Electron Impact Ionization Of Helium Research Articles

RF plasma simulations using the TOMATOR 1D code: a case study for TCV helium ECRH plasmas

The 1-dimensional reaction-diffusion-convection code TOMATOR-1D describes plasma production by RF waves inside a tokamak using the Braginskii continuity and heat balance equations. The model simulates self-consistent radial density and temperature profiles for magnetised plasma mixtures of hydrogen and helium. The model reproduces the density profiles of X2 electron cyclotron resonance heating (ECRH) plasmas on TCV and proposes a Bohm-like poloidal magnetic field dependent scaling for anomalous diffusion and a convection scaling that results from drifts in the toroidal magnetic field configuration. A relation is proposed between the anomalous diffusion and the outward convection in toroidal plasmas. It is found that the EC absorption efficiency decreases at higher power, which is understood from the acceleration of electrons beyond the optimal energy for the electron impact ionisation of helium. A dramatic increase of the absorption efficiency is seen at intermediate vertical magnetic field values of which results in the highest density plasmas. Losses along the field lines in the vertical direction become dominant at higher fields which effectively reduces the plasma density in these discharges. To arrive at predictive capabilities towards ECRH plasmas on JT-60SA and ITER, the proposed scalings, subsuming dependencies on the torus major radius and the toroidal field strength, need to be validated in a multi-machine study.

Infrared Spectroscopy of Water and Zundel Cations in Helium Nanodroplets.

Here, we show that electron impact ionization of helium (He) droplets doped with water molecules and clusters yield water and Zundel cations embedded in the droplets consisting of a few thousand helium atoms. Infrared spectra in the OH-stretching range were obtained using the release of the cations from the droplets upon laser excitation. The spectra in He droplets appear to have about a factor of 10 narrower bands and similar matrix shifts as compared to those obtained via tagging with He and Ar atoms. The results confirm the calculated structure of the free Zundel ion, where the proton is equidistant from the two water units. The effect of the He environment on the spectra of ions is discussed. The signal shows nonlinear laser pulse energy dependence consistent with the evaporation of the entire droplet upon multiple absorptions of infrared photons. This conclusion is supported by the model calculations of the efficiency of the cations' release vs laser flux.

Electron impact ionization of helium, neon and argon at intermediate energies

With a relatively simple distorted-wave model, in which the ejected and the scattered electrons see different residual systems depending on their energies, in this paper we show the calculation of the CPE cross sections for electron impact ionization of He, Ne and Ar. Combined with the ‘maximum interference’ exchange approximation, this model gives results which for Ne and Ar are in better agreement with the experiment than the other existing theoretical models.

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.

Propensity for distinguishing two free electrons with equal energies in electron-impact ionization of helium

We report a combined experimental and theoretical study on the electron-impact ionization of helium at ${E}_{0}\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}70.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ and equal energy sharing of the two outgoing electrons (${E}_{1}={E}_{2}=23\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$), where a double-peak or dip structure in the binary region of the triple differential cross section is observed. The experimental cross sections are compared with results from convergent close-coupling (CCC), $B$-spline R-matrix-with-pseudostates (BSR), and time-dependent close-coupling (TDCC) calculations, as well as predictions from the dynamic screening three-Coulomb (DS3C) theory. Excellent agreement is obtained between experiment and the nonperturbative CCC, BSR, and TDCC theories, and good agreement is also found for the DS3C model. The data are further analyzed regarding contributions in particular coupling schemes for the spins of either the two outgoing electrons or one of the outgoing electrons and the $1s$ electron remaining in the residual ion. While both coupling schemes can be used to explain the observed double-peak structure in the cross section, the second one allows for the isolation of the exchange contribution between the incident projectile and the target. For different observation angles of the two outgoing electrons, we interpret the results as a propensity for distinguishing these two electrons---one being more likely the incident projectile and the other one being more likely ejected from the target.

Electron impact ionization of helium: test of screening potential in the final state

The triple differential cross sections for electron impact ionization of a helium atom is studied using the 70.6 eV incident electron. The initial state which includes both bound and continuum wave functions is calculated using the Hartree–Fock approximation. The final state continuum electron wave functions are computed in the potential of the core. The interaction potential between the two continuum electrons in the final state is approximated with the screening potential determined variationally. It is found that there is qualitative agreement between the present triple differential cross section results and the available experimental and the other theoretical data for equal and unequal sharing of 46 eV excess energy by the two final state continuum electrons. It is concluded that the screening potential which is an approximation to the actual Coulomb interaction potential between the two final state continuum electrons can produce qualitative agreement with the experiment.

Double Differential Cross-Section Measurements for Electron Impact Ionization of Helium

Double Differential Cross-Section Measurements for Electron Impact Ionization of Helium

NonperturbativeB-splineR-matrix-with-pseudostates calculations for electron-impact ionization of helium

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Momentum transfer dependence of the triple differential cross section for electron impact ionization of helium at incident energy of 150 eV

The momentum transfer dependence of the cross section for electron impact ionization of helium at an incident energy of 150 eV is investigated by using the BBK model and the DS3C model. The results of the present work are compared with experimental data. The structure of the cross section is analyzed, and the exchange effect is discussed.

Theoretical study of shield effect in the momentum transfer dependence of the cross section

The momentum transfer dependence of the cross section for electron impact ionization of helium for (e,2e) process is investigated by use of BBK model and modified BBK model. The results of the present work are compared with experimental data. The structure of the cross section is analysed, and the effective shield is discussed in detail in the final state.

Strong Dynamical Screening of the ( e, 2e) Processes for Electron Impact Ionization of Helium in the Perpendicular Plane Symmetric Geometry

We apply BBK, DS3C model and the new modified Sommerfeld parameters of the present DS3C model to calculation of the triple differential cross sections (TDCS) for electron impact ionization of helium in the perpendicular plane symmetric geometry. The results of the present are compared with the measurements and those of other theoretical models. It was found that the present results give a better description for the experimental data and the dynamical screening effects are strong in this geometry. The influence of the Sommerfeld parameters on the triple differential cross sections is also analyzed and discussed in the perpendicular plane geometry.

Theoretical study on (e,2e) process for helium in perpendicular geometry

The triple differential cross section for electron impact ionization of helium in the perpendicular geometry is calculated by use of BBK model and modified BBK model. The results of the present work are compared with experimental data. The structure of the cross section is analysed and the exchange effects are discussed systematically.

Three-body coupling for electron impact ionization of helium in the perpendicular plane geometry

Dynamically screened three-Coulomb-wave (DS3C) model is applied to study single ionization of helium by 102 eV electron impact. Fully differential cross sections (FDCS) are calculated at different scattering angle of (8°, 10°, 15°, 20°) in the perpendicular plane asymmetric geometry. The results are compared with experimental data and theoretical predictions from a three-Coulomb wave function (3C) approach, convergent close-coupling calculation (CCC), as well as second-order distorted-wave Born method (DWB2). It is shown that three-body coupling effects are important for the perpendicular plane geometry.

A theoretical study on final channel screening and exchange effects in large energy loss geometry

The triple differential cross sections for electron impact ionization of helium in the special geometry of coplanar large energy loss are calculated by using the BBK model and the modified BBK model. The calculated results are compared with the experimental results of Catoire and the present experimental results of Stevenson. The previous theoretical deduction is proved and the exchange effects are analyzed and discussed.

Fully differential cross sections for 102eV electron-impact ionization of helium in non-coplanar geometry

DS3C model is applied to study single ionization of helium by 102eV electron impact. Fully differential cross sections are calculated at different scattering angles of θ1 (8°，10°，15°，20°) for the perpendicular plane geometry. The results of 3C and CCC model were compared with experimental data, which sho ws that DS3C model can give a successful description of the (e，2e) process for noncoplanar geometry qualitatively，indicating the existence of strong dynamic correlations effect between the three particles in the final channel for noncoplanar geometry.

Investigation of the closure and simplified Green's function approximations in second-order distorted-wave calculations for the (e, 2e) processes

Triple-differential cross sections for electron impact ionization of helium at 102 eV are presented for both coplanar and perpendicular plane asymmetric geometries within the framework of second-order distorted-wave theory. The closure approximation and the simplified Green's function approximation, which are currently used in the evaluation of the second-order amplitudes, are investigated and the accuracy of the approximations is examined. It is found that second-order effects are significant for this incident energy and the simplified Green's function approximation is generally more valid than the closure approximation.

Experimental and theoretical momentum transfer dependence of the He (e, 2e) cross section for incident electron energies 150 eV and 488 eV

The relative triple differential cross section for the electron impact ionization of helium has been measured and calculated for incident energies of 150 eV and 488 eV, and an ejected electron energy 34.5 eV. At both incident energies coplanar (e, 2e) experiments were carried out for a fixed pair of ejected directions +90°, − 90° and a range of scattering angles −30° → +30°, and also for a fixed pair of ejected directions +75°, − 105° and a range of scattering angles −34° → +18°. The kinematics of these experiments cover a momentum transfer range 0.38 → 3 au. The data are presented directly as pairs of (e, 2e) scattered electron angular distributions, and in the manipulated form of their sum, difference and the ratio difference/sum. These are compared with up to five types of theoretical calculations. Good, but not perfect, agreement is found between experiment and calculations that include significant post-collision interaction effects. All calculations predict an almost identical oscillatory feature, in the difference and ratio of the cross sections, which is observed in the experimental data for incident energy 150 eV and is due to the fact that the cross section passes through zero close to, but not exactly at, 0° scattering.

A theoretical study on double differential cross sections for electron impact ionization of helium at low energies

In the present work we have studied the double differential cross sections for single ionization of a helium atom by electron impact at low energies. The double differential cross sections of the secondary ejected electron have been obtained by integrating the triple differential cross sections over the entire angular range of the primary scattered electron. The double differential cross sections for electron impact ionization of helium at incident energies of 26.3，28.3，30.3，32.5，34.3，36.5 and 40.7eV have been calculated in the BBK and DS3C model. The present DS3C results are compared with the absolute measurements，and reasonably good agreement has generally been found except for low incident energies (e.g.，26.3eV) and low ejected angles. In addition，the direct and exchange amplitudes are also considered. It has also been shown how the exchange effects contribute to the cross sections.

A theoretical study on (e, 2e) process for helium in large energy loss and close to minimum momentum transfer geometry

The triple differential cross section for electron impact ionization of helium in the special geometry of coplanar large energy loss and close to minimum momentum transfer are calculated by use of BBK model and modified BBK model. The structure of the cross section is discussed.The results of the present work are compared with current experimental data. It is found that both BBK and modified BBK models are in good agreement with the experimental results of binary-to-recoil peak ratio and their angular distribution.

Electron-impact ionization of helium with large energy transfer

We consider the recently measured case of 730 eV electron-impact ionization of the ground state of helium with 205 and 500 eV coplanar outgoing electrons by Catoire et al. J. Phys. B 39 2827 (2006)]. These measurements, which are on a relative scale, show some unexpected structure and variation from the second-order distorted-wave Born approximation R-matrix and Brauner-Briggs-Klar theories. Using the convergent close-coupling method we provide an improved agreement with experiment, but some discrepancies still remain.