Velocity Distribution Of Atoms Research Articles

Production of warm ions in electron beam generated E × B plasma

Several recent experiments have demonstrated low-damage processing of 2D materials, such as graphene and single crystal diamond, using electron beam (e-beam) generated plasmas with applied crossed electric and magnetic (E × B) fields. The low damage of these sensitive materials is commonly attributed to the low energy of ions incident to the substrate surface and the ion confinement in E × B fields. In this work, measurements of atom and ion velocity distribution functions in an e-beam E × B plasma at sub-mTorr argon pressures using a laser-induced fluorescence diagnostic revealed the presence of a warm population of ions with temperatures of ∼ 1 eV that are sufficient to destroy the ion confinement in E × B fields and drive the ion flux by cross field diffusion in the direction opposite to the applied electric field, toward the plasma-bounded walls or substrate. Thus, it is this nearly ambipolar diffusion process that is responsible for the flux of charged particles impinging on the wall/substrate surface.

Detection of atomic vortex beam using the near-field diffraction method

We theoretically introduce the method for detecting the atomic vortex (AV) of a helium atomic beam. The AV detection is based on near-field diffraction. The simulations relied on the Feynman path integral. The Gaussian beam and velocity distribution of helium atoms are taken into account. Gravitational field is also included in this study. This research can be useful in the field of matter-wave optics.

Sharp Focusing of an Atomic Beam with the Doppler and Sub-Doppler Laser Cooling Mechanisms in a Two-Dimensional Magneto-Optical Trap

The focusing of an atomic beam with the use of a two-dimensional magneto-optical trap in order to increase the number of atoms in the region of their laser cooling and localization near an atom chip is discussed. Two regimes of the interaction of atoms with a focusing laser field are considered: (i) the Doppler interaction regime, which occurs at small detunings of the laser field from the atomic resonance, and (ii) the sub-Doppler interaction regime, which occurs at large detunings of the laser field from the atomic resonance. The efficiency of focusing in the first case is low because of the momentum diffusion. It has been shown that the momentum diffusion in the sub-Doppler cooling mechanism is insignificant and, as a result, the broadening of the transverse velocity distribution of atoms is small. The sharp focusing of the atomic beam is possible in this interaction regime.

Improving Thermal Conductivity and Tribological Performance of Polyimide by Filling Cu, CNT, and Graphene

The thermal conductivity, mechanical, and tribological properties of polyimide (PI) composites filled by copper (Cu), carbon nanotube (CNT), graphene nanosheet (GNS), or combination were investigated by molecular dynamics simulation (MD). The simulated results suggested that Cu can improve thermal stability and thermal conductivity, but it reduces mechanical properties and tribological properties. CNT and GNS significantly improved the thermal and tribological properties at low content, but they decreased the properties at high content. In this study, the modification mechanism, friction, and wear mechanism of different fillers on polyimide were revealed by observing the frictional interface evolution process from the atomic scale, extracting the atomic relative concentration, the temperature and velocity distribution at the friction interface, and other microscopic information.

Slow wind belt in the quiet solar corona

The slow solar wind belt in the quiet corona, observed with the Metis coronagraph on board Solar Orbiter on May 15, 2020, during the activity minimum of the cycle 24, in a field of view extending from 3.8 R⊙ to 7.0 R⊙, is formed by a slow and dense wind stream running along the coronal current sheet, accelerating in the radial direction and reaching at 6.8 R⊙ a speed within 150 and 190 km s−1, depending on the assumptions on the velocity distribution of the neutral hydrogen atoms in the coronal plasma. The slow stream is separated by thin regions of high velocity shear from faster streams, almost symmetric relative to the current sheet, with peak velocity within 175 and 230 km s−1 at the same coronal level. The density–velocity structure of the slow wind zone is discussed in terms of the expansion factor of the open magnetic field lines that is known to be related to the speed of the quasi-steady solar wind, and in relation to the presence of a web of quasi-separatrix layers, S-web, the potential sites of reconnection that release coronal plasma into the wind. The parameters characterizing the coronal magnetic field lines are derived from 3D MHD model calculations. The S-web is found to coincide with the latitudinal region where the slow wind is observed in the outer corona and is surrounded by thin layers of open field lines expanding in a non-monotonic way.

Spatially resolved measurement of helium atom emission line spectrum in scrape-off layer of Heliotron J by near-infrared Stokes spectropolarimetry

For plasma spectroscopy, Stokes spectropolarimetry is used as a method to spatially invert the viewing-chord-integrated spectrum on the basis of the correspondence between the given magnetic field profile along the viewing chord and the Zeeman effect appearing on the spectrum. Its application to fusion-related toroidal plasmas is, however, limited owing to the low spatial resolution as a result of the difficulty in distinguishing between the Zeeman and Doppler effects. To resolve this issue, we increased the relative magnitude of the Zeeman effect by observing a near-infrared emission line on the basis of the greater wavelength dependence of the Zeeman effect than of the Doppler effect. By utilizing the increased Zeeman effect, we are able to invert the measured spectrum with a high spatial resolution by Monte Carlo particle transport simulation and by reproducing the measured spectra with the semiempirical adjustment of the recycling condition at the first walls. The inversion result revealed that when the momentum exchange collisions of atoms are negligible, the velocity distribution of core-fueling atoms is mainly determined by the initial distribution at the time of recycling. The inversion result was compared with that obtained using a two-point emission model used in previous studies. The latter approximately reflects the parameters of atoms near the emissivity peak.

Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Methods based on laser-induced fluorescence spectroscopy are widely used for spatially resolved non-intrusive diagnostics of atomic or molecular densities and velocity distributions in plasma applications. With regard to electric space propulsion, one focus is on the investigation of rare gases such as xenon or krypton, which are currently the favored propellants in gridded ion- and Hall-effect thrusters. For gridded ion engines, diagnostics of neutral atoms is of interest since charge-exchange processes between neutrals and ions are the main driver of accelerator grid erosion, which limits the lifetime of a gridded ion thruster. Extending the capabilities of the advanced electric propulsion diagnostics platform which has been developed by the IOM and partners, single- and two-photon absorption laser-induced fluorescence diagnostics have been set-up recently at our institute. Both experimental set-ups, and as a series of first applications, measurements of krypton neutrals in the plume of the radiofrequency ion thruster RIT-10 (ArianeGroup GmbH), and xenon neutrals within the discharge chamber of a gridded radiofrequency ion source developed at IOM, are presented.

Transition from ballistic to thermalized transport of metal-sputtered species in a DC magnetron

Tunable diode-laser induced fluorescence technique has been optimized to accurately measure the titanium (Ti)-sputtered atom velocity distribution functions (AVDFs) in a magnetron discharge operating in DC mode. The high spatial and spectral resolution achieved reveals some features of the transport of the metal-sputtered atoms and their thermalization. The two groups of thermalized and energetic atoms have been very well separated compared to previous works. Hence, the fitting of the energetic atom group shows dumping from modified Thompson to Gauss distribution when the product pressure-distance from the target increases. In parallel, sputtered metal transport from the target has been simulated using the Monte Carlo collision (MCC) approach. Direct comparison between numerical and experimental results led to an improved cross-section for Ti–Ar momentum transfer, based on the ab initio formulas of the interaction potential derived from noble gas interaction. The accuracy of the experimental data enabled the numerical parametric study of the angular distribution and cut-off energy for the initial distribution of sputtered atoms to reveal the precise characterization of the initial conditions. A very good overall agreement is obtained for measured and calculated AVDFs. Comparison between the measured and modeling results emphasized the major role played by the argon (Ar) ions, not only in the sputtering process, but in the neutral metal transport by the gas rarefaction near the target. The microscopic description provided by the MCC model clearly reveals different transport regimes: ballistic, diffusive and back-scattering, which provide new insight into the thermalization of sputtered species in the intermediate pressure range.

Generalization of the Menegozzi and Lamb maser algorithm to the transient superradiance regime

ABSTRACT We investigate the application of the conventional quasi-steady state maser modelling algorithm of Menegozzi & Lamb (ML) to the high field transient regime of the one-dimensional Maxwell-Bloch (MB) equations for a velocity distribution of atoms or molecules. We quantify the performance of a first order perturbation approximation available within the ML framework when modelling regions of increasing electric field strength, and we show that the ML algorithm is unable to accurately describe the key transient features of R. H. Dicke’s superradiance (SR). We extend the existing approximation to one of variable fidelity, and we derive a generalization of the ML algorithm convergent in the transient SR regime by performing an integration on the MB equations prior to their Fourier representation. We obtain a manifestly unique integral Fourier representation of the MB equations which is $\mathcal {O}\left(N\right)$ complex in the number of velocity channels N and which is capable of simulating transient SR processes at varying degrees of fidelity. As a proof of operation, we demonstrate our algorithm’s accuracy against reference time domain simulations of the MB equations for transient SR responses to the sudden inversion of a sample possessing a velocity distribution of moderate width. We investigate the performance of our algorithm at varying degrees of approximation fidelity, and we prescribe fidelity requirements for future work simulating SR processes across wider velocity distributions.

Two-photon absorption laser induced fluorescence measurements of absolute neutral deuterium density, temperature, and bulk flow in Proto-MPEX

Neutral particle control is critical for fusion fueling and confinement. Neutral diagnostics for fusion-relevant plasmas are commonly restricted to line-integrated or ex situ methods. A non-perturbative, two-photon absorption laser induced fluorescence (TALIF) diagnostic is implemented on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) to probe neutral atomic deuterium in a fusion-relevant plasma at 1 cm intervals along the radius of the vacuum vessel. The diagnostic is situated ∼20 m from the vacuum vessel, and a signal is collected along the laser injection axis, requiring only one line-of-sight. TALIF measurements are absolutely calibrated using xenon and krypton. Absolute atomic densities derived from xenon calibration are compared to absolute atomic densities derived from krypton calibration. Here, preliminary measurements of absolute atomic deuterium density, temperature, and local bulk flow dependence on radial location and input power in Proto-MPEX are presented. Neutral atomic deuterium velocity distribution functions are measured throughout a one-second plasma pulse with a time resolution of 250 ms.

Preparing narrow velocity distributions for quantum memories in room-temperature alkali-metal vapors

Quantum memories are a crucial technology for enabling large-scale quantum networks through synchronisation of probabilistic operations. Such networks impose strict requirements on quantum memory, such as storage time, retrieval efficiency, bandwidth, and scalability. On- and off-resonant ladder protocols on warm atomic vapour platforms are promising candidates, combining efficient high-bandwidth operation with low-noise on-demand retrieval. However, their storage time is severely limited by motion-induced dephasing caused by the broad velocity distribution of atoms comprising the vapour. In this paper, we demonstrate velocity selective optical pumping to overcome this decoherence mechanism. This will increase the achievable memory storage time of vapour memories. This technique can also be used for preparing arbitrarily shaped absorption profiles, for instance, preparing an atomic frequency comb absorption feature.

Evolution of the velocity distribution of atoms under the action of the bichromatic force

We study numerically the evolution of the velocity distribution of atoms under the action of the bichromatic force. The comparison of the time dependencies of the distribution width and the average acceleration of atoms reveal the correlation of these quantities. We show that the estimation of the momentum diffusion coefficient on the basis of the analogy between the interaction of atoms with the counter-propagating bichromatic waves and the interaction of atoms with the counter-propagating sequences of the $\pi$-pulses roughly corresponds to the results of numerical calculations. To separate the influence of the momentum diffusion on the evolution of atomic momentum distribution from the influence of the time-dependent Doppler shift, we study the motion of a ``heavy'' atom, for which the velocity change during the interaction of an atom with the field can be neglected. Provided that the parameters of the atom-field interaction are optimal, we show that the momentum diffusion coefficient is proportional to the intensity of the laser radiation. We used the Monte Carlo wave-function method for the numerical simulation of the atomic motion.

Relaxation and Recombination of Antiprotons and Positrons in a Strong Magnetic Field

A physical model is proposed, which makes it possible to determine the velocity distributions of antihydrogen atoms formed as a result of three-body recombination for the given parameters of an antiproton–positron plasma. Particle velocity relaxation is accounted for using the Derbenev–Skrinskii approach. The recombination rate is calibrated from the available results of molecular dynamics calculations. Good agreement with experimental results is achieved. The model makes it possible to estimate the efficiency of trapping of atoms in experimental conditions for obtaining antihydrogen.

Dynamics of Optical Pumping Processes in Coated Cells Filled with Rb Vapour

A better theoretical understanding of the atom – surface interactions and of the experimental consequences and applications to a reproducible method for the construction of reliable coated absorption cells is the main scope of our research. In this paper we report on the influence of diode laser intensity and of diode frequency scanning rate on the spectrum of the D2 line of Rb atoms, contained in paraffin or PolyDiMethylSiloxane (PDMS) coated glass cells. The anti-relaxation coating of inner walls of the glass cells allows to extend the optical orientation of atoms induced by the light to the entire cell volume and atomic velocity distribution. Moreover, in order to further investigate the role of atoms adsorbed in the anti-relaxation coatings, the D2 line spectra are compared for two cases: without and with illumination of the walls of the cell by an ultraviolet lamp. A theoretical model based on rate equations is introduced in order to analyze the experimental results. The extracted behaviour is a simple method to test the conditions of a coated surface. This finds applications in all cases where timely assessment of an organic coating layer is critical, from vapour spectroscopy to magneto-optical trapping of rare and radioactive species.

Nanostructured Alkali-Metal Vapor Cells

Atom-light interactions in nano-scale systems hold great promise for novel technologies based on integrated emitters and optical modes. We present the design architecture, construction method, and characterization of an all-glass alkali-metal vapor cell with nanometer-scale internal structure. Our cell has a glue-free design which allows versatile optical access, in particular with high numerical aperture optics, and incorporates a compact integrated heating system in the form of an external deposited ITO layer. By performing spectroscopy in different illumination and detection schemes, we investigate atomic densities and velocity distributions in various nanoscopic landscapes. We apply a two-photon excitation scheme to atoms confined in one dimension within our cells, achieving resonance line-widths more than an order of magnitude smaller than the Doppler width. We also demonstrate sub-Doppler line-widths for atoms confined in two dimensions to micron-sized channels. Furthermore, we illustrate control over vapor density within our cells through nano-scale confinement alone, which could offer a scalable route towards room-temperature devices with single atoms within an interaction volume. Our design offers a robust platform for miniaturized devices that could easily be combined with integrated photonic circuits.

Analytic theory for Bragg atom interferometry based on the adiabatic theorem

High-fidelity Bragg pulses are an indispensable tool for state-of-the-art atom interferometry experiments. In this paper, we introduce an analytic theory for such pulses. Our theory is based on the pivotal insight that the physics of Bragg pulses can be accurately described by the adiabatic theorem. We show that efficient Bragg diffraction is possible with any smooth and adiabatic pulse shape and that high-fidelity Gaussian pulses are exclusively adiabatic. Our results give strong evidence that adiabaticity according to the adiabatic theorem is a necessary requirement for high-performance Bragg pulses. Our model provides an intuitive understanding of the Bragg condition, also referred to as the condition on the "pulse area". It includes corrections to the adiabatic evolution due to Landau-Zener processes as well as the effects of a finite atomic velocity distribution. We verify our model by comparing it to an exact numerical integration of the Schr\"odinger equation for Gaussian pulses diffracting four, six, eight and ten photon recoils. Our formalism provides an analytic framework to study systematic effects as well as limitations to the accuracy of atom interferometers employing Bragg optics that arise due to the diffraction process.

Characterization of a supersonic gas jet via laser-induced photoelectron ionization

We describe the characterization of a pulsed supersonic rare gas beam which is intended to serve as an ultracold neutral atom target for the production of an ultrashort ion pulse via femtosecond photoionization. The velocity distribution of atoms entrained in the beam is measured and characterized by temperatures TII and T⊥ in directions along and perpendicular to the beam propagation, respectively. It is shown that T⊥ values in the mK regime are achieved at distances of the order of 1000 mm from the nozzle. Moreover, the center beam density at this position is measured to be of the order of 1011 atoms/cm3. Both findings are essential for the intended application and confirm the targeted beam specifications. Comparison with theoretical estimates reveals the well-known skimmer interference effect, which is found to reduce the beam density by more than one order of magnitude.

Modeling Cusp weak collision Kernel for the photon return of sodium beacon.

Sodium beacon adaptive optics (AO) system has been proved to be a highly productive tool for improving the resolving power of large-aperture ground-based telescope imaging. The performance of the AO system is mainly limited by photon return of the sodium beacon, which is determined by the coupling efficiency that characterizes the interaction rate between sodium laser and sodium atoms. The interaction processing is strictly influenced by the collisions of sodium atoms with other molecules (N2, O2). Most of the existing collision kernels are assumed as the "memoryless" hard collision, which is completely velocity reset in a Maxwellian distribution of the sodium atoms after scattering. To be more realistic, we adopt a more practical "memory" Cusp weak collision kernel, considering the velocity distribution of sodium atoms after collisions are correlated with the velocity before collision. By solving the Bloch equations, the processing for the interaction between sodium laser and sodium atom with Cusp kernel is established, and the coupling efficiency of sodium beacon with different collision kernel by analyzing the population is obtained. The researching results show that, for "memoryless" kernel, comparing to Cusp kernel with shaping parameter (s) of 100, the coupling efficiency is larger than 56% at best case; for sodium laser with 12% power detuned to D2b line and at a power density ranges from 10 to 100 W/m2, the coupling efficiency of "memoryless" kernel is nearly the same as "memory" Cusp kernel with s of 10, 100 and 3 Cusp kernel.

Orbital angular momentum transfer via spontaneously generated coherence

We study the orbital angular momentum (OAM) transfer from a weak Laguerre–Gaussian (LG) field to a weak plane wave in two closed-loop three-level V-type atomic systems. In the first scheme, the atomic system has two non-degenerate upper levels where the corresponding transition is excited by a microwave plane wave. It is analytically shown that the microwave field induces an OAM transfer from an LG field to a generated third field. It is demonstrated that the efficiency of the OAM transfer decreases when the thermal velocity distribution of atoms is considered. In the second scheme, we consider a three-level V-type atomic system with two near-degenerate excited states and study the effect of the quantum interference due to the spontaneous emission on the OAM transfer. It is found that spontaneously generated coherence (SGC) induces the OAM transfer from the LG field to the weak planar field, and the OAM transfer does not occur in the absence of the SGC. The suggested models offer a rather simple method for the OAM transfer that can be used in quantum information processing and data storage.

Characterization of transport of titanium neutral atoms sputtered in Ar and Ar/N2 HIPIMS discharges

In this work we report on the investigation of the transport behavior of Ti neutral atoms sputtered in a reactive high power impulse magnetron sputtering device used for TiN coating deposition. The time-resolved tunable diode laser induced fluorescence (TR-TDLIF), previously developed to study the transport of tungsten atoms, was improved to measure Ti neutral atom velocity distribution functions. We find that the TR-TDLIF signal has to be fitted using three Gaussian distributions, corresponding to the energetic, thermalized, and quasi-thermalized (atoms with non-zero mean velocity) atom populations. The ability to distinguish populations of atoms and to determine their corresponding deposited flux and energy may be of great interest to control film properties as desired for targeted applications. From the fitting, the vapor transport parameters (flux and energy) are calculated and studied as a function of distance from the target, pressure, and percentage of nitrogen in an Ar/N2 gas mixture. The study focuses on the effect of added nitrogen on the transport of sputtered atoms.