Spectroscopic study of a brush cathode plasma in a magnetic field

The brush cathode helium discharge in the magnetic field has been operated stably at discharge currents larger than those without magnetic field. The diameter of the plasma column has been determined by the configuration of the magnetic field. The measurements of the spectral intensities of the recombination continuum followed by the 2³S-n³P series reveals that the electron density is 1·8 × 10¹³ cm^-3 and the electron temperature is 0·17 eV at a discharge current of 500 mA and a pressure of 0·9 torr for a magnetic flux density of 1·3 kG. The principal quantum number for line merging is 20.     

Use of the 447 nm He I line to determine electron concentrations

The helium 447 nm complex line has been excited in a wall-stabilized arc fed at atmospheric pressure by pure He, He-H₂, He-Ne-H₂ or He-Ar-H₂ mixtures. Photoelectric endon observations of the central part of the arc channel were made with high spatial (1/600) and spectral (53,000) resolution.A collection of 88 helium 447 nm line profiles, of which 75 were recorded simultaneously with H_β and Ne I or Ar II line profiles, yielded information about the electron concentration (2 X 10²⁰-2 X 10²² m^-3), temperature and relative ion composition of the plasma. Plots have been made of the forbidden to allowed peak separations (S), forbidden to allowed relative intensities of the peaks (F/A) and dips (i.e. the minimum intensities between lines) to allowed peak intensities (D/A) as functions of electron concentration, temperature and ionic composition in different plasmas.The peak separations depend only on the electron concentration. Other characteristic line-profile parameters (F/A and D/A) show weak ion motion with a strong electron-concentration influence. We propose simple formulae, which may be useful for practical determinations of the electron concentrations in helium-containing plasmas with an accuracy of ±15% and without taking into account either the chemical composition of the plasma or the temperature.     

Change of phonon dispersion curves due to interstitials in Al

An experimental investigation of the profiles of the 4471 Å and 4922 Å lines in the afterglow of an high density plasma of helium produced by laser is reported. The measured profiles of these two neutral helium lines and their forbidden components are given at electron densities between 5 · 10¹⁶ cm^?3 and 2 · 10¹⁷ cm^?3 and for electron temperature between 3 and 4 eV.     

Langmuir probe and spectroscopic studies of RF generated helium-nitrogen mixture plasma

This paper reports the effect of helium percentage variation in a capacitive RF helium-nitrogen mixture plasma on various plasma parameters and concentration of nitrogen active species (N₂(C³Π _u) and N₂ ⁺(B²Σ _u ⁺)). Langmuir probe is used for determination of electron energy distribution functions, effective electron temperature, plasma potential and electron density. Optical emission spectroscopy is used for determination of electron temperature from Boltzmann's plot of He–I lines and the relative changes in the concentration of active species by measuring the emission intensities of nitrogen (0-0) bands of the second positive and the first negative systems. The results demonstrate that electron temperature, electron density and concentration of active species increase significantly with increase in helium percentage in the mixture and RF power.     

Negative hall effect of hot holes in silicon

Hot electron effects of low density holes in silicon were studied in the helium temperature with electric fields up to 5000 V cm^?1. Hall current of electron polarity was observed in high electric fields at low magnetic fields. The phenomenon is ascribed to the re-entrant energy surface of holes in silicon.     

Field evaporation experiments with a magnetic sector atom-probe FIM

A magnetic sector atom-probe FIM has been successfully operated for dc field evaporation of tip materials such as Rh, W, Ir, Mo and Ti. A limited number of evaporated metal ions were clearly identified forming a line spectrum. Field evaporation of Rh in the presence of ³He and ⁴He gases showed that the formation of the helium compound (RhHe)²⁺ is quite sensitive to He gas pressure; no helium compound were observed below 5 × 10^?7 Torr and all ions detected as helium compound above 5 × 10^?5 Torr at 78 K.     

Generation of harmonics of femtosecond radiation from the surface of aluminum targets

The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (λ=1.06 μm, t=475 fs, and I～2×10¹⁷ W cm^?2) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I<10¹⁷ W cm^?2). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered.     

Spatial-temporal measurements of magnetic fields in laser-produced plasmas

The results of an investigation of the electromagnetic wave polarization, probing high-temperature laser plasma, as well as spatial-temporal structure of the magnetic fields, electron density, current density, and electron drift velocity are presented. To create the plasma, plane massive Al targets were irradiated with the second harmonic of a phoenix Nd laser at intensities up to 5·10¹⁴ W/cm². It was shown that the magnetooptical Faraday effect is the main mechanism responsible for the changing polarization of the probing wave. Magnetic fields up to 0.4 MG with electron densities ∼10²⁰ cm⁻³ were measured. Analysis of the magnetic field spatial distribution showed that the current density achieved the value ∼90 MA/cm² on the laser axis. The radial structure of the magnetic field testified to the availability of the reversed current in the laser plasma. The spatial and temporal resolutions in these experiments were equaled to ∼5 μsec and ∼50 psec, respectively. Translated from Preprint No. 35 of the Lebedev Physics Institute, Moscow, 1993.     

Role of neon in a decaying high-purity helium plasma

The evolution of the electron and atomic and molecular metastable densities and the radiation of the decaying plasma of helium with a 10^–5-fraction of neon additive is experimentally studied. A model of elementary processes in He–Ne plasma is constructed, which describes the formation and destruction of HeNe⁺ and Ne₂ ⁺ molecular ions and their contribution to the formation of the afterglow spectrum by the electronion recombination. The various criteria influence of neon on the parameters of the decaying plasma are studied. The possibility of determining the amount of neon in helium by measuring the relative intensities of helium molecular bands and neon spectral lines in the afterglow is considered.     

Extreme accumulation layers on ZnO surfaces due to He+ ions

Accumulation layers with excess surface electron densities of up to 10¹⁴ cm^?2 have been produced on the (001) face of ZnO single crystals by an electrical discharge in helium atmosphere. Hall effect measurements show that at these extreme surface electron densities both the electron density and the mobility are temperature independent from room temperature down to 1.6 K.     

Reversal of sign of the Hall coefficient in n-InSb in the magnetic freeze out regime

Change in sign of the Hall coefficient has been observed in n-InSb with carrier concentrations ranging from 2 × 10¹⁴ cm^?3 to 6 × 10¹⁴ cm^?3, which occurs at liquid helium temperatures and magnetic fields above 30 kG. The influence of the compensation ratio, temperature, and geometry has been investigated.     

Ionization of helium atoms under the effect of the antineutrino magnetic moment

Differential cross sections for inelastic antineutrino interaction with a helium atom are calculated. It is shown that, in the energy-transfer range extending up to 1 keV, the cross sections in question are considerably enhanced in the electromagnetic-interaction channel in relation to the cross sections for elastic scattering on a free electron. Absolute cross-section values are of interest in searches for the antineutrino magnetic moment, provided that its value in Bohr magneton units falls within the range (10^?13?10^?12)_µB.     

Raman spectroscopy studies of antiferromagnetic FeCO3 and related carbonates

Raman spectroscopy experiments were performed on antiferromagnetic siderite (natural FeCO₃). Weak lines at room temperature (in addition to the expected vibrational lines) were found to be seven well defined excitations at liquid helium temperature. Polarization tensor components of these new lines were examined at temperatures varying between room and liquid helium temperature. Frequency decreases upon cooling were observed for three of the lines (the greatest change occuring near the Néel temperature, 38 K). By comparison with infrared spectra, variable temperature Raman spectra and impurity analysis of two related crystals (antiferromagnetic MnCO₃ and CaMg(CO₃)₂ containing 6% iron), new explanations for two (741 and 1735 cm^?1) of three previously observed lines and for one (870cm^?1) of the remaining four are presented. The three variable frequency lines (440,1175 and 1225 cm^?1) are considered magnetic excitations between trigonal field, spin-orbit, and exchange split states of the ferrous ion. The frequency decreases upon cooling may be due to unquenched orbital angular momentum resulting in an exchange interaction of a non-Heisenberg form. Symmetry distortion due to magnetic ordering upon cooling may cause the infrared 741 cm^?1 vibration to become Raman-active.     

Elektronendichten und Elektronentemperaturen bei Verdichtungsstößen

Electromagnetically driven shock-waves of about Mach 50 in Helium have been spectroscopically analysed. Shape and relative intensities of the HeII-3203 and HeI-5876 lines and continuum have been measured during the first microsecond following the luminous front. From 0.15 μsec up to 0.85 μsec after luminous onset electron density decreases from 2.5·10¹⁷cm^?3 to 10¹⁷cm^?3. The electron temperature drops from 4.5 eV to 4.0 eV. Values as calculated byRankine-Hugoniot coincide with experimental results within 10%. From HeI-5876 and HeI-3889 precursor radiation and no corresponding radiation from HeII-lines it is to be concluded that the signals are due to secondary emission.     

Hall coefficient and conduction mechanism in intermediate concentration n-Ge at helium temperatures

Hall coefficient measurements for intermediate concentration n-type Ge were carried out at liquid helium temperatures. The measurements show that the Hall coefficient and mobility increase with decreasing temperature down to 1.7 K and with increasing magnetic field up to 25 KG. These behaviours are opposite to what was observed in low concentration samples. We conclude that the thermal activated localised hopping motion does not exist in our concentration level, 6 × 10¹⁶ cm^?3, but rather the delocalised quasi-free carriers still dominate the overall conduction for temperature as low as 1.7 K. A model is suggested to explain the Hall mobility behaviour. The model based on the decrease of the dominant scattering mechanism, ionised impurity scattering in our case, as the temperature is lowered and when the magnetic field is increased. From the Hall coefficient behaviour at 4.2 and 1.7 K as well as the resistivity measurements, we found no effect of magnetic field on the unique activation energy existing in this concentration level.     

Cyclotron mechanism of electron acceleration in subpicosecond laser plasma

The effect of ultrastrong magnetic fields generated in a relativistic-intensity subpicosecond laser plasma on the acceleration of fast electrons was studied. It is shown that resonance electrons can continuously accumulate energy from the circularly polarized laser field in the presence of a longitudinal magnetic field. For the linear polarization and a transverse magnetic field, energy accumulation has a pulse-periodic character, and the electron trajectories correspond to electron rotation in the Larmor orbit in a quasi-stationary magnetic field, while the energy strongly oscillates. In both cases, electron energy may attain values higher than 100 MeV for intensities of 10²⁰ W/cm².     

Rapid current-penetration in turbulent heating of a high-density plasma

Radial distribution of electron temperature as well as that of plasma current has been measured in the skin phase in turbulent heating of a high-density plasma (n ? 2 × 10¹⁴m^?3). Anomalously rapid disappearance of the current skin is observed, while the skin profile of the electron temperature remains longer near the plasma core, indicating that the plasma current is redistributed without electron heat transfer across the magnetic field.     

Langmuir Probe Diagnostics of a Low Temperature Non-Isothermal Plasma in a Weak Magnetic Field

This article presents measurements by a cylindrical Langmuir probe in the plasma of a DC cylindrical magnetron discharge át the pressure 1.5 Pa that aim at the experimental assessment of the influence of a weak magnetic field to the estimation of the electron density when using conventional methods of probe data interpretation. The probe data was obtained under the presence of a weak magnetic field in the range 1.10^?2?5.10^?2 T. The influence of the magnetic field on the electron probe current is experimentally assessed for two cylindrical probes with different radii, 50 μm and 21 μm. This assessment is based on comparison of the values of the electron density estimated from the electron current part with the values of the positive ion density estimated from the positive ion current part of the probe characteristic respectively by assuming that at the magnetic field strengths used in the present study the probe positive ion currents are possible to be assumed as uninfluenced by the magnetic field. For interpretation of the probe positive ion current two theories are used and compared to each other: the radial motion model by Allen, Boyd and Reynolds [10] and Chen [11] and the model that accounts for the collisions of positive ions with neutrals in the probe space charge sheath that we call Chen-Talbot model [8]. At lower magnetic field 3 · 10^?2 T the positive ion density values interpreted by using the Chen-Talbot model [8] are in better agreement with the values of electron density compared to those obtained by using the theory [10,11]; therefore the model [8] is used for calculation of the positive ion density from the probe data at higher magnetic fields. The comparison of the positive ion and electron density values calculated from the same probe data at higher magnetic fields shows that up to the magnetic field strength 4 . 10^?2 T with the probe 100 μm and up to 5 . 10^?2 T with the probe 42 μm in diameter respectively the decrease of the magnitude of the electron current at the space potential due to the magnetic field does not exceed the error limits that are usual for Langmuir probe measurements (absolute error ±20%).     

Low-lying energy levels in 166Ho

The properties of some low-excited states of ¹⁶⁶Ho have been studied using thin, high resolution Ge detectors. The relative intensities of γ-lines were determined. The directional angular correlation measurements for the 28.23–54.24 keV cascade are in agreement with the 1^?(M1)2^?(E2)0^? spin sequence. The magnetic moment μ = 0.068 ± 0.010 μ_N of the 54.24 keV level was determined by integral perturbed angular correlation (IPAC) in an external magnetic field. This value can be explained by the configuration mixing due to the Coriolis interaction.     

Three-phonon assisted cyclotron harmonic transitions in the magnetophotoconductivity of n-InSb

The CO₂ laser-induced magnetophotoconductivity of n-InSb (10¹⁴ cm^-3) at liquid helium temperatures exhibits resonant structure which depends upon the photon energy and the magnetic field. The resonant peaks are explained on the basis of a simple model in which an electron absorbs a photon and emits successively three LO phonons.