Radiation of the Plasma of a Transverse Discharge in a Helium–Krypton–Elegas Mixture

We carried out a spectroscopic investigation of the degradation of the active medium of a pulsed-periodic KrF emitter based on a He/Kr/SF₆ mixture (P = 10–150 kPa) with pumping by a transverse volumetric discharge. The plasma radiation spectra in the range 200–620 nm at different stages of degradation of the working mixture and the dynamics of the radiation of inert gases as well as of the products of decomposition of SF₆ molecules in the plasma are studied. It is shown that since the number of discharge pulses is 10⁴, rather effective formation of excited sulfur molecules is observed which decompose with emission in the spectral range 260–550 nm. This can be employed for developing a wideband lamp based on the system of KrF(B–X; D–X), S₂(B–X), and S₂(f–a) bands.     

Generation of picosecond pulses in solid-state lasers using new active media

Results are reported of investigations aimed at generating nanosecond radiation pulses in solid-state lasers using new active media having broad gain lines. Passive mode locking is accomplished for the first time in a BeLa:Nd³⁺ laser at a wavelength 1.354 m, and in a YAG:Nd³⁺ laser on a 1.32–m transition. The free lasing and mode-locking regimes were investigated in an alexandrite (BeAl²O₄:Cr³⁺) laser in the 0.72–0.78–m range and in a synchronously pumped laser on F 2- centers in LiF in the 1.12–1.24–m region. The features of nonlinear perception of IR radiation by the eye, using a developed picosecond laser on F₂ ^– centers, are investigated for the first time.Translated from Lazernye Sistemy, pp. 67–86, 1982.     

The influence of superradiant relaxation on associative ionisation yield of laser excited potassium atoms

The influence of superradiant relaxation of excited 7P _1/2 potassium atoms on their associative ionisation yield has been investigated. The excitation was carried out by a laser pulse in a tube containing potassium vapour with concentration 10¹⁴ cm^–3. A seven-fold drop of ionisation yield was found and spontaneous luminescence pulse distortions by superradiance (SR) were measured in the wide interval of excited atoms densities N _e ⁰ 10⁸–10¹³ cm^–3. The values of associative ionisation rate constant and SR threshold atomic densities are obtained, and the shapes and quantum yields of SR pulses are evaluated from the experimental results.     

ГЕНЕРАТОР ВЫСОКОВОЛ ЬТНЫХ ИМПУЛЬСОВ МАЛО Й ДЛИТЕЛЬНОСТИ С МАЛЫМ ВРЕМЕНЕМ НАРАСТАНИЯ И ЕГО ПРИМЕНЕНИЕ ДЛЯ И ЗМЕРЕНИЯ ПОДВИЖНОСТ ЕЙ ЭЛЕКТРИЧЕСКИХ ЗАРЯД ОВ В ГАЗАХ

The paper describes a generator of square-wave power pulses with a front of 2×10^–8 second and length of 0·6×10^–6 sec for a capacity load of 70 pF, with continuously regulable amplitude from 1 to 9 kV. The current in the pulses >100 A. The delay in the high-power pulse with respect to the triggering pulse does not exceed (0·09–0·15)×10^–6 sec. The generator is intended for operating spark counters of relativistic particles although its range of use may be much greater. The paper points out its use for studying the mobilities of electric charges in gas mixtures of spark counters.     

Generation of Submillimeter-Wave Radiation with GaAs Tunnett Diodes and InP Gunn Devices in a Second or Higher Harmonic Mode

Efficient second-harmonic power extraction was demonstrated recently with GaAs tunnel injection transit-time (TUNNETT) diodes up to 235 GHz and with InP Gunn devices up to 325 GHz. This paper discusses the latest theoretical and experimental results from second-harmonic power extraction at submillimeter-wave frequencies and explores the potential of using power extraction at higher harmonic frequencies to generate continuous-wave radiation with significant power levels at frequencies above 325 GHz. Initial experimental results include output power levels of more than 50 W at 356 GHz from a GaAs TUNNETT diode in a third-harmonic mode and at least 0.2–5 W in the frequency range 400–560 GHz from InP Gunn devices in a third or higher harmonic mode. The spectral output of these submillimeter-wave sources was analyzed with a simple Fourier-transform terahertz spectrometer and, up to 426 GHz, with a spectrum analyzer and appropriate harmonic mixers. Initial experimental results from a GaAs/AlAs superlattice electronic device at D-band (110–170 GHz) and J-band (170–325 GHz) frequencies are also included.     

Nonresonant tripling and sum-frequency mixing in Hg

Nonresonant frequency tripling and sum-frequency mixing of pulsed dye-laser radiation generates in Hg vapor continuously tunable coherent vuv radiation in the wavelength range _vuv=132-185nm. Laser pulse powers of 1–2 MW produce vuv light pulses of 0.6–1.5 W (2–4.8×10⁹ photons/pulse).     

High-power source of coherent picosecond light pulses tunable from 0.41 to 12.9 μm

We report a high-power source of coherent picosecond light pulses based on optical parametric generation and amplification in LiB₃O₅ and AgGaS₂ crystals. The spectral range of this continuously tunable source covers the visible, near-infrared and medium-infrared spectrum from 0.41 to 12.9 m. An optical parametric generator and amplifier, consisting of two type-I phase-matched LiB₃O₅ crystals and a diffraction grating, is pumped by the third harmonic of a picosecond Nd:YAG laser and provides spectrally narrow, high-power pulses from 0.41 to 2.4 m. Energy conversion efficiencies up to 16 percent are achieved. The pulse duration is about 14 ps, the bandwidth between 10 and 30 cm^–1. The tuning range is extended to 12.9 m by mixing the infrared output between 1.16 and 2.13 m with the fundamental of the Nd:YAG laser in type-I-phase-matched AgGaS₂ crystals. Up to 25 percent of the pulse energy at 1.064 m is converted into parametric infrared pulses. Bandwidths between 3 and 8 cm^–1 and a pulse duration of approximately 19 ps are measured for these pulses. We also observe a retracing behaviour in the tuning curve of AgGaS₂ not reported before.     

Brightness-Spectrum Characteristics of the Surface Laser Plasma of Polymeric Targets in the VUV Spectral Region

The brightness-spectrum and energy parameters in the VUV range of the spectrum (h 9–70 eV) of the surface laser plasma of polymeric targets in vacuum are determined experimentally. Using the experimental technology of double open and closed ionization chambers (as radiation detectors) requiring no absolute energy calibration in a wide range of regulation parameters of the effect of standard laser frequencies, we obtained new data on multifactor radiative gasdynamic processes of effect which are needed to determine emission and mass-output characteristics of the targets irradiated under vacuum.     

Absorption of subpicosecond ultraviolet laser pulses in high-density plasma

The absorption of 250 fs KrF laser pulses incident on solid targets of aluminum, copper and gold has been measured for normal incidence as a function of laser intensity in the range of 10¹²–10¹⁴ W cm^–2 and as a function of polarization and angle of incidence for the intensity range of 10¹⁴–2.5×10¹⁵ W cm^–2. As the intensity increases from 10¹² W cm^–2 the reflectivity at normal incidence changes from the low-intensity mirror reflectivity value to values in the range of 0.5–0.61 at 10¹⁴ W cm^–2. For this intensity maximum absorption of 63–80% has been observed for p-polarized radiation at angles of incidence in the range of 54°–57°, increasing with atomic number. The results are compared with the expected Fresnel reflectivity from a sharp vacuum-plasma interface with the refractive index given by the Drude model and also to numerical calculations of reflectivity for various scale length density profiles. Qualitative agreement is found with the Fresnel/Drude model and quantitative agreement is noticed with the numerical calculations of absorption on a steep density profile with normalized collision frequencies, v/, in the range of 0.13–0.15 at critical density and normalized density gradient scale lengths, L/₀, in the range of 0.018–0.053 for a laser intensity of 10¹⁴ W cm^–2.At 2.5×10¹⁵ W cm^–2 a small amount of preplasma is present and maximum absorption of 64–76% has been observed for p-polarized radiation at angles of incidence in the range of 45°–50°.Dedicated to Prof. Dr. Rudolf Wienecke on the occasion of his 65^th birthdayOn leave from: Department of Electrical Engineering, University of Alberta, Edmonton, T6G 2G7, Canada     

The effects of slippage and diffraction in long wavelength operation of a free electron laser

The Free-Electron Laser user facility FELIX produces picosecond optical pulses in the wavelength range of 5–110m. The proposed installation of a new undulator with a larger magnetic period would allow extension towards considerably longer wavelengths. This would result in the production of extremely short, far-infrared pulses, with a duration of a single optical period or even less. In order to investigate the pulse propagation for free-electron lasers operating in the long wavelength limit, a three-dimensional simulation code was developed. Using the FELIX parameters, with the addition of a long-period undulator, the effects of slippage, diffraction losses, changes in the filling factor, as well as the effects of the optical cavity geometry were studied for wavelengths up to 300m, with electron pulses in the ps regime. It is shown that slippage effects are less restrictive for long wavelength operation than the increasing losses due to optical beam diffraction.     

An Electrodischarge Source of Ultraviolet Radiation on Chlorine Molecules

We investigated the characteristics of a transverse glowing discharge and of a volume discharge in a system of sphereplane electrodes in chlorine of low pressure (P = 40–160 Pa). The formation of a single plasma domain has been revealed. It is a selective source of UV radiation at electronicvibrational transitions of the chlorine molecule. Due to the development of the discharge plasma instability in chlorine, the radiation of the plasma is represented in the form of pulses with a repetition rate f = 1–50 kHz on increase in the mean discharge current from 2 to 50 mA. For this range of discharge currents, the dependence of the brightness of the radiation bands for molecular chlorine with = 200 and 257 nm has a character which increases with the current. The plasma of the chlorinebased domain can be used in a source of shortwave radiation source having no quartz discharge tube (or any other tube transparent in the UV–VUV range of wavelengths) which contracts the plasma and needs cooling and when in the source of the lamp pumping there are no commutators (thyratrons, tacitrons, or spark gaps).     

Dynamics of light-induced diffraction gratings in silicon excited by picosecond pulses

On the basis of a numerical solution of the system of continuity equations we analyze the formation and decay of light-induced diffraction gratings in silicon under excitation by picosecond pulses. The concentration of nonequilibrium carriers generated at excitation-pulse intensities I₀ 10¹⁰ W/cm² reaches values of 10¹⁹–10²⁰ cm^–3. Estimates of the relaxation time of diffraction gratings from widely used formulas are shown to be too low because of the nonlinearity of the evolution of nonequilibrium carriers. The internal electric fields are calculated. The dynamics of diffraction gratings that are nonuniform over the depth of the sample have been studied experimentally as a function of the intensity of the excitation pulses. The experimental data accord with the results of numerical calculations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 53–58, March, 1990.     

Control of susceptibility-related image contrast by spin-lock techniques

Macroscopic magnetic field inhomogeneities might lead to image distortions, while microscopic field inhomogeneities, due to susceptibility changes in tissues, cause spin dephasing and decreasing T₂ relaxation time. The latter effects are especially observed in the trabecular bone and in regions adjacent to air-containing cavities when gradient-echo sequences are applied. In conventional MRI, these susceptibility-related signal voids can be avoided by applying spin-echo (SE) techniques. In this study, an alternative method for the examination and control of susceptibility-related effects by spin-lock (SL) radiofrequency pulses is presented: SL pulses were applied in two different susceptibility-sensitive sequence types: (a) between the jump and return 90° pulses in a 90°_x−τ−90°_−x magnetization-prepared Fast Low Angle Shot (FLASH) sequence and (b) between the 90° pulse and the 180° pulse in an asymmetric SE sequence. The range of Larmor frequencies used for spin locking can be determined for different B₁ amplitudes of the SL pulses, allowing control of image contrast by the amplitude of the SL pulses.     

The effect of the diffusion time and pulse gradient duration ratio on the diffraction pattern and the structural information estimated from q-space diffusion MR: experiments and simulations

q-Space diffusion MRI (QSI) provides a means of obtaining microstructural information about porous materials and neuronal tissues from diffusion data. However, the accuracy of this structural information depends on experimental parameters used to collect the MR data. q-Space diffusion MR performed on clinical scanners is generally collected with relatively long diffusion gradient pulses, in which the gradient pulse duration, δ, is comparable to the diffusion time, Δ. In this study, we used phantoms, consisting of ensembles of microtubes, and mathematical models to assess the effect of the ratio of the diffusion time and the duration of the diffusion pulse gradient, i.e., Δ/δ, on the MR signal attenuation vs. q, and on the measured structural information extracted therefrom. We found that for Δ/δ  1, the diffraction pattern obtained from q-space MR data are shallower than when the short gradient pulse (SGP) approximation is satisfied. For long δ the estimated compartment size is, as expected, smaller than the real size. Interestingly, for Δ/δ  1 the diffraction peaks are shifted to even higher q-values, even when δ is kept constant, giving the impression that the restricted compartments are even smaller than they are. When phantoms composed of microtubes of different diameters are used, it is more difficult to estimate the diameter distribution in this regime. Excellent agreement is found between the experimental results and simulations that explicitly account for the use of long duration gradient pulses. Using such experimental data and this mathematical framework, one can estimate the true compartment dimensions when long and finite gradient pulses are used even when Δ/δ  1.     

Nonlinear power attenuation of the radiation of a pulsed CO2 laser in the atmosphere near the ground

Conclusion Thus, as a result of the natural investigations performed on the propagation of short (microsecond) pulses of CO₂ laser radiation in the near-ground atmosphere, quantitative data are obtained on the nonlinear power attenuation of powerful radiation by the atmosphere in the range of energy densities w = 3–18 J/cm² as a function of the meteorological conditions and the turbulent state of the atmosphere. It has been shown that the principal mechanism of nonlinearity of the medium along a 100–250 m path is the origin of low-threshold optical breakdown centers on solid aerosol particles. The initiation conditions and the main characteristics of an extended laser flash in the atmosphere have been investigated.Future work of interest is to obtain statistically justified dependences of the nonlinear power attenuation and distortions of the structure of beams of powerful pulsed radiation along extended paths on the basic opticometeorological parameters of the state of the atmosphere.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 5–13, February, 1983.     

Plasma radiation emitted by dense mixtures of rare gases excited by an electron beam and by a discharge

An experimental investigation was made of the radiation emitted from a plasma formed by rare gases (He, Ne, Ar, Kr, Xe) excited by a beam of electrons with different parameters and by discharge pulses (P= 0.1–0.5 atm, I=1 kA, U=10–15 kV, C=0.25 F). An investigation was made of the spectral composition and time characteristics of the radiation, and of their dependences on the excitation conditions. New molecular bands were observed in the near-ultraviolet and visible ranges. The uniquely large widths of these bands could be of interest in the amplification of light in a wide spectral range.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 56–73, January, 1992.     

A Pulsed-Periodic, Low-Pressure Volume Discharge in Chlorine and in a Xenon/Chlorine Mixture

The electrical and optical characteristics of a self-initiated, low-pressure, pulsed-periodic discharge in chlorine and in a xenon/chlorine mixture are investigated. A volume discharge not bounded by dielectric walls was triggered in a spherical anode–plane cathode system of electrodes on supply of a constant positive-polarity voltage to the anode. The discharge existed in the form of a unit domain. The spatial, spectral (in the range 150–350 nm), and time characteristics (voltage, current, and photocurrent of total radiation of plasma in the spectral range 200–700 nm) of the volume discharge are investigated. Optimization of the pressure and of the structure of the working medium is carried out to obtain the maximum brightness of UV–VUV radiation of the bands of the Cl₂(D–A), Cl₂ ^**, and XeCl(D, B–X) molecules. The results obtained are of interest for being used in a pulsed-periodic, excimer-halogen low-pressure lamp.     

Reflection of narrow light beams by a scattering medium

A calculation is made of the doubly back scattered radiation from the intersection volume and the close-range volume. A comparison of the results for singly and doubly scattered radiation with experimental data enables the limits of applicability of the equations to be determined. It is shown that for scattering coefficients in fog of < 0.06 m^–1 the scattered radiation field is caused by single scattering but when increases to 0.6 m^–1 only the doubly scattered radiation from the intersection and close-range volumes need be considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 95–100, April, 1974.     

Gas formation in liquid in initial stages of a pulsed discharge

Gas formation in electrolytes with=10^–2–10^–4–1cm^–1, distilled water = 10^–5-1.5 ·10^{–6 –1}cm^–1, and chemically pure n-hexane in the initial stages of formation of discharge with rectangular voltage pulses of 0.67 and 1.85 sec duration is investigated. The experimental results are compared with the results of approximate calculations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 42–47, November, 1972.The authors thank V. V. Ryumin for taking part in the discussion of the results and V. V. Lopatin for participation in the experiments with electron-optical light amplifier.