Concentration Of Helium Atoms Research Articles

PiC/MC-simulation of a low-pressure discharge in helium initiated by a relativistic electron beam in a coaxial chamber with a magnetic cusp

A gas discharge is simulated and numerically studied. The discharge is excited by a tubular relativistic electron beam in gaseous helium in a coaxial gas-discharge chamber. A magnetic field in the form of a cusp was imposed on the chamber. The code based on the PiC/MC-method was used. Electron beam dynamics, time evolution of phase portraits of particles, physical parameters, and spatial-time characteristics of plasma were calculated. It is shown that a dense plasma appears in the cavity under the action of an electron beam in the form of a thin-walled tube of a large length; its concentration can be nHe+≈2×1010 cm−3. At the same time, it turned out that plasma is considerably non-uniform in length; its denser part corresponds to the parts where beams stop. Time dependence of the charge compensation of electrons on the helium atom concentration in the chamber was calculated. The dependence turned out to be decreasing since the rate of the discharge development is increasing with the growth of the helium pressure in the cavity.

Current–voltage characteristics and mechanisms of electron emission from cold cathodes in a helium discharge

The abnormal discharge (AD) in pure helium and in helium containing molecular impurities was investigated. The obtained results were compared with the parameters of wide-aperture AD and different variants of ‘open’ discharge. It was shown that in all of the investigated types of discharges the current–voltage characteristics (CVCs) and, correspondingly, the emission properties of cold cathodes are determined mainly not by their material, but by the doping of cathodes with atoms of working gases and the purity of experimental conditions. With the impurity content less than 10−4% of the helium atom concentration, the CVCs begin to acquire S-shaped form, which is associated with a change in the electron emission mechanism. It was shown that the diversity of the CVCs is caused by uncertainty in the values of the secondary electron emission coefficients γ and the electron multiplication coefficient α in the cathode layer at reduced electric field strength E/N 103 Td. The reproducibility of CVCs and, correspondingly, emission properties of cathodes can be ensured by high purity of the working gas and by maintenance of the cathode doping only by the working gas atoms.

Particle‐in‐cell/Monte Carlo‐simulation of the discharge in helium initiated by a relativistic electron beam in the chamber with a magnetic mirror

AbstractA gas discharge is simulated and numerically studied in the gaseous helium in the region of the squeezed state of a single tubular magnetized relativistic electron beam at its partial reflection from a magnetic mirror. The code based on the particle‐in‐cell/Monte Carlo method was used. The electron beam dynamics, time evolution of phase portraits of particles, physical parameters, and spatial–time characteristics of plasma were calculated. It is shown that dense plasma appears in the cavity under the action of an electron beam in the form of a thin‐walled long cylinder of a large length; its electron concentration can be several units of 1011 cm−3. At the same time, it turned out that plasma is considerably inhomogeneous in length; its more dense part corresponds to the part of the squeezed state of a beam. The time dependence of the charge compensation of electrons on the helium atom concentration in the chamber was calculated. The dependence turned out to be decreasing since the rate of discharge development is increasing with the growth of the helium pressure in the cavity.

Atomistic simulations to study the effect of helium nanobubble on the shear deformation of nickel crystal

So far, many theoretical and experimental studies have investigated the detrimental effect of helium nanobubble on the mechanical properties of nuclear structural materials. In this article, efforts have been made to study the effect of helium nanobubble on the onset of plastic deformation in a single crystal Ni. The onset of plastic deformation has been quantified as a function of the orientation of slip planes with shear force and configuration of helium nanobubbles. Two configurations were generated for helium nanobubble, one with radius fixed where the He to Ni vacancy ratio was varied. In contrast, the ratio was kept constant in another configuration, and the nanobubble radius was varied. From the simulation, it was predicted that irrespective of the orientation of Ni crystal with respect to shear forces, helium nanobubble has a detrimental effect on the mechanical properties of the single-crystal Ni. Compared to the size of helium nanobubble, the higher concentration of helium atoms in the nanobubble has a more detrimental effect on mechanical strength. The focus of nuclear scientists is to develop future generation nuclear reactors and predict the safe life cycle of existing nuclear structures, which are in use for a long time. Helium nanobubbles change the mechanical behavior of underlying nuclear material in an effective manner. The results presented in this article will help in developing a better understanding of the deformation in irradiated Ni crystal.

PIC/MC-Simulation of Helium Ionization by a Relativistic Electron Beam With a Distributed Virtual Cathode

A gas discharge in gaseous helium in the vicinity of a distributed virtual cathode (VC) formed in a tubular magnetized relativistic electron beam when it is reintroduced into a cylindrical cavity is simulated and numerically investigated. The “KARAT” code, based on the particle-in-cell (PIC)/Monte Carlo (MC) method, was used. The dynamics of the electron beam, the time evolution of the phase space distributions of the particles, the physical parameters, and the space–time characteristics of the plasma are calculated. It is shown that, under the action of an electron beam, dense plasma appears in the cavity in the form of a thin-walled cylinder of great length, the concentration of which can reach 1013 cm−3. It turned out that plasma is substantially inhomogeneous along its length and its denser section corresponding to the location of the distributed VC. The instant of charge overcompensation of electrons is shown to be a function of the concentration of helium atoms in the cavity. The instant of charge balance was reached more rapidly with increased helium pressure, which indicates an increase in the rate of discharge development.

Influence of the ionization-energy losses of high-energy bismuth ions on the development of helium blisters in silicon

Understanding the behavior of helium in solids under conditions of intense ionizing radiation is of particular interest in solving many problems of nuclear, fusion, and space materials science and also in microelectronics. The observed effect of suppressing the formation of helium blisters on the surface of helium ion-doped silicon as a result of irradiation with high-energy bismuth ions is reported in this publication. It is suggested that a possible decrease in the concentration of helium atoms in silicon is due to their radiationinduced desorption from the area of doping in terms of the high-impact ionization of bismuth ions.

Statistical analysis of the dynamics of secondary electrons in the flare of a high-voltage beam-type discharge

In the present work, results of investigations into the dynamics of secondary electrons with helium atoms in the presence of the reverse electric field arising in the flare of a high-voltage pulsed beam-type discharge and leading to degradation of the primary electron beam are presented. The electric field in the discharge of this type at moderate pressures can reach several hundred V/cm and leads to considerable changes in the kinetics of secondary electrons created in the process of propagation of the electron beam generated in the accelerating gap with a grid anode. Moving in the accelerating electric field toward the anode, secondary electrons create the so-called compensating current to the anode. The character of electron motion and the compensating current itself are determined by the ratio of the field strength to the concentration of atoms (Е/n). The energy and angular spectra of secondary electrons are calculated by the Monte Carlo method for different ratios E/n of the electric field strength to the helium atom concentration. The motion of secondary electrons with threshold energy is studied for inelastic collisions of helium atoms and differential analysis is carried out of the collisional processes causing energy losses of electrons in helium for different E/n values. The mechanism of creation and accumulation of slow electrons as a result of inelastic collisions of secondary electrons with helium atoms and selective population of metastable states of helium atoms is considered. It is demonstrated that in a wide range of E/n values the motion of secondary electrons in the beam-type discharge flare has the character of drift. At E/n values characteristic for the discharge of the given type, the drift velocity of these electrons is calculated and compared with the available experimental data.

Helium nanobubble release from Pd surface: An atomic simulation

Abstract

17.3: Improvement of the Speed of Address Discharges in Ne‐Xe‐He Discharge Gases for ACPDPs

AbstractThe time lag of address discharges is discussed in terms of its relation with discharge gas composition. We measured the discharge time lag in ACPDPs with Ne‐Xe‐He mixed gases. As the concentration of helium increases, the address discharge time lag becomes shortened because helium atoms reduce mostly the formative time lag of the discharge. The formative time lag depends on the ion drift velocity at the initial stage of discharge formation. This ion drift velocity becomes lager with higher helium atom concentration.

Behavior of helium gas in the LHD vacuum chamber

In general, helium gas does not remain on vacuum chamber walls because of its small activation energy. However, outgassing of helium gas from the walls has been observed in the LHD plasma vacuum chamber with a very long time constant after helium glow discharge cleaning (GDC), and absorption of helium atoms has been observed in plasma discharge experiments using helium gas. The helium partial pressure before the daily experiments was determined only by the gas species of the last GDC. No dependence has been found between the helium partial pressure and the species of the fueling gas of the last plasma experiment fueling gas. Considering that the outgassing rate of the helium gas is almost the same each morning after He GDC, the retention of helium atoms in the wall after the GDC is almost at the same level. The concentration of helium atoms in the wall before the daily experiments is estimated. The outgassing rate after the GDC is 2×10 −4 Pa m 3/s and the concentration is 4.7×10 16 atoms/cm 2. These results are of the same order as in another experiments. During helium gas plasma experiments, about a half of the amount of the inlet gas disappears with the missing particles remaining in the wall. The stainless steel wall, which is saturated with He GDC, may still have the capacity to trap high energy helium atoms. However, the energy dependence of trapping helium atoms presently is not clear.

Thermodynamic properties of helium bubbles in aged plutonium

A qualitative model of the thermal properties of helium gas bubbles in aged plutonium metal is developed. Elasticity theory and plasticity are used to calculate the gas pressure inside the bubble. The chemical potential of a gas atom inside a bubble or a vacancy is estimated relative to an interstitial position. It is shown that the bubble expansion is controlled by plastic yield, with the gas pressure of the order of the yield stress σ y, in particular p=2/3 σ y. When the metal is at low temperatures (at or below 300 K), surface tension is not operative, since there is no diffusion to move material. When the metal is heated, surface tension becomes operative, but the bubble does not shrink, because elastic forces are reversed. Helium atoms are readily incorporated into existing bubbles. They also have a strong tendency to enter vacancies. This increases the equilibrium vacancy concentration as vacancy–helium complexes. While insignificant at room temperature (300 K), this tendency is important when the metal is heated: when the helium atom concentration increases, helium can migrate into bubbles. The effective pressure of 10 000 atm is then large enough to enable bubbles to grow, overcoming surface tension for bubbles of 5 nm diameter. When the metal is at room temperature, heterogeneous nucleation is required, most likely at grain boundaries.

The formation of helium bubbles in 316L SS irradiated with helium ions at different temperatures

Specimens of 20% cold worked 316L SS were irradiated with 2.5 MeV 4He + at room temperature, 500 and 600°C separately. The bubble structures in bulk were investigated in a transmission electron microscope with cross sectional technique. As the irradiation temperature is raised from room temperature to 500°C, the bubbles grow slowly. When the irradiated temperature is varied from 500 to 600°C, the bubbles grow very fast. This means there is a turning point of bubbles evolution between 500 and 600°C. At the same temperature, the bubble size and density tend to saturation with the increasing of helium atom concentration and displacement per atom (dpa).

Transitions between fine-structure components of neon in inelastic collisions of neon and helium atoms

UDC 542.43 In the deactivation of laser states in gas-discharge lasers, a significant rule is played by nonradiative transitions between energy levels in inelastic collisions between atoms of the active material and also collisions with impurity atoms. Such transitions occur both within a single multiplet and between states with different multiplicity, under the condition that the energy difference between the initial and final states be no more than a few kT. Further increase in the energy difference leads to a sharp decrease in energy-transmission cross section. If the energy gap is small, however, the working-gas pressure is a few torr, the probability of nonradiative transitions between fine-structure components of the atoms may be of the same order as the probability of optically resolved transitions. This leads to additional redistribution of the excitation between the energy states, which considerably influences the operating conditions of gas-discharge lasers. Inelastic processes begin to play a special role in collisions of strongly excited atomic particles [i], since in this case there is an increase not only in the density of energy states but also in the purely geometric cross section of the atoms participating in the collisions (Oam ~ n2). The cross section of nonradiative transition between the energy levels of the atom increases here. The need for detailed study of the process of nonradiative excitation transmission is also a consequence of the presence of considerable difficulties in the theory of atomic collisions in considering the deviations from LS coupling, for which there exists an experimentally confirmed conservation rule for the total spin moment of the colliding particles (the Wigner rule). The inadequacy of experimental data on the transmission of excitation between energy states described by other forms of interaction does not permit an unambiguous conclusion regarding the presence of definite transition conditions for the case of jZ and jj coupling. The measurable cross sections of nonradiative transitions offer the possibility of estimating the exchange-interaction potential of the colliding particles and finding the distance between atoms at which inelastic processes with transmission of excitation occur. In a theoretical consideration of the transitions between fine-structure components, this allows the wave functions of the initial and final states of the quasimolecules constituted by the colliding atoms to be correctly chosen and also allows all types of interaction to be taken into account in the Hamiltonian of the given system. The absence of a sufficient amount of experimental data on the inelastic-transi tion cross section considerably retards the development of theoretical research in this field. The process of nonradiative excitation transmission may occur in collisions between atoms of the same or different kinds. Therefore in investigating theactive medium of a gasdischarge laser (GDL) consisting of a mixture of gases, it is necessary to distinguish between the possible channels of particle interaction. In the present work, experimental results are given on the excitation transmission between the 2pS3p levels of neon in collisions of neon and helium atoms in gas-discharge plasmaz Ne*(2pi)+He(0) ~-Ne*(2p~)q-He(0) , where Ne* (2pi,k) are the initial (i) and final (k) excited states of the neon atom; He(0) is a helium atom in the ground state; AEik is the energy distance between the levels i and k. This process plays the main role in the nonradiative transmission of excitation between levels of the group 2pS3p in inelastic collisions of atomic particles, since the concentration of helium atoms in the plasma is higher than that of neon atoms.

Performance of the modified orbitron pump

The conventional orbitron pump is modified to improve the speeds for inert gases. Pumping speed measurements were made on the modified orbitron pump by using the two‐gauge calibrated conductance method for helium, argon, and xenon with the temperatures of the pump body at 288 K (15 °C), 323 K (50 °C), and 418 K (145 °C). Improvements in the maximum speed in comparison with the conventional orbitron pump are by factors varying from 1.15 to 1.88. Increase in speed for helium with pressure is attributed to the concentration of helium atoms at some trapping sites in titanium films. Increase in speed for helium at 323 K (50 °C) temperature indicates that the pumping of helium is governed by a diffusion‐controlled process before the entrapment. Deterioration in the speeds for argon and xenon at 323 K (50 °C) and for helium at 418 K (145 °C) is believed to be due to thermal release of the trapped gas.

Physical phenomena and analytical applications of helium microwave discharges

The intensity of atomic emission from a microwave-induced helium gas discharge as a function of pressure in the range 13–130 mbar is described. Two of the spectra studied were given by excited helium atoms, and one by (a species of) an excited triplet helium molecule (He 2 *). The pressure dependence of the concentration of helium atoms in the triplet metastable state was studied by absorption spectroscopy. After introduction of known quantities of mercury, chlorine, and iodine into the helium plasma, the emission was measured for some intense lines in the visible and near-u.v. Comparison of the data suggests that the atoms of these elements can be excited by helium atoms and molecules to levels at which they emit light in the visible and near-u.v. The use of a helium discharge tube for the detection of single elements in gas chromatographic fractions is described. Selectivity is greatly improved by wavelength modulation. The method allows a highly sensitive and selective determination of nanogram amounts of organic compounds which contain the elements sought, including stable isotopes such as deuterium, carbon-13 and nitrogen-15.

Measurement of metastable helium atom concentration by pulse discharge in a cathode field

Measurement of metastable helium atom concentration by pulse discharge in a cathode field

Effect of helium gas bubbles on the creep ductility of an austenitic alloy

Post-irradiation transmission electron microscopy has been performed on a split heat of a high temperature precipitation-hardened alloy (A-286) containing 0.001 wt % and 0.010 wt % natural boron. After annealing at 760, 845 and 955 °C, foils were prepared from sections taken from close to the fracture of specimens creep-tested to failure. The observations were compared with those previously obtained for undeformed specimens. p]Helium gas bubbles were found to be arranged in spherical shells around coarse precipitates believed to be TiN. Any effect of creep deformation on bubble size was masked by the annealing treatment, and the bubble size did not differ systematically from that observed for the undeformed specimens. The measured increase in creep resistance with increasing helium atom concentration is explained in terms of a reduction of dislocation mobility. It is suggested that the reduced ductility and associated reduction in strain rate sensitivity after irradiation may be very sensitive to the gas bubble distribution. The observation of bubbles on dislocations lying in the boundaries may be particularly significant.