Estimation of the electron density and radiative energy losses in a calcium plasma source based on an electron cyclotron resonance discharge

The parameters of a calcium plasma source based on an electron cyclotron resonance (ECR) discharge were calculated. The analysis was performed as applied to an ion cyclotron resonance system designed for separation of calcium isotopes. The plasma electrons in the source were heated by gyrotron microwave radiation in the zone of the inhomogeneous magnetic field. It was assumed that, in such a combined trap, the energy of the extraordinary microwave propagating from the high-field side was initially transferred to a small group of resonance electrons. As a result, two electron components with different transverse temperatures—the hot resonance component and the cold nonresonance component—were created in the plasma. The longitudinal temperatures of both components were assumed to be equal. The entire discharge space was divided into a narrow ECR zone, where resonance electrons acquired transverse energy, and the region of the discharge itself, where the gas was ionized. The transverse energy of resonance electrons was calculated by solving the equations for electron motion in an inhomogeneous magnetic field. Using the law of energy conservation and the balance condition for the number of hot electrons entering the discharge zone and cooled due to ionization and elastic collisions, the density of hot electrons was estimated and the dependence of the longitudinal temperature T _e∥ of the main (cold) electron component on the energy fraction β lost for radiation was obtained.     

Nonlinear Thomson scattering of a relativistically strong tightly focused ultrashort laser pulse

The problem of nonlinear Thomson scattering of a relativistically strong linearly polarized ultrashort laser pulse tightly focused into a spot with a diameter of D _F ? λ (where λ is the laser wavelength) is solved. The energy, spectral, and angular distributions of radiation generated due to Thomson scattering from test electrons located in the focal region are found. The characteristics of scattered radiation are studied as functions of the tightness of laser focusing and the initial position of test particles relative to the center of the focal region for a given laser pulse energy. It is demonstrated that the ultratight focusing is not optimal for obtaining the brightest and hardest source of secondary electromagnetic radiation. The hardest and shortest radiation pulse is generated when the beam waist diameter is ?10λ.     

Suppression of longitudinal losses in a gas-dynamic trap by using an ambipolar mirror

The efficiency of utilizing ambipolar mirrors for suppression of longitudinal losses of particles and energy in a gas-dynamic trap (GDT) was investigated. An additional relatively small axisymmetric mirror cell was installed in one of the facility ends. Hydrogen or deuterium atomic beams with an energy of 22 keV and equivalent current density of up to 1 A/cm² were injected into the additional cell at an angle of 90° to the facility axis. Trapping of the beams with a total power of 800 kW by the plasma in the additional cell leads to the formation of a hot ion population with an anisotropic velocity distribution, a mean energy of 13 keV, and a density of up to 4.5 × 10¹³ cm⁻³. It is shown that the confinement of hot ions in the additional cell is determined by classical processes, such as charge exchange on the beam atoms and collisional deceleration by electrons, in spite of the onset of Alfvén ion-cyclotron instability at fast ion densities higher than 2.5 × 10¹³ cm⁻³. The effect of ambipolar confinement manifests itself in that, at hot ion densities higher than 3 × 10¹³ cm⁻³, the flux density of ions escaping from the trap in the mode with beam injection decreases fivefold as compared to that without injection. In this case, the density of the Maxwellian plasma component in the central cell is about 2.5 × 10¹³ cm⁻³. The efficiency of suppression of longitudinal particle losses by the ambipolar mirror substantially exceeds estimates obtained for both collisional (gas-dynamic) and collisionless (adiabatic) confinement modes. Qualitatively, this is because, in the GDT experiments, the mode of warm plasma confinement is transitional between the gas-dynamic and adiabatic modes and the use of an ambipolar mirror facilitates a transition from the lossy gas-dynamic mode into a nearly adiabatic one.     

Kinetics of the reactions involving CF2 and CF in a pure tetrafluoromethane plasma: II. Production and loss of CF2 and CF in the processes of fluorocarbon polymerization

Mechanisms for the production and loss of CF₂ and CF radicals in a glow discharge in pure CF₄ are investigated by the time-resolved laser-induced fluorescence method. The fluorocarbon polymerization processes are shown to contribute significantly to the production of radicals both in the plasma volume and on the surface of the discharge tube. The effective frequencies of both the volume and surface processes of radical production and loss are determined. An analysis of these frequencies allowed us to study the polymerization mechanism in a CF₄ plasma at a high relative concentration of F atoms and low ion energy. It is shown that, at elevated pressures, when the density of C_xF_y polymer particles in the plasma volume becomes comparable with the density of simple fluorocarbon radicals, the electron-impact dissociation of these particles is the main channel for the production of CF₂ and CF radicals. Another source of CF₂ and CF radicals is related to the reactions of C_nF_2m+1 unsaturated fluorocarbon particles both in the plasma volume and on the surface of a fluorocarbon film arising on the discharge tube wall. The C_xF_y fluorocarbon polymer particles form both in the discharge volume and on the fluorocarbon filmsurface also in the course of the film destruction. At lowered pressures, the main channel for the production of CF₂ and CF is the direct electron-impact dissociation of CF₄ molecules, whereas the loss of these radicals at the tube wall is the main loss channel. The probabilities of the heterogeneous losses of CF₂ and CF radicals on the heavily fluorinated surface of the fluorocarbon film at low ion energies are determined. Under these conditions, the surface recombination of the Fch chemisorbed fluorine atoms and CF _x ^ph physisorbed radicals with the production of an activated complex is shown to be the most probable mechanism for the heterogeneous losses of CF₂ and CF. The approximate activation energies for the production of F_ch · CF ₂ ^ph and F_ch · CF^ph surface complexes are found to be 750±70 K and 1030±100 K, respectively.     

Nonstationary model of an axisymmetric mirror trap with nonequilibrium plasma

The DOL nonstationary model intended to describe plasma processes in axisymmetric magnetic mirror traps is considered. The model uses averaging over the bounce period in order to take into account the dependence of plasma parameters on the coordinate along the facility axis. Examples of calculations of trap parameters by means of the DOL code based on this model are presented. Among the features of the DOL model, one can single out two points: first, the capability of calculating the terms of the collision integral with the use of a non-Maxwellian scattering function while evaluating the distribution function of fast ions and, second, concerning the background plasma, the capability of calculating the longitudinal particle and energy fluxes in confinement modes with the particle mean free path being on the order of the trap length. The influence of the scattering function approximation used to calculate the collision integral on the solution to the kinetic equation is analyzed. The dependences of plasma parameters on the power of heating injectors and the length of the fast-ion turning zone are presented as calculation examples. The longitudinal profile of the fusion reaction rate in the case of a trap with a long fast-ion turning zone is shown to depend strongly on the input parameters of the model.     

Generation of dense multicharged ion flows from an ECR plasma confined in a quasi-gas-dynamic magnetic cusp trap

The possibility of generating dense multicharged ion beams with a current density as high as ～1 A/cm² from an ECR plasma confined in a quasi-gas-dynamic cusp trap is studied both theoretically and experimentally. The most important advantages of this type of ion source are that the plasma in the cusp is stabile against MHD perturbations and that a trap intended to operate at fairly high pump-field frequencies (above 30 GHz) is relatively inexpensive. A theoretical model of confinement of a high-density nonequilibrium ECR plasma (T _e ? T _i ) in an open magnetic trap is proposed and results are presented from model experiments with an ～30-cm-long cusp trap (here, by the cusp length is meant the volume of a paraxial magnetic tube divided by the area of its cross sections in magnetic mirrors) pumped by a pulsed microwave field with a frequency of 37.5 GHz and power of 100 kW. The possibility of achieving a quasi-gas-dynamic regime of plasma confinement of an ECR plasma in a cusp trap is demonstrated. Ion beams with a average ion charge number of 2–4 (depending on the sort of working gas) and current densities unprecedented for ECR sources are obtained. Good agreement between theoretical and experimental results makes it possible to reliably predict the ion beam parameters that can be achieved at even higher microwave frequencies.     

Machine Learning Methods for X-Ray Scattering Data Analysis from Biomacromolecular Solutions

Small-angle x-ray scattering (SAXS) of biological macromolecules in solutions is a widely employed method in structural biology. SAXS patterns include information about the overall shape and low-resolution structure of dissolved particles. Here, we describe how to transform experimental SAXS patterns to feature vectors and how a simple k-nearest neighbor approach is able to retrieve information on overall particle shape and maximal diameter (D_max) as well as molecular mass directly from experimental scattering data. Based on this transformation, we develop a rapid multiclass shape-classification ranging from compact, extended, and flat categories to hollow and random-chain-like objects. This classification may be employed, e.g., as a decision block in automated data analysis pipelines. Further, we map protein structures from the Protein Data Bank into the classification space and, in a second step, use this mapping as a data source to obtain accurate estimates for the structural parameters (D_max, molecular mass) of the macromolecule under study based on the experimental scattering pattern alone, without inverse Fourier transform for D_max. All methods presented are implemented in a Fortran binary DATCLASS, part of the ATSAS data analysis suite, available on Linux, Mac, and Windows and free for academic use.     

Magnetic field excitation during electron beam injection from the Intercosmos-25 satellite (APEX)

Results of active experiments on electron beam injection from the Intercosmos-25 satellite into the ionospheric plasma are presented. A quasistatic magnetic field and the VLF-wave magnetic component are excited when an unmodulated electron beam with a current of I _be ?0.1 A and energy of ? _be =mv ²/2?10 keV is injected into the ambient plasma. The magnetic field excitation is attributed to the onset of plasma gradient instabilities.     

Surfatron accelerator in the local interstellar cloud

Taking into account results of numerous experiments, the variability of the energy spectra of cosmic rays (protons and helium nuclei) in the energy range of 10 GeV to ~10⁷ GeV is explained on the basis of a hypothesis of the existence of two variable sources close to the Sun. The first (soft) surfatron source (with a size of ~100 AU) is located at the periphery of the heliosphere. The second (hard) surfatron source (with a size of ~1 pc) is situated in the Local Interstellar Cloud (LIC) at a distance of <1 pc. The constant background is described by a power-law spectrum with a slope of ~2.75. The variable heliospheric surfatron source is described by a power-law spectrum with a variable amplitude, slope, and cutoff energy, the maximum cutoff energy being in the range of E _СН/Z < 1000 GeV. The variable surfatron source in the LIC is described by a power-law spectrum with a variable amplitude, slope, and cut-off energy, the maximum cut-off energy being E _СL/Z ≤ 3 × 10⁶ GeV. The proposed model is used to approximate data from several experiments performed at close times. The energy of each cosmic-ray component is calculated. The possibility of surfatron acceleration of Fe nuclei (Z = 26) in the LIC up to an energy of E _CL ~ 10¹⁷ eV and electron and positrons to the “knee” in the energy spectrum is predicted. By numerically solving a system of nonlinear equations describing the interaction between an electromagnetic wave and a charged particle with an energy of up to E/Z ~ 3 × 10⁶ GeV, the possibility of trapping, confinement, and acceleration of charged cosmic-ray particles by a quasi-longitudinal plasma wave is demonstrated.     

Discharge dynamics and the production of active particles in a cathode-directed streamer

The emission spectroscopy technique is used to analyze a cathode-directed streamer discharge in air at atmospheric pressure in point-plane geometry at interelectrode distances of up to 100 mm and a high-voltage pulse amplitude of 18 kV. The densities of molecules in the N₂(C ³Π_u, v=0), N ₂ ⁺ (B ²Σ _u ⁺ , v=0) and NO(A ²Σ⁺, v=0) states are determined, and the reduced electric field in the streamer head is estimated. It is shown that the increase in the average electric field in the discharge gap substantially intensifies the production of active particles in the discharge plasma and makes the plasma more homogeneous. This effect is only related to the increase in the fraction of regions with a high electric field in the discharge gap and, as a result, the reduction of the discharge energy losses via rapidly thermalized degrees of freedom. The active particles are only produced in the streamer head, including the case in which the interelectrode gap is bridged by the streamer channel.     

HIV Fusion Peptide Penetrates, Disorders, and Softens T-Cell Membrane Mimics

This work investigates the interaction of N-terminal gp41 fusion peptide (FP) of human immunodeficiency virus type 1 (HIV-1) with model membranes in order to elucidate how FP leads to fusion of HIV and T-cell membranes. FP constructs were (i) wild-type FP23 (23 N-terminal amino acids of gp41), (ii) water-soluble monomeric FP that adds six lysines on the C-terminus of FP23 (FPwsm), and (iii) the C-terminus covalently linked trimeric version (FPtri) of FPwsm. Model membranes were (i) LM3 (a T-cell mimic), (ii) 1,2-dioleoyl-sn-glycero-3-phosphocholine, (iii) 1,2-dioleoyl-sn-glycero-3-phosphocholine/30 mol% cholesterol, (iv) 1,2-dierucoyl-sn-glycero-3-phosphocholine, and (v) 1,2-dierucoyl-sn-glycero-3-phosphocholine/30 mol% cholesterol. Diffuse synchrotron low-angle x-ray scattering from fully hydrated samples, supplemented by volumetric data, showed that FP23 and FPtri penetrate into the hydrocarbon region and cause membranes to thin. Depth of penetration appears to depend upon a complex combination of factors including bilayer thickness, presence of cholesterol, and electrostatics. X-ray data showed an increase in curvature in hexagonal phase 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, which further indicates that FP23 penetrates into the hydrocarbon region rather than residing in the interfacial headgroup region. Low-angle x-ray scattering data also yielded the bending modulus K_C, a measure of membrane stiffness, and wide-angle x-ray scattering yielded the S_xray orientational order parameter. Both FP23 and FPtri decreased K_C and S_xray considerably, while the weak effect of FPwsm suggests that it did not partition strongly into LM3 model membranes. Our results are consistent with the HIV FP disordering and softening the T-cell membrane, thereby lowering the activation energy for viral membrane fusion.     

Direct Nanoscale Characterization of Deep Levels in AgCuInGaSe2 Using Electron Energy‐Loss Spectroscopy in the Scanning Transmission Electron Microscope

A new experimental framework for the characterization of defects in semiconductors is demonstrated. Through the direct, energy‐resolved correlation of three analytical techniques spanning six orders of magnitude in spatial resolution, a critical mid‐bandgap electronic trap level (E_V + 0.56 eV) within Ag_0.2Cu_0.8In_1?xGa_xSe₂ is traced to its nanoscale physical location and chemical source. This is achieved through a stepwise, site‐specific correlated characterization workflow consisting of device‐scale (≈1 mm²) deep level transient spectroscopy (DLTS) to survey the traps present, scanning probe–based DLTS (scanning‐DLTS) for mesoscale‐resolved (hundreds of nanometers) mapping of the target trap state's spatial distribution, and scanning transmission electron microscope based electron energy‐loss spectroscopy (STEM‐EELS) and X‐ray energy‐dispersive spectroscopy for nanoscale energy‐, structure, and chemical‐resolved investigation of the defect source. This first demonstration of the direct observation of sub‐bandgap defect levels via STEM‐EELS, combined with the DLTS methods, provides strong evidence that the long‐suspected Cu_In/Ga substitutional defects are indeed the most likely source of the E_V + 0.56 eV trap state and serves as a key example of this approach for the fundamental identification of defects within semiconductors, in general.     

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density N _e and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N₂. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C ³Π _u state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of N _e and E. A kinetic model of the processes governing the steady-state density of the C ³Π _u nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.     

Optical characteristics and parameters of gas-discharge plasma in a mixture of mercury dibromide vapor with neon

Results are presented from studies of the optical characteristics and parameters of plasma of a dielectric barrier discharge in a mixture of mercury dibromide vapor with neon—the working medium of a non-coaxial exciplex gas-discharge emitter. The electron energy distribution function, the transport characteristics, the specific power losses for electron processes, the electron density and temperature, and the rate constants for the processes of elastic and inelastic electron scattering by the working mixture components are determined as functions of the reduced electric field. The rate constant of the process leading to the formation of exciplex mercury monobromide molecules is found to be 1.6 × 10^?14 m³/s for a reduced electric field of E/N = 15 Td, at which the maximum emission intensity in the blue-green spectral region (λ_max = 502 nm) was observed in this experiment.     

The Role of ATP in Mechanically Stimulated Rapid Closure of the Venus's Flytrap

When the midribs of untreated traps of Dionaea muscipula are frozen in liquid nitrogen after rapid closure, they contain significantly less ATP than those frozen before closure. Exogenous ATP causes a significant increase in the rate of mechanically stimulated trap closure. Illuminated traps close faster than those kept in the dark. The traps of plants placed in 100% O₂ close much faster than do air controls, while 100% CO₂ inhibits closure. It is concluded that ATP is probably the native source of potential energy for contraction of the trap's midrib, and that if the endogenous ATP titer is increased by oxidative phosphorylation or an exogenous source, the trap will close faster.     

Molecular Cloning,Overexpression and Characterization of a Novel Water Channel Protein from Rhodobacter sphaeroides

Aquaporins are highly selective water channel proteins integrated into plasma membranes of single cell organisms; plant roots and stromae; eye lenses, renal and red blood cells in vertebrates. To date, only a few microbial aquaporins have been characterized and their physiological importance is not well understood. Here we report on the cloning, expression and characterization of a novel aquaporin, RsAqpZ, from a purple photosynthetic bacterium, Rhodobacter sphaeroides ATCC 17023. The protein was expressed homologously at a high yield (∼20 mg/L culture) under anaerobic photoheterotrophic growth conditions. Stopped-flow light scattering experiments demonstrated its high water permeability (0.17±0.05 cm/s) and low energy of activation for water transport (2.93±0.60 kcal/mol) in reconstituted proteoliposomes at a protein to lipid ratio (w/w) of 0.04. We developed a fluorescence correlation spectroscopy based technique and utilized a fluorescent protein fusion of RsAqpZ, to estimate the single channel water permeability of RsAqpZ as 1.24 (±0.41) x 10⁻¹² cm³/s or 4.17 (±1.38)×10¹⁰ H₂O molecules/s, which is among the highest single channel permeability reported for aquaporins. Towards application to water purification technologies, we also demonstrated functional incorporation of RsAqpZ in amphiphilic block copolymer membranes.     

Limiting Parameters of the Plasma Opening Switch

Implementing programs for nuclear fusion research and X-ray generation requires the creation of superpower generators based on plasma opening switches (POSs) capable of commutating currents as high as several tens of megaamperes at output voltages of up to 5 MV and higher. The physical mechanisms limiting the POS voltage are investigated. It is shown that, as the generator voltage U _g increases, the voltage multiplication factor k = U_POS/U_g (where U_POS is the POS voltage) decreases. An explanation for such a dependence is proposed, and the maximum value of the POS voltage is estimated. A POS design that enables operating in the above current and voltage ranges is considered. The design is based on applying an external magnetic field to the POS interelectrode gap, increasing the initial generator voltage, and decreasing the linear (along the perimeter of the outer electrode) density of the charge passing through the POS during the conduction phase.     

Laboratory modeling of nonstationary processes in space cyclotron masers: First results and prospects

Results are presented from laboratory modeling of the dynamics of space cyclotron masers. A selfoscillatory mode of cyclotron instability in the nonequilibrium plasma of an ECR discharge in a magnetic mirror trap is found. The plasma comprises two electron populations: the background population with a density of N _e ～ 10¹³–10¹⁴ cm^?3 and temperature of T _e ≈ 300 eV and the energetic population with a density of N _e ～ 10¹⁰ cm^?3 and temperature of T _e ≈ 10 keV. Quasi-periodic pulsed precipitation of energetic electrons from the trap, accompanied by microwave bursts at frequencies below the electron gyrofrequency in the center of the trap, is detected. The study of the microwave plasma emission and the energetic electrons precipitated from the trap shows that the precipitation is related to the excitation of whistler-mode waves propagating nearly parallel to the trap axis. The observed instability has much in common with phenomena in space magnetic traps, such as radiation belts of magnetized planets and solar coronal loops. The experimental results demonstrate the opportunity of laboratory modeling of space cyclotron masers. The main tasks and possibilities of such modeling are discussed.     

Simulation of the Plasma Afterglow in the Discharge Gap of a Subnanosecond Switch Based on an Open Discharge in Helium

The phenomenon of subnanosecond electrical breakdown in a strong electric field observed in an open discharge in helium at pressures of 6–20 Torr can be used to create ultrafast plasma switches triggering into a conducting state for a time shorter than 1 ns. To evaluate the possible repetition rate of such a subnanosecond switch, it is interesting to study the decay dynamics of the plasma remaining in the discharge gap after ultrafast breakdown. In this paper, a kinetic model based on the particle-in-cell Monte Carlo collision method is used to study the dynamics of the plasma afterglow in the discharge gap of a subnanosecond switch operating with helium at a pressure of 6 Torr. The simulation results show that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only while the plasma density remains higher than 10¹² cm^?3; the main mechanism of the further plasma decay is diffusion of plasma particles onto the wall. Therefore, the effect of recombination in the plasma bulk is observed only during the first 10–20 μs of the afterglow. Over nearly the same time, plasma electrons become thermalized. The afterglow time can be substantially reduced by applying a positive voltage U_c to the cathode. Since diffusive losses are limited by the ion mobility, the additional ion drift toward the wall significantly accelerates plasma decay. As U_c increases from 0 to +500 V, the characteristic time of plasma decay is reduced from 35 to 10 μs.