Charge exchange of medium energy H and He ions emerging from solid surfaces

Charge exchange of medium energy H and He ions emerging from clean solid surfaces is studied extensively using a toroidal electrostatic analyzer with an excellent energy resolution. The charge distributions of He ions scattered from sub-monolayers near a surface are non-equilibrated, resulting in a surface peak even for poly-crystal solids. By solving simultaneous rate equations numerically, we derive electron capture and loss cross sections for Ni and Au surfaces. Based on a free electron gas model, non-equilibrated He⁺ fractions dependent on emerging angle reveals uniform electronic surfaces for metals and corrugated surfaces for Si and graphite with covalent bonds. It is also found that equilibrium charge fractions of H⁺ are independent of surface materials (Z₂) and in contrast equilibrium He⁺ fractions depend pronouncedly on Z₂. The data obtained are compared with semi-empirical formulas.     

Attainment of equilibrium charge distributions in fast ion beams passing through solid films

The thicknesses T_s of carbon targets needed for equilibration of the charge distribution in H, He and N ions beams have been calculated in the energy range E = (0.03–100) MeV/amu. The calculations have shown that for N ions at E = 0.1 MeV/amu the value of T_s is 20 times T_g in a gas target and that the difference decreases with increasing energy.     

Electron capture in Iq+ (q = 10–41) + He collisions at low energies

One electron capture processes in I^q+ (q = 10–41) +He collisions at low energies have been investigated. It is found that total cross sections for one-electron capture processes increase roughly linearly with increasing charge q of the incident ions and also increase with the square of the crossing radius R_c of the diabatic potential energy curves where the electron transfer takes place. These smooth increases are in contrast to those observed for ions with low charge q < 10.     

Single electron capture by state-selected multiply charged Arq+ ions (q = 3,4)

State-selective single electron capture by multiply charged Ar^q+ ions (q = 3, 4) has been studied in rare gas targets with the aid of double translational spectroscopy. The energy gain spectra have been measured using different projectile beams, either extracted from an electron beam ion source or produced by electron capture reactions in an additional preparation cell. The influence of metastable projectile states as well as the reaction channel identification in complex systems are discussed.     

Monte Carlo study of ion-induced backward and forward secondary electron emission from thin Al foil

A direct Monte Carlo program has been developed to calculate the backward (γ_b) and forward (γ_f) electron emission yields from 20 nm thick Al foil for impact of C⁺, Al⁺, Ar⁺, Cu⁺ and Kr⁺ ions having energies in the range of 0.1-10 keV/amu. The program incorporates the excitation of target electrons by projectile ions, recoiling target atoms and fast primary electrons. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. The calculated backward electron emission yield and the Meckbach factor R = γ_f/γ_b are compared with the available experimental data, and a good agreement is found. In addition, the effect of projectile energy and mass on the longitudinal and lateral distribution of the excitation points of the electrons emitted from front and back of Al target has been investigated.     

Fast ion collisions with C60 in vapour phase and collective excitation: Comparison with other gaseous targets

The single and double ionization of a free C₆₀ molecule in collisions with fast heavy (F and Si) ions is investigated using a recoil ion time-of-flight mass spectrometer. The projectile charge state (q_p) dependence has also been investigated. A linear q_p-dependence has been explained in terms of a plasmon excitation model. In addition, continuum electron spectroscopy has been used to detect the electron emission from fullerenes. The measured electron angular distribution for the fullerene target is compared with that for a gaseous target at a fixed electron energy. The ratio of forward-to-backward cross section for C₆₀ is quite different from that for Ne.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.     

The ion-induced electron yield as a function of the target material

The total (backward) electron yield from 27 elemental, noncrystalline solids under bombardment by H⁺, H₂⁺ and H₃⁺ ions at 100 keV/proton was measured. It is observed that the yield exhibits an oscillatory dependence on the atomic number Z₂ of the target material which is correlated with the periods of the periodic system.     

TEM investigation of irradiation damage in single crystal CeO2

In order to understand the evolution of radiation damage in oxide nuclear fuel, 150-1000 keV Kr ions were implanted into single crystal CeO₂, as a simulation of fluorite ceramic UO₂, while in situ transmission electron microscopy (TEM) observations were carried out. Two characteristic defect structures were investigated: dislocation/dislocation loops and nano-size gas bubbles.The growth behavior of defect clusters induced by 1 MeV Kr ions up to doses of 5 × 10¹⁵ ions/cm² were followed at 600 °C and 800 °C. TEM micrographs clearly show the development of defect structures: nucleation of dislocation loops, transformation to extended dislocation lines, and the formation of tangled dislocation networks. The difference in dislocation growth rates at 600 °C and 800 °C revealed the important role which Ce-vacancies play in the loop formation process. Bubble formation, studied through 150 keV Kr implantations at room temperature and 600 °C, might be influenced by either the mobility of metal-vacancies correlated with at threshold temperature or the limitation of gas solubility as a function of temperature.     

Formation of complex Al–N–C layer in aluminium by successive carbon and nitrogen implantation

The results of Auger electron spectroscopy and transmission electron microscopy of the surface layer of aluminium after successive implantation by carbon and nitrogen ions are presented in this work. The energy of implanted ions is 40 keV. The implantation dose varies in the range (3.3–6.5) × 10¹⁷ ions/cm². The findings show that successive implantation leads to the formation of two main layers in aluminium. The first layer is AlNC_x (0 < x < 0.5) layer with violated hcp. AlN structure, where carbon atoms form bonds with nitrogen atoms. The second layer contains disoriented Al₄C₃ precipitates and carbon atoms migrated from the first layer. The mechanism of migration is discussed.     

In situ studies of ion irradiated inverse spinel compound magnesium stannate (Mg2SnO4)

Magnesium stannate spinel (Mg₂SnO₄) was synthesized through conventional solid state processing and then irradiated with 1.0 MeV Kr²⁺ ions at low temperatures 50 and 150 K. Structural evolutions during irradiation were monitored and recorded through bright field images and selected-area electron diffraction patterns using in situ transmission electron microscopy. The amorphization of Mg₂SnO₄ was achieved at an ion dose of 5 × 10¹⁹ Kr ions/m² at 50 K and 10²⁰ Kr ions/m² at 150 K, which is equivalent to an atomic displacement damage of 5.5 and 11.0 dpa, respectively. The spinel crystal structure was thermally recovered at room temperature from the amorphous phase caused by irradiation at 50 K. The calculated electronic and nuclear stopping powers suggest that the radiation damage caused by 1 MeV Kr²⁺ ions in Mg₂SnO₄ is mainly due to atomic displacement induced defect accumulation. The radiation tolerance of Mg₂SnO₄ was finally compared with normal spinel MgAl₂O₄.     

Triple-probe Diagnostic Measurements in Plasma of GLAST Spherical Tokamak

Triple-probe has been developed and operated successfully to characterize ECRH-assisted argon as well as hydrogen microwave plasmas in GLAST Spherical Tokamak. This technique enables to determine transient plasma parameters such as floating potential, electron temperature and electron number density in rapidly time-varying plasmas. An effective electron heating mechanism is applied to produce microwave plasma by injecting radiofrequency (RF) radiation at a frequency of 2.45 GHz in the presence of resonant toroidal magnetic field. Plasma parameters and corresponding fluctuations are measured as a function of time in different gas fill pressures for various applied magnetic fields. The results demonstrate the dependence of plasma parameters such as V _f , T _e , n _e and their fluctuations on gas fill pressure during the pre-ionization phase of the GLAST operation. Plasma behavior is observed to be closely depending on the coupling of RF power during microwave discharge. Additionally, the hydrogen plasma shows pronounced fluctuations in comparison with argon plasma with some decrease in electron temperature and densities.     

A novel (SiC)1−x(AlN)x compound synthesized using ion beams

The formation of a novel (SiC)_1−x(AlN)_x compound (x=0.2) at low temperatures within the miscibility gap of the SiC/AlN phase diagram by hot, high-dose co-implantation of N⁺ and Al⁺ ions into 6H–SiC substrates is investigated. The compound layers have been studied by Rutherford backscattering spectrometry/ion channelling (RBS/C), Auger electron spectroscopy (AES), polarized infrared reflection spectroscopy (PIRR) and cross sectional electron microscopy (XTEM) and the temperature dependence of their fabrication has been examined. An optimum temperature window has been established within which the structure of the synthesized material retains good crystallinity during implantation.     

(n,l)−Subshell electron capture cross sections in collisions of C4+, N5+ and O6+ with atomic hydrogen

By means of VUV-photon spectroscopy we have determined absolute Subshell selective electron capture cross sections σ_nlfor collisions of helium-like carbon, nitrogen and oxygen ions with atomic hydrogen, in the velocity range 0.1–0.5 a.u.The atomic hydrogen beam target was produced by means of a radio-frequency discharge source, and absolutely calibrated by measuring atomic and molecular radiation in the visible region produced by electron impacts.For all systems studied we find that the total capture cross section (σ_t = Σ_nlσ_nl depends only weakly on the impact velocity, and is in good agreement with published results from other authors. In contrast to σ_t, the Subshell selective cross sections σ_nl are strongly velocity dependent.Our results are compared with recent calculations by Fritsch and Lin; the agreement is quite satisfactory.     

Analytical Study of Quantum Magnetic and Ion Viscous Effects on p11B Fusion in Plasma Focus Devices

In this article we studied the feasibility of proton-boron (p¹¹B) fusion in plasmoids produced by plasma pinch devices like plasma focus facility as commercially sources of energy. In plasmoids fusion power for 76 keV < T_i < 1,500 keV exceeds bremsstrahlung loss (W/P_b = 5.39). In such situation gain factor and the ratio of Te to Ti for a typical 150 kJ plasma focus will be 7.8 and 4.8 respectively. Also with considering the ion viscous heating effect W/P_b and T_i/T_e will be 2.7 and 6 respectively. Strong magnetic field will reduces ion–electron collision rate due to quantization of electron orbits. While approximately there is no change in electron–ion collision rate, The effect of quantum magnetic field makes ions much hotter than electrons which enhances the fraction of fusion power to bremsstrahlung loss.     

Monte Carlo study of ion-induced secondary electron emission from SiO2

Here we describe a recently developed direct Monte Carlo program to study kinetic electron emission from SiO₂ target. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons), subsequent transport and escape of these electrons from the target surface. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. In order to demonstrate the capabilities of this program, we report a study on the kinetic electron emission from SiO₂ induced by fast (1-10 keV) rare gas ions. The calculated kinetic electron yield for various ion energies and masses is in good agreement with the predictions of most frequently applied theoretical model. In addition, the effects of projectile energy, mass and impact angle on the depth distribution of electron excitation points and average escape depth of the outgoing electrons were investigated. It is important to mention that the existing experimental techniques are not capable to measure these parameters.     

Fine-structure effects on the polarization of Kα1 radiation from heavy, highly-charged ions

Radiative decay of heavy, few-electron ions following electron capture is studied within the framework of the density matrix theory combined with the multiconfiguration Dirac-Fock approach. Special attention is paid to the linear polarization of the decay X-ray photons. Detailed calculations have been carried out, in particular, for the Kα₁ (1s2p_3/2^1,3P_1,2 →  1s²¹S₀) transition in helium-like uranium U⁹⁰⁺ ions. For this line, the characteristic radiation is almost unpolarized as a result of the superposition of its fine-structure ¹P₁ → ¹S₀ and ³P₂ → ¹S₀ components which are strongly linearly polarized in perpendicular directions.     

The fragmentation of sputtered cluster ions and their contribution to secondary ion mass-spectra

The emission of cluster ions Ta⁺_n Nb⁺_n, Cu⁺_n and Ag⁺_n (n ⩽ 15) from samples of Ta, Nb, Cu, Ag due to projectiles has been investigated. These clusters have been found to be emitted from the surface in an excited state. On the whole way from the target to the ion collector of the mass-spectrometer these ions can undergo monomolecular fragmentation as a result of excitation energy redistribution inside the ion with the formation of stable (energetically advantageous) fragmented ions with a smaller number of atoms. The quantity of these fragmented ions is so large that the ion composition usually recorded by secondary ion mass spectrometry (SIMS) is determined to a great extent by monomolecular fragmentations of “parent” ions and does not reflect the original composition of the latter ones as emitted from the target.     

Utilizing RF Electric Field for Injection-Trapping High Energy Molecular Ions in Mirror Field, (I)

A method of using rf electric field in a device for high-energy injection is described, and the behavior of the molecular ions injected into a system combining rf and DC magnetic fields is investigated by calculation and observation.

In this method, some of the injected molecular ions are decelerated by the rf field, and trapped in a small region between the mirror fields, where they are dissociated by collision with the background gas or with the plasma particles, and are thus trapped in the form of atomic ion. The relation between the rf frequency ω and the Larmor frequencies ΩN⁺ ₂, ΩN⁺ ₁ is given by ω=N/2· ΩN⁺ ₂=N/4· ΩN⁺ ₁, where N is the number of rf electrodes. The number N is chosen so that N/2 is an integer and N/4 a non-integer.

Calculation shows that the ions injected in a certain favorable phase are stably decelerated despite their initially possessing a precessional motion. This is proven by observation of actual electron behavior.     

Radiation damage of α-Al2O3 in the HVEM: I. Temperature dependence of the displacement threshold

The temperature dependence of the apparent displacement threshold energy has been measured for single crystal α-Al₂O₃ in a high voltage electron microscope, over the temperature range 320–1120 K. Below 570 K the threshold energy for observable damage remains constant at 390 ± 10 keV, then rises to a peak of 440 ± 20 keV at 695 K followed by a rapid fall to ~200 keV at 870 K and above. Measurement of the displacement threshold energy, using the optical absorption and luminescence of oxygen and aluminium electron irradiation induced vacancies at several temperatures, gives a value of 440 ± 25 keV for the oxygen ion and 175 ± 25 keV for the aluminium ion. These results show that oxygen and aluminium ions have widely different displacement energies, E_d, of ~75 and 18 eV respectively.