Range Of Electron Densities Research Articles

Measurements of the effective electron density in an electron beam ion trap using extreme ultraviolet spectra and optical imaging.

In an electron beam ion trap (EBIT), the ions are not confined to the electron beam, but rather oscillate in and out of the beam. As a result, the ions do not continuously experience the full density of the electron beam. To determine the effective electron density, n e,eff, experienced by the ions, the electron beam size, the nominal electron density n e, and the ion distribution around the beam, i.e., the so-called ion cloud, must be measured. We use imaging techniques in the extreme ultraviolet (EUV) and optical to determine these. The electron beam width is measured using 3d → 3p emission from Fe xii and xiii between 185 and 205 Å. These transitions are fast and the EUV emission occurs only within the electron beam. The measured spatial emission profile and variable electron current yield a nominal electron density range of n e ∼ 1011-1013 cm-3. We determine the size of the ion cloud using optical emission from metastable levels of ions with radiative lifetimes longer than the ion orbital periods. The resulting emission maps out the spatial distribution of the ion cloud. We find a typical electron beam radius of ∼60 μm and an ion cloud radius of ∼300 μm. These yield a spatially averaged effective electron density, n e,eff, experienced by the ions in EBIT spanning ∼ 5 × 109-5 × 1011 cm-3.

Forbidden component of the Be II 436.1 nm line recorded from pulsed gas discharge plasma

We report results of the experimental study of the singly charged beryllium spectral line 436.1 nm, transition , and forbidden component, transition , which is for the first time identified in this work. Beryllium lines are recorded from gas discharge, after ablation of a beryllium oxide discharge tube, running in pulsed regime with the following gases: helium with traces of hydrogen, argon with traces of hydrogen and pure krypton. The ratio of line intensities and wavelength separation of the Be II 436.1 nm line and neighbouring line located at the blue wing are followed in the electron density range (1.16–6.4) × 1022 m−3 determined from the hydrogen Balmer beta line () in the electron temperature interval between 10500 K and 15500 K. The functional dependence of the wavelength separation range and peak intensity ratio of these lines upon electron number density suggests the complex profile of the forbidden and allowed line, which can be used for diagnostics of low-temperature beryllium containing plasmas.

Modeling and experimental characterization of the plasma produced by a velvet cathode in a linear induction accelerator

A high power electron diode is used to generate an intense high quality pulsed electron beam (2.6 kA, 4 MeV) in order to produce a bremsstrahlung radiation pulse for the first axis of the flash radiography facility named Epure. The plasma created by surface flashover of the velvet cathode provides a zero work function emitter used to produce the space-charge-limited electron beam. In order to optimize the design of diodes and to better understand the plasma dynamics, we have developed a 0D Collisional-Radiative Model (CRM) which describes the time-dependent evolution of the density of the plasma species in three plasma compositions: 100% H, 95% H + 5% C, and 64% H + 31% O + 5% C. The non-equilibrium electron energy distribution functions (EEDFs) are pre-calculated for a large range of electron mean energies for each mixture. EEDFs and cross sections of electron impact processes are then used to tabulate the electron rate coefficients needed to simulate the time-dependent plasma species densities. The evolution of the electron mean energy is estimated through the electron beam current density itself inferred from particle-in-cell calculations experimentally validated. Visible emission spectroscopy measurements were performed in order to investigate the plasma composition, to measure the Hα/Hβ intensity ratio, and to estimate the electron density. It was shown that the electron density in our setup is between 1014 and 1016 cm−3. Experimental results were compared to the 0D CRM. Results from the time-dependent CRM show that for the three studied plasma compositions, maximum electron densities range from 0.9 × 1014 cm−3 to 1.7 × 1014 cm−3 and that the electron energy averaged over the time interval of the electronic beam pulse is about 3.5 eV. An estimate of the electron energy inside the plasma during the relaxation provides values ranging from 0.10 eV to 0.12 eV based on comparison between the calculated and measured Hα/Hβ intensity ratio averaged over 5 μs. Moreover, main reactions involved in the evolution of the electron density were pointed out.

Path integral approach for Stark broadening of Lyman lines in hydrogen plasma

New results for Lyman lines from hydrogen plasmas are presented using the path integral approach. The influence of plasma components (electrons and ions) on the radiator is analysed separately. The ionic contribution is treated within the path integral approach, while the electronic contribution is estimated by the standard collision operator. The Stark effect, including the ion quadrupole contribution, is considered. The time‐dependent ionic microfield is treated within the path integral approximation using the model microfield method (MMM). The comparison with the quantum statistical approach is performed using a wide range of temperatures (T = 104–107 K) and electron densities (Ne = 1023–1026 m−3). Good agreement is mainly obtained for low density and high temperature.

Simulation of Physical Phenomena in the Ionosphere and Magnetosphere of the Earth on Krot Plasma Device. Some Results and Prospects

Krot device is a unique installation of the scientific infrastructure of the Russian Federation. It is a source of highly uniform low-temperature plasma that takes up to several tens of cubic meters. The setup makes it possible to perform both scaling laboratory simulations of ionospheric and magnetospheric phenomena in the approximation of unbound plasma, as well as plasma tests of full-size samples of the onboard equipment of spacecraft (SC). The simulation results of the dynamics and interaction of small-scale thermal plasma irregularities occurring during ionospheric heating experiments are presented. The impedance of the small-size models of RESONANCE and STRANNIK SC electric antennas in plasma is measured. The possible use of free-space calibration of antennas in the magnetospheric portions of the orbit in the ELF and VLF ranges is confirmed. The efficiency of a new plasma parameters resonance sensor for the TRABANT SC in the ionospheric range of electron densities is shown.

Zeeman Splitting of Electron Spectrum in HgTe Quantum Wells Near the Dirac Point

The Zeeman splitting of the conduction band in the HgTe quantum wells both with normal and inverted spectrum has been studied experimentally in a wide electron density range. The simultaneous analysis of the Shubnikov–de Haas oscillations in low magnetic fields at different tilt angles and of the shape of the oscillations in moderate magnetic fields gives a possibility to find the ratio of the Zeeman splitting to the orbital one and anisotropy of g-factor. It is shown that the ratios of the Zeeman splitting to the orbital one are close to each other for both types of structures, with a normal and inverted spectrum and they are close enough to the values calculated within kP method. In contrast, the values of g-factor anisotropy in the structures with normal and inverted spectra are strongly different and for both cases differ significantly from the calculated ones. We assume that such disagreement with calculations is a result of the interface inversion asymmetry in the HgTe quantum well, which is not taken into account in the kP calculations.

Finite-T correlations and free exchange-correlation energy of quasi-one-dimensional electron gas

We have studied the effect of temperature on static density–density correlations and plasmon excitation spectrum of quasi-one-dimensional electron gas (Q1DEG) using the random phase approximation (RPA). Numerical results for static structure factor, pair-correlation function, static density susceptibility, free exchange-correlation energy and plasmon dispersion are presented over a wide range of temperature and electron density. As an interesting result, we find that the short-range correlations exhibit a non-monotonic dependence on temperature T, initially growing stronger (i.e. the pair-correlation function at small inter-electron spacing assuming relatively smaller values) with increasing T and then weakening above a critical T. The cross-over temperature is found to increase with increasing coupling among electrons. Also, the [Formula: see text] peak in the static density susceptibility [Formula: see text] at T = 0 K smears out with rising T. The free exchange-correlation energy and plasmon dispersion show a significant variation with T, and the trend is qualitatively the same as in higher dimensions.

Electron-density-sensitive Line Ratios of Fe xiii– xvi from Laboratory Sources Compared to CHIANTI

Abstract We present electron-density-sensitive line ratios for Fe xiii– xvi measured in the spectral wavelength range of 200–440 Å and an electron density range of (1–4) × 1013 cm−3. The results provide a test at the high-density limit of density-sensitive line ratios useful for astrophysical studies. The measurements were performed on the National Spherical Torus Experiment-Upgrade, where electron densities were measured independently by the laser Thomson scattering diagnostic. Spectra were collected with a flat-field grazing-incidence spectrometer, which provided a spectral resolution of up to 0.3 Å, i.e., high resolution across the broad wavelength range. The response of the instrument was relatively calibrated using spectroscopic techniques in order to improve accuracy. The line ratios are compared to other laboratory sources and the latest version of CHIANTI (8.0.2), and an agreement within 30% is found.

X-ray observations of emission from medium Z He-like ions in C-Mod tokamak plasmas

X-ray spectra of n = 3–1 transitions in He-like ions (and satellites) from calcium, argon and chlorine have been measured in the core of Alcator C-Mod tokamak plasmas using high wavelength resolution x-ray spectrometer systems. The intensity ratio of the intercombination line y3 (1s3p 3P1–1s2 1S0) to the resonance line w3 (1s3p 1P1–1s2 1S0) is found to be much larger than what is expected if collisional excitation out of the ground state is considered as the only population mechanism for the upper levels. This suggests that recombination and cascades from higher levels with n 4 are important. Modeling with the MARIA code is in good agreement with the observations, demonstrating the importance of recombination population of the upper level for y3. The intensity ratio y3/w3 has been studied over a large range of core electron temperature and density, and radial position in the plasma. The observed ratio decreases with increasing Te, increases with increasing Z and is independent of ne, in agreement with modeling.

Zeeman splitting of electron spectrum in HgTe quantum wells near the Dirac point

AbstractThe Zeeman splitting of the conduction band in the HgTe quantum wells both with normal and inverted spectrum has been studied experimentally in a wide electron density range. The simultaneous analysis of the Shubnikov–de Haas oscillations in low magnetic fields at different tilt angles and of the shape of the oscillations in moderate magnetic fields gives a possibility to find the ratio of the Zeeman splitting to the orbital one and anisotropy of g -factor. It is shown that the ratios of the Zeeman splitting to the orbital one are close to each other for both types of structures, with a normal and inverted spectrum and they are close enough to the values calculated within kP method. In contrast, the values of g -factor anisotropy in the structures with normal and inverted spectra are strongly different and for both cases differ significantly from the calculated ones. We assume that such disagreement with calculations is a result of the interface inversion asymmetry in the HgTe quantum well, which is not taken into account in the kP calculations.

In-depth study of intra-Stark spectroscopy in the x-ray range in relativistic laser–plasma interactions

Intra-Stark spectroscopy (ISS) is the spectroscopy within the quasistatic Stark profile of a spectral line. The present paper advances the ISS-based study of the relativistic laser–plasma interaction from our previous paper (Oks et al 2017 Opt. Express 25 1958). By improving the experimental conditions and the diagnostics, it provides an in-depth spectroscopic study of the simultaneous production of the Langmuir waves and of the ion acoustic turbulence at the surface of the relativistic critical density. It demonstrates a reliable reproducibility of the Langmuir-wave-induced dips at the same locations in the experimental profiles of Si XIV Ly-beta line, as well as of the deduced parameters (fields) of the Langmuir waves and ion acoustic turbulence in several individual 1 ps laser pulses and of the peak irradiances of 1–3 × 1020 W cm−2. Besides, this study employs for the first time the most rigorous condition of the dynamic resonance, on which the ISS phenomenon is based, compared to all previous studies in all kinds of plasmas in a wide range of electron densities. It shows how different interplays between the Langmuir wave field and the field of the ion acoustic turbulence lead to distinct spectral line profiles, including the disappearance of the Langmuir-wave-induced dips.

Indication of band flattening at the Fermi level in a strongly correlated electron system

Using ultra-high quality SiGe/Si/SiGe quantum wells at millikelvin temperatures, we experimentally compare the energy-averaged effective mass, m, with that at the Fermi level, mF, and verify that the behaviours of these measured values are qualitatively different. With decreasing electron density (or increasing interaction strength), the mass at the Fermi level monotonically increases in the entire range of electron densities, while the energy-averaged mass saturates at low densities. The qualitatively different behaviour reveals a precursor to the interaction-induced single-particle spectrum flattening at the Fermi level in this electron system.

Stark shift of the CI 2479 Å spectral line of carbon

The Stark shift of the CI 2479 A emission line of atomic carbon is measured in a broad range of electron densities between ~6 × 1016 and ~4 × 1018 cm–3. It is established that the shift is linear and its value is significantly lower than that predicted by theory. The line shape is found to be dominated by the Stark effect at Т = 32 kK and electron concentration N e ≈ 3.9 × 1018 cm–3. The electron-impact and ion widths of this contour are in good agreement with the Grim data, while its total shift to longer wavelengths is found to be half that calculated theoretically.

Plasma sheath: An equivalent nonlinear mirror between electron density and transmitted electromagnetic signal

An experiment on the propagation of electromagnetic (EM) signals in continuous time-varying plasma is designed to establish the nonlinear mirror between electron density and transmission coefficient. The nonlinearity is confirmed from the theoretical and experimental results. The amplitude and phase can be considered nonlinear functions of electron density when the complex interaction between plasma and EM waves is ignored. Results show that amplitude and phase distributions are asymmetrical when electron density follows symmetric distribution. The skewness of amplitude is positive, whereas the skewness of phase is negative. The nonlinear degree is closely related to the ratio of plasma frequency to the incident wave frequency and the range of electron density. The conclusions are crucial to the modeling of plasma sheath channels and understanding the blackout problem.

(Keynote) Interface Control for High Performance N-Channel Ge FETs

The demonstration of high electron mobility Ge n-MOSFETs was not an easy task. To overcome such big challenge, three carrier scattering mechanisms in Ge MOS inversion layer such as Coulomb scattering, phonon scattering, and surface roughness scattering were carefully investigated. The first step was to improve the peak electron mobility by reducing the interface states density (Dit ). We succeeded in the reduction of Dit (<1011 eV-1cm-2) at Ge/GeO2 interface by employing the high-pressure O2 (HPO) oxidation [1]. This is quite reasonable thermodynamically [2]. However, there still remained two critical challenges. One was how to suppress the significant degradation of high-Ns electron mobility. The other one was how to keep electron mobility high in thin EOT region [3]. It is generally considered that high-Ns electron mobility is dominated by the interface roughness scattering. Since we demonstrated that Ge(111) was the best surface orientation for electrons to achieve the highest peak electron mobility, we challenged to realize the atomically flat Ge(111) surface by using H2 annealing [4]. Then, MOSFETs on Ge(111) with and without atomically flat surface were fabricated with HPO. N-channel MOSFETs fabricated by HPO on atomically flat Ge(111) exhibited greatly improved high-Ns electron mobility [5]. EOT reduction was also challenging because HPO should naturally form thicker GeO2. This fact was a big hurdle on Ge FET technology. Meanwhile, it was interestingly found that the low-temperature HPO (LT-HPO) reduced the oxidation rate [6]. This fact enabled us to achieve ~2 nm EOT GeO2 on Ge with superior interface and bulk GeO2. Another way to reduce EOT is obviously to replace HPO-GeO2 with other high-k oxides which are appropriate for Ge. We have fortunately found that Y2O3 is very friendly with Ge [7]. Furthermore, in our recent experiments Y2O3-doped GeO2 (YGO) is much better than pure Y2O3 on Ge. Significantly improved electron mobility in n-channel Ge FETs in a wide range of electron density has been achieved, when YGO in addition to H2 annealing has been used for the interfacial layer in gate stacks [8]. Poor electron mobility in n-channel Ge FETs is not true now. We can engineer the Ge interface through understanding of thermodynamics in gate stack formation and of kinetics of both surface planarization and oxidation. Thus, we can safely conclude that Ge is very promising not only for p-channel but also for n-channel FETs.

Dynamics of Surface Streamer Plasmas at Atmospheric Pressure: Mixtures of Argon and Methane

A nonequilibrium atmospheric streamer discharge was investigated as a means to seed a large-gap arc breakdown. The dynamics of the streamer were analyzed with high speed imaging, photodiode light intensity, and current–voltage measurements. The temporal evolution of the discharge included a localized surface corona and a positive surface streamer. With the addition of an impurity gas (methane), the ionization was suppressed, which inhibited surface streamer propagation. The electron temperature was determined from time and spatially averaged spectra, coupled with a collisional–radiative model. The electron temperature in argon was measured at 1.25 eV for an electron density range of 1019 – 1020 m –3. Partial local thermodynamic equilibrium calculations showed that the Ar II 4p states followed a Boltzmann distribution with an excitation temperature of 0.7 eV. The gas temperature was estimated at 815 K from a black-body distribution. The velocity of the surface streamer in argon was estimated at ~100 km/s with a diameter of $\sim 500~\mu \text{m}$ .

High-field electron transport in doped ZnO

Current–voltage characteristics have been measured for ZnO:Ga and Zn:Sb epitaxial layers with electron densities ranging from cm−3 to cm−3. Two-terminal samples with coplanar electrodes demonstrate virtually ohmic behavior until thermal effects come into play. Soft damage of the samples takes place at high currents. The threshold power (per electron) for the damage is nearly inversely proportional to the electron density over a wide range of electron densities. Pulsed voltage is applied in order to minimize the thermal effects, and thus an average electric field of 150 kV cm−1 is reached in some samples subjected to 2 ns voltage pulses. The results are treated in terms of electron drift velocity estimated from the data on current and electron density under the assumption of uniform electric field. The highest velocity of ∼ cm s−1 is found at an electric field of ∼100 kV cm−1 for the sample with an electron density of cm−3. The non-ohmic behavior due to hot-electron effects is weak, and the dependence of the electron drift velocity on the doping resembles the variation of mobility.

Experimental Stark parameters of Mn I lines in the y6P° → a 6 S multiplet under conditions of “long” laser plasma

The Stark broadening parameters for the components of the y6P° → a 6 S Mn I multiplet have been determined under conditions of “long” laser plasma: the electron impact width and line shift due to the quadratic Stark effect have been measured in the range of electron densities of (4.8–45) × 1016 cm–3. The presence of positive Stark shifts of the multiplet components (from +0.1 to +0.15 A) has been demonstrated for the first time. The Stark widths are found to increase in the multiplet with a decrease in J of the upper level and significantly decrease with an increase in the plasma temperature. The d/w ratio is identical for all multiplet components; it amounts to 0.35–0.65, depending on the observation conditions. The dependences of this ratio on the plasma temperature and electron density have also been considered.

A novel electron mirror pulse compressor

An electron mirror-based pulse compressor design has been developed for improving the temporal resolution of dynamic/ultrafast transmission electron microscopes and ultrafast electron diffraction cameras. The improvement will enable electron microscopes and diffraction cameras to better resolve the dynamics of reactions in the areas of solid state physics, chemistry, and biology. The design utilizes a combination of mirror optics and a magnetic beam separator, which exploits the symmetry inherent in reversing the electron trajectory in the mirror in order to compress the pulse. This system can also simultaneously correct the spherical and chromatic aberration of the objective lens for improved spatial resolution. For pulsed experiments with a practical bunch charge, the correction of the chromatic aberration coefficient counters the spread in the electron energies induced by the space charge of the pulse to make possible the probing of the sample with high spatial resolution. The pulse compressor can accommodate pulses with a range of electron densities and energy spreads. Furthermore, it is designed to fit into both ultrafast electron diffraction cameras and dynamic/ultrafast transmission electron microscopes. Consequently, this instrument is suitable for enhancing the study of the structure, composition, and bonding states of new materials at ultrafast time scales.

Zeeman splitting of conduction band in HgTe quantum wells near the Dirac point

The Zeeman splitting of the conduction band in the HgTe quantum wells both with normal and inverted spectrum has been studied experimentally in a wide electron density range. The simultaneous analysis of the Shubnikov-de Haas (SdH) oscillations in low magnetic fields at different tilt angles and of the shape of the oscillations in moderate magnetic fields gives a possibility to find the ratio of the Zeeman splitting to the orbital one and anisotropy of g-factor. It is shown that the ratios of the Zeeman splitting to the orbital one are close to each other for both types of structures, with a normal and inverted spectrum and they are close enough to the values calculated within kP method. In contrast, the values of g-factor anisotropy in the structures with normal and inverted spectra are strongly different and for both cases differ significantly from the calculated ones. We assume that such disagreement with calculations is a result of the interface inversion asymmetry in the HgTe quantum well, which is not taken into account in the kP calculations.