强流二极管绝缘体滑闪监测及其影响

 以闪光二号加速器为研究平台, 建立了微分环阵列和Rogowski线圈同时监测二极管电流的方法,监测了二极管绝缘体表面的滑闪现象。根据电流探头测量结果的差异,分析了绝缘体滑闪对电子束流参数的影响。二极管绝缘体出现滑闪,位置附近的微分环波形严重畸变,其它位置的微分环和Rogowski线圈测量结果基本一致。采用距离滑闪位置较远的微分环结果处理二极管束流参数,相对于不出现滑闪时的结果,束流强度和总能量没有明显的变化。绝缘体滑闪沿面局部放电,能量损失较小,尚未对电子束流造成较大影响。     

Ultraviolet flashes in the equatorial region of the Earth

Intense ultraviolet flashes with a duration of 1 to 64 ms have been detected by the UV detector (wavelengths 300–400 nm) that operates at the Universitetsky-Tatiana MSU satellite. Most flashes occur in the equatorial region of the Earth. The nature of the observed flashes is discussed.     

Terrestrial gamma-ray flashes and neutron pulses from direct simulations of gigantic upward atmospheric discharge

Gamma-ray pulses are calculated from 2D numerical simulations of upward atmospheric discharge in a self-consistent electric field using the multigroup approach to the kinetics of relativistic runaway electrons. Thecalculated numbers and spectra of γ-rays are consistent with the data on terrestrial γ-ray flashes observed aboard spacecraft. The calculated runaway electron flux is concentrated mainly within the domain of the blue jet fluorescence. This proves that exactly this domain is a source of the γ-ray flashes. One γ-ray flash generates ∼10¹⁴-10¹⁵ photonuclear neutrons. The text was submitted by the authors in English     

On amplifications of photonuclear neutron flux in thunderstorm atmosphere and possibility of detecting them

The reliability of communications reporting observations of neutron flux enhancements in thunderstorm atmosphere is analyzed. The analysis is motivated by the fact that the employed gas-discharge counters on the basis of reactions ³He(n, p)³H and ¹⁰B(n; ⁴He, γ)⁷Li detect not only neutrons but any penetrating radiations. Photonuclear reactions are capable of accounting for the possible amplifications of neutron flux in thunder-storm atmosphere since in correlation with thunderstorms γ-ray flashes were repeatedly observed with spectra extending high above the threshold of photonuclear reactions in air. By numerical simulations, it was demonstrated that γ-ray pulses detected in thunderstorm atmosphere are capable of generating photonuclear neutrons in numbers sufficient to be detected even at sea level.     

Dynamics of microcracks and time dependences of surface deformation of a heterogeneous body (granite) under an impact

The dynamics of fractoluminescence flashes and the time dependences of surface deformation of granite with different sizes of feldspar grains under an impact on samples by a metal pin have been studied with a 10 ns resolution. A band at ??1.9 eV has been observed in the fractoluminescence spectra, which means that, under the influence of mechanical stresses, Si-O-Si bonds are broken and ?? SiO^?? free radicals are formed. The fractoluminescence has the form of flashes with a duration of ??10 ns. It has been assumed that each of them corresponds to the nucleation of a microcrack. From the flash intensities and the elastic wave velocity, the linear size of microcracks has been estimated to be from ??8 to 30 ??m. Microcracks are mainly generated during passage of a deformation wave through feldspar grains. An impact causes the appearance of eigenvibrations of the entire sample, and cracking of grains gives rise to eigenvibrations of grains.     

Lightning effects at high altitudes: sprites, elves, and terrestrial gamma ray flashes

A fascinating set of newly discovered complex phenomena indicate that thunderstorms and lightning discharges are strongly coupled to the overlying upper atmospheric regions. Lightning discharges at cloud altitudes (<20 km) affect altitudes >40 km either via the release of intense electromagnetic pulses (EMPs) and/or the production of intense quasi-static electric (QE) fields. The intense transient QE fields of up to 1 kV·m⁻¹, which for positive CG discharges is directed downwards, can avalanche accelerate upward-driven runaway MeV electron beams, producing brief (1 ms) flashes of gamma radiation. A spectacular manifestation of these intense fields is the so-called ‘Sprites’, large luminous discharges in the altitude range of 40 km to 90 km, which are produced by the heating of ambient electrons for a few to tens of milliseconds following intense lightning flashes. The so-called ‘Elves’ are optical flashes which last much shorter (<1 ms) than sprites, and are typically limited to 80–95 km altitudes with much larger (up to 600 km) lateral extent, being produced by the heating, ionization, and optical emissions due to the EMPs radiated by both positive and negative lightning discharges. To cite this article: U.S. Inan, C. R. Physique 3 (2002) 1411–1421.     

Spatial and Angular Characteristics of the Atmospheric Showers Initiated by Photons of Energy E γ = 1018–1022 eV in the Magnetosphere of the Earth

Spatial-angular characteristics are calculated for the atmospheric electromagnetic showers generated by primary photons of ultrahigh energies (E = 10¹⁸–10²² eV) coming from outer space. Calculations carried out for vertical and oblique showers in view of the interaction of photons and electrons with the dipole magnetic field of the Earth (formation of electron-positron pairs, synchrotron radiation) and taking into account the Landau–Pomeranchuk–Migdal effect in the processes of interaction of electrons and photons with matter in the meteorological atmosphere.     

Oscillatory disruptions in the Alcator C tokamak

Periodic disruptions were observed which persisted for as long as 400 ms during plasma discharges. Soft X-ray emissions, T_e, n_e, impurity radiation and plasma position showed the pulsations. The abrupt collapse in X-ray signal (1–2 ms; period ∽10 ms) encompassed a large fraction of the plasma cross section. Plasma conditions associated with the onset of this pattern are examined.     

Excitation of X rays by electrons accelerated in air in the wake wave of a laser pulse

The characteristics of X rays of a laser plasma generated in the interaction of a femtosecond pulse with solid targets in an air atmosphere have been investigated. It has been shown that the mechanism for the generation of X rays in the interaction of short intense laser pulses with solid targets in a gas atmosphere is attributed to the generation of fast electrons in the region of the filamentation of a laser pulse. It has been proven experimentally that under such conditions, the solid target irradiated by laser radiation of even a low density of about 10¹⁵ W/cm² very efficiently emits ∼10-keV photons. It has been shown theoretically that the maximum energy of accelerated electrons can reach ɛ_max ∼ 100–200 keV under these conditions. This means that the proposed method can provide characteristic radiation with the energy of photons much higher than 10 keV.     

Comparison of focusing optics for femtosecond X-ray diffraction

We characterize and compare four different types of focusing optics for hard X-rays, suitable for femtosecond X-ray diffraction experiments, usinga tabletop femtosecond laser-based plasma source. We demonstrate a 23 μmfocus with a toroidally bent Ge single crystal. A maximum flux of 7× 10⁸ photons/(s mm²) is generated in a 32 μm focus using a multi-layer mirror. An elliptical glass capillary yields the highest number of photons per Bragg angle [2× 10⁵ photons/(s mrad)]. The largest number of photons[3× 10⁶ photons/s] per second is obtained in the 105 μm focus of a poly-capillary optical lens system. All numbers are given for characteristic Cu K _α photons.This revised version was published online in May 2005. The Article Category was removed.This revised version was published online in August 2005 with a corrected cover date.     

Slow intermolecular redistribution of vibrational energy in SF6 gas excited by a tea CO2 laser

Infra-red fluorescence (IRF) spectra of SF₆ excited by the 944.2 cm^-1 line of a pulsed CO₂ laser were observed at various times after the time of the laser excitation. Each spectrum showed a strong IRF peak of the v₃ mode which was red shifted relative to the room temperature fundamental (948 cm^-1) by an amount which depended, apart from the level of excitation, on the different times employed. For a strong excitation with 〈n〉 ≈ 11 photons absorbed per molecule, a significant decrease of red shift versus time was observed, indicating mainly excitation losses by IRF emission. For weak excitation with 〈n〉 ≈ 1.4, almost an constant red shift versus time is observed. This result, and the previous finding that at weak excitation a nonthermal energy distribution in the ensemble of molecules exists, leads to the conclusion that intermolecular redistribution of vibrational energy in SF₆ is slow, and does not exceed the observed fluorescence duration (～1 ms).     

Radial scanning diagnostics of bremsstrahlung and line emission in T-10 plasma

The paper describes the scanning spectroscopic diagnostics designed for measurement of line integrated plasma radiation in two visible spectral ranges. This diagnostic system is aimed at measuring the bremsstrahlung absolute values and profile with high spatial resolution. The bremsstrahlung absolute values are used to determine the value and radial distribution of effective plasma ion charge Z _eff(r) in T-10 discharges. The importance of Z _eff measurement is due to its strong influence on plasma heating, confinement, and stability. The spatial distribution of emission for one of the chosen spectral lines is measured simultaneously with bremsstrahlung. The spatial resolution of measurements is ~1 cm, and the temporal resolution is up to 10 ms. The spectral equipment and methods for its calibration are described. Examples of line integrated brightness distribution in a “continuum window” of 5236 ± 6 Å and brightness of the lines C⁵⁺ (5291 Å), He¹⁺ (4686 Å), and D_β (4861 Å) are given. Flattening of the bremsstrahlung brightness profile in the central region of the plasma column in some discharges with sawtooth oscillations in the T-10 is observed. The measured effective ion charge profiles in ohmic discharges with high plasma density and low discharge currents demonstrate accumulation of light impurities at the column axis; as a consequence, quenching of sawtooth oscillations in some discharges is observed. The developed diagnostics provides necessary data for investigation of heat, particle, and current transport in the plasma of the T-10. Successful application of the obtained data on Z _eff(r) for investigation of geodesic acoustic modes of plasma oscillations in the T-10 should be specially noted.     

Photon-trap spectroscopy of size-selected free cluster ions: “direct” measurement of optical absorption of Ag+ 9

An absorption spectrum of size-selected free cluster ions has been measured “directly” via extinction of light without relying on photodepletion/dissociation spectroscopy. The novel technique employs an ion trap and an optical cavity; cluster ions stored in an ion trap interact with photons trapped in a cavity. The storage lifetime of photons in the cavity provides “direct” observation of extinction of light (photon-trap spectroscopy, which is a generalized scheme of cavity ring-down spectroscopy). The first measurement is performed on ultraviolet absorption of Ag₉ ⁺. Temperature dependence of the spectrum is presented by cooling the ion trap down to 10 K.     

Effect of residual atmospheric pressure on the development of electrostatic discharges at the surface of protective glasses of solar cells

Electrostatic discharges obtained upon the irradiation of K-208 glass with 40-keV electrons at a flux density φ of 10¹⁰ to 2 × 10¹¹ cm^–2 s^–1 are studied. The residual pressure p _v in the vacuum chamber is varied from 5 × 10^–5 to 5 × 10^–3 Pa. Structural changes in the sample surfaces are studied by atomic-force microscopy. Depending on the pressure level, two types of discharges are observed in experiments at 3 × 10¹⁰ ≤ φ ≤ 1.2 × 10¹¹ cm^–2 s^–1: a microprojection at the glass–ionized-residual-atmosphere surface and a discharge which develops along the irradiated surface. It is found that at 5 × 10^–5 ≤ p _v ≤ 3 × 10^–4 Pa and 8 × 10¹⁰ ≤ φ ≤ 10¹¹ cm^–2 s^–1, discharges of the first type appear at the beginning of exposure; that is, an increase in microprojections is observed. Further, surface discharges propagate through these microprojections. At 10^–3 ≤ p _v ≤ 5 × 10^–3 Pa and 10¹⁰ ≤ φ ≤ 5 × 10¹⁰ cm^–2 s^–1, on the contrary, discharges of the second type are realized at the beginning. These discharges result in the appearance of channels with inhomogeneities on the glass, at which subsequently discharges of the first type occur. It is determined by calculations that in the region adjacent to the exposed glass surface, secondary electrons accelerated in a field of charge accumulated in the glass make the main contribution to the ionization of gases.     

Broadband downconversion based near-infrared quantum cutting via cooperative energy transfer in YNbO4:Bi3+, Yb3+ phosphor

YNbO₄:Bi³⁺, Yb³⁺ phosphor was prepared to study the quantum cutting process of converting one ultraviolet photon into two near-infrared photons. An intense near-infrared emission of Yb³⁺:²F_5/2 → ²F_7/2 around 1 μm was observed under the ultraviolet excitation belonging to the broadband absorption of the [NbO₄]^3- group and the Bi³⁺ ion. The photoluminescence spectra and decay lifetime measurements indicate efficient energy transfer from Bi³⁺ to Yb³⁺ ions, which is attributed to be of a cooperative energy transfer mechanism. The YNbO₄:Bi³⁺, Yb³⁺ phosphor with optimized doping concentration may be applicable in improving the efficiency of silicon-based solar cells.     

Characteristics of a multi-keV monochromatic point x-ray source based on vacuum diode with laser-produced plasma as cathode

Temporal, spatial and spectral characteristics of a multi-keV monochromatic point x-ray source based on vacuum diode with laser-produced plasma as cathode are presented. Electrons from a laser-produced aluminium plasma were accelerated towards a conical point tip titanium anode to generate K-shell x-ray radiation. Approximately 10₁₀ photons/pulse were generated in x-ray pulses of ∼18 to ∼28 ns duration from a source of ∼300 μm diameter, athυ = 4.51 keV (K _α emission of titanium), with a brightness of ∼10²⁰ photons/cm²/s/sr. This was sufficient to record single-shot x-ray radiographs of physical objects on a DEF-5 x-ray film kept at a distance of up to ∼10 cm.     

Atmospheric gamma-ray and neutron flashes

Gamma-ray pulses are calculated from 2D numerical simulations of an upward atmospheric discharge in a self-consistent electric field using the multigroup approach to the kinetics of relativistic runaway electrons (REs). Computed γ-ray numbers and spectra are consistent with those of terrestrial γ-ray flashes (TGFs) observed aboard spacecrafts. The RE flux is concentrated mainly within the domain of the Blue Jet fluorescence. This confirms that exactly the domain adjacent to a thundercloud is the source of the observed γ-ray flashes. The yield of photonuclear neutrons is calculated. One γ-ray pulse generates ∼10¹⁴–10¹⁵ neutrons. The possibility of the direct deposition of REs to the detector readings and the origin of the lightning-advanced TGFs are discussed. The text was submitted by the authors in English.     

Characteristics of electron cyclotron resonance plasma formed by lower hybrid current drive grill antenna

A 3.7 GHz system, which is meant for LHCD experiments on ADITYA tokamak, is used for producing ECR discharge. The ECR discharge is produced by setting the appropriate resonance magnetic field of 0.13 T, with hydrogen at a fill pressure of about 5 × 10⁻⁵ Torr. The RF power, up to 10 kW (of which ∼50% is reflected back), with a typical pulse length of 50 ms, is injected into the vacuum chamber of the ADITYA tokamak by a LHCD grill antenna and is used for plasma formation. The average coupled RF power density (the RF power/a typical volume of the plasma) is estimated to be ∼5 kW/m³. When the ECR appears inside the tokamak chamber for the given pumping frequency (f = 3.7 GHz) a plasma with a density (n _e) ∼ 4 × 10¹⁶ m⁻³ and electron temperature ∼8 eV is produced. The density and temperature during the RF pulse are measured by sets of Langmuir probes, located toroidally, on either side of the antenna. H_α signals are also monitored to detect ionization. An estimate of density and temperature based on simple theoretical calculation agrees well with our experimental measurements. The plasma produced by the above mechanism is further used to characterize the ECR-assisted low voltage Ohmic start-up discharges. During this part of the experiments, Ohmic plasma is formed using capacitor banks. The plasma loop voltage is gradually decreased, till the discharge ceases to form. The same is repeated in the presence of ECR-formed plasma (RF pre-ionization), formed 10 ms prior to the loop voltage. We have observed that (with LHCD-induced) ECR-assisted Ohmic start-up discharges is reliably and repeatedly obtained with reduced loop voltage requirement and breakdown time decreases substantially. The current ramp-up rates also decrease with reduced loop voltage operation. These studies established that ECR plasma formed with LHCD system exhibits similar characteristics as reported earlier by dedicated ECR systems. This experiment also addresses the issue of whether ECR plasma formed with grill antenna exhibits similar behavior as that formed by single waveguide ECR antenna. Our experimental observations suggest that the characteristics of (LHCD system-induced) ECR-assisted Ohmic start-up discharges show similar properties, reported earlier with normal ECR-assisted Ohmic start-up discharges and hence LHCD system may be used as ECR system at reduced toroidal magnetic field for other applications like wall conditioning.      

Untersuchung der Elektronenkomponente von Einzellawinen mit dem Photomultiplier

With a photomultiplier single electron avalanches of 10³ carriers can be observed in nitrogen. 2.5% of all photons (in the range 3000≦λ≦7000 Å) of an avalanche produce a photoelectron at the cathode of the photomultiplier. The influence of the variable solid angle on the photomultiplier pulse is given. Trouble by noise is calculated and it is shown that the information depends on the number of photoelectrons only. The electron components of avalanches in methane, nitrogen, and mixtures have an exponential rise and the measured rise time constant τ_beob is in accordance with the theoretical value 1/αυ_. For carrier numbers >10⁶ space charge influence is observed, as given by theory. The quantum efficiencyQ per ionizing impact is found to beQ～10^?3 in vapours andQ～1 in gases. Values of the electron drift velocity in mixtures of N₂ and CH₄ are given.     

Mechanism of catalytic reduction of NO by H2 or CO on a Pd foil; Role of chemisorbed nitrogen on Pd

The adsorption of NO and its reaction with H₂ over polycrystalline Pd were investigated using flash desorption technique and ultraviolet photoelectron spectroscopy under 10^?5 Pa pressure range of reactants and surface temperatures between 300 and 900 K. NO was adsorbed dissociatively onto the Pd surface above 500 K, and the heat of dissociative adsorption was ca. 126 kJ/mol. Atomic nitrogen was observed to accumulate on the Pd surface during the NO-H₂ reaction, whose desorption rate exhibited second order kinetics and is expressed as follows: V_d = 10^{?9.8 ± 0.3}exp(?67(kJ/mol)/RT) (cm²/atom·s). Hydrogenation of the adsorbed nitrogen proceeded rapidly at 485 K. It was confirmed from these results that formation of N₂ and NH₃ in the NO-H₂ reaction proceeds through this atomically adsorbed nitrogen. Pd-N bond energy and enthalpies of some intermediate states of the NO-H₂ reaction were estimated.