Comparison of probe and microwave measurements of electron density in plasmatic cylinder

The paper deals with a comparison of the measurement of the electron density of a plasmatic cylinder in a weak magnetic field (radius of cyclotron rotation of ionsr _Hi is larger than radius of probea) parallel to the axis of the plasma by means of a cylindrical Langmuir probe and a microwave interferometer in the X band. A Langmuir probe is used to measure the electron density distribution across the cross-section of the plasmatic cylinder, i.e. in the plane perpendicular to the axis of the cylinder. The measured density distribution is used to determine the mean electron density over the cylinder cross-section. This mean electron density is compared with the density found with a microwave interferometer. In evaluating the measurements the concentration of the microwave energy through the antennas of the interferometer in the cross-section of the plasmatic cylinder is considered. The difference of the mean electron density found by the microwave method from the mean density found by the probe method was in the least favourable case 30%. In conclusion, the paper gives a comparison of the probe and microwave measurements of the plasma density in a stronger magnetic field, whenr _Hi a.The authors thank J. Teichmann, C. Sc., R. Klíma, C.Sc. and other members of the department for discussions of the results, V. Kopecký for aid in the probe measurements and the director of the institute, Ing. J. Váa, for his interest in the experiments.     

Small-signal modulation characteristics for 1.5 μm lattice-matched InGaNAs/GaAs and InGaAs/InP quantum well lasers

The small-signal modulation characteristics of 1.5 m lattice-matched InGaNAs/GaAs and InGaAs/InP quantum well lasers and their temperature dependence have been calculated. It is found that the maximum bandwidth of the InGaNAs/GaAs quantum well lasers is about 2.3 times larger than that of the InGaAs/InP quantum well lasers due to the high differential gain which results from the large electron effective mass in the dilute nitride system. The slope efficiency for the 3 dB bandwidth as a function of optical density is twice as large for InGaNAs/GaAs as for InGaAs/InP quantum well lasers.     

An energy-transport model for semiconductors derived from the Boltzmann equation

An energy-transport model is rigorously derived from the Boltzmann transport equation of semiconductors under the hypothesis that the energy gain or loss of the electrons by the phonon collisions is weak. Retaining at leading order electron-electron collisions and elastic collisions (i.e., impurity scattering and the elastic part of phonon collisions), a rigorous diffusion limit of the Boltzmann equation can be carried over, which leads to a set of diffusion equations for the electron density and temperature. The derivation is given in both the degenerate and nondegenerate cases.     

Double-λ hypernuclear production in high-energy heavy-ion collisions

The production of double- hypernuclei was studied regarding three different mechanisms: (1) coalescence of a nuclear fragment from peripheral collisions and two A particles, (2) coalescence of two A particles, neutrons and protons due to central collisions, and (3) conversion of a hypernucleus. The production probability is estimated for several projectile-target combinations at 5 and 14.5 GeV/nucleon.     

An Experimental Study of a Radial Coaxial Gaseous Discharge

Experimental measurements of a radial coaxial gaseous discharge were made using an adjustable Langmuir probe and a microwave interferometer. Excellent agreement of these results is observed when inhomogeneities of both the electron number density and temperature are accounted for and the electronneutral collisions are assumed to be "hard-sphere" collisions.     

High-intensity lasers and controlled fusion

The use of high-intensity lasers to cause ignition in inertial confinement fusion is presented, with emphasis on current experimental programs and physical concepts that are at the forefront of the field. In particular, we highlight the issues of fast electron transport through dense materials, an essential element of the Fast Ignitor concept.Received: 22 November 2002, Published online: 5 August 2003PACS: 52.20.Fs Electron collisions - 52.38.Kd Laser-plasma acceleration of electrons and ions     

Electron-Cyclotron Current Drive under Conditions of Quasi-Transverse RF-Beam Launch

We consider the linear mechanism of electron-cyclotron current drive (CD) in the case of quasi-transverse launch of microwave power from the low-field side of a toroidal system for plasma confinement. In this specific scenario, the electromagnetic-field energy is absorbed mainly by low-energy thermal electrons under conditions of electron-cyclotron resonance (ECR) with allowance for both the Doppler shift and the relativistic dependence of the electron mass. Under these conditions, degradation of the CD efficiency or even current reversal with respect to the direction of microwave-beam launch can occur. The kinetic equation is analyzed by the method of adjoint equation for the current, which allows for the accurate treatment of electron-electron collisions for an arbitrary effective charge of the ion component.     

Spectral Properties of the Radiation of Injection Lasers at Prethreshold Excitation Levels

The spectral properties of the radiation of injection lasers at low (prethreshold) levels of excitation were investigated experimentally by the method of optical heterodyning of the Doppler signal and also with the use of a KSVU23 spectral setup. It is established that these lasers may serve as sources of the optical radiation of TE and TMpolarizations with a retuned length of coherence within the range 1–10 mm in spectrometric facilities of photoheterodyne type. A procedure for determining the inversion current in an injection laser is suggested.     

Estimates for the efficient production of antihydrogen by lasers of very high intensities

Starting from the non-linear relativistic equations of motion for charged particles in the very high intensity fields of laser radiation, the maximum kinetic energy _kin of the resulting oscillation is derived exactly. In non-relativistic conditions _kin agrees with the well-known valuee ² E _v ²/(2m₀²|n|), showing a dependence on the rest massm ₀ of the particle. In the relativistic case, the mass dependence vanishes. The multipole radiation is calculated on the basis of Sommerfeld's formula for relativistic conditions. It is shown that this radiation is not important for oscillation energies up to =70m ₀ c ² for electrons in neodymium glass laser radiation and up to higher values for CO₂ lasers and for protons. With the limitationm ₀ c ² < _kin < , the formula for _kin is used to calculate the pair production (a) for singly oscillating particles in vacuum without collisions and (b) for plasmas with collisions. Taking into account the local increase of the effective electric laser field near the cut-off density due to the decrease of ¦n¦ (n is the complex refractive index), there is the possibility of efficient proton pair production at intensities of 10¹⁹ W cm^–2 for neodymium glass lasers and of 10¹⁷ W cm^–2 for CO₂ lasers, besides electron pair production.     

软X射线激光探针诊断高$lt;i$gt;Z$lt;/i$gt;材料等离子体

激光辐照靶产生的等离子体电子密度的诊断对于惯性约束聚变、高能量密度物理等相关领域的研究具有重要意义,特别是高Z材料等离子体临界面附近的电子密度分布信息的测量. 利用软X射线激光作为探针是诊断等离子体电子密度分布的一种重要方法,但在诊断激光辐照高Z材料产生的等离子体研究中,遇到了高Z材料等离子体自发辐射过大的问题,难以开展. 为此,针对软X射线激光的特点,发展了多种具体的实验技术. 通过综合利用这些技术,大大的抑制了待测等离子体自发辐射对信号的影响,使得软X射线激光探针诊断高Z材料等离子体成为可能. 作为典型例子,实验诊断了激光辐照金平面靶的等离子体,获得了清晰的实验图像,表明相关的技术是有效和可行的. 关键词： 等离子体诊断 激光探针技术 软X射线激光 Z材料等离子体')" href="#">高Z材料等离子体     

Optically-Pumped Far-Infrared Laser Lines in Hydrazine, Methanol, Heavy Water, and Ammonia: New Laser Lines and Frequency Measurements

A far-infrared laser cavity designed to favor short-wavelength laser lines was used to generate optically-pumped far-infrared laser radiation. New far-infrared laser lines were discovered in hydrazine, heavy water, ammonia, and several short-wavelength lines previously discovered in methanol were observed. Wavelength, frequency, and relative intensity measurements were performed on laser lines in the wavelength range 42.4 to 253.7 m. Each far-infrared frequency measurement was obtained by mixing the far-infrared radiation with radiation from two reference CO₂ lasers and from a microwave synthesizer in a metal-insulator-metal diode. The pump laser was a high-Q Fabry Perot resonator oscillating on 275 grating-selected laser lines including regular, sequence, and hot band lines.     

Observable radiation zeroes in HERA interactions

Radiation zeroes were discovered over 10 years ago and heralded as a direct probe of gauge couplings and fractional quark charges. Observations of such zeroes have eluded proton-antiproton collider experiments so far. We here point out that radiation zeroes appear in electron-quark collisions and conclude that they should be observable at HERA in the subprocesseqeq. Choosing suitable cuts, we expect 660 (430) events/year in the electron (positron) channel of the neutral current single jet plus direct photon events at HERA, i.e. we expect 12 (4) events/bin/year in the electron (positron) channel over a large fraction of the range of cos (10 of the 30 bins we use).     

Kinetics issues of CuBr lasers with added hydrogen

A computational kinetics model of the CuBr laser with added hydrogen is presented. The space-time behaviour of the lasers and lasants is described in detail. Besides the discharge field, it is confirmed that the dissociation/recombination of the CuBr molecule plays an important role in the plasma electron temperature. The radial field, which peaks at 30% of the discharge field and has two opposite directions along the radius, results in the skin of electron density and the black centre of the large-bore laser beam.     

Physical grounds of high-sensitive room temperature detection of infrared radiation involving spherical excitons at impurities in crystals

The possibility of the IR-radiation detecting in crystals of direct-gap semiconductors, caused by effects of IR-quenching of probe visible-range radiation within the region of a crystal relative transparency, is studied both theoretically and experimentally. The comparison of some mechanisms investigated allows to conclude that the most probable explanation of the IR-quenching effect, experimentally observed in the CdS crystal, is the mechanism of probe radiation absorption with photon energy deficit with respect to exciton resonance, which is eliminated due to exchange interaction of a free exciton in the intermediate state with spherical excitons localized on manyelectron atoms of impurity.     

Probing interface properties of nanocomposites by third-order nonlinear optics

We describe the exploitation of third-order nonlinear optical response — particularly nonlinear absorption and the nonlinear index of refraction — to probe interface dynamics, modifications and relaxation processes in granular materials consisting of metal quantum dots embedded in such dielectrics as fused silica and sapphire. Many features of these materials can be interpreted in terms of the quantum-mechanical model of the particle-in-a-box. Electronic and thermal relaxation processes in these novel nanocomposites are dominated by interactions of conduction-band electrons at the boundary between the quantum dot and its surrounding host material. Experimental examples presented include measurements of thermal and electronic relaxation rates, dephasing due to electron collisions at the nanocluster surface, effects of local structural order, changes in the saturation parameter due to chemical modification of the substrate, and one-and two-dimensional heat-transfer effects.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994.     

Optical bistability produced by resonant Joule's heating of semiconductors

The thermal hysteresis of Joule's absorption of infrared radiation in then-type semiconductor magnetoplasma is analysed. Such bistability is connected with the temperature dependence of the relaxation time of free electrons, their density being constant. The resonant Joule's absorption bistability of the pumping infrared wave near a Langmuir resonance of free electrons is discussed as well as the hysteresis cross-modulation of an infrared probing wave by means of a pumping microwave. The possibilities of smooth detuning of the hysteresis regime due to a magnetic-field control and low energy thresholds of these regimes are illustrated for InSb thin films in relatively weak magnetic fieldsH1–2 kG.     

Femtosecond time-resolved reflection second-harmonic generation on polycrystalline copper

Optical Second-Harmonic Generation (SHG) in reflection from a polycrystalline copper surface in air was studied using femtosecond time-resolved pump and probe measurements at =625 nm. The observed time dependence of second-harmonic yield from the probe beam demonstrates, that SHG is a very sensitive technique for measuring transient electron temperatures of metals even when these are covered by an oxide layer. For polycrystalline copper, an electron-phonon energy transfer time of 2 ps was observed, corresponding to a coupling constant of 3.75×10¹⁷ W/m³ K at average lattice temperatures of about 500 K. The analysis of experimental data indicates that the time dependence of SHG is governed by the linear dielectric function which, in turn, is affected by the electron temperature. There is no evidence for a temperature dependence of the nonlinear susceptibility ⁽²⁾.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Relativistic filamentation of laser beams in plasma

We describe the results of a numerical code which models the relativistic selffocussing of high-intensity laser beams in plasmas by the nonlinear relativistic dependence of the optical constants on laser intensity. The plasma dynamics of 10¹³ W Nd glass lasers of 30 m initial beam diameter in nearly cut-off density plasmas is followed for a few picoseconds interaction time and 25 m depth into the plasma. Rapid relativistic selffocussing down to a beam diameter of one micron in a distance of the order of the original beam diameter is observed, as well as the production of GeV ions moving against the laser light.     

Dynamic cesium D2-line holograms written with a single-frequency semiconductor laser

Conclusion Semiconductor lasers, being tunable radiation sources, can be successfully used to write, and investigate the properties of, resonant dynamic holograms in the near IR, which is the range of the spectra and of their applications. Such lasers can be used to form transmission and reflection DH with specified properties (energetic, spectral, spatiotemporal). DH written in the bulk of a medium under optical pumping of hyperfine sublevels of the ground state or under selective action of the writing light field on individual atom-velocity groups can be used for conversion of low-intensity (10^–2 W/cm²) rapidly varying optical signal with nearly planar wave fronts. A volume containing vapor can be heated (if cesium vapor is used) at near-room temperature. Reflection DH, whose features are absence of angle selectivity and a practically uniform SFC, are of interest because they can be used to transform optical signals having wide spatial-frequency spectra and process the images in real time, but have a lower sensitivity and require appreciable heating of a vapor-filled volume. Promising for practical tasks is the us of higher-power lasers to record resonant DH (1 W) [45], and also the possibility of using non-single-mode emitters to tune of the longitudinal modes to resonance with the absorption line [33].We conclude by pointing out a trend close to the topic of the present article — resonant interferometry in the near IR —a spectrum region in which, in our opinion, the use of semiconductor lasers is beneficial [46].All Union Institute of Optophysical Measurements, Moscow. Vavilov State Optical Institute, St. Petersburg