Polarization bremsstrahlung of a hydrogen-like ion in a single crystal

Polarization bremsstrahlung (PB) that arises when a fast hydrogen-like ion is scattered in a single crystal is investigated theoretically. Four types of the process are analyzed that are caused by virtual excitation of electrons of the target and of the incident particle (IP), as well as by a coherent and incoherent interaction between the IP and the single crystal. The spectral, angular, and velocity (of the IP) characteristics of PB are calculated with regard to the spectral function of a photodetector. Optimal observation conditions and regions in which different types of PB are dominant are determined, and the dependence of these regions on the charge of the IP nucleus and of the target atoms is revealed.     

飞秒激光辐照铝靶产生快电子发射的实验和模拟研究

采用实验和数值模拟研究了飞秒激光辐照铝靶产生的快电子发射.实验中,在主脉冲前加上一个预脉冲产生预等离子体,然后主脉冲与预等离子体作用产生快电子.在激光反射方向附近,实验测量的快电子束发射与数值模拟的结果高度地一致;在靶背面,发射的快电子的数目小于数值模拟的结果,原因在于快电子在靶内输运受到电荷分离场和碰撞的影响;在数值模拟中未出现的,沿靶表面发射的快电子束,是由表面准静态电磁场的禁闭效应产生.     

飞秒激光辐照铝靶产生快电子发射的实验和模拟研究

采用实验和数值模拟研究了飞秒激光辐照铝靶产生的快电子发射。实验中,在主脉冲前加上一个预脉冲产生预等离子体,然后主脉冲与预等离子体作用产生快电子。在激光反射方向附近,实验测量的快电子束发射与数值模拟的结果高度地一致;在靶背面,发射的快电子的数目小于数值模拟的结果,原因在于快电子在靶内输运受到电荷分离场和碰撞的影响;在数值模拟中未出现的,沿靶表面发射的快电子束,是由表面准静态电磁场的禁闭效应产生。     

强激光与固体靶作用产生的表面电子加速和辐射研究

利用单电子在固体靶表面准静态电磁场中运动的模型和非线性汤姆孙散射理论,研究了以大角度斜入射的强激光照射在固体靶表面产生的沿靶面方向发射的高能超热电子的运动及其产生的电磁辐射脉冲. 数值模拟表明,靶表面的电子在靶面附近的准静态电磁场和反射的激光场中作振荡. 当电子振荡频率接近激光频率时,电子被有效加速,被加速的电子主要沿靶面方向运动并产生向前的阿秒脉冲辐射. 讨论了电子在加速前的不同初始速度分布对辐射脉冲的时间和空间特性的影响,模拟了不同初始状态的多电子相干辐射脉冲的频谱特性. 关键词： 表面准静态电磁场 超热电子 阿秒脉冲 相干辐射     

Polarization bremsstrahlung of heavy charged particles in a polycrystal

Polarization bremsstrahlung (PBS) of a fast ion scattered in a polycrystalline medium is calculated and analyzed with allowance for the contributions from the coherent and incoherent channels of the process. It is shown that scattering of a projectile from the crystal lattice of the target is responsible for typical features of the PBS spectrum. For example, PBS is suppressed (as compared to radiation emitted by a single atom) in the low-frequency part of the spectrum, where coherent PBS dominates. In the intermediate spectral region, a step structure is formed as a result of “elimination” of the contribution from the reciprocal lattice vector with a preset magnitude to the coherent component of the process. Finally, incoherent PBS dominates in the high-frequency part of the spectrum and the process occurs as in the case of a single atom. These spectral peculiarities of PBS are determined by the structure of the target and depend on the velocity of the projectile and the emission angle, and can be observed in experiments dealing with radiation emitted by fast charged particles in thin polycrystalline films.     

Peak structure of polarized bremsstrahlung upon scattering by coupled atomic electrons

Peculiarities of polarized bremsstrahlung (PB) of relativistic electrons produced upon scattering by coupled atomic electrons are discussed; in this case, PB is regarded as scattering of virtual photons of the electromagnetic field of a fast charge by atomic electrons with their coupling taken into account. In this case, the atomic electron during scattering can acquire recoil energy only through separate portions, as a result of which the spectrum of scattered photons degenerates into a series of narrow peaks.     

Contribution of the second-order born approximation to the coherent bremsstrahlung cross section by relativistic electrons and positrons in crystals

Equations for the cross section and polarization of the coherent bremsstrahlung emitted by relativistic electrons and positrons in crystals are obtained taking into account the contribution from the second-order Born approximation. The radiation cross section and polarization in the field of the atomic plane is considered as a function of the charge sign of the particle.     

The role of electron correlations in the double ionization of helium by fast protons

We consider the double ionization (DI) of a helium atom by fast protons and study the role of electron correlations in this process. We develop a quantum-mechanical approach that takes into account the interaction of the emitted electrons in continuum and the dynamical charge screening of the ejected particles, which depends on their ejection kinematics. The interaction of the emitted electrons between themselves and with the core is described in the model of approximate 3C functions, while the dynamical charge screening is described by introducing effective charges of the emitted electrons and the ion core, which are determined by the particle momenta. The derived closed analytical expressions for the differential ionization cross sections have been applied to the case of a coplanar particle ejection geometry at various momenta q transferred to the atom. Analysis of our calculations has shown that the developed model describes adequately the available experimental data. Including the dynamical charge screening has a significant effect on the DI cross section and improves considerably the agreement between theory and experiment.     

Bombarding insulating foils with highly energetic ions

Insulating (MYLAR), semi-insulating (MYLAR-Au) and conducting foils have been bombarded by very energetic 64 MeV u^{?1 78}Kr³²⁺ ions. The velocity spectra of fast electrons emitted in the backward and forward directions have been measured and analyzed as a function of the elapsed time in the run. A shift of binary encounter and convoy electrons emitted in the forward direction toward lower velocities has been observed with insulating targets. No such shift occurs with metallic targets. The surface potential evolves with time (i.e. ion fluence) both at forward and backward emission angle. It is shown that strong bulk charging of insulating targets leads to a positive potential as high as 9 kV before charge breakdown.     

Atomic analogue of photonuclear configurational splitting

It is shown that the polarized bremsstrahlung (PB) emitted upon the scattering of a relativistic charged particle’s Coulomb field by atomic electrons yields direct information on the electronic structure of a medium. Indeed, in the X-ray photon energy range 1–10 keV, PB exhibits collective features and the PB intensity sharply increases. At higher photon energies, the coherence of shell electrons rapidly collapses, which is accompanied by a drastic decrease in the PB intensity; hence, the position of the PB spectrum kink allows estimation of the atomic size.     

Charge-constrained coherent pion states produced in nuclear matter

In a recent article, Bloss, Hone and Scalapino have used a simplified model Hamiltonian and Keldysh field theoretic methods to approximately calculate the power radiated as Cherenkov charged pions whose emission is triggered when a fast nucleon traverses nuclear matter. The model used by these authors conserves charge by flipping the isospin state of the fast nucleon alone for each charged pion emitted. Since it is the nuclear matter excited by the fast nucleon which should be largely reponsible for emitting the Cherenkov charged pions, we investigate an alternate model that allows all the disturbed nuclear matter to change its charge state when a charged pion is emitted—as an idealization, the emitting matter is assumed to have an infinite number of charge states, in contrast to the two of the fast nucleon. This modified model is shown to be exactly solvable in terms of charge-constrained coherent pion states; the power radiated as charged pions is found to be approximately twice that calculated by Bloss et al. from their model.     

Electrical detection of spin coherence in silicon

Experimental evidence is presented showing that photocurrents in silicon can be used as highly sensitive readout probes for coherent spin states of localized electrons, the prime candidates for quantum bits in various semiconductor based quantum computer concepts. Conduction electrons are subjected to fast Rabi oscillation induced by means of pulsed electron spin resonance. The collective spin motion of the charge carrier ensemble is reflected by a spin-dependent recombination rate and therefore by the sample conductivity. Because of inhomogeneities, the Rabi oscillation dephases rapidly. However, a microwave induced rephasing is possible causing an echo effect whose intensity contains information about the charge carrier spin state and the coherence decay.     

Neutralization of multiply charged ions at a surface

We report on emission processes induced by particle-solid interaction involving ions with a large potential (i.e., high ion charge state) and low kinetic energy. After an introduction into existing neutralization models for ion scattering at a metal surface a detailed discussion on the electron emission processes is presented.The number of electrons emitted per incident ion is shown to be proportional to the potential energy only within a restricted parameter field involving charge state and ion velocity. The kinetic energy distribution of emitted electrons is dominated by low-energetic electrons (30 eV), while inner shell holes of the projectile ion can initiate high-energetic characteristic Auger electrons. The presence of inner shell holes is also of importance for the charge state of highly charged ions being scattered at surfaces whereas normally the charge state distribution of scattered ions depends on the impact parameter only.The influence of the primary ion charge state on the sputtering yield of insulating surfaces is seen for the charge state of sputtered particles, whereas the total sputtering yield seems to be insensitive. This question is still subject to controversy, however.Photon emission dependent on the charge state of the impinging ion has been observed up to now only for extremely highly charged ions as hydrogenlike Ar or Kr.     

双荧光层靶Kα线强度比诊断靶内超热电子温度

 利用双荧光层复合靶产生的Kα特征线强度比诊断了靶内超热电子的温度,即通过实验测量复合靶中两种不同材料荧光层辐射出的Kα特征线强度比,结合ITS3.0程序模拟结果,对超热电子温度进行诊断。将诊断结果与实验中利用电子磁谱仪测量的超热电子温度进行了比较,二者基本一致。结果表明,选取适当的荧光层靶厚,可以利用双荧光层复合靶产生的Kα特征线强度比对靶内的超热电子温度进行诊断。     

Polarization of Photons under Conditions of Interference between the Parametric Radiation and Coherent Bremsstrahlung

X rays emitted by relativistic electrons traversing a system of parallel atomic planes of a crystal are considered. The spectral, angular, and polarization characteristics of emitted photons are studied theoretically simultaneously for two coherent emission mechanisms, namely, the coherent bremsstrahlung and parametric X rays. Based on the results obtained, an optimal design of the experiment on studying the effect of interference of these emission mechanisms on the polarization of photons is calculated. The developed experimental facility is described.     

Diagnostics of atoms encapsulated into fullerenes by the polarization bremsstrahlung from accelerated electrons

It is shown that the polarization bremsstrahlung (PB), which arises during scattering of coherent electromagnetic fields of fast charged particles by atomic electrons and resonantly reacts to changes of the density of a substance, allows one to diagnose the states of atoms that are encapsulated in fullerene. As a result of modeling, by comparing the spectra of an empty fullerene and fullerene with an encapsulated atom and using difference fitting, it is possible to detect compressions or expansions of an electron shell of the atom that is encapsulated in a fullerene. The calculations are fulfilled for the model approximation of the P@C₆₀ endohedral compound.     

Spin-polarized electrons in collisions of multicharged nitrogen ions with a magnetized Fe(001) surface

We report on the first spin-resolved energy spectra for the emission of electrons during grazing scattering of 150 keV multicharged nitrogen ions from a magnetized Fe(001) surface. A substantial spin polarization for KLL Auger electrons emitted in the final stage of the neutralization sequence during the interaction of multicharged ions with a metal surface is observed. We conclude from our data that the projectile L shell is dominantly populated by electrons from the conduction band of the target. For low energy electrons we find an increase of their spin polarization with an increase of the projectile charge.     

Model description of surface charging during ultra-fast pulsed laser ablation of materials

We present a model describing the dynamical mechanisms responsible for generating fast ion ejection under ultra-short pulsed laser irradiation. The model is based on a simplified drift–diffusion approach describing the evolution of the laser-generated charge carriers, their transport, and the electric field generated as a result of quasi-neutrality breaking in the irradiated target. The importance of different processes in generating the non-thermal material-ejection mechanisms is discussed. A common frame is applied to dielectrics, semiconductors, and metals and different dynamical behaviour is observed. The modelling results are in good agreement with fs pump–probe studies and measurements of the velocity distributions of the emitted ions.     

Investigation of fast pitch angle scattering of runaway electrons in the EAST tokamak

This paper reports that an experimental investigation of fast pitch angle scattering(FPAS) of runaway electrons in the EAST tokamak has been performed.From the newly developed infrared detector(HgCdTe) diagnostic system,the infrared synchrotron radiation emitted by relativistic electrons can be obtained as a function of time.The FPAS is analysed by means of the infrared detector diagnostic system and the other correlative diagnostic systems(including electron-cyclotron emission,hard x-ray,neutrons).It is found that the intensity of infrared synchrotron radiation and the electron-cyclotron emission signal increase rapidly at the time of FPAS because of the fast increase of pitch angle and the perpendicular velocity of the energetic runaway electrons.The Parail and Pogutse instability is a possible mechanism for the FPAS.     

Study of forward accelerated fast electrons in ultrashort Ti <Emphasis Type="BoldItalic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> sources

The results of an experiment aimed at studying hot electrons emerging from a target rear side in ultrashort laser-based [ _]Kα sources are described. In particular, forward accelerated fast electrons propagating through a Ti foil are found to be emitted in a cone perpendicular to the target surface. The energy of these electrons is estimated as well as their divergence. A comparison of the experimental findings with the results of a PIC simulation is also reported, aimed at identifying the physical processes responsible for the production of this forward propagating electron population. PACS 52.38.-r; 52.38.kd; 52.38.Ph