tungsten plasmas using time-of-flight mass spectroscopy

Tungsten (W) is an important material in tokamak walls and divertors.The W ion charge state distribution and the dynamic behavior of ions play important roles in the investigation of plasma-wall interactions using laser-ablation-based diagnostics such as laser-induced breakdown spectroscopy and laser-induced ablation spectroscopy.In this work,we investigate the temporal and spatial evolutions of differently charged ions in a nanosecond-laser-produced W plasma in vacuum using time-of-flight mass spectroscopy.Ions with different charge states from 1 to 7 (W+ to W7+) are all observed.The temporal evolutions of the differently charged ions show that ions with higher charge states have higher velocities,indicating that space separation occurs between the differently charged ion groups.Spatially-resolved mass spectroscopy measurements further demonstrate the separation phenomenon.The temporal profile can be accurately fitted by a shifted Maxwell-Boltzmann distribution,and the velocities of the differently charged ions are also obtained from the fittings.It is found that the ion velocities increase continuously from the measured position of 0.75 cm to 2.25 cm away from the target surface,which indicates that the acceleration process lasts through the period of plasma expansion.The acceleration and space separation of the differently charged ions confirm that there is a dynamic plasma sheath in the laser-produced plasma,which provides essential information for the theoretical laser-ablation model with plasma formation and expansion.     

Trapping of energetic hydrogen ions in reactive metals

A model is proposed to describe the trapping of a beam of energetic ions of hydrogen isotopes by targets of reactive metals in which the incident ions dissolve exothermically. The model considers the ions free to diffuse inside the metal, and to escape from the surface at a rate determined from the known solubility data.It has been shown that in the case of semi-infinite geometry the trapping efficiency can be expressed in analytic form as a function of dose. However, in the two dimensional case appropriate for beam experiments, radial diffusion becomes important and a numerical solution has been found to be necessary. The model has been compared with experimental results on the trapping of 18 keV deuterons by targets of Nb, Zr, Ti and Pd in the temperature range 230–1000 K and in general good agreement has been obtained.     

Extreme ultraviolet spectral characteristic analysis of highly charged ions in laserproduced Cu plasmas

This paper reports the results of spectral measurements and a theoretical analysis of the temporal and spatial evolution of laser-produced Cu plasma in vacuum in the range of 8–14 nm. The time dependence of the extreme ultraviolet band spectrum at different positions near the target surface was obtained and found to be dominated by three broad-band features. The 3p and 3d excitations of Cu⁵⁺–Cu⁹⁺ ions were calculated using the Hartree–Fock theory with configuration interact...