Superradiation from non-ideal plasmas in electric field

An effect of external static electric field on emission of radiation from non-ideal plasmas of erosion focus has been experimentally observed. An order-of-magnitude increase in radiation intensity for spectral interval hv = 40 ? 350 eV with electric field increasing up to 10³V/cm has been found for plasmas with $\text{ξ}\text{Z}\text{?1}$, where ξ is the number of electrons in Debye sphere, Z = 2 ? 3 is the stage of ionization. The energy emitted has been several times higher than the black body energy for the same spectral interval at maximum electric field achieved. The effect vanishes at $\text{ξ}\text{Z}\text{?10}$.     

Hot electron plasmas trapped in helical magnetic surfaces

Experimental studies on nonneutral (pure electron) plasmas of finite temperature, trapped in helical closed magnetic surfaces have been conducted. The helical electron plasmas are produced with thermal electrons launched from the outside of the last closed flux surface (LCFS). About 150 μs after the electron injection, the plasmas reach equilibrium state. Around the LCFS, a steep gradient of plasma space potential φ _s is formed. The corresponding radial electric field is about 2.5 kV/m. On the other hand, around the magnetic axis of helical magnetic surfaces, φ _s is almost constant, indicating that there are little electrons there. The volume-averaged electron density is on the order of 10¹³ m^–3, smaller than the Brillouin density limit. The confinement time seems to be limited by a disruptive instability, and is so far about 1.5 ms.      

碰撞辐射稳态等离子体电荷态分布的一种扩展模型

通过等离子体中离子激发-退激发平衡关系,构造一种离子激发组态之间满足的非Boltzmann分布,利用这个分布作为权函数对原子过程速率系数进行平均,构造出一个扩展的碰撞辐射稳态模型.利用该模型计算从低Z到高Z元素等离子体平均离子电荷随电子温度的变化.进而研究非Boltzmann分布对平均电离度和激发组态相对密度的影响.结果表明,它对相对激发组态密度的影响非常显著. 关键词： 扩展的碰撞辐射稳态 概率分布 能级动理学     

Characteristics of the backscattering of moderate-energy electrons by solids having different atomic numbers

The coefficient of backscattering of primary electrons is discussed as a function of their energy and atomic number Z. The amplitude of the wave function obtained in the first Born approximation and the Thomas-Fermi atom model are used to calculate the constant for screening of the electric field of the nucleus by atomic electrons. The theoretically calculated integral backscattering coefficients of primary electrons are compared with the experimental values in the range 12<Z<92 for primary electron energies of 10 and 40 keV. Possible applications of these results are indicated. Zh. Tekh. Fiz. 69, 40–45 (June 1999)     

Generation of lower and higher harmonics in different plasma media with small <Emphasis Type="Italic">Z</Emphasis>

The generation of lower (third) and higher harmonics of femtosecond laser radiation in plasmas produced by laser ablation of different targets with a small atomic number Z (B, Be, Li) has been investigated. The high (10⁻³) efficiency of third-harmonic generation was observed in plasma produced on the boron surface. Efficient third-harmonic generation was also observed in beryllium plasma using femtosecond pulses of Ti:sapphire laser radiation (λ = 790 nm) and its second harmonic (395 nm). We could tune the higher harmonics generation spectrum by tuning the crystal converter when using 395-nm radiation to be converted. It is shown that, in plasmas formed on targets with small Z, the conversion efficiency and limiting generated harmonic order depend on the delay between the ablation pulse and the pulse to be converted.     

Cross-Field Diffusion by Charge Changing Processes

The particle transport induced by ionization, recombination and charge exchange processes in magnetically confined plasmas is analyzed. For ions with charge numbers Z ≧ 2 a random walk is caused by the fluttering of the gyro-radius as a result of successive recombination and ionization events. The corresponding diffusion coefficient, however, is very small and may be neglected even in the case of high-energy alpha particles being produced in fusion plasmas. Single and neutral particles are subjected to joint transport across the magnetic field due to their unimpeded motion during the short atomic phases. It is found that this mechanism is more important than classical transport over a wide temperature and density range. The corresponding diffusion coefficient D₁ = f(Te)Ti/mine is independent of the magnetic field strength and shows an electron temperature dependence which can be expressed in terms of the rate coefficients for ionization, recombination and charge exchange. The latter effect leads to a strong reduction of the diffusion process in hydrogen plasmas. Diffusion coefficients comparable with anomalous values of about 1m²/s are therefore reached only for cool (divertor) plasmas with T < 5eV.     

Theory of gyro devices with thin electron beams (large-orbit gyrotrons)

It is shown for a precisely aligned electron beam of small thickness in a large-orbit gyrotron that high cyclotron harmonics (up to s⩽5) can be selectively excited with an electronic efficiency that is acceptable for some applications (1–10%). When the quality of the electron beam is high, the selective properties are maintained even for modes with high radial indices, raising hopes that this method can be used to obtain coherent radiation not only in the millimeter but also in the entire submillimeter wavelength range. A method for taking into account the space-charge effects is developed, and it is shown that the corresponding effects can be important at relatively small values of the electron pitch factor. The results of the analysis and preliminary experiments reveal the possibility of creating high-power compact sources of submillimeter radiation for the spectroscopy of various media, the diagnostics of dense plasmas, and some other applications. Fiz. Tekh. Poluprovodn. 68, 91–98 (October 1998)     

Quantum relativistic theory of an electron in terms of local observables

Dirac equation is reformulated in terms of real local observables, which are mean values of the wave function . The quadrivector current is shown to be a function of the potential vector and of other local observables. The equations describe the evolution of a four dimensional system T, X, Y, Z, and of two scalars, in the coordinate system ct, x, y, z. The current is proportional to the T vector. The Z vector is associated with the spin of the electron. Energy and gauge transformations correspond to rotations in the plane (X, Y). In the presence of a static field, the (real) solutions of the equations appear as eigenfunctions associated with energy eigenvalues. Received 7 September 1998     

Electron transport anomaly in a plasma in an intense radiation field

It is shown that in an intense radiation field the electron friction force resulting from electron-ion collisions becomes an electron accelerating force on account of absorption of radiation by electrons. It is pointed out that the thermal conductivity increases. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 19–22 (10 July 1996)     

X-ray converters for radiation treatment of thin films

The energy absorbed in thin films of selected materials bombarded by x rays emitted in the braking of low-energy electrons (E ₀<500 keV) in converters with various atomic numbers (Z=29–73) is calculated by the Monte Carlo method. The program takes into account both of the K-shell ionization mechanisms that lead to emission of characteristic photons as a result of electron impact and as a result of the photoelectric effect, and the characteristic radiation is shown to make a large contribution to the absorbed energy in thin films. Calculations show that the proper choice of material and thickness of the converter affords a two-to fivefold increase in the energy of the x radiation absorbed in thin films of semiconductor materials. Zh. Tekh. Fiz. 68, 99–101 (November 1998)     

Deep impurity-center ionization by far-infrared radiation

An analysis is made of the ionization of deep impurity centers by high-intensity far-infrared and submillimeter-wavelength radiation, with photon energies tens of times lower than the impurity ionization energy. Within a broad range of intensities and wavelengths, terahertz electric fields of the exciting radiation act as a dc field. Under these conditions, deep-center ionization can be described as multiphonon-assisted tunneling, in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field. The field dependence of the ionization probability permits one to determine the defect tunneling times and the character of the defect adiabatic potentials. The ionization probability deviates from the field dependence e(E) ∝ exp(E ²/E _c ² ) (where E is the wave field, and E _c is a characteristic field) corresponding to multiphonon-assisted tunneling ionization in relatively low fields, where the defects are ionized through the Poole-Frenkel effect, and in very strong fields, where the ionization is produced by direct tunneling without thermal activation. The effects resulting from the high radiation frequency are considered and it is shown that, at low temperatures, they become dominant. Fiz. Tverd. Tela (St. Petersburg) 39, 1905–1932 (November 1997)     

Annihilation of a relativistic positron and K-electron to yield a photon and second K-electron

We study the annihilation of a fast positron and a K-electron resulting in the emission of a photon and a second K-electron. It is assumed that all electrons and positrons move in the Coulomb field of the nucleus and that the Coulomb parameter αZ is much less than unity (α=1/137 is the fine-structure constant and Z is the atomic number). The electron-electron interaction, which is responsible for the ejection of the electron by the atom, is taken into account in the first order of perturbation theory. We calculate the differential and total cross sections of the process and construct the ratio of the cross sections of double and single ionization as a function of the energy of the incident positron. Finally, we establish the high-energy limit of this ratio, equal to 0.34/Z ². Zh. éksp. Teor. Fiz. 113, 786–804 (March 1998)     

Associated production of Higgs bosons with Z bosons by charged leptons in strong external fields

The probabilities of the associated production of a Higgs boson with a Z boson by a charged lepton in the field of a plane electromagnetic wave of arbitrary intensity and in a constant crossed field are obtained. The behavior of the cross section of the process as a function of the particle energies and the external field intensity is investigated for various values of the Higgs boson mass. It is shown that there is a logarithmic increase in the photoproduction cross section at superhigh energies up to a value significantly exceeding the cross section of the reaction e ⁺+e ⁻→Z+H, which is presently regarded as the most probable channel for the production of Higgs bosons. Zh. éksp. Teor. Fiz. 113, 1979–1990 (June 1998)     

Stability of Ultraviolet-Cutoff¶Quantum Electrodynamics with Non-Relativistic Matter

We prove that the quantum-mechanical ground state energy of a system consisting of an arbitrary number, M, of static nuclei of atomic number ≤Z and of an arbitrary number, N, of Pauli electrons interacting with the quantized, ultraviolet-cutoff radiation field is bounded below by $-K.M, where K is a finite constant depending on Z, on the finestructure constant α and on the ultraviolet cutoff Λ, with , as , and K' independent of Λ. Received: 4 September 1996/ Accepted: 9 April 1997     

Double ionization and excitation ionization in the Compton scattering of high-energy photons for metastable states of heliumlike ions

Double ionization and excitation ionization in Compton scattering for heliumlike ions in metastable states are investigated. The electron energy distribution for double ionization and the total cross sections for both processes are calculated. The calculations are carried out in the zeroth order of perturbation theory with respect to electron-electron interaction, using Coulomb wave functions as the first approximation. The resulting equations are valid only in the high-energy nonrelativistic range. It is assumed that Z≫1, but αZ≪1 (Z is the charge of the nucleus, and α is the fine-structure constant). Zh. éksp. Teor. Fiz. 116, 1889–1902 (December 1999)     

Derivation of the vacuum longitudinal fieldB (3) from the Dirac equation of the electron in the electromagnetic field

By solving the Diras equation for the motion of an electron (c) in the circularly polarized electromagnetic field it is shown that the intrinsic electron spin forms an interaction Hamiltonian with a time independent fieldB ⁽³⁾ of electromagnetic radiation in the vacuum. In the same way as intrinsic spin is a fundamental property of the electron,B ⁽³⁾ is therefore a fundamental and intrinsic property of the vacuum electromagnetic field.     

Instability of electromagnetic waves in transversely magnetised semiconductor-plasmas

Using the hydrodynamic model of plasmas the general dispersion relation is derived in the collisiondominated regime when a d.c. magnetic field is applied (Y-axis) transversly to the propagation vector k (Z-axis), and the d.c. electric field is inclined to the Z-axis in the X-Z plane. The dispersion relation is solved for intrinsic and extrinsic semiconductors to explore the possibility of wave instability. The threshold conditions of wave oscillations are obtained. In n-InSb the frequency of the oscillation attains a maximum value when the electron cyclotron frequency is equal to the electron collision frequency. In intrinsic InSb instability is possible only in the long wavelength region for E₀ ? 10 kVm^?1 when B₀> 0.2 T, while for lower values of B₀, E₀ should be greater 20kVm^?1. The energy dependent collision frequency has a significant effect on the threshold frequency of oscillation.     

Nonlinear properties of a weakly collisional plasma at low radiation intensities

In this paper we evaluate theoretically the modification of the distribution function of slow subthermal electrons heated by radiation in a completely ionized plasma. The new solution we have found to the kinetic equation under conditions typical of weak-collisional plasmas can be used to predict new nonlinear behavior arising from perturbations of the nonuniform electron density and the coefficient of nonlocal heat transfer that depend on the intensity of the radiation heating the plasma, which is absorbed via inverse bremsstrahlung. It is predicted that this new nonlinear behavior manifests itself at unexpectedly small radiation intensities. Zh. éksp. Teor. Fiz. 113, 1299–1312 (April 1998)     

Ionization of hydrogen and helium atoms by highly charged fast ions in collisions with small momentum transfer

Ionization of hydrogen and helium atoms is studied for the case of “soft” collisions with highly charged fast ions with v≲Z≪v² and v≫v ₀, where Z is the ion charge, v is the collision velocity, and v ₀∼1 is the characteristic velocity of the electron in the ground state of the atom. Analytical expressions are derived for the singly and doubly differential cross section for ionization of a hydrogen atom accompanied by the ejection of a slow electron v _e≲v ₀, where v _e is the velocity of the ejected electron with respect to the recoil ion). The results are generalized to the case of single ionization of helium. It is shown that soft collisions provide the main contribution to the hydrogen ionization cross section and for all practical purposes determine the cross section for single ionization of helium. The asymmetry in the angular distribution of the ejected slow electrons and the properties of momentum exchange in such collisions are discussed. Finally, a formula for the cross section for single ionization of helium is proposed. Zh. éksp. Teor. Fiz. 112, 1966–1977 (December 1997)