Low-frequency transition radiation from a short laser pulse at the plasma boundary

The transition radiation that appears when a short laser pulse crosses the vacuum-plasma boundary and that is attributable to an vortex electric current produced at the plasma boundary by averaged ponderomotive forces has been considered for the first time. The spectral, angular, and spatiotemporal characteristics of the transition radiation in a vacuum are analyzed. It is shown that under typical conditions of present-day laser plasma experiments, the frequency of the transition radiation lies in the terahertz range and its power can reach several megawatts.     

An escape of fast electrons from a plasma cluster in subcritical electric field

A behaviour of electrons with high initial thermal velocities in plasma clusters is discussed in the paper. These electrons from the tail of the Maxwell velocity distribution can be accelerated into considerable energies by means of an external electric field. Both numerical and analytical estimates of the energy of the escaping electrons and an estimate of the total energy transferred by the electrons from the cluster are treated in the paper. The calculations are performed for a situation usual in coaxial and rail accelerators of the plasma clusters.     

X-ray radiation from nonlinear Thomson scattering of an intense femtosecond laser on relativistic electrons in a helium plasma

We have generated x-ray radiation from the nonlinear Thomson scattering of a 30 fs/1.5 J laser beam on plasma electrons. A collimated x-ray radiation with a broad continuous spectrum peaked at 0.15 keV with a significant tail up to 2 keV has been observed. These characteristics are found to depend strongly on the laser strength parameter a(0). This radiative process is dominant for a(0) greater than unity at which point the relativistic scattering of the laser light originates from MeV energy electrons inside the plasma.     

Influence of an electric field on the magnetoabsorption in a field of resonant laser radiation

The frequency dependence of the coefficient of interband magnetoabsorption of a weak electromagnetic wave propagating in a constant electric field and in a field of resonant laser radiation at a frequency equal to the cyclotron frequency (infrared magnetic resonance) is calculated. The specific features observed in interband absorption of the electromagnetic wave in a uniform electric field are considered for the case in which the frequency of laser radiation is equal to the confinement frequency in a parabolic quantum well (infrared quantum-well resonance).     

次稠密等离子体对激光与锥形靶相互作用的影响

利用三维粒子模拟程序模拟了强激光在锥形靶内的传播情况.发现锥内次稠密等离子体的存在使激光在锥顶部的最大聚焦强度有所降低，产生的相对论电子的最大能量和数目增加.激光在锥壁激发起强的电流和磁场，次稠密的存在还使锥内产生强的准静态磁场，磁场的存在使相对论电子速度分布在垂直激光传播方向上表现出各向同性. 关键词： 超强激光脉冲 锥形靶 快点火 粒子模拟     

Amplification of non-Cerenkovian magnons by electrons in a laser radiation field

The rate of change of the magnon population due to scattering by conduction electrons in the presence of an electromagnetic wave is calculated. It is found that non-Cerenkovian high-frequency magnons may be amplified over a very narrow band of magnon wavenumbers.     

Analysis of characteristic X-ray generation induced by laser plasma electrons accelerated by an electric field

The results of experiments devoted to the study of spectral, spatial, and time characteristics of a spectrally bright point x-ray source based on a vacuum diode with a laser-plasma cathode and a titanium needle anode with a photon energy approximately equal to 4.5 keV are presented. The experimental results revealed a considerable difference between the electron emission from laser plasma in a strong electric field and the explosive electron emission and demonstrated the effectiveness of laser plasma as an electron source. The optimization of the laser radiation power density, the accelerating voltage, and the interelectrode spacing made it possible to create a point x-ray source whose spectral brightness exceeds available sources in the class of small-size pulse x-ray instruments (tubes with explosive cathodes). It has been proved experimentally that the maximum contrast of the characteristic lines of the anode material is attained in the case of an optimal choice of accelerating voltage. The x-ray source has the following parameters: (1) spectral brightness of the K-lines of titanium of the order of 10²¹ photons/cm² s sr keV; (2) emitting region size of 250 mm; and (3) laser pulse duration less than 20 ns.     

Observation of optical transition radiation from electron beams generated by laser plasma accelerator

Laser plasma accelerators (LPAs) have made great progress, achieving electron beam with energy up to 1 GeV from a centimeter scale capillary plasma waveguide. Here, we report the measurement of optical transition radiation (OTR) from the capillary-based LPA electron beams. Transition radiation images, produced by electrons passing through two separate foils (located at 2.3 m and 3.8 m away from the exit of the LPA) were recorded with a high resolution imaging system, respectively. Two magnetic quadrupole lenses were placed right after the capillary to focus and collimate the electron beams. Significant localized spikes appeared in the OTR images when the electron beam was focused by the magnetic quadrupole lenses, indicating the coherence of the radiation and the existence of ultrashort longitudinal structures inside the electron beam.     

快电子发散角的实验测量

采用飞秒激光辐照铜靶,利用电子角分布仪和LiF热释光探测器测量了快电子发射的发散角.实验结果显示,快电子的发散角与激光入射角密切相关,随着激光入射角增加,快电子的发散角逐渐减小.在相同入射角条件下,加上预脉冲将导致快电子的发散角变小.这个结果为获取较小发散角的快电子束提供了实验参考.     

Influence of fast electrical discharge on expansion of laser produced plasma

For some applications of laser produced plasma it may be advantageous to implement some degree of control on the ion flux and velocity distribution in the plasma expansion. We have shown how a fast, high current discharge in a laser produced plasma leads to a dramatic increase in the plasma ionization and expansion velocity. These observations are consistent with a magnetohydrodynamic discharge model whereby the self-induced magnetic field causes radial compression of an open-ended plasma column.     

Diffracted transition radiation of relativistic electrons in an artificial periodic structure

A theory concerning the coherent X-ray radiation of relativistic electrons crossing an artificial periodic medium in Laue scattering geometry has been constructed. Expressions describing the spectralangular characteristics of radiation in the Bragg scattering direction have been obtained and studied. Radiation is considered in analogy with that in the crystal medium as a result of the coherent summation of the contributions of two radiation mechanisms, in particular, parametric X-ray radiation (PXR) and diffracted transition radiation (DTR). It is shown that the DTR yield from a layered target can exceed the particle radiation yield in a single-crystal radiator by one order or more under similar conditions.     

Escape into vacuum of fast electrons generated by oblique incidence of an ultrashort, high-power laser pulse on a solid target

The possibility that fast electrons can escape in a direction close to the trajectory of a reflected ultrashort laser pulse at extremely high laser radiation fluxes is examined analytically and numerically. Analytic estimates are made of the feasibility of forming electron bursts in the plasma and of their subsequent motion. The self-consistent, collisionless motion of a plasma acted on by specified incident and reflected ultrashort laser pulses is modeled in two dimensions by the particle-in-cell method. It is shown that a substantial number of electrons located in the subcritical region are gathered into bunches by the resultant forces and escape to the vacuum in a direction different from the normal to the target surface within a narrow range of solid angles. This demonstrates the feasibility of laser acceleration of an electron burst during reflection of an ultrashort laser pulse from a solid target. Zh. éksp. Teor. Fiz. 116, 1184–1197 (October 1999)     

Preheating of a target by laser radiation through plasma and polymer aerogel

Processing of X-ray and optical streak-camera images after brightness amplification 5 proves unambiguously the existence of a preheating of the foil in two-layer targets (polymer aerogel plus Al foil). We observed this effect earlier at the accuracy limit of the experiment. Preheating may be explained by the initial transmission of laser radiation by an inhomogeneous plasma and aerogel remnants. The fraction of laser energy that passed through a microheterogeneous plasma to the foil or shell is small in comparison with the total energy in the pulse, but it comes to the target (foil or shell) at the initial period of the laser pulse and can be significantly affected by the dynamics of plasma and the efficiency of spherical cryogenic thermonuclear targets.     

等离子体中相对论电子的同步曲率辐射特性研究

根据同步曲率辐射理论推导了在等离子体环境中,不同磁场条件下的相对论性电子的吸收系数和发射系数表达式，计算了电子的发射强度，并且在此基础上研究了同步曲率辐射机制的脉泽效应.研究了两种磁场位型，第一种是强度均匀但弯曲的磁场，第二种是偶极磁场，结果发现了一些偶极磁场下特有的辐射特性.考虑到在天体的环境下电子具有的不同的能谱分布，分别选用了三种典型的能谱分布(幂率分布，高斯分布，热分布)进行了研究，通过计算负吸收和脉泽放大效应在发射强度上的表现后， 发现在某些天体物理环境中，同步曲率辐射在等离子体中的确存在脉泽放大效应.这些研究结果对太阳系中行星外层辐射的研究和宇宙中的射电高亮温度等问题的研究可能提供有益的帮助. 关键词： 同步曲率辐射 负吸收 脉泽效应     

Hard X radiation and fast particles in laser plasma experiments at laser intensities of up to 5×1018 W/cm2 on the target surface

Results are presented from an investigation of the hard X-ray spectrum and the parameters of fast particles in experiments on the interaction of laser pulses with solid targets in the PROGRESS-P facility at laser intensities of up to 5×10¹⁸ W/cm² on the target surface. The maximum energy of fast electrons obtained from direct measurements is found to be 8–10 MeV.