Resonant transition radiation in the X-ray region from a low emittance 855 MeV electron beam

The interference of transition radiation coherently produced from a periodic stack of four polyimide foils of 7.2 μm thickness and a separation of 162 μm was investigated. This stack has been brought into the low emittance (3 π nm rad) electron beam of the 855 MeV Mainz Microtron MAMI. Transition radiation was observed in the energy range from 2 to 15 keV with a LN₂-cooled pin photodiode. A good energy resolution of 0.8 keV and angular resolution of 0.15 mrad was achieved simultaneously allowing for the first time to quantitatively study the interference pattern. Good agreement with theoretical calculations is found. Prospects to exploit transition radiation in the x-ray region from a low emittance electron beam as a high brilliant radiation source are discussed.     

Tunable terahertz radiation from arbitrary profile dielectric grating coated with graphene excited by an electron beam

In this paper, the enhanced terahertz radiation transformed from surface plasmon polaritons, excited by a uniformly moving electron bunch, in a structure consisting of a monolayer graphene supported on a dielectric grating with arbitrary profile is investigated. The results show that the grating profile has significant influence on the dispersion curves and radiation characteristics including radiation frequency and intensity. The dependence of dispersion and radiation characteristics on the grating shape for both the symmetric and asymmetric gratings is studied in detail. Moreover, we find that, for an asymmetric grating with certain profile, there exist two different diffraction types, and one of the two types can provide higher radiation intensity comparing to the other one. These results will definitely facilitate the practical application in developing a room-temperature, tunable, coherent and miniature terahertz radiation source.     

Features of the transition radiation of the electron in the vacuum ultraviolet range

The transition radiation (TR) of an electron in the vacuum ultraviolet range near the direction of total quantum reflection from the medium-vacuum interface has been investigated. The effect of a substantial increase (tens times) in the TR angular density has been predicted. The substantial dependence of lightness of this effect on the susceptibility of the dielectric target, the exit angle, and the energy of the emitting electron has been found.     

On the transition radiation and bremsstrahlung from a relativistic electron with a nonequilibrium field

The problem of the transition radiation from an electron with a nonequilibrium self-field appearing in view of the sharp scattering of the electron has been considered. It has been shown that the state of the electron with the nonequilibrium field is manifested in the suppression of transition radiation and in the oscillatory dependence of its characteristics on the distance from a plate on which radiation occurs to the scattering point. The problem of the measurement of the characteristics of bremsstrahlung under the conditions when macroscopic transverse distances are responsible for the scattering process has been considered. It has been shown that the results of the measurement in this case significantly depend on the size of the detector and on its position with respect to the scattering point.     

Transition radiation on a thin lens

Transition radiation of a relativistic charged particle intersecting a thin biconvex lens has been analyzed. The trajectory of the uniformly moving particle coincides with the optical axis of the lens. The intensity of the transition radiation in the focal plane of the lens has been estimated.     

A possibility to produce elliptically polarized transition radiation

Transition radiation produced when a charged particle successively crosses two anisotropically conducting planes is considered. The first plane has the form of a two-dimensional array (grating) made of thin metal wires parallel to the x and y axes. The second plane is a one-dimensional array whose wires are parallel to the y axis. The planes are parallel to each other and perpendicular to the particle trajectory. Transition radiation is shown to be elliptically polarized, the degree of ellipticity and rotation direction being dependent on the direction of radiation, the distance between the arrays, and the particle velocity.     

超热电子的产生与输运背向辐射的测量

报道了在20TWfs激光器上采用电子磁谱仪和光学CCD积分成像相机分别对激光固体靶相互作用在靶背方向产生的超热电子能谱及其光学渡越辐射进行的测量。能谱测量结果显示:超热电子能谱呈双温类麦克斯韦分布,拟合的温度分别为90和280keV,平均温度为185keV,这与已知的温度定标率较好地吻合。光学渡越辐射测量结果显示:光学渡越辐射是由于超热电子输运穿越固体靶所致,而辐射区域呈圆盘状、有发散角、有光强分布。如果考虑超热电子的产生和加热机制,则占主导地位的加热机制是共振吸收对电子的加热。     

Simulation of the transition radiation detection conditions in the ATLAS TRT detector filled with argon and krypton gas mixtures

Performance of the Transition Radiation Tracker (TRT) at the ATLAS experiment with argon and krypton gas mixtures was simulated. The efficiency of transition radiation registration, which is necessary for electron identification, was estimated along with the electron identification capabilities under such conditions.     

Transition radiation of elastic waves at the interface of two elastic half-planes

Radiation of elastic waves is studied that is emitted by a point load that crosses the interface of two elastic half-planes. It is assumed that the load has a constant magnitude, moves along a straight line normal to the interface, and has a constant speed that is smaller than the minimum shear wave speed in the half-planes. In this case the mechanism of excitation of elastic waves is conventionally referred to as transition radiation. The adopted model allows to obtain an analytical expression for the elastic field excited by the load in the frequency-wavenumber domain. Using this expression, the energy of transition radiation is derived in a closed form. It is shown that transition radiation of the body waves occurs at any non-zero velocity of the load. Additionally, transition radiation of interface waves may occur provided that parameters of the half-planes allow existence of Stoneley waves. A parametric analysis of the directivity diagram of radiated body waves is accomplished focusing on dependence of the diagram on the load speed, load direction, and parameters of the half-planes. Using parameters that allow radiation of interface waves, the energy of this radiation is compared to that of the body waves. It is shown that the energy of the interface waves is greater unless the load velocity is close to the lowest body wave velocity.     

Observation of frequency-locked coherent terahertz Smith-Purcell radiation

We report the observation of enhanced coherent Smith-Purcell radiation (SPR) at terahertz (THz) frequencies from a train of picosecond bunches of 15 MeV electrons passing above a grating. SPR is more intense than other sources, such as transition radiation, by a factor of Ng, the number of grating periods. For electron bunches that are short compared with the radiation wavelength, coherent emission occurs, enhanced by a factor of Ne, the number of electrons in the bunch. The electron beam consists of a train of Nb bunches, giving an energy density spectrum restricted to harmonics of the 17 GHz bunch train frequency, with an increased energy density at these frequencies by a factor of Nb. We report the first observation of SPR displaying all three of these enhancements, NgNeNb. This powerful SPR THz radiation can be detected with a high signal to noise ratio by a heterodyne receiver.     

Zero infrared reflectance anomaly in doped silicon lamellar gratings. I. From antireflection to total absorption

Zero-reflectance phenomenon for a binary lamellar grating on n-Si substrate irradiated by normally incident TE polarized plane electromagnetic wave of wavelength 10.6 μm is studied. The treatment is performed in the strong diffraction regime, where the structural dimensions and the wavelength are of the same order of magnitude, using data on the IR dielectric function of bulk doped silicon and a version of rigorous coupled-wave analysis. The evolution of normal reflectance zeros with increasing electron concentration from dielecric to metallic-like n-Si is traced. It is shown that the groove height undergoes sharp increase and the period shrinks when plasma wavelength becomes equal to the radiation wavelength. This marks the transition from the antireflection to the total absorption regime where most of incident power is absorbed in the grating region. The cavity-resonance origin of total absorption and satellite peaks in the spectral response are discussed.     

Temperature and orientation dependence of surface relief gratings based on dye-doped polymer film with the interface of nematic liquid crystals

The formation of surface relief grating on dye-doped polymer film with the interface of nematic liquid crystals has been investigated by means of the holographic technique. The first-order diffraction efficiency of surface relief grating depends on the temperature and the orientation of molecular director in the interface of nematic liquid crystals. The diffraction efficiency is roughly independent of thermal fluctuations of molecular director in the most part of nematic temperature range and apparently drops near the transition temperature. The morphology of surface relief grating demonstrates that the surface modulation is larger for molecular director parallel to the groove direction. The experimental result also shows that the first-order diffraction efficiency is dependent on the surface modulation of surface relief grating.     

Unelastische Elektronenstreuung am 1,78 MeV- und 11,4 MeV-Niveau des Si28

TheE2 transition at 1.78 MeV and the strongM1 transition at (11.42±0.02) MeV (measured excitation energy) in Si²⁸ have been studied by inelastic electron scattering at the Darmstadt linear accelerator. Primary energies between 30 and 56 MeV, and scattering angles from 104° to 165° were used. In Born approximation, the following radiation widths to the ground state have been deduced: (1.21±0.17) · 10^?3eV (1.78 MeV,E2), and (32.4±4.5)eV (11.4 MeV,M1). Transition radii have been determined from the dependence of transition probability on momentum transfer.     

Tunable Bragg gratings for fiber lasers

A tuning method of a fiber Bragg grating by the bending compression-tension is considered. With this method, a relative tuning range of the resonance wavelength of the grating of about 5% has been obtained. The changes in the shape of the spectrum and in the grating reflectivity during the tuning process are studied. The reflectivity of the fiber Bragg grating has been found to considerably increase with increasing degree of compression. It is shown that such an increase is caused by compression-induced changes in the refractive index modulation amplitude in the grating. The fiber Bragg grating developed has been applied for tuning the radiation wavelength of an ytterbium fiber laser. The tuning range of the laser has been found to be 45 nm around the central wavelength 1080 nm.     

Observation of the Talbot effect for ultrasonic waves

The diffraction of ultrasonic radiation on an amplitude diffraction grating in the near-field area (Fresnel diffraction) has been studied. The effect of self-imaging of the grating (Talbot effect) has been detected for ultrasonic radiation at distances from the grating in the range from z = 0 to z = 2L _T, where L _T is the Talbot length. The fractional Talbot effect, i.e., the ultrasonic image of the grating with the period d/2, has been observed.     

甚长波量子阱红外探测器光栅耦合的研究

采用平面波展开的散射矩阵方法研究n型甚长波量子阱红外探测器的二维衍射光栅，并同时从实验方面研究了其红外透射光谱.研究表明，n型量子阱器件的光栅耦合是传输场和倏逝场共同作用的结果.对于n型量子阱红外探测器的光栅耦合，光栅周期、光栅深度和占空比三者之间相互影响；要达到好的光学耦合效果，需要根据量子阱器件的峰值探测波长选择合适的光栅参数. 关键词： n型量子阱红外探测器 二维光栅 光耦合     

HL-2A装置的约束改善和边界净输入功率分析

在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、H_α辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

HL-2Aװ�õ�Լ�����ƺͱ߽羻���빦�ʷ���

在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、Hα辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

超热电子的产生与输运背向辐射的实验研究

报道了在100TW fs激光器上采用电子磁谱仪和光学CCD积分成像相机分别对激光-固体靶相互作用在靶背方向产生的超热电子能谱及其光学渡越辐射进行的测量。能谱测量结果显示：超热电子能谱呈单温类-麦克斯韦分布,拟合的温度为107keV;光学渡越辐射(OTR)测量结果显示：OTR是由于超热电子输运穿越固体靶所致,而辐射区域呈圆盘状、有发散角、有光强分布;如果考虑超热电子的产生和加热机制,则占主导地位的加热机制是共振吸收对电子的加热。