Role of excited states in coherent harmonic bremsstrahlung in a photoionized plasma

It is established for a photoionized plasma formed in the barrier-suppression ionization regime that the preliminary population of the excited states of the ionizing atoms plays an important role. It is established that in this case an anomalously strong (several orders of magnitude) increase occurs in the efficiency of generation of the harmonics of the pump radiation. It is shown that a relative decrease of the harmonics generation efficiency occurs with time as a result of collisions of the electrons produced by ionization.     

X-ray amplification by laser controlled coherent bremsstrahlung

A coherent x-ray generation scheme is proposed which involves characteristics of free electron lasers and atomic high harmonic generation schemes. A thin solid layer or any other periodic atomic structure with limited dimensions is exposed to a short, superintense laser pulse. The electrons are extracted from the layer due to the extreme force and penetrate periodically through the ionic structure. Consequently, thousands of harmonics of the laser radiation field are shown to coherently amplify along the interaction length. The small signal gain of the generated x-ray radiation exceeds that arising from the multiphoton Compton process in plasmas and is competitive with that in the leading x-ray free electron lasers.     

Theory of coherent bremsstrahlung

Abstract

We review the phenomenon of coherent bremsstrahlung (CB) of electrons in a crystal. This well-studied process in a new light after the realization by Andersen [J. U. Andersen, Nucl. Inst. and Methods 170, 1 (1980)] that CB and the phenomenon of channeling radiation (CR) are two aspects of the same physical process. These two types of radiation may be described in a unified framework by the use of Bloch functions for the electron in the crystal. The kinematics of CB is discussed from this viewpoint and a model calculation for CB in a strained-layer superlattice is presented.     

Channeling radiation and coherent bremsstrahlung

We show that calculations based on the use of Bloch functions in at least two-dimensions predict both coherent brems-strahlung peaks (k_CB) and channeling radiation peaks (k_CR) in the spectrum of photons emitted by electrons traversing a crystal when moving nearly parallel to a crystal plane; and that furthermore, “sidebands” (e.g., k_CB ± k_CR) are predicted when the electrons move nearly parallel to two intersecting crystal planes.     

Studies of advanced soft X-ray laser and coherent higher harmonic generation using intense subpicosecond laser produced high density plasma

Current activities on our research of soft X-ray lasers and higher harmonic generations using intense, subpicosecond lasers are reviewed. Especially for soft X-ray lasers we experimentally demonstrate that by longitudinally pumping 2-mm-long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-X-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submilliradian order, and only requires pump laser energy of 150 mJ. Simulations show that the pedestal in the main pump pulse can generate electron density and gain profiles with large spatial gradients, which result in the selective amplification of low-order transverse modes. The present result is the demonstration of an efficient and alternative method of improving the spatial coherence of X-ray lasers with amplified spontaneous emission medium, with possibilities of becoming an excellent tool to explore various application experiments. In higher harmonic generation, topics related to observation of blue shift due to collisionless absorption process is described.     

High harmonic generation in a laser plasma

The results of investigation of high harmonics of radiation of a Ti:sapphire laser propagating through a laser plasma generated on the surface of different targets are presented. For most of the targets, the intensity distribution of the high harmonics generated is found to form a plateaulike pattern similar to that observed in the case of gas jets. The generation of high harmonics (up to the 65th harmonic, λ = 12.24 nm) is caused by the interaction of femtosecond laser radiation with ions. The conversion efficiency in the plateau region varies from 10^?7 to 8 × 10^?5 depending on the target. The main restriction on the conversion efficiency and the peak intensity of the harmonics generated is caused by the self-defocusing of the femtosecond radiation due to free charge carriers formed as a result of tunnel ionization.     

Exclusive measurement of coherent proton-deuteron bremsstrahlung

For the first time a high-precision proton-deuteron bremsstrahlung experiment has been performed in which all the different exit channels have been distinguished separately. High-precision cross sections and analyzing powers in one of the outgoing channels, namely, the coherent bremsstrahlung with a proton and a deuteron in the final state, are presented at 190 MeV incoming proton beam energy and are compared to calculations based on the low-energy theorem. The results of the calculations vary considerably calling for a fully microscopic calculation. However, using a recipe including the initial- and final-state interactions, the predictions come close to the data.     

High-order harmonic generation in laser plasma: Recent achievements

Recent studies of high-order harmonic generation of laser radiation in laser-produced plasma show new attractive developments in this field. Those include generation of extended harmonics in plasma plumes, new approaches in application of two-color pump, generation of extremely broadened harmonics, further developments in harmonic generation in clusters (fullerenes, carbon nanotubes, in-situ produced nanoparticles), destructive interference of harmonics from different emitters, resonance-induced enhancement of harmonics, applications of high pulse repetition rate lasers for the enhancement of average power of generating harmonics, observation of quantum path signatures, etc. We review some of these recent developments.     

On polarization of transition bremsstrahlung in a weakly gyrotropic plasma

The degree of polarization of transition bremsstrahlung (TBS) is calculated in a plasma with weak gyrotropy. The cases of weak and strong Faraday rotation in a radiation source are discussed. The distribution of polarization is shown to have no azimuthal symmetry, if the particle gyroradius is larger than the source size. We calculate the polarization degree of TBS for an ensemble of fast particles and show that polarization exceeds the respective value for transition radiation in the presence of nonthermal inhomogeneities of the plasma density.A. F. Ioffe Physical and Technical Institute, Russian Academy of Sciences, St. Petersburg. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 9, pp. 887–892, September, 1995.     

Maximum entropy principle and coherent harmonic generation using a single-pass free-electron laser

Single-pass free-electron lasers constitute an example of systems with long-range interactions. The light-particle interplay leading to the power growth and successive relaxation towards a quasi-stationary state is governed by the Vlasov equation. A maximum entropy principle inspired to Lynden-Bell's theory of “violent relaxation" for the Vlasov equation can be invoked to analytically characterize the behaviour of the saturated system. In particular, we here concentrate on the case of coherent harmonic generation obtained from an externally seeded free-electron laser and provide a simple strategy to predict the laser intensity as well as the final electron-beam energy distribution.     

Second harmonic generation of a self-focused Hermite-Gaussian laser in plasma

Second harmonic generation (SHG) using intense Hermite-Gaussian laser beam (HGLB) propagating through the plasma for mode-indices m = 0 and m = 1 is reported in the present work. Ponderomotive force induced density perturbation beats with the oscillatory velocity of electrons at incident laser frequency, generate the second harmonic nonlinear current that give rise to SHG. Using paraxial approximations, we have derived the coupled equations for the beam width parameter of HGLB and second harmonic's normalized amplitude. Resonance condition is obtained by considering wiggler magnetic field which providing an extra momentum to the second harmonic photon and this result a significant increase in the amplitude of SHG. Our analysis shows the prominent rise in normalized amplitude of second harmonic on increasing the value of the intensity of fundamental laser pulse, normalized wiggler magnetic field and normalized density of plasma. It is notified that the gain of SHG is more prominent for m = 1. Dependency of laser and plasma parameters on SHG is also reported in the current work.     

High-order harmonic enhancement using the quasi-phase-matching in a laser plasma

The quasi-phase-matching of the high-order harmonics generating in the multiple plasma jets produced on the surfaces of perforated Ag, V, and Mn targets is demonstrated. The 20× and 30× enhancement factors of quasi-phase-matched harmonics of 804 nm radiation in the range of the 35th harmonic in the cases of silver and vanadium plasmas were achieved.     

Electron-electron bremsstrahlung in a non-equilibrium plasma

Electron-electron bremsstrahlung in a weakly ionized uniform non-stationary plasma is investigated. The respective problem is solved analytically for two initial electron-distributions: maxwellian with the temperatureT ₀, and the delta distribution. In the case of initial maxwellian distribution the radiation intensity is proportional to ^3/2, where=T+(T ₀–T) exp (–s),s=2mv ₁ t/M,v ₁ is the momentum transfer frequency, and the other symbols are standard. In the case of initial delta distribution the radiation intensity is proportional to [1 -exp (–s)]^3/2. The proportionality factors are known, and are expressed in terms of mathematical and physical constants.     

Properties of coherent bremsstrahlung from electrons in thick single crystals

A technique for calculating the frequency and polarization of the coherent bremsstrahlung from relativistic electrons in single crystals of arbitrary thickness is proposed and implemented. It accurately includes multiple scattering of the emitting particles and the angular distribution and collimation of the radiation. The spectral properties of the collimated beam of radiation from electrons with energies of 900 MeV in thick (0.1 r.l.) single crystals of diamond, silicon, and tungsten are compared with the calculations. When the radiation is tightly collimated the dependence of the frequency and polarization properties of the radiation on the multiple scattering of the emitting particles in the crystals becomes weaker.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 21–43, June, 1991.     

Total second harmonic emission from a laser produced plasma

The total and specular light emitted at 2ω by a laser produced plasma (τ = 30 ps, λ = 1 μm) have been measured versus intensity (10¹²-10¹⁴ W/cm²), polarization and angle of incidence of the laser beam. The possibility for second harmonic to arise from resonant absorption, through a rippled critical surface, or from parametric decay instability is discussed.     

用于激光等离子体诊断实验的二倍频探针光系统

为了满足基准物理实验的要求,准确地探测出靶面的等离子体的电子温度、密度、电子离子漂移速度等参数,在星光Ⅱ激光装置上发展了一束二倍频激光作为探针光。通过模拟实验已经证实了该探针光的二倍频总能量大于5J,焦斑尺寸小于100μm,可以满足激光与等离子体相互作用的高功率要求。目前,该探针光系统已经用于用于激光等离子体诊断实验的常规运行。     

正态分布随机数发生器及其在模拟光传输软件中的应用

利用计算机高级语言提供的伪随机数发生器,通过“高斯筛选”和“求平均值”两种方法,开发出满足正态分布的随机数发生器。将其应用于分析大型激光装置光路稳定性的模拟软件中,成功地完成了对“神光Ⅲ原型装置四程放大器”的光路稳定性分析。