Investigations on novel XUV-laser schemes

Short pulse laser generated plasmas interacting with a pulsed gas jet are studied to aid the realization of novel XUV laser schemes based on recombination supported by charge exchange pumping. Interaction of C⁴⁺, O³⁺ and O⁴⁺ ions with hydrogen and nitrogen indicates strong charge exchange pumping of C³⁺(3p-2s, 3d-2p) lines at 31.2 and 38.4 nm, and O²⁺ (2p3s-2p²) and O³⁺ (2s3d-2s²2p) lines at 37.4 and 23.8 nm. PACS 52.50.Jm; 52.25.Nr     

Towards XUV lasers with charge transfer pumping

Detailed studies of charge exchange pumping of ions in femtosecond laser-produced plasmas colliding with a pulsed gas jet are presented. Strong selective excitation of XUV ionic transitions in the reaction C⁴⁺ + H → C³⁺ + H⁺ is observed. Dependences of line intensities on various experimental parameters are reported which are in good agreement with the theory of charge transfer processes. Analyses of experimental data provide evidence that an efficient charge exchange pumping is realized at densities of reagents well in excess of 10¹⁶ cm⁻³, which is essential for the realization of XUV lasers. In preliminary investigations of the reaction C⁶⁺ + H → C⁵⁺ (n = 3, 4) + H⁺ a strong increase of line intensities at 13.5 and 18.2 nm is reported. Analysis of lasing in Na-like ions with an example for Chlorine promises even more efficient pumping as compared with the before analyzed hydrogen like ions.     

Optimization of C<Superscript>5+</Superscript> Balmer-α line intensity at 182 Å from laser-produced carbon plasma

Parametric dependence of the intensity of 182 Å Balmer-α line (C⁵⁺; n = 3 → 2), relevant to xuv soft X-ray lasing schemes, from laser-produced carbon plasma is studied in circular spot focusing geometry using a flat field grating spectrograph. The maximum spectral intensity for this line in space integrated mode occurred at a laser intensity of 1.2 × 10¹³ W cm^?2. At this laser intensity, the space resolved measurements show that the spectral intensity of this line peaks at ～1.5 mm from the target surface indicating the maximum population of C⁵⁺ ions (n = 3), at this distance. From a comparison of spatial intensity variation of this line with that of C⁵⁺ Ly-α (n = 2 → 1) line, it is inferred that n = 3 state of C⁵⁺ ions is predominantly populated through three-body recombination pumping of C⁶⁺ ions of the expanding plasma consistent with quantitative estimates on recombination rates of different processes.     

6.4 GHz ECR ion source at VECC

The 6.4 GHz ECR ion source that was indigenously developed a few years ago has been operating continuously for injecting oxygen and neon beams to the cyclotron since 1997. VEC-ECR is a single stage high magnetic field ion source provided with a negatively biased electron repeller placed on the axis, near the injection mirror point. The supply of cold electrons and use of low mass mixing gas improve the stability of ECR plasma. Very recently, the effect of aluminum oxide coating on the copper plasma chamber wall has been studied. The plasma chamber wall was coated with aluminum by vacuum evaporation method and then exposed to oxygen gas to form aluminum oxide. It was noticed that the process substantially shifts the charge state distribution to the higher charge state with an enhancement of ion current by an order of magnitude. With the aluminized plasma chamber, the VEC-ECR can now produce 12 μA of O⁷⁺, 6.5 μA of Ar¹²⁺, 1.5 μA of Kr²⁰⁺ and 1.0 μA of Xe³¹⁺.     

Formation of nanostructured carbon on steel targets irradiated by a high-power ion beam

The effect of the preliminary high-power ion beam treatment of steel targets (12KhN3A, St3, St20, St45) on the morphology and structure of a carbon layer produced by DC discharge plasma enhanced chemical vapor deposition using a C₂H₂/H₂ gas mixture has been investigated. It has been found that carbon nanotubes are formed on the St3 and St20 surface irradiated by the high-power ion beam with the ion current densities 50 and 100 A/cm².     

Density functional investigation of metal encapsulated X@C12Si8 heterofullerene (X=Li, Na, K, Be, Mg, Ca, Al, Ga)

The stability and the possible application of our recently reported SiC heterofullerenes inspire the investigation of their further stabilization through ion encapsulation. The endohedral complexes X@C₁₂Si₈, where X=Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺, Al³⁺, and Ga³⁺, are probed at the MPWB1K/6-311G? and B3LYP/6-311G* levels of theory. The optimized geometries show the expanding or contracting capability of C₁₂Si₈ in order to accommodate metal ion guests. The inclusion energies indicate the stability of the complexes compared to the components. Meanwhile, the calculated binding energies show the stabilization of C₁₂Si₈ through the inclusion of Be²⁺, Mg²⁺, Al³⁺, and Ga³⁺. The host-guest interaction that is probed through NBO atomic charges supports the obtained results. This study refers to “metal ion encapsulation” as a strategy for stabilization of SiC heterofullerenes.     

A numerical model for lithium plasma process in a hybrid microwave ion source

Lithium ions are widely used in many scientific fields; in order to get these ions, it is necessary to study lithium plasma process thoroughly. Recently, a hybrid ⁷Li³⁺ ion source has been designed and tested at Peking University (PKU). To understand the lithium plasma behaviour inside the plasma chamber and to provide some guidelines for ion source optimization to generate ⁷Li³⁺, a numerical model based on the plasma equilibrium equations is developed in this work, which is helpful not only for our ion source, but also for understanding the physical process of lithium plasma from ECR ion sources with different frequencies. This model can describe the density and fraction of lithium ions in various system parameters. The dependences of the Li⁺, Li²⁺, and Li³⁺ ion density and fraction on electron temperature, gas pressure, microwave power, surface ionizer, and the magnetic field are investigated systematically.     

Folded tandem ion accelerator facility at trombay

The folded tandem ion accelerator (FOTIA) project at BARC has been commissioned. The analysed carbon beams of 40 nA(3⁺) and 25 nA(4⁺), at terminal voltage of 2.5 MV with N₂ + CO₂ as insulating gas, were obtained. The beams were characterized by performing the Rutherford back scattering (RBS) on gold, tin and iron targets. The beam energy of 12.5 MeV for ¹²C⁴⁺ was consistent with the terminal voltage of 2.5 MV. The N₂ + CO₂ mixture is being replaced by SF₆ gas in order to achieve 6 MV on the terminal. In this paper, some of the salient features of the FOTIA and its present status are discussed.     

Studies of basic limitations on production, transport and acceleration of the high intensity polarized H− beam in the RHIC polarized ion source

Basic limitations on the high-intensity polarized H^? ion beam production and transport were experimentally studied in charge-exchange collisions of the neutral atomic hydrogen beam in the Na-vaporjet ionizer cell. The energy dependence of space-charge effects on the beam instabilities and losses were studied and described in the model of synthetic H⁺ — H^? beam transport. A xenon gas admixture to the H^? ion production cell and beam transport line greatly improves the space-charge compensation, which is also successfully described in the simulations. These studies are the part of the polarized source upgrade project for RHIC.     

Optical properties of trivalent chromium ions in the LiNbO<Subscript>3</Subscript> crystal

The formulas of the crystal-field theory have been adapted to a system with the symmetry group C _3v. A simple method has been proposed for including the polarization of the local environment of the Cr³⁺ impurity ion in LiNbO₃. A model dependent on one parameter has been proposed for a distortion of the niobium octahedron due to the incorporation of the trivalent chromium ion. This parameter has been determined from experimental data. The parameters of the intraionic and interionic interactions have been obtained for the Cr³⁺ ion in the lithium and niobium positions of the crystal lattice of lithium niobate.     

Formation of close-to-target products in reactions induced by <Superscript>12</Superscript>C ions on tin isotopes at the energy of 2.2 GeV per nucleon

Cross sections for charge-exchange reactions induced by the interaction between ¹²C ions of energy E_{¹² C} = 2.2E_{^{12} C} = 2.2 GeV per nucleon and tin targets enriched in the isotopes ^118,120,124Sn were measured by the induced-activity method. The cross sections for products whose charge numbers were in excess of the target charge number (Sb and Te) were determined. The shape of the isotope distribution of Sb products was indicative of the evaporative character of neutron emission in the formation of final-state products. The dependence of cross sections for charge-exchange reactions on the nucleonic composition of the target was considered. The contribution of electromagnetic excitation to the cross section for the reaction ¹²⁴Sn(¹²C, x)¹²⁴Sn was estimated.     

Energetic Characteristics of Oxygen Plasma

A low-temperature oxygen plasma is studied using the thermocouple, probe and spectral method in the range of 2–8 Torr pressure, 30–100 mA discharge currents, 0, 5, 10 m/s pumping velocity. The gas temperature, concentrations of electrons and oxygen atoms in the 3 p⁵P state, and the longitudinal electric field are measured. Radial distributions of gas temperature and electron concentration are obtained. Ranges of pressures and currents are found in which considerable increases in electric field, discharge voltage, temperatures of gas and electrons are observed at pumping switched on. An estimate of the rate constant of the steplike ionization through the O₂(a¹Δg) state is given. The concentration of atomic oxygen in the ground state and excitation constant of the triplet O(3p⁵P) by electron impact are obtained from the solution of the balance equations and our experimental data.     

Modernized active spectroscopic diagnostics (CXRS) of the T-10 tokamak

This work presents the results of modernization of the CXRS (charge exchange recombination spectroscopy) diagnostics [1] at the T-10 tokamak. The relevance of this work is due to the importance of measurements of the ion temperature and nuclei density of the working gas and impurities for analysis of transport processes in the plasma ion component. Measurements of radial profiles of the ion temperature are extremely important for investigating the geodesic acoustic mode behavior which is conducted at the T-10 [2]. The modernized scheme of CXRS measurements, as well as the design and operational features of the spectrometer created for the new diagnostics, is described. Principles of data recording and further processing are considered in detail; attention is given to the problem of calibration of the whole complex of equipment. The performed changes in diagnostics allow the measurements to be taken simultaneously in three spectral intervals: in the region of the beam line H_α, the CXRS line of carbon ion C⁵⁺, and the CXRS line of one of the hydrogen-like ions: He¹⁺, Li²⁺, N⁶⁺, O⁷⁺ or Ne⁹⁺. This makes it possible to measure the density profiles of two plasma impurities simultaneously, as well as the ion temperature from CXRS lines of different elements. The modernized diagnostics significantly broadens the possibilities of studying the physics of transport processes and quasi-coherent modes of plasma oscillations at the T-10.     

Dusty photoresonant plasma with coulomb collisions

We have studied the charging of dust particles in a dense photoresonant sodium plasma with electron and ion densities as high as 10¹⁶ cm^?3 produced by laser pumping of the resonance level of Na, which was a small admixture (up to 1%) in an argon buffer gas. We show that the charge of dust particles with a radius of 10 mm at maximum reaches 3 × 10⁵ electron charges and that the potential of the dust particles at a low electron bulk loss rate agrees well with the orbital motion limited (OML) model data. The behavior of the electric field near a dust particle was found to be nonmonotonic. We established that the distribution of the potential near a solitary charged dust particle agrees well with the Debye one, but the screening length proves to be much larger than even the electron Debye length; the discrepancies are largest at the afterglow stage of the photoresonant plasma, when the sodium ion with a low recombination coefficient is the main plasma ion. We determined the domain of parameters for a dense plasma where an ensemble of dust particles can crystallize.     

Nonlinear transmission coefficient of ytterbium-holmium fiber at the wavelength 978 nm

An experimental and theoretical investigation of the nonlinear transmission coefficient of ytterbium-holmium codoped silica fiber (YHF) at continuous-wave 978-nm pumping is reported. An analysis of the fiber absorption and luminescence spectra under 978-nm pumping reveals a variety of processes participating in the mixed (Yb³⁺, Ho³⁺) system: the energy transfer Yb³⁺ → Ho³⁺, the up-conversion and excited-state absorption in Ho³⁺, and the luminescence quenching due to the presence of Yb³⁺-Yb³⁺ ion pairs. These processes are shown to notably affect the transmission coefficient of YHF, which is reflected in a pronounced modification of its dependences on pump power, fiber length, and dopant concentrations, in comparison with the case of purely ytterbium-doped fiber (YF). A modeling of the experimentally measured dependences of the YHF transmission coefficient on pump power and fiber length allows us to obtain the coefficients addressing the energy transfer process Yb³⁺ → Ho³⁺ in YHF. A comparison of YHF and YF at the same pumping conditions reveals that, in YHF, a considerable part of the pump power absorbed in the Yb³⁺ subsystem is transferred into the Ho³⁺ subsystem; this results in an effective population of its ⁵ I ₇ state and makes YHF promising for 2-μm lasing and amplifying.     

Substrate effects on the analysis of biomolecular layers using Au, Au3 and C60 bombardments

Effects of platinum silicon, graphite and PET substrates on the secondary ion yield of sub-monolayer and multilayer samples of Cyclosporin A following 20 keV Au⁺, Au₃⁺and C₆₀⁺ impacts have been investigated. The obtained results of sub-monolayer samples show that platinum enhances the yield of the pseudo-molecular ion following Au⁺ and Au₃⁺ impacts due to the high density of the substrate that enables the energy of the primary ions to be deposited near the surface. C₆₀⁺ impacts on sub-monolayer samples are less effective, but there is an enhancement on PET substrates. Impacts of 20 keV Au⁺ and Au₃⁺ are not very efficient on multilayer samples. 20 keV C₆₀⁺ impacts enhance the yields significantly, especially for the relatively high molecular weight [M+H]⁺ ion.     

Energy levels and interaction between Eu3+-ions at lattice sites of symmetryC 2 and symmetryC 3i in Y2O3

Besides the known energy levels of Eu³⁺ at lattice sites of symmetryC ₂ in Y₂O₃, some transitions of this ion at lattice sites of symmetryC _3i in the same host crystal have been found and it was possible to construct the energy level scheme in part. Evidence for an energy transfer from Eu³⁺(C _3i ) to Eu³⁺(C ₂) is given, the strength of which is measured as a function of the Eu₂O₃ concentration. The results lead to the assumption of a fixed distanceR ₀ for the interaction responsible for the energy transfer. A temperature-independent value ofR ₀=8.7 Å was found. The full explanation of the underlying quantum mechanical mechanism requires further experiments.     

The study of a Tm:YLF laser pumped by a Raman shifted Erbium fiber laser at 1678 nm

A Tm:YLF laser pumped by a Raman shifted Er-fiber laser at 1.678 μm was studied at two Tm³⁺ ion concentrations equal to 1.5% and 5%. At output powers up to 460 mW the measured lasing efficiency at a wavelength of ~ 1.93 μm was as high as ~ 50%. The lasing performance was compared with that obtained under pumping by a 792-nm laser diode. The temporal structure of the laser pulse was recorded and the beam propagation factor M² was measured for all pumping conditions.     

Non-radiative energy transfer from Sm3+→Nd3+ in sodium borate glass

A study of energy transfer from optically excited Sm³⁺ to Nd³⁺ in borate glass has been performed. Contrary to the observations made by Cabezas and DeShazer, the Sm³⁺ → Nd³⁺ energy transfer has been observed as non-radiative. Energy transfer probabilities (P_da) and transfer efficiencies (η_T) have been calculated from our measurements of donor fluorescence intensity and decay times. The mechanism governing the transfer is electrostatic dipole-dipole in nature, contrary to the conclusions made by Nakazawa and Shionoya. At low acceptor (Nd³⁺) concentrations a linear dependence of P_da on acceptor concentration (C) has been observed which suggests the migration of excitation energy among donors. At high acceptor (Nd³⁺) concentration a plot of P_da vs (C_o + C)², where C_o is donor ion concentration, presents a linear dependence which is consistent with the Fong-Drestler theory of dipole-dipole energy transfer mechanism and interaction of one donor (Sm³⁺) with two acceptors (Nd³⁺).