Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Stimulated Brillouin scattering of CO2 radiation in compressed xenon

Stimulated Brillouin scattering (STBS) and phase conjugation of CO₂ laser radiation have been demonstrated experimentally for the first time in compressed xenon (59 atm at 21°C) located inside the low-Q cavity of this laser. The nonlinear medium was exposed to the action of counterpropagating focused multimode radiation beams. The difference between the frequencies of the longitudinal cavity modes was set at the frequency of the acoustic wave (v _s=32.2±0.3 MHz) excited as a result of STBS by 9.584 μm radiation. The duration of the radiation pulse τ _L was close to the acoustic phonon lifetime (τ _L<τ _ph≈3× 10⁻⁶s). The excitation of STBS was manifested experimentally as the locking of longitudinal modes, an increase in power and energy, and also an increase in the duration of the lasing pulse and a reduction in the divergence to the diffraction limit. Zh. éksp. Teor. Fiz. 116, 1941–1946 (December 1999)     

Effect of photoexcitation on the electrical performance of ZnS: Mn thin-film electroluminescent structures

It is shown that the kinetics of the charge and current passing through a thin-film electroluminescent emitter, as well as the I-V characteristics of the emitter, greatly diverge under blue, red, and IR pulsed illumination with photon energies of ≈2.6, ≈1.9, and ≈1.3 eV, respectively, and a photon flux density of 4×10¹⁴–3×10¹⁵ mm⁻² s⁻¹. Results obtained indicate that, during the operation of the emitter, deep centers associated presumably with V _Zn ²⁻ zinc vacancies and V _S ⁺ and V _S ²⁺ sulfur vacancies exchange charge. These centers lie above the valence band by ≈1.1, ≤1.9, and ≤1.3 eV, respectively. Their concentrations are estimated as (3–4)×10¹⁶ cm⁻³ for V _Zn ²⁻ and V _S ⁺ and ≈1.5×10¹⁶ cm⁻³ for V _S ²⁺ . It is demonstrated that positive and negative space charges forming in the near-anode and near-cathode regions of the phosphor layer specify the electric performance of the emitters.     

Influence of the axial magnetic field on the current instability and on the collective acceleration of ions in plasma diodes

It is shown that, while suppressing transverse electron motion, the axial magnetic field with an induction of up to 6.8 × 10⁻² T in the gap of a plasma diode has no significant effect on the current instability and on the acceleration of ions at electron beam currents of ≤40 A. The increase in both the critical current and the period of current oscillations is related to an increase in the plasma density after applying the magnetic field. The maximum energy of the accelerated magnesium ions decreases by ≈25% at an induction of 1.7 × 10⁻² T and does not depend on the magnetic field in the range (1.7–6.8) × 10⁻² T.     

Improvement of the efficiency of a glow discharge-based ion emitter with oscillating electrons

Ion emission from the plasma of a low-pressure (≈5×10⁻² Pa) glow discharge with electrons oscillating in a weak (≈1 mT) magnetic field is studied in relation to the cold hollow cathode geometry. A hollow conic cathode used in the electrode system of a cylindrical inverted magnetron not only improves the extraction of plasma ions to ≈20% of the discharge current but also provides the near-uniform spatial distribution of the ion emission current density. The reason is the specific oscillations of electrons accelerated in the cathode sheath. They drift in the azimuth direction along a closed orbit and simultaneously move along the magnetic field toward the emitting surface of the plasma. A plasma emitter with a current density of ≈1 mA/cm² over an area of ≈100 cm² designed for an ion source with an operating voltage of several tens of kilovolts is described.     

Normal-metal hot-electron bolometer with Andreev reflection from superconductor boundaries

This paper describes the design and experimental testing of a high-sensitivity hot-electron bolometer based a film of normal metal, exploiting the Andreev reflection from superconductor boundaries, and cooled with the help of a superconductor-insulator-normal metal junction. At the measured thermal conductivity, G≈6×10⁻¹² W/K, and a time constant of τ=0.2 μs, and a temperature of 300 mK, the estimated noise-equivalent power NEP=5×10⁻¹⁸ W/Hz^1/2, assuming that temperature fluctuations are the major source of noise. At a temperature of 100 mK, the thermal conductivity drops to G≈7×10⁻¹⁴ W/K, which yields NEP=2×10⁻¹⁹ W/Hz^1/2 at a time constant of τ=5 μs. The microbolometer has been designed to serve as a detector of millimeter and FIR waves in space-based radio telescopes. Zh. éksp. Teor. Fiz. 115, 1085–1092 (March 1999)     

Effect of gas pressure on amplitude and duration of electron beam current in a gas-filled diode

The parameters of an electron beam generated in helium in the pressure range p = 10⁻⁴−12 atm are studied. Nanosecond high-voltage pulses are applied to a gap between a tubular cathode and planar anode, which is made of 45-μm-thick AlBe foil. Behind the anode, an electron beam is detected at a helium pressure of 12 atm. The pressure dependence of the beam current amplitude shows three peaks at p ≈ 0.01, ≈ 0.07, and ≈ 3 atm. The beam-induced glow of a luminescent film placed behind the foil and the discharge glow at different helium pressures in the gas-filled diode are photographed.     

Coherent backscattering of electromagnetic waves in a magnetised plasma

Summary A microwave coherent backscattering experiment has been carried out on Mirabelle, a weakly ionised plasma device, with the objective of measuring the electron density fluctuation level. The experiment is a preliminary step in order to prepare the detection system for a microwave stimulated backscattering experiment. The incident electromagnetic wave is focused in front of a plane grid which excites ion acoustic or electron Bernstein waves inducing fluctuations in the plasma. The backscattering signal is collected by the launching circuit and detected by homodyne mixing. The typical ratio of the scattered power to the incident power is about 10⁻¹² and the relative density fluctuations are of the order of δn _e/n _e=10⁻³ against a background electron density ofn _e=1–5·10⁹ cm⁻³. The backscattering measurement is compared with Langmuir probe measurements. The spectral width of the backscattered signal has also been studied, by taking into account effects due to the incident wave focusing and plasma wave damping. The authors of this paper have agreed to not receive the proofs for correction     

Fast electron energy deposition in aluminium foils: Resistive vs. drag heating

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 10¹⁹ Wcm^-2 laser pulses interacting with Al foils of thicknesses 10-200 μm. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ≈407 nm (the second harmonic of the laser light) and ≈500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (T _h ^tail ≈ 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (T _h ^bulk=500 ± 100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2ω₀ emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 μm. Beyond that depth, because of the beam divergence, the incident current goes under 10¹² Acm^-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 μm depth.     

Formation of two-dimensional current sheets under high initial pressure conditions

The possibilities of current-sheet formation in two-dimensional magnetic fields with a null line as well as the characteristic features of the plasma dynamics under high initial pressure conditions (helium, P ₀≈300 mtorr) are investigated for the first time. It is shown that current-sheet formation and efficient compression of the plasma into a sheet require that the magnetic field gradient be sufficiently large. A brightly emitting compact region with electron density N _e∼9×10¹⁶ cm⁻³, an order of magnitude higher than the gas atom density, was observed to form at the center of the layer. Zh. éksp. Teor. Fiz. 114, 1202–1214 (October 1998)     

Search for cosmic γ-ray bursts in the (1÷50) GeV energy range

Summary A search for cosmic gamma-ray bursts in the GeV energy range has been performed by means of the EAS-TOP Extensive Air Shower array at Campo Imperatore (Gran Sasso Laboratories) during the period March–December 1990. In 2566.5 hours of measurement the obtained upper limit to the rate of bursts of amplitude >2% of the cosmic-ray intensity and time duration τ=1 s, isR≤7.9y⁻¹ (90% c.l.). Assuming for γ-rays a differential energy spectrumS(E ₀ )≈E ₀ ^−2.5 , the corresponding upper limit to the energy flux of γ-rays with energy >5 GeV in bursts of duration τ≤1 s is Φ<8.3·10⁻⁵erg cm⁻².     

Formation of a shock wave in aerogel irradiated with a high-current pulsed electron beam

The propagation of pressure jumps excited by a high-current pulsed electron beam in SiO₂ aerogels with density ranging from 0.025 to 0.25 g/cm³ is investigated using a laser differential interferometer and optical methods. Spallation on the back side of the aerogel targets is observed and the velocity of the spallation fragments is measured. The expansion velocity of the aerogel in the direction of the electron beam is determined. The parameters of the shock adiabat are established in a wide range of aerogel porosities. The depth of the energy-deposition zone of the electron beam is determined experimentally as a function of the aerogel density in the range from 0.015 to 0.25 g/cm³. A model describing highly porous materials which reflects the fractal properties of highly porous aerogels is developed on the basis of the experimental data. Numerical calculations of the observed phenomena are performed. Zh. Tekh. Fiz. 69, 18–25 (December 1999)     

Determination of the profile of energy release from a high-power electron beam in an aerogel

Optical methods are used to investigate the dynamics of the interaction of a high-current electron beam with an aerogel (a highly porous transparent dielectric with a low density ρ=0.36 g/cm³). The measured profile of the glow of the aerogel and the pattern of its expansion are compared with the results of a numerical simulation. The influence of the space charge on the profile of the energy absorption from the high-current relativistic electron beam is discussed. Zh. Tekh. Fiz. 67, 26–32 (November 1997)     

Near IR laser light visualizators using nonlinear GaSe and other layered crystallites

Two methods of preparation of the devices for visualization of pulsed and continuous near-IR (near infrared) are described and the results of conversion of pulsed and continuous IR (800–1360 nm) laser radiation into the visible range of spectra (400–680 nm) by using a transparent substrate covered with the particles (including nanoparticles) of effective nonlinear materials of GaSe _x S_{1 − x} (0.2 ≤ x ≤ 0.8) are presented. Converted light can be detected in transmission or reflection geometry as a visible spot corresponding to the real size of the incident laser beam. Developed device structures can be used for checking if the laser is working or not, for optical adjustment, for visualization of distribution of laser radiation over the cross of the beam and for investigation of the content of the laser radiation. Low energy (power density) limit for visualization of the IR laser pulses with 2–3 ps duration for these device structures are: between 4.6–2.1 μJ (3 × 10⁻⁴−1 × 10⁻⁴ W/cm²) at 1200 nm; between 8.4–2.6 μJ (4.7 × 10⁻⁴−1.5 × 10⁻⁴ W/cm²) at 1300 nm; between 14.4–8.1 μJ (8.2 × 10⁻⁴–4.6 × 10⁻⁴ W/cm²) at 1360 nm. Threshold damage density is more than 10 MW/cm² at λ = 1060 nm, pulse duration τ = 35 ps. The results are compared with commercially existing laser light visualizators.     

密度渐变多层碳气凝胶靶型的制备

 以间苯二酚-甲醛为原料,结合自制活动式微模具成型工艺制备不同厚度和密度的碳气凝胶薄片,采用密度为10 mg·cm^-3的SiO₂溶胶为“粘合剂”,获得单元薄片厚度在100～580 μm,密度在50～400 mg·cm^-3范围内变化的5层密度渐变碳气凝胶靶型。重点研究了该特殊靶型内部C/SiO₂气凝胶层间界面情况。采用场发射扫描电镜（FESEM）,X射线相衬成像仪等对靶型整体结构及碳气凝胶单元薄片表面和内部微观结构进行了表征。结果表明：胶粘层SiO₂气凝胶厚度约为15 μm,厚度一致,远小于碳气凝胶层厚度且与碳气凝胶薄片的胶粘程度较好,界面平整,靶结构均匀。     

Extra-liganding effects in Zn-octaethylporphyrin solutions in a temperature interval of 300-77 K

It is established experimentally for the first time that Zn-octaethylporphyrin in a solution of dried methyl cyclohexane is in unliganded form in the temperature interval of 293-77 K. This form is characterized by the shortest-wave position of absorption and luminescence spectra, the independence of the degrees of fluorescence and phosphorescence polarizations from the excitation wavelength, and high phosphorescence time (τ_T=126 μsec) and quantum yield (φp=0.085) at 77 K. Adding electrodonor components to the methyl cyclohexane or going over to alcohol-containing polar media leads to the formation of a monoliganded form of ZnOEP, for which we observe a bathochromic shift of ≈150–350 cm⁻¹ for Q(0,0) absorption and luminescence bands, a splitting of ≈50–100 cm⁻² for the Q(0, 0) absorption band, and phosphorescence quenching (τ_T=60 μsec, φp=0.05-0.03 at 77 K). The effects revealed are explained by a decrease in the symmetry of a ZnOEP molecule in extra liganding and by an increase in the probability of nonradiative activation for a T₁-state due to the extra ligand being involved in the exchange of electron-excitation energy and to the enhancement of a spin-orbital coupling as a result of the disturbance of the planarity, of a macrocycle. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 900–907, November–December, 1998.     

Probe measurements in a discharge with liquid nonmetallic electrodes in air at atmospheric pressure

Discharges with liquid nonmetallic electrodes of much interest for applications are investigated. It is found that a dc discharge between two streams of tap water in air at atmospheric pressure is stable at a currentof 40≤I≤100 mA. The discharge exists in the diffuse (volume) form with a relatively low current density (∼0.2 A/cm²) and a high (above one kilovolt) voltage drop across the air gap (∼1 cm) between the water electrodes. The current density and voltage depend only slightly on the discharge current. Probe measurements show that three regions can be distinguished in the discharge: two electrode regions (1–2 mm in length) and a discharge column with a constant electric field of ≈0.8 kV/cm (i.e., E/N≈20 Td, because the gas in the discharge is heated up to 1500–2000 K). The average electric field strength near the electrodes is E≈2–3×10³ V/cm (E/N≈60–80 Td). The charged particle density in the column is n ∼ 10¹² cm⁻³. The probe measurements of n agree with the previous microwave absorption measurements. The water vapor concentration in the column is also estimated from probe measurements.     

The temperature and light intensity dependence of photorefraction in LiNbO3

In the paper the dependence of the photorefraction (PhR) in LiNbO₃ and LiNbO₃−Fe (0.1 wt%, 0.3wt%) crystals on light intensity (within 10¹⁶–10²³ quanta·cm⁻²·s⁻¹ at wavelengths 496.5 nm and 600 nm) and temperature (in the region 100–500 K) is studied. For all the crystals the limiting values of PhR are similar and atT=293 K Δn _sat ^lim ≈3·10⁻³. In LiNbO₃ the temperature dependence of PhR in the range 100–500 K requires to take into account at least two trapping centres.     

Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/Al_xGa_1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 10¹¹ cm⁻² the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 139–144 (10 August 1997)     

Target plasma formation by UHF power

The properties of plasma injected into an open magnetic trap of uniform field from an independent UHF source have been investigated. Plasma is created in the UHF source at the frequency of 2400 MHz (power input 150 W) in the electron cyclotron resonance (ECR) regime at the pressure of neutral argon (10⁻⁵−10⁻²) torr. It is established that a rather quiescent target plasma with controlled density within the range of (2 × 10⁸−2 × 10¹²) cm⁻³ and temperature 2–3eV is accumulated in the trap. It turned out that plasma lifetime in the trap is determined by a classical mechanism of particle escape at the expense of collisions, at fixed value of magnetic field in the trap it practically is not changed with the variation of neutral gas pressure and reaches the value ≈ 4×10⁻³ s at the magnetic field strength in the trap equal 1600 Oe.