Helicity Cross Sections Research Articles

Design of a magnet to study quench propagation in a Cable-In-Conduit-Conductor filled with stagnant superfluid helium

In the dark matter Axion research context, the MADMAX project acts as a figurehead in the physics research field. The goal of the project is to discover axion as dark matter. For this, it is necessary to design and manufacture a dipole, composed of 18 coils, that generates a Figure of Merit of 100 T2m 2. The MADMAX coils have several specific features though, as the use of Cable-In-Conduit-Conductor (CICC) with a copper profile that serves as thermal stabilizer and the use of stagnant superfluid helium inside the CICC conduit. With a 28 mm2 helium cross-section and a conductor of several hundred meters before reaching the helium bath, the quench dynamics appear then as a challenge of the design phase. To experimentally investigate the quench propagation in the MADMAX coils, we designed a MADMAX-like solenoidal prototype based on numerical simulations made with THEA® called MACQU (MAdmax Coil for Quench Understanding). This numerical study allowed analysing the physics behind the quench phenomenon and define the relevant current range for the experimental quench studies. Nevertheless, as THEA® has never been used for a CICC filled with superfluid helium, the results will have to be benchmarked by the quench testing campaign on MACQU. The different calculations made with THEA® show that the quench propagation is divided into two different phases: after the initiation, the propagation is at a constant speed of around 5 m/s, followed by an acceleration phase where the propagation speed reaches 40 m/s, at 17 kA. Among the different possible phenomena, we have found that the friction forces term, as defined in the THEA code, are responsible for this two-phase propagation because they initiate the pre-heating of the magnet before the quench, reducing then the local temperature margin.

Evaluating the Effects of Metal Adduction and Charge Isomerism on Ion-Mobility Measurements using m-Xylene Macrocycles as Models.

Gas-phase ion-mobility spectrometry provides a unique platform to study the effect of mobile charge(s) or charge location on collisional cross section and ion separation. Here, we evaluate the effects of cation/anion adduction in a series of xylene and pyridyl macrocycles that contain ureas and thioureas. We explore how zinc binding led to unexpected deprotonation of the thiourea macrocyclic host in positive polarity ionization and subsequently how charge isomerism due to cation (zinc metal) and anion (chloride counterion) adduction or proton competition among acceptors can affect the measured collisional cross sections in helium and nitrogen buffer gases. Our approach uses synthetic chemistry to design macrocycle targets and a combination of ion-mobility spectrometry mass spectrometry experiments and quantum mechanics calculations to characterize their structural properties. We demonstrate that charge isomerism significantly improves ion-mobility resolution and allows for determination of the metal binding mechanism in metal-inclusion macrocyclic complexes. Additionally, charge isomers can be populated in molecules where individual protons are shared between acceptors. In these cases, interactions via drift gas collisions magnify the conformational differences. Finally, for the macrocyclic systems we report here, charge isomers are observed in both helium and nitrogen drift gases with similar resolution. The separation factor does not simply increase with increasing drift gas polarizability. Our study sheds light on important properties of charge isomerism and offers strategies to take advantage of this phenomenon in analytical separations.

Experimental investigation of heat transfer and pressure drop inside elliptic tube with inserted helical coils

The paper deals with an experimental study of the influence of varying helical coil cross section diameter and pitches of helical coil on heat transfer and friction characteristics in an elliptical tube. The present work studies the effects of copper helical coils of circular cross section formed with different diameters of 2 and 6 mm at varying pitches (20, 45, 135 and 225 mm) inside elliptic tube. In the experiments, cold air at ambient condition is passed through the uniform heat flux circular tube for Reynolds numbers in a range of 11000 to 37500. The experimental results demonstrate that the use of helical coil inserts in elliptic tube leads to a higher heat transfer rate than that of the plain surface tube. Also, increasing helical coil cross section diameter and decreasing pitches of the helical yield a better heat transfer than the plain tube. The enhancement efficiencies of the helical coil cross section 6 mm over wire cross section 2 mm diameter at a pitch of 20 mm is 2.3% at Re=11000. The results are also correlated in the form of Nusselt number, friction factor and enhance efficiency as a function of Reynolds number, pitches and diameter of helical coil

Pathways in the molecular fragmentation for the C2H5OH/He electrical discharge

ABSTRACTThis study focuses on the glow discharge generated with a gases mixture of Ethanol (C2H5OH) and Helium (He), at different concentrations maintained at a total pressure of 2.0 Torr. We used optical emission spectroscopy (OES) to analyze the discharge mixture at different concentrations of Helium. Single Langmuir probe data was used to determine the Electron Energy Distribution Function (EEDF). For the total C2H5OH/He mixture plasma concentrations, the EEDF has a Maxwellian distribution function. A decrease in He concentration results in significant changes in the EEDF, this behavior is related to the increase in the C2H5OH percentage must increase the energy loses of the electrons in the inelastic collision with C2H5OH producing a significant change in the EEDF, therefore, the EEDF pattern results in an increase of electron–molecule reaction rates. The rise in electron temperature for increasing Helium percentage is explained by the decreasing electron energy loss in the inelastic collisions with C2H5OH molecule. It observes a decrease of electron density ne as a function of the Helium percentage, which can be related to the ratio between ionization cross sections of Helium and C2H6O molecule. The active species are generated in the electron-molecule processes, which are associated with electron impact dissociation of C2H5OH and Helium electronic impact excitation in the gas phase. The emission optical spectra (OES) show changes in the intensity of the most important peaks of the plasma mixture, which indicates the dependence in the formation of the plasma as a function of the percentage of the gases. The changes in the intensities of the same observed species are due to different processes of excitation and ionization energies of the system, in addition to the increase of He metastable states He I. Hydrogen is the main product obtained from the decomposition of C2H5OH.

Partial Photoionization Cross Section of Collinear eZe Helium: Numerical Confirmation of Semiclassical Predictions

Based on the semiclassical theory of chaotic scattering, Tanner et al. [J. Phys. B 40, F157 (2007)] proposed the fluctuation in the partial photoionization cross section of helium below the double-ionization threshold would show the same characteristics as in the total cross section, predicting that the Fourier spectrum of the fluctuation reveals peaks at the classical actions of closed triple collision orbits and the amplitude of the fluctuation decreases algebraically as the energy approaches the double-ionization threshold. In that paper, however, the predictions were not clearly confirmed due to the lack of experimental data with sufficient accuracy. So instead, we calculate the partial photoionization cross sections of collinear eZe helium for the energy range from the single-ionization threshold I20 to I32 in order to numerically confirm the predictions. Analysis of the fluctuation in the partial cross section shows that the predictions are indeed valid. Our findings mean that the fluctuation in the partial photoionization cross section can be described by classical triple collision orbits in the semiclassical limit. Thus it explains in a natural way the mirroring and mimicking structures observed in cross section signals for different ionization channels.

Iteration based calculation of position and orientation of grinding wheel for solid cutting tool flute grinding

End mill cutting tools are widely used in machining of curved surface parts in many areas, e.g. aerospace, automobile, and energy. The parameters of helical groove cross section, influencing chip removal capacity, include rake angle, core radius and edge width. Towards a parametrical control algorithm of the cutting tool flute, this paper proposed an iteration based method. Within the context, the cutting tool parameters and grinding wheel are set firstly. Then, three angle parameters, α,β, and θ in the grinding coordinate system, are looped. In each loop, one of the target flute parameters is compared with the pre-set values, and if the parameter is within the range, the calculation goes the next loop until the parameters are in the target range. Since the loop number is so high that the computing time is too long, the patterns between the flute parameters and looped parameters are used to improve the calculation speed. The method is implemented by using C#, and validated by a set of numeric simulation based on Matlab®.

A theoretical study on collision mechanisms for low energy electron impact ionization of helium in the perpendicular geometry

Under the condition of ten different incident energies ranging from 3 eV to 80 eV above the ionization potential of helium and the outgoing electrons having equal energies, by making use of 3C model and modified 3C model, the triple differential cross sections of electron-impact single ionization of the ground state of helium in the perpendicular geometry are calculated. The result is compared with corresponding experimental result to systematically investigate the influences of various screening effects on the triple differential cross sections for helium. The collision mechanisms of the triple differential cross sections are explored. The result shows that the effects of dynamic screening in the final state can directly affect the structures of the triple differential cross sections at lower incident energy, which will unavoidably affect the angular distribution and relative amplitude of side peaks at angles =90 and =270. The screening effects of residual electron in the final state of He+ have a similar significant effect on the amplitude of triple differential cross section of helium and angular distributions and relative amplitudes of side peaks at angles =90 and =270. When the incident energy is over 84.6 eV, the screening effect of residual electron in the final state of He+ has a slight effect on the amplitude of triple differential cross section, which can be overlooked. But the effects of dynamic screening in the final state on side peaks at angles =90 and =270 need considering. In addition, by taking advantage of DS3C-Z model, the results of collision mechanism of various peaks at angles =180, =90 and =270 show that the middle peak at angle =180 is produced by a process called triple scattering mechanism and then the side peaks at angles =90 and =270 are produced by a process called double scattering mechanism. Such a collision mechanism has a direct influence on the generation and variation law of triple differential cross sections.

EUV two-photon-ionization cross sections of helium from the solution of the time-dependent Schrödinger equation, and comparison with measurements using free-electron lasers

Two recent experimental papers reported the first measurements of absolute two-photon-ionization cross sections $\ensuremath{\sigma}(2)$ of helium, for EUV wavelengths, using free-electron laser (FEL) pulses [Sato et al., J. Phys. B 44, 161001 (2011); Fushitani et al., Phys. Rev. A 88, 063422 (2013)]. The wavelengths correspond to transitions that are off resonance as well as on resonance with the $1s2p$ and $1s3p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}^{o}$ Rydberg states. Inspection of their results reveals considerable discrepancies, while their comparison with theoretical results obtained earlier from time-independent calculations, one perturbative and two nonperturbative ones, cannot lead to secure conclusions as to the true values of $\ensuremath{\sigma}(2)$. We examined this prototypical problem by implementing a time-dependent approach, which utilizes the nonperturbative solution of the time-dependent Schr\odinger equation. This solution was obtained in terms of the state-specific expansion approach, in an upgraded version where the coupling matrix elements are computed using the full electric operator of the multipolar Hamiltonian. The $\ensuremath{\sigma}(2)$ were obtained for pulses of 300 fs, as in the 2011 FEL experiment. Their computation was achieved by fitting the time-dependent ionization survival probability to ${e}^{\ensuremath{-}\mathrm{\ensuremath{\Gamma}}t}$, where $\mathrm{\ensuremath{\Gamma}}$ is the rate of ionization. The wavelengths and intensities are those of the FEL experiments, as well as others, such as the wavelengths 52.22 and 51.56 nm, for which the $1s4p\phantom{\rule{0.28em}{0ex}}{}^{1}{P}^{o}$ and $1s5p\phantom{\rule{0.28em}{0ex}}{}^{1}{P}^{o}$ levels are on resonance with the initial $^{1}S$ state. Apart from the predictions for these wavelengths, the paper contains characteristic comparisons among all the results on these EUV $\ensuremath{\sigma}(2)$, experimental and theoretical. In general, the trends predicted by nonperturbative methods are confirmed by the FEL measurements. However, discrepancies exist among the absolute numbers. Furthermore, comparison among the results of the three nonperturbative approaches (present time-dependent and two earlier time-independent ones published in 1999 and 2005) indicates an overall consistency, although quantitative differences for individual cases are apparent.

Higgs boson form factor effects intt¯production byW−W+andZZfusion

We study the fusion processes $W^-W^+\to t\bar t$ and $ZZ\to t\bar t$ observable at a future $e^-e^+$ collider and we discuss their sensitivity to an $Htt$ form factor which may be due to compositeness, in particular when the $H$ and the top quark have common constituents. We make an amplitude analysis and illustrate which helicity amplitudes and cross sections for specific final $t\bar t$ polarizations are especially sensitive to this form factor.

Toward a Rational Design of Highly Folded Peptide Cation Conformations. 3D Gas-Phase Ion Structures and Ion Mobility Characterization.

Heptapeptide ions containing combinations of polar Lys, Arg, and Asp residues with non-polar Leu, Pro, Ala, and Gly residues were designed to study polar effects on gas-phase ion conformations. Doubly and triply charged ions were studied by ion mobility mass spectrometry and electron structure theory using correlated ab initio and density functional theory methods and found to exhibit tightly folded 3D structures in the gas phase. Manipulation of the basic residue positions in LKGPADR, LRGPADK, KLGPADR, and RLGPADK resulted in only minor changes in the ion collision cross sections in helium. Replacement of the Pro residue with Leu resulted in only marginally larger collision cross sections for the doubly and triply charged ions. Disruption of zwitterionic interactions in doubly charged ions was performed by converting the C-terminal and Asp carboxyl groups to methyl esters. This resulted in very minor changes in the collision cross sections of doubly charged ions and even slightly diminished collision cross sections in most triply charged ions. The experimental collision cross sections were related to those calculated for structures of lowest free energy ion conformers that were obtained by extensive search of the conformational space and fully optimized by density functional theory calculations. The predominant factors that affected ion structures and collision cross sections were due to attractive hydrogen bonding interactions and internal solvation of the charged groups that overcompensated their Coulomb repulsion. Structure features typically assigned to the Pro residue and zwitterionic COO-charged group interactions were only secondary in affecting the structures and collision cross sections of these gas-phase peptide ions. Graphical Abstract ᅟ.

Molecular Structures and Ion Mobility Cross Sections: Analysis of the Effects of He and N2 Buffer Gas.

An empirically observed correlation between ion mobility cross sections in helium and nitrogen buffer gases was examined as a function of temperature, molecular size, and shape. Experimental cross sections were determined for tetraglycine, bradykinin, angiotensin 2, melittin, and ubiquitin at 300 K and in the range from 80 to 550 K on home-built instruments and calculated by the projection superposition approximation (PSA) method. The PSA was also used to predict cross sections for larger systems such as human pancreatic alpha-amylase, concanavalin, Pichia pastoris lysyl oxidase, and Klebsiella pneumoniae acetolactate synthase. The data show that the ratio of cross sections in helium and nitrogen depends significantly on the temperature of the buffer gas as well as the size and shape of the analyte ion. Therefore, the analysis of the data indicates that a simple formula that seeks to quantitatively relate the momentum transfer cross sections observed in two distinct buffer gases lacks a sound physical basis.

PossibleΣ*(12−)in the initial-state polarizedγN→K+Σ*(1385)→K+πΛreaction near threshold

By using an effective Lagrangian method, we study the effects of a newly proposed $\Sigma^*(1/2^-)$ state with mass around 1380 MeV in the initial-state polarized $\gamma N\rightarrow K^{+} \Sigma^*(1385) \rightarrow K^{+} \pi \Lambda$ process near threshold. The theoretical predictions for the helicity cross sections $\sigma_{3/2}$, $\sigma_{1/2}$ as well as their ratios, and the angular distributions of $\pi$ in the $\pi\Lambda$ center-of-mass system are given. It is found that assuming $\Sigma^*(1/2^-)$ exists or not, these physical quantities are distinctly different. So our results could be useful for the investigation of the existence of $\Sigma^*(1/2^-)$ when the experimental data are available in the future.

Influence of screening effect on double differential cross section and single differential cross section for (e, 2e) process of helium

The double differential cross sections for electron impact single ionization of helium at incident energies of 32.5, 36.5 and 40.7 eV are calculated by using the DS3C model. The calculated results are compared with the experimental date. The exchange effects are discussed. The single differential cross section for electron impact single ionization of helium is obtained by integrating the double differential cross sections over the angle of the ejected electron. The single differential cross sections of helium at incident energies of 32.5, 36.5, 40.7 and 50 eV are calculated by using the 3C model and the DS3C model. The calculated results are compared with the experimental date. The structure of the cross section is analysed and the contribution of exchange effect to the corss section is discussed.

One-photon double ionization of helium: A heuristic formula for the cross section

Without a formal derivation, we propose a formula for the total and single-differential cross sections in the problem of one-photon double ionization of an atom. The formula is benchmarked against accurate experimental data for the total cross section of helium. Furthermore, a direct comparison with ab initio calculations for the double ionization of Li${}^{+}$ suggests that the framework is valid for the entire helium isoelectronic sequence. To this end, we introduce a formula for the double ionization of lithium as well as for the triple ionization of lithium and beryllium.

Thermalization of positronium in helium: A numerical study

In this paper we present a numerical study of positronium (Ps) thermalization in pure helium (He). Recent measurements of Ps thermalization yielded data that were analyzed to produce the scattering cross-sections in helium by using energy balance equations with an assumption of a Maxwell–Boltzmann distribution (MBD) function for Ps. We have applied a Monte Carlo code to test the cross-sections. As our code was developed without any approximations for the energy distribution function we have effectively also tested the assumptions and the validity of the simple theory based on Maxwellian distributions. We present the simulation results using the simulation technique that is limited only by the accuracy of the available cross-sections. We calculate thermalization profiles for several theoretical and measured cross-sections. Also, the temporal evolution of energy distributions has been shown along with diffusion coefficients and spatial ranges of penetration. Thermalization of the initial distribution is rapid and the data follow relatively closely, those calculated in recent experiment, which supports the choice of MBD and the obtained cross-section. However the distribution function most of the time deviates from the MBD due to strong scattering. Finally, we applied the same procedure to analyze Ps thermalization in water vapor.

Unreliability of the autocorrelation function as evidence for Ericson fluctuations

In a recent combined experimental and theoretical paper for the total photoionization cross section of helium up to the single-ionization threshold ${I}_{13}$, we had shown that Ericson fluctuations were absent, although the Ericson regime had already been reached. In the present paper, we study the autocorrelation functions of the total photoionization cross section below ${I}_{9}\text{--}{I}_{13}$ and find that these are essentially identical to that of an artificial spectrum with Ericson fluctuations. Consequently, we have to conclude that the autocorrelation function alone---although its Lorentzian-like shape for small displacements $\ensuremath{\varepsilon}$ has been used by some authors---is a necessary but is not a sufficient proof for the existence of Ericson fluctuations. In the present case of He, the absence of Ericson fluctuations is a consequence of a strong hierarchy between the transitions belonging to different series with approximate quantum numbers that still exist in this region; this causes the spectra to be dominated by the principal series.

Semiclassical fully differential ionization cross sections of helium with negatively charged fast projectiles

Abstract Fully differential cross sections are calculated for the ionization of helium by negatively charged fast projectiles using a semiclassical model developed previously for the ionization of atoms by positive projectiles. The method is tested in the case of 1 keV electron and 500 keV antiproton projectiles. The semiclassical results show reasonable agreement with the experiments and other theories. The origin of the obtained structures has been investigated by a partial wave analysis.

Coherent pion production by neutrinos

In this paper we present quantitative results for coherent pion production by neutrinos scattered off nuclei within the framework developed by Gounaris, Kartavtsev, and Paschos. The method is based on partially conserved axial-vector current and uses helicity cross sections for the scattering of weak gauge bosons on nuclei. The process relies on experimental data for elastic pion-nucleus scattering. A detailed analysis of the differential and integrated cross sections is presented for neutral and charged currents, with special emphasis on the regions of integrations. The results are extended to energies of 10.0 GeV and are compared with experimental data.

Multi-ionization of helium by slow highly charged ions

The double to single ionization ratios ${R}_{21}$ of helium bombarded by slow $({v}_{\text{Bohr}}lvl4{v}_{\text{Bohr}})$ ${\text{C}}^{2+}$ and ${\text{C}}^{3+}$ ions are measured by the time-of-flight and coincidence technique. When the velocity is about $3{v}_{\text{Bohr}}$, the values of ${R}_{21}$ by ${\text{C}}^{q+}$ reach a maximum and are found to be 2 times the values for bare ions such as ${\text{He}}^{2+}$ and ${\text{Li}}^{3+}$. A simple model is presented to estimate the single and double ionization cross sections of helium by various ions, and to present the scenario of multi-ionization in the low to intermediate energy range. The calculation results by the model are in excellent agreement with the experimental data.

Identification of higher order contributions in three-dimensional (e,2e) cross-sections for helium

Fully differential cross-sections for single ionization of helium by 102 eV and by 1 keV electron impact have been obtained using an advanced reaction microscope. The data cover a large range of emission angles for a low-energy ( E ≤ 15 eV) electron and different scattering angles for the fast electron. Significant electron emission out of the projectile scattering plane in between the binary and the recoil lobes is observed. The experimental data are compared with theoretical predictions from a three-Coulomb wave function model.