STABILITY NUMBER IN SUBCLASSES OF P5-FREE GRAPHS

Two new hereditary classes of P5-free graphs where the stability number can be found in polynomial time are proposed. They generalize several known results.     

Powerful Cherenkov oscillators with 2D distributed feedback

The feasibility of using 2D distributed feedback based on 2D planar and coaxial Bragg structures for generating spatially coherent radiation from rectilinear ribbon and tubular electron beams is studied. One-section and sectional Cherenkov masers are analyzed. In the former design, a 2D Bragg structure acts as a resonator and a periodic slow-wave system simultaneously. In the latter (sectional) design, radiation is synchronized in a 2D Bragg structure that is placed at the cathode end of the interaction space and couples longitudinal and transverse (azimuthal) wave flows. The wave is amplified by the electron beam mainly in the fairly long middle section. The output (collector) part contains a standard 1D Bragg structure that partially reflects the amplified radiation toward the cathode and closes the feedback circuit. It is shown that dissipation introduced into the 2D Bragg structure of the sectional design makes it possible to increase one of the transverse sizes of the system to ∼10³ wavelengths with the energy exchange efficiency and one-frequency masing mode stability remaining the same. With such an overdimension, the millimeter-wave radiation integral power may reach a gigawatt level.     

Generation of terahertz radiation by the optical breakdown induced by a bichromatic laser pulse

Experimental results are presented on generation of terahertz radiation by the air breakdown induced by a high-intensity laser pulse having not only a fundamental component, but also a second-harmonic one. A theoretical explanation of the experimental data is proposed, based on a model of field ionization of a gas by a bichromatic laser.     

Gargano,Lewinter and Malerba问题的解

§1　IntroductionLet G be a graph with vertex-set V(G) ={ v1 ,v2 ,...,vn} .A labeling of G is a bijectionL:V(G)→{ 1,2 ,...,n} ,where L (vi) is the label of a vertex vi.A labeled graph is anordered pair (G,L) consisting of a graph G and its labeling L.Definition1.An increasing nonconsecutive path in a labeled graph(G,L) is a path(u1 ,u2 ,...,uk) in G such thatL(ui) + 1相似文献   

Development of a compact hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range

The key elements of a mobile hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range have been designed, produced, and approved in model experiments. These elements are a compact THz oscillator based on an all-fiber femtosecond laser system and a unit for recovering electrodynamic characteristics of layered objects from scattered THz radiation spectra. Generation of 250-fs optical pulses at a wavelength of 1.03 µm with energy of 0.3 µJ and a repetition frequency of 1MHz is demonstrated and the efficiency of optical-THz conversion is found to be 5×10^?6. The proposed algorithm is constructed based on an iterative procedure and can be used for dispersive and absorbing media. It has higher operating speed in comparison with the algorithms for solving inverse problems, which are based on functional minimization methods.     

Chaotic generation in a megawatt backward-wave tube

Experimental and theoretical studies on the self-modulation mode of generation in a high-power BWT with the electrodynamic system representing a slightly corrugated waveguide are presented. The BWT is fed by an electron beam with the energy 150 keV and the current 150 A. The system operates at the E ₀₁ mode with the mean frequency 8.7 GHz. Dynamic chaos is obtained by a three-fold increase in the length of the interaction space in comparison with the prototype exhibiting stationary generation. The stationary generation was changed to periodic sinusoidal self-modulation and then to chaotic self-modulation as the current increases from 6 to 60 A. The generation mode is simplified when the current ranges from 70 to 90 A and becomes complicated again for the current exceeding 100 A. Experimental observations are in good agreement with the results of simulation predicting a certain simplification of the self-modulation mode at the currents 70–90 A owing to the effect of high-frequency space charge. Under the conditions of chaotic generation, the mean power was as high as 2 MW at the relative spectral width of the signal 4% and the total duration of the microwave pulse 10 μs.     

翼型模型振动的实验研究与气动特性分析

通过在NF-3低速风洞专门研制的翼型模型及相应的俯仰和沉浮振动机构,选用NACA0012翼型进行大迎角下不同频率的振动实验,研究了模型振动平均状态下对其气动力特性的影响情况,并在N-S方程基础上对振动流场进行了初步分析。实验与计算研究的结果表明:在临近定常失速迎角的大迎角条件下,翼型的振动可以引起旋涡分离,导致翼型升力减小和失速迎角的提前;就所讨论的两种振动模式而言,俯仰振动的影响大于沉浮振动。所以,为了提高飞机模型,尤其是大展弦比飞机模型的风洞实验精准度,在模型设计和加工时要特别注意加强机翼弦向的扭转刚度。     

Experimental study of the possibility to increase the pitch factor of an intense relativistic helical electron beam

We present the results of experiments on the formation of a high-quality relativistic helical electron beam (HEB) in a magnetron-injection gun. It is shown that suppression of parasitic excitation of microwaves in the input part of the transportation channel allows eliminating high-voltage breakdowns in the gun and achieving greater beam compression. A modulation of the electron beam current at the frequency of longitudinal electron oscillations between the cathode and the magnetic mirror in the trap, which is related to the instability of the helical electron beam, has been observed for the first time. The modulation depth can reach tens percent. Pickup of reflected electrons by a special diaphragm makes it possible to increase the achievable pitch factor, eliminate the beam-current modulation and, as a result, form although with a current loss on the diaphragm, an HEB with record-breaking values of the pitch factor, which exceed 2. For a moderate HEB compression, when the portion of reflected electrons is relatively low, their pickup by the diaphragm allows one to form a beam in which the total energy of the transverse motion of electrons conserves despite the loss of part of the current. After the optimization, a beam with an electron energy of 300 keV, a current of 100 A, a pitch factor of 1.5, and a velocity spread of 20% is obtained for a 15% loss of the current on the diaphragm. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 855–863, October 2008.     

Relativistic carcinotron with a thermionic injector of electrons

An experimental study directed to the determination of a mechanism for microwave breakdown in a relativistic X-band carcinatron is presented. An electron beam was generated using a thermionic cathode, which provided a stable beam geometry. The use of this cathode decreased the probability of breakdown caused by electron bombardment of the slow-wave structure. An important part played in microwave breakdown are molecules absorbed on the slow-wave structure surface. It is shown that the presence of these molecules, an implementation of conditions for secondary-electron resonant discharge (SERD), can result in a very fast (during 10-20 ns) limitation of the radiation pulsewidth. To remove the adsorbed molecules, heat degassing of the slow-wave structure and a collector of electrons was applied, going on continuously, during a working day of the device. By degassing and choice of slow-wave structure material, output radiation power of the device was increased by a factor of ten (up to 5 MW at the pulsewidth of 10 μs)