Fast Helium Atoms Research Articles

Studies of Dynamics of Neutral Component of Current Sheet Plasma, Based on Spectral Broadening of Helium Line He I 5876 Å

Spectroscopy methods were used for studying the dynamics of neutral plasma component in current sheets formed in two-dimensional (2D) and three-dimensional (3D) magnetic configurations during discharge in helium. It has been ascertained that when the current sheet is formed in the 2D magnetic field, flows of fast suprathermal helium atoms appear in it, which are directed along the current sheet width (the largest of the sheet transverse dimensions). It is shown that helium atoms can acquire the directed energy Wx due to the resonance charge exchange of accelerated ions in the current sheet plasma. The energy of directed motion of helium atoms can reach Wx ≈ (480 ± 120) eV, which is ~20 times higher than the temperature of helium atoms Ta ≈ (20 ± 2) eV at the same times. During the current sheet formation in the 3D magnetic configuration, fast helium atoms were not observed.

Limit characteristics of switches based on planar open discharge

The limit characteristics of switches based on open discharge (OD) with counter-propagating electron beams were studied. Such switches (kivotrons) are able to generate high-voltage pulses with a sub-nanosecond leading edge. Separately obtained limit values of the switch parameters are as follows: current density up to 1000 A cm−2, operating voltage up to 100 kV, switching time down to 100 ps, the degree of pulse compression up to 100, a pulse repetition frequency up to 100 kHz (in burst mode), and an efficiency of more than 0.9. It was shown that the fast current development arises when the discharge self-sustaining mode is caused by the photoemission from the cathodes due to the resonant radiation emitted by fast helium atoms that have a large Doppler shift with respect to the line center. As a result, the emitted radiation reaches the cathodes without absorption by the helium gas. The presence of a fast atom group with an energy determined by the applied voltage was experimentally demonstrated. The dynamics of the plasma decay was studied. It is shown that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only when the plasma density remains higher than 1012 cm−3. The main mechanism of the further plasma decay is the diffusion of plasma particles onto the wall. Therefore, the effect of recombination in the plasma bulk is observed only during the first 10–20 μs of the afterglow. The results of the lasing characteristics of a copper vapor laser excited by pulses generated by an OD-based switch are presented.

Effect of secondary electron emission on subnanosecond breakdown in high-voltage pulse discharge

The subnanosecond breakdown in open discharge may be applied for producing superfast high power switches. Such fast breakdown in high-voltage pulse discharge in helium was explored both in experiment and in kinetic simulations. The kinetic model of electron avalanche development was developed using PIC-MCC technique. The model simulates motion of electrons, ions and fast helium atoms, appearing due to ions scattering. It was shown that the mechanism responsible for ultra-fast breakdown development is the electron emission from cathode. The photoemission and emission by ions or fast atoms impact is the main reason of current growth at the early stage of breakdown, but at the final stage, when the voltage on discharge gap drops, the secondary electron emission (SEE) is responsible for subnanosecond time scale of current growth. It was also found that the characteristic time of the current growth τS depends on the SEE yield of the cathode material. Three types of cathode material (titanium, SiC, and CuAlMg-alloy) were tested. It is shown that in discharge with SiC and CuAlMg-alloy cathodes (which have enhanced SEE) the current can increase with a subnanosecond characteristic time as small as τS = 0.4 ns, for the pulse voltage amplitude of 5- 12 kV..

On the mechanism of the picosecond switching phenomenon in devices based on the open discharge with counter-propagating electron beams

The subnanosecond breakdown stage in the kivotron, a switching device with counter-propagating electron beams based on the open discharge in helium, was experimentally studied. It was shown that the fast discharge stage arises when the discharge self-sustaining regime is ensured by the photoelectron emission from of the cathodes due to the resonant radiation, emitted by fast helium atoms that have large Doppler shifts with respect to the line center; as a result, the emitted radiation reaches the cathodes without imprisonment by the helium gas. Since the cross-section for the excitation of a helium atom with another (fast) helium atom increases rapidly with the energy of the fast atom, the duration of the breakdown stage strongly depends on the working voltage. The transient characteristic is modulated by microwave oscillations of ~4.4 GHz frequency generated during the discharge of kivotron self-capacitance through its self-induction. An increase in working pressure leads to suppression of oscillations. A switching time of 80 ps was achieved in a discharge circuit loaded to a resistance RL ⩾ 50 Ω. On decreasing the value of RL down to 10 Ω, the switching time increases to about 100 ps at 1.5 kA current. A minimum switching time that can be achieved via kivotron design optimization is estimated to be about 35 ps.

Generation of high-voltage pulses with a subnanosecond leading edge in an open discharge. II. Switching mechanism

The mechanism of fast switching in open-discharge-based devices is clarified by analyzing the feature of the I–V characteristic of a quasi-stationary open discharge: at a voltage of 3–4 kV, the characteristic sharply rises, obeying the law j ~ U y with y > 10 (j is the current density). Such a run of the curve is explained by the fact that at U > 3 kV helium atom excitation by fast helium atoms becomes the main reason for VUV radiation. Fast helium atoms result from the resonance charge exchange between He+ ions moving from the anode to the cathode. In the coaxial design and in the sandwich design consisting of two accelerating gaps in which electrons move toward each other, multiple oscillations of electrons take place. This favors the generation of fast atoms and, accordingly, resonance VUV photons. Switching times as short as 0.5 ns are achieved. The minimal switching time estimated from experimental data equals 100 ps.

Young-Type Interference for Scattering of Fast Helium Atoms from an Oxygen Covered Mo(112) Surface

In studies on two structures of oxygen adsorbates on Mo(112), we demonstrate the potential of fast atom diffraction to derive the surface unit cell size and its symmetry. Helium atoms with energies of 1-2 keV are scattered from an adsorbate covered Mo(112) surface along low indexed surface directions under grazing angles of incidence. From the observed diffraction patterns, the lateral periodicity of the surface structures is derived. In addition to the periodic lattice, information on the structure within the unit cell can be obtained from double slit type of interference.

Fast-atom diffraction at surfaces

Fast helium atoms diffracted at alkali-halide surfaces under grazing angles of incidence exhibit intriguing diffraction patterns. The persistence of quantum coherence is remarkable, considering high surface temperatures and high (keV) kinetic energies of the incident atoms. Dissipative and decohering effects such as the momentum transfer between the incident helium atoms and the surface influence the diffraction patterns and control the width of the diffraction peaks, but they are weak enough to preserve the visibility of the diffration patterns. We perform an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the (110) direction. Our results agree well with recent experimental diffraction data.

Superstructures of oxygen and sulphur on a Fe(110) surface via fast atom diffraction

Fast helium atoms and hydrogen molecules with energies from 400 eV up to several keV are grazingly scattered from a Fe(110) surface covered by oxygen and sulphur atoms forming $c(2\ifmmode\times\else\texttimes\fi{}2)$ and $c(1\ifmmode\times\else\texttimes\fi{}3)$ superstructures, respectively. For scattering along low-index azimuthal directions we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the evaluation of those diffraction patterns we derive the widths of low-indexed axial channels, the corrugation of the interaction potential across these channels, and the normal positions of adsorbed atoms above the Fe lattice. Our analysis is based on semiclassical models using hard-wall approximation as well as individual potentials for the interaction of projectiles with the surface. By comparing the results of different models, we discuss the robustness of the information on the geometrical structure of the surfaces.

Influence of inelastic processes on fast-atom-surface diffraction

Diffraction of fast helium atoms at alkali-halide surfaces under grazing angles of incidence shows intriguing diffraction patterns. The persistence of quantum coherence is remarkably strong, even though high surface temperatures and high (keV) kinetic energies of the incident atoms would strongly suggest the dominance of dissipative and decohering processes. The main source of decoherence is the excitation or absorption of surface vibrations upon impact. The momentum transfer between the surface and the incident helium atom depends on the amplitude of the thermal vibrations of the surface atoms and the energy of the incident particle. We present an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the (110) direction, and compare with recent experimental diffraction data.

Influence of the polarization in grazing scattering of fast helium atoms from LiF(001) surfaces

Grazing scattering of neutral atoms from insulator surfaces is investigated in the intermediate velocity range, in which interference effects have recently been observed. To describe this process we introduce a distorted-wave method based on the use of the eikonal wave function, which takes into account the phase of the scattering state along the classical projectile path. The eikonal theory is applied to evaluate the angular distribution of few-keV helium atoms after impinging on a LiF(001) surface along low-index crystallographic directions. The interest focuses on the role played by the projectile polarization produced by cations and anions of the crystal surface. For the considered collision system we found a polarization channel, corresponding to the direction ⟨110⟩, which is affected by this effect, while for incidence in the direction ⟨100⟩ the polarization contribution is nearly negligible. The proposed eikonal approach, including polarization effects, provides angular projectile spectra in good agreement with the experimental data.

A quasi-coincidence measurement of the polarization of radiation emitted from the 3 3P state of fast helium atoms populated by He + −He charge transfer collisions

The relative circular polarization of photons emitted in the 2 3S-3 3P He I transition near 3889 Å has been measured. The 3 3P state of fast He atoms was populated by He +-He charge transfer collisions at 3 keV. Radiation from the fast atoms was collected as a function of scattering angle by selecting scattered atoms with an angle defining aperture. Measurement of the relative circular polarization stokes parameter S I permitted a direct comparison with theoretical predictions of the magnitude of this parameter. The data appear to be in basic agreement with the predictions of this theory, indicating that the quasi-coincidence or impact parameter selection method can be useful and accurate means to study anisotropic excitation.

Polarized photon emission following He +He charge capture collisions

The polarization of 3889 Å photons from the 2 3S3 3P transition of fast helium atoms produced by charge capture in a He +He collision has been measured as a function of the reduced angle τ= Eθ. An impact parameter selection method was employed with ions having energies between 2 and 3 keV, and at scattering angles of a few degrees. Circular polarizations, and in some cases linear polarization, of the light was measured. Large circular polarization, corresponding to collisional orientation, were observed. The results are discussed with reference to other measurements, and to the electron promotion model.

Foil Angle Dependence of Anisotropic Excitation at Large Inclination Angles

The dependence of orientation and alignment parameters on foil inclination angle represents an important means of testing theories of the production of excited level anisotropy. Such dependences have been extended to angles of 80 degrees or more, usually at a constant incident ion energy. However, the increased target thickness arising from the foil inclination can significantly alter the mean energy and velocity distribution of the emerging ions. The energy dependence of the anisotropy generation process is sufficiently large to alter the angular dependence in certain cases. A study of the orientation and alignment of the 4d 1D2 level of fast helium atoms at several incident energies illustrates the significance of foil thickness effects.

Single and double electron loss by fast helium atoms in gases

Measurements of single and double electron-loss cross sections for He atoms in the gases ${\mathrm{H}}_{2}$, He, ${\mathrm{N}}_{2}$, Ar, and Kr are reported in the energy range from 0.1 to 4 MeV. The cross sections are determined by the initial-growth method, and the neutral beam is formed by charge transfer neutralization of ${\mathrm{He}}^{+}$ in a He gas. The results on single electron loss in ${\mathrm{H}}_{2}$ and He targets have been discussed in an earlier paper and compared with theoretical Born-approximation calculations. The present results are compared with those from other experiments and with available theoretical estimates.

The Effect of Anomalous Low Friction of Polymers by Bombing with Accelerated Helium Atoms

Abstract New effect that is a drastic drop to anomalous low level (< 0.002) of friction cosfficient value in vacuum for steel-polymer friction pares is described. The effect was found by combing polyethylene and polypropylene surface with fast helium atoms simultaneously with friction process. The effect disappears spontaneously after irradiation is ended.

First Born approximation cross sections for electron loss from fast hydrogen atoms passing through atomic helium and fast helium atoms passing through atomic hydrogen and helium

The first Born approximation is used to calculate cross sections for one-electron loss from fast atoms A passing through atomic targets B for the following colliding partners (A, B): (H, He), (He, H) and (He, He). Full account is taken of double transitions and the calculations are performed using length and velocity formulation values of the helium matrix elements. Comparison is made with available experimental data, and for the case (H, He) excellent agreement is obtained at high impact energies. Further experimental measurements are desirable for (He, H) and (He, He).

Metastable atoms in fast helium beams

The process of one-electron loss during the passage of fast helium atoms through gaseous targets has been investigated. Apparent cross sections σ01 are shown to be dependent on the conditions of formation of the fast helium atoms.

Helium Repulsive Potential from Collision Cross Section Measurements

Total collision cross sections have been measured for fast helium atoms, having velocities corresponding to 300–1000 electron volts, scattered in room temperature helium gas. Using classical theory, attempts were made to find a repulsive intermolecular potential which could account for the experimental results. A hard sphere model or a Lennard-Jones type potential were both unsatisfactory, whereas exponential type potentials of the form V(r)=A exp (−arn) seemed promising. For the distances involved in the scattering experiments, 0.55 to 1.05A, the specific repulsive potential which predicted total collision cross sections within the experimental error was V(r)=11.3 exp (−4.63r12)×10−10erg.