Effect of the parameters of the plasma channel produced by a high-current relativistic electron beam in dense gaseous media on the beam transportation stability

Results are presented from experimental studies of the time evolution of the plasma channel produced by a high-current electron beam (with an electron energy of E _e = 1.1 MeV, a beam current of I _b = 24 kA, and a pulse duration of t = 60 ns) in helium, nitrogen, neon, air, argon, krypton, xenon, and humid air (air: H₂O) at pressures from 1 to 760 Torr. It is shown that, in gases characterized by a small ratio of the collision frequency to the gas ionization rate u _i , the electron beam produces a broad high-conductivity plasma channel, such that R _b/R _p < 1, where R _b and R _p are the beam and channel radii, respectively. As a result, large-scale resistive hose instability is suppressed.     

Development of a fast risetime pulsed relativistic electron beam generator for the excitation of shorter wavelength lasers

We have developed a compact and simple high current relativistic electron beam (REB) generator for the excitation of lasers in the shorter wavelength region. This REB generator mainly consists of a Tesla transformer-type high-voltage generator, N₂ gas insulated Blumlein-type pulse-forming line and cold-cathode electron gun to generate 800 keV, 12 kA pulsed electron beams with 6 ns current duration and approximately 4 ns current risetime. The design and operational characteristics along with discussions of the excitation capabilities of the gas laser media are presented in this paper.     

Two-pulse generation of high-current relativistic electron beams and their transportation in the gaseous medium of a plasmochemical reactor

Experimental results on two-pulse generation and transportation of high-current relativistic electron beams (REBs) through the gaseous medium of a plasmochemical reactor (PR) are presented. The generation of two consecutive high-current REB pulses with a duration of 60 ns was achieved at the Tonus accelerator with modified schemes of high-voltage pulse formation. The first version of the formation scheme enabled pulse powers of 2 and 4.0–9.6 GW with a time interval between the pulses of 500 ns. The second version enabled one to generate pulses with powers of 1.8 and 16 GW and time interval between the pulses of 160 μs. The transportation parameters of an REB injected into a 1.4-m-long PR filled with an N₂: O₂ gas mixture are studied. The conductance of the plasma produced under the action of the electron beams is measured. It is shown that the schemes proposed provide more efficient (by 35–45%) transportation of the REBs in the reactor volume as compared to single-pulse high-current REBs of the same power and pulse duration.     

On distribution over the cathode surface of the current due to positive ions in discharge with oscillating electrons

The mechanisms of processes determining distribution of positive ions on the cathode surface in a discharge with oscillating electrons are studied. At low pressures P≤5×10⁻⁵ torr over the entire range of anode length variation l _a=0.5–11 cm, the ion current distribution over the cathode radius J _i(r) features a single maximum in the center and drops steeply with distance from the axis. At pressures P=1–4×10⁻⁴ torr, the distribution J _i(r) for short anodes (l _a<6 cm) is similar to the previous one but, for long anodes (l _a=6–10 cm), new maxima at higher J _i values have been detected. A physical explanation for the obtained results is proposed.     

Compact X-ray radiograph based on a plasma gun

The results of the experiments on the formation of a plasma emitter with small spatial dimensions for pulsed radiography in the soft X-ray spectral range are presented. Emitting hot plasma was formed as a result of compression of the plasma jet by a current pulse with amplitude I _m = 215 kA and rise time T _fr = 200 ns. For the jet formation, we used a plasma gun based on the arc discharge (I _m = 8.5 kA and T _fr = 6 μs) initiated by breakdown over the surface of a dielectric in vacuum. The experiments were carried out with aluminum, tin, copper, and iron plasma jets. A single emitter, i.e., point Z-pinch (PZ-pinch), was formed when an interelectrode gap of a high current generator of 1.3–1.5 mm was used. The smallest spatial dimensions of the emitting region were obtained with the use of aluminum and tin. For a tin jet, the diameter of the emitting region was 7 ± 2 μm and its height was 17 ± 2 μm. The emission pulse duration at half-height was 2–3 ns. The total emission yield per pulse in the spectral range 1.56–1.90 keV was 30–50 mJ for the aluminum pinch and 10–30 mJ for the tin pinch. The developed method makes it possible to carry out radiographic examination of microobjects (including biological ones) 1–1000 μm in thickness, with spatial (10–20 μm) and time (2–3 ns) resolution.     

Low threshold current density, low resistance oxide-confined VCSEL fabricated by a dielectric-free approach

We present the fabrication process and experimental results of 850-nm oxide-confined vertical cavity surface emitting lasers (VCSELs) fabricated by using dielectric-free approach. The threshold current of 0.4 mA, which corresponds to the threshold current density of 0.5 kA/cm², differential resistance of 76 Ω, and maximum output power of more than 5 mW are achieved for the dielectric-free VCSEL with a square oxide aperture size of 9 μm at room temperature (RT). L–I–V characteristics of the dielectric-free VCSEL are compared with those of conventional VCSEL with the similar aperture size, which indicates the way to realize low-cost, low-power consumption VCSELs with extremely simple process. Preliminary study of the temperature-dependent L–I characteristics and modulation response of the dielectric-free VCSEL are also presented.     

Critical Parameters of the Pumping Scheme of Ar+8 Lasers Excited by Z Pinches in Long Capilaries

The present study is focused on the demonstration of the most critical parameters of the pumping scheme of a table‐top Ar⁺⁸‐laser excited by discharges with relatively low current and voltage (I ≤ 20 kA, U ≤ 200 kV) in long (L ～ 0.5 m) capillaries. The most critical parameters of the pumping scheme were analyzed and then adjusted experimentally. The table‐top size is attributed to the use of a low‐inductance co‐axial discharge configuration that decreases the voltage and current necessary for laser excitation. Low inductance is achieved by using a capillary, water‐capacitor and water spark‐gap placed into a chamber filled with deionized water. The capillary z‐pinch is produced by the water capacitor, which is pulse‐charged by a six‐stage Marx generator, optimized for the low‐inductance discharge configuration. Optimization is performed by adjusting the value of the charging inductance and the peak charging voltage with a water spark‐gap. At the optimal conditions laser pulses with a Gaussian‐like intensity distribution and divergence angle ～ 1 mrad and energy ～ 10 μJ are generated. The physical method for generation of a laser beam with such parameters is based on the use of a long (L = 0.45 m) capillary plasma (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Subnormal glow discharge in a Xe/Cl2 mixture in a narrow discharge tube

Electrical and optical characteristics of a subnormal glow discharge in a short (L=10 cm) discharge tube with an inner diameter of 5 mm are investigated. The dependences of the discharge current-voltage characteristic, the energy deposition in the discharge, the plasma spectral characteristics in the 130-to 350-nm wavelength range, the emission intensities of the XeCl(D-X) 236-nm and XeCl(B-X) 308-nm bands, and the total emission intensity in the range 180–340 nm on the pressure and composition of the Xe/Cl₂ mixture are studied. Two modes of glow discharge are shown to exist: the low-current mode at a discharge current of I _ch ≤2 mA and the high-current mode at I _ch >2 mA. The transition from one mode to another occurs in a stepwise manner. The increase in the chlorine content causes the discharge voltage and the energy deposition in the plasma to increase. At low pressures of the Xe/Cl₂ mixture (P≤0.7 kPa), stationary strata form in the cathode region. The lower the discharge current, the greater the volume occupied by the strata. This longitudinal discharge acts as a powerful source of continuous broadband emission in the range 180–340 nm, which forms due to overlapping the XeCl(D, B-X) and Cl ₂ ^* bands with edges at λ=236, 308, and 258 nm. The intensity of the 236-nm band is at most 20% of the total intensity of UV radiation. The maximum power of UV radiation (3 W at an efficiency of 8%) is attained at a xenon partial pressure of 250–320 Pa and a total pressure of the mixture of 2 kPa.     

Evaluation of dew-water density by the Monte Carlo method

To construct a physical model of water vapor condensation with the formation of dew water, a geometrically based Monte Carlo method was developed. The hit and geometrical density functions of random space filling with identical spheres were determined. The parameters of these functions is the minimum allowable approach OO _min of spheres and their excess in comparison with the space capacity. The specificity of dew-water molecules and hydrogen bonds (HBs) between them appears at 2/3L _b < OO _min < 2L _b, where L _b is the HB length. As the approach OO _min = L _b, the random filling density does not exceed the packing density in the sphere models of I _h and I _c ices. The densities characteristic of the sphere model of water molecule packing in these ices are achievable at OO _min ≈ 0.8L _b and/or a significant HB kink, as well as at vapor supersaturation (excess of molecules over the space capacity).     

Non-steady-state photo-EMF in nanostructured GaN and polypyrrole within porous matrices

We report an experimental investigation of the non-steady-state photoelectromotive force in nanostructured GaN within porous glass and polypyrrole within chrysotile asbestos. The samples are illuminated by an oscillating interference pattern created by two coherent light beams and the alternating current is detected as a response of the material. Dependences of the signal amplitude versus temporal and spatial frequencies, light intensity, and temperature are studied for two wavelengths λ=442 and 532 nm. The conductivity of the GaN composite is measured: σ=(1.1–1.6)×10⁻¹⁰ Ω⁻¹ cm⁻¹ (λ=442 nm, I ₀=0.045–0.19 W/cm², T=293 K) and σ=(3.5–4.6)×10⁻¹⁰ Ω⁻¹ cm⁻¹ (λ=532 nm, I ₀=2.3 W/cm², T=249–388 K). The diffusion length of photocarriers in polypyrrole nanowires is also estimated: L _D=0.18 μm.     

Control of gigawatt REB pulse duration using gas injection

A technique for controlling the voltage and current pulse duration of a gigawatt relativistic electron beam (REB) was investigated. The pulse duration of a beam of 250 keV and 10 kA was controlled from 60 ns to 10 ns by injecting gases (air,, argon, and hydrogen) into the diode gap at a pressure ranging from 10^-5 torr to 10^-1. The observed dependence of the pulse duration on the nature and pressure of the gas is explained in terms of volume ionization of the gas by beam electrons. It is concluded that the pulse duration is governed by the time at which the plasma density created by the beam reaches a critical value of the order of beam electron number density     

XUV generation from plasma produced by a XeCl excimer laser on a Cu target

Summary We present a detailed study of XUV and soft X-ray emission from Cu plasma produced by an excimer laser at intensitiesI _L≦8·10¹¹ W/cm². The XeCl excimer laser (ψ≈308 nm) delivers pulses with energyE _L≈2.3 J, temporal durationt _L≈100 ns and brightnessB≧10¹⁴ W/cm² sr. We recorded a spectral conversion efficiency η=0.5% eV⁻¹ forI _L=4·10¹¹W/cm² in the aluminium window at 73eV with a harder X-ray tail around ≈400eV. We also measured the dependence of X-ray signal on laser intensity and viewing angle. Experimental results have been compared with some analytical laser-plasma interaction models.     

Mechanism for the influence of the geometric parameters on the electrical characteristics of Penning ion sources

A steady-state Penning ion source is studied experimentally. Depending on the geometric parameter l _a (the anode length to diameter ratio) and pressure, maxima are observed in the discharge current and in the ion beam current extracted from an aperture at the center of the cathode. It is shown that at pressures of the order of 10⁻⁴ Torr, two maxima appear in these currents: one, for short discharge gaps with l _a =1–1.5, corresponds to a diverging ion beam, and the other, for longer anodes with l _a =4–5, to a collimated ion beam. At pressures of the order of 10⁻⁵ Torr, only one maximum appears in these currents, for short anodes l _a =2–3 with a diverging ion beam. A physical explanation is proposed for these findings. Zh. Tekh. Fiz. 68, 29–32 (September 1998)     

Collisional effect on the Weibel instability in the limit of high plasma temperature

The Weibel instability (WI) of relativistic electron beam (REB) penetrating an infinite collisional plasma was studied in the following models: (i) REB model, where the total equilibrium distribution function f ₀($ \vec p $ \vec p ) is approximated by nonrelativistic background electron and REB distribution functions and (ii) relativistic monoenergetic beam (RMB) model, where f ₀($ \vec p $ \vec p ) is approximated by nonrelativistic background electrons and RMB distribution functions.     

Fine structure of LiI (n=4–7)d 2 D-terms by beam-foil quantum-beat spectroscopy

L-subshell fluorescence yields of lead are determined in a series of experiments where lead atoms in lead tetraethyl, Pb(C₂H₅)₄, in a small admixture to the propane in proportional counter are ionized in theL-shell by a beam of monochromatic X-rays. The technique of selective photoexcitation of consecutive subshells is applied and fluorescence events are resolved into subshell contributions. The subshell fluorescence yields are determined asω _LI=0.16±0.04,ω _LII=0.407±0.010 andω _LIII=0.347±0.005.     

Laser-induced spark for simultaneous ignition and fuel-to-air ratio measurements

This work investigates the use of laser-induced gas breakdown for simultaneously igniting and measuring fuel-to-air ratio of CH₄–air and H₂–air combustible mixtures. The fuel-to-air ratio is determined using the measured spectral peak ratio I_o,Hα/I_o,OI. Sparks are produced using a single-mode, Q-switched Nd–YAG laser. The laser produces a beam of 6 mm in diameter at the wavelength of 1064 nm and pulse duration of 5.5 ns. The beam optics is designed to have mainly a beam splitter and a focusing lens. The beam splitter is coated to reflect the laser beam and transmit emission lines with wavelengths from 600 to 900 nm which are then collected by a fiberoptic cable and detected by an imaging spectrometer–detector assembly. The results showed a linear dependence of the spectral peak ratio on the equivalence ratio that can be generally expressed by φ=a(I_o,Hα/I_o,OI)+b, where a and b are the parameters that depend on the gas pressure. Using the least-square curve fitting technique to fit the experimental data, a calibration curve for calculating the equivalence ratio as a function of the ratio of (I_o,Hα/I_o,OI) was generated.     

Results for neutral to charged current cross section ratios from ν and\bar v nucleus interactions below 30 GeV

Using the freon filled bubble chamber SKAT in the (anti)neutrino wide band beam of the Serpukhov accelerator we determine the neutral to charged current cross section ratios for neutrinos and antineutrinos below 30GeV. From these ratios we calculate in leading order a mixing parameter of the standard model of Θ _w =0.215±0.029.     

An investigation of electron and oxygen ion damage in Si npn RF power transistors

Abstract

The effects of 8 MeV electrons and 60 and 95 MeV oxygen ions on the electrical properties of Si npn RF power transistors have been investigated as a function of fluence. The dc current gain (h _FE), displacement damage factor, excess base current (Δ I _B=I _Bpost?I _Bpre), excess collector current (Δ I _C=I _Cpost?I _Cpre), collector saturation current (I _CS) and deep level transient spectroscopy trap signatures of the irradiated transistors were systematically evaluated.     

Characteristic features of the formation of the resistive state in a Bi2Sr2CaCu2Oy crystal

The effect of a magnetic field H⊥(ab) on the transverse current-voltage characteristics (IVCs) of the mixed state of a single crystal of the layered superconductor Bi₂Sr₂CaCu₂O_y (BSCCO) is investigated. It is established that in a wide range of temperatures and fields above the irreversibility line the initial part of the IVC is described by the law V∝I ^γ with γ≃1. As the current increases further, this law is replaced by a section where V∝exp(I). It is established that the multivalued, multibranch characteristics, interpreted as a manifestation of an internal Josephson effect, do not change appreciably when the crystal passes into a state with nonzero linear resistance. The character of the dependence of the characteristic switching current on the first resistive branch, I _J (H,T), is determined. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 543–548 (25 October 1999)     

Initiation of combustion of a methane-air mixture in a supersonic flow behind a shock wave during laser excitation of O<Subscript>2</Subscript> molecules

The possibility of initiating detonation of CH₄ + air in a supersonic flow behind an oblique shock wave under the exposure of the mixture to laser radiation with wavelengths λ_I=1.268 μm and 762 nm is analyzed. It is shown that this irradiation leads to excitation of O₂ molecules to the a ¹Δ_g and b ¹Σ _g ⁺ states, which intensifies the chain mechanism of combustion of CH₄/O₂ (air) mixtures. Even for a small value of the laser radiation energy absorbed by an O₂ molecule (∼0.05–0.1 eV), detonation mode of combustion in a poorly inflammable mixture such as CH₄/air can be realized at a distance of only 1 m from the primary shock wave front for relatively small values of temperature (∼1100 K) behind the front under atmospheric pressure.