超强飞秒激光与固体靶产生的超热电子加热机制

 在SILEX-1激光器上测量了超强飞秒激光与Ta靶相互作用产生的出射超热电子能谱及角分布,研究了出射超热电子加热机制。激光脉宽为 30 fs,激光功率密度为8.5×10¹⁸ W/cm²。靶前法线方向超热电子温度为550 keV。从实验结果可知：共振吸收是靶前法线方向超热电子主要加热机制,这与靶前存在大密度标长预等离子体的实验条件吻合。靶厚为6～50 μm时,靶后超热电子沿法线方向出射;靶厚为2 mm时,该发射峰消失。     

Angular distribution of electrons in the field of a short laser pulse of relativistic intensity

Based on the dynamics of the electron in the field of a laser pulse of relativistic intensity, the electron emission angles with respect to the laser pulse propagation axis are calculated. The angular distribution of accelerated electrons is analyzed together with their energy spectrum. It is shown that fast electrons forming the high-energy spectral region are emitted into a fixed angular cone.     

Untersuchung verschiedener Elektronenemissionsprozesse auf Mehrfachemission

A comparison of the pulse hight distribution of several electron emission processes is drawn. Thermionic electrons, photoelectrons, field electrons and exoelectrons are found to be emitted as single electrons.     

Vacuum heating evaluation for plasmas of exponentially decreasing density profile

Ultra-short pulse lasers have opened a regime of laser-plasma interaction where plasmas have scale lengths shorter than the laser wavelength and allow the possibility of generating near-solid density plasmas. The interaction of high-intensity laser beams with sharply bounded high-density and small scale length plasmas is considered. Absorption of the laser energy associated with the mechanism of dragging electrons out of the plasma into the vacuum and sending them back into the plasma with the electric field component along the density gradient, so called vacuum heating, is studied. An exponentially decreasing electron density profile is assumed. The vector potential of the electromagnetic field propagating through the plasma is calculated and the behaviour of the electric and magnetic components of the electromagnetic field is studied. The fraction of laser power absorbed in this process is calculated and plotted versus the laser beam incidence angle, illumination energy, and the plasma scale length.     

Study of x-ray emission enhancement via a high-contrast femtosecond laser interacting with a solid foil

We observed the increase of the conversion efficiency from laser energy to Kalpha x-ray energy (eta(K)) produced by a 60 fs frequency doubled high-contrast laser pulse focused on a Cu foil, compared to the case of the fundamental laser pulse. eta(K) shows a strong dependence on the nonlinearly modified rising edge of the laser pulse. It reaches a maximum for a 100 fs negatively modified pulse. The hot electron efficient heating leads to the enhancement of eta(K). This demonstrates that high-contrast lasers are an effective tool for optimizing eta(K), via increasing the hot electrons by vacuum heating.     

On a source of outgoing electrons in a pulsed gas discharge

It has been shown that explosive electron emission is delayed by 10^?10 s with respect to field emission in a pulsed subnanosecond discharge in atmospheric air. A pulse of outgoing electrons is observed for approximately the same time in air. Correspondingly, field emission is a source of these electrons. Owing to the sharp nonlinearity of the emission current density as a function of the electric field j(E), the real duration of the current pulse of the outgoing electrons is equal to about 10^?11 s.     

Messungen zur Ansprechwahrscheinlichkeit und Impulshöhenverteilung eines Sekundärelektronenvervielfachers

The pulse height distribution of a CuBe-secondary electron multiplier has been investigated under the action of single electrons. For an energy interval from 150 eV to 1000 eV the maximum pulse height increases with increasing electron energy (the energy corresponding to maximum yield being 365±10 eV). This shows the statistics of secondary electron emission as being dependant on primary electron energy.     

稀薄空气中圆柱腔体内系统电磁脉冲的混合模拟

系统电磁脉冲广泛存在于强电离辐射环境中,且难以有效屏蔽.为了评估稀薄空气对系统电磁脉冲的影响,本文基于粒子-流体混合模拟方法,建立了三维非稳态模型,计算并分析了稀薄空气等离子体的特性以及其与电磁场响应的相互作用.结果表明,压力越高,光电子发射面附近的次级电子数密度越高,轴向分布的梯度越大,腔体中部的电子数密度在20 Torr(1 Torr=133 Pa)下出现峰值,而电子温度随压力升高单调递减.腔体内的稀薄空气等离子体阻碍了空间电荷层的产生,电场响应峰值比真空条件下的低了一个数量级,电场脉冲宽度也显著降低.光电子运动特性决定了电流响应的峰值,压力升高,到达腔体末端的电流先增加再减小.而等离子体电流会抑制总电流的上升速率,并使电流响应出现拖尾.最后,将数值模拟结果与电子束模拟系统电磁脉冲的实验结果进行比较,验证了本文混合模拟模型的可靠性.本研究所采用的混合模拟方法相比于粒子云网格-蒙特卡罗碰撞方法,大幅减小了计算消耗.     

稀薄空气中圆柱腔体内系统电磁脉冲的混合模拟

系统电磁脉冲广泛存在于强电离辐射环境中,且难以有效屏蔽.为了评估稀薄空气对系统电磁脉冲的影响,本文基于粒子-流体混合模拟方法,建立了三维非稳态模型,计算并分析了稀薄空气等离子体的特性以及其与电磁场响应的相互作用.结果表明,压力越高,光电子发射面附近的次级电子数密度越高,轴向分布的梯度越大,腔体中部的电子数密度在20 Torr(1 Torr=133 Pa)下出现峰值,而电子温度随压力升高单调递减.腔体内的稀薄空气等离子体阻碍了空间电荷层的产生,电场响应峰值比真空条件下的低了一个数量级,电场脉冲宽度也显著降低.光电子运动特性决定了电流响应的峰值,压力升高,到达腔体末端的电流先增加再减小.而等离子体电流会抑制总电流的上升速率,并使电流响应出现拖尾.最后,将数值模拟结果与电子束模拟系统电磁脉冲的实验结果进行比较,验证了本文混合模拟模型的可靠性.本研究所采用的混合模拟方法相比于粒子云网格-蒙特卡罗碰撞方法,大幅减小了计算消耗.     

激光尾场加速电子的密度梯度注入的解析处理

在激光尾场加速电子的机理中, 引入适当的密度梯度容易实现并控制电子注入到等离子体波中实现加速. 迄今对密度梯度注入的理论研究主要采用粒子模拟. 本文发展了一种解析处理密度梯度注入的方法, 分析了密度下降区域中电子的注入和加速. 给出了一维密度梯度中电子注入发生的条件, 采用哈密顿力学得到了线性条件下密度梯度区的电子相空间分界线(separatrix); 讨论了密度梯度区电子注入的时间对电子进入密度均匀区持续加速的影响. 用粒子模拟验证了分析结果.     

NMR diffusometry and the short gradient pulse limit approximation

In NMR diffusometry, one often uses the short gradient pulse (SGP) limit approximation in the interpretation of data from systems with restricted diffusion. The SGP limit approximation means that the gradient pulse length, delta, is so short that the spins do not diffuse during the pulse duration, but this condition is rarely met. If the length scale of the pores corresponds to the molecular mean square displacement during the gradient pulse, the measured echo intensities become a function of the gradient pulse length. Here, we have studied highly concentrated emulsions to show how the length of the gradient pulse influences NMR diffusion experiments. We have focused on molecules confined to one pore and molecules that can migrate through the porous system. For the former the echo decays give smaller pores than the actual case and for the latter we show large changes in echo decay depending on the gradient pulse length, everything else being equal.     

Effects of inverse bremsstrahlung in laser-induced plasmas from a graphite surface

The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation     

Electron energy distributions in a metal-polymer-vacuum system

The energy distributions of electrons emitted from a metal coated with a polymer (polydiphenylene phthalide) is studied experimentally using field electron spectroscopy. A considerable decrease in the electron work function for the metal-polymer-vacuum system as compared to pure metal is observed. Analysis of the energy distributions of emitted electrons shows that the distribution in the case with the polymer is broader and displaced towards low energies, and its high-energy edge is slightly extended. The effect of emission voltage on the shape of the energy distribution of emitted electrons is studied. A model is proposed to explain the substantial decrease in the effective electron work function in the case when the metal electrode is coated with a polymer film.     

Measurements of electron transport in foils irradiated with a picosecond time scale laser pulse

The heating of solid foils by a picosecond time scale laser pulse has been studied by using x-ray emission spectroscopy. The target material was plastic foil with a buried layer of a spectroscopic tracer material. The laser pulse length was either 0.5 or 2 ps, which resulted in a laser irradiance that varied over the range 10(16)-10(19) W/cm(2). Time-resolved measurements of the buried layer emission spectra using an ultrafast x-ray streak camera were used to infer the density and temperature conditions as a function of laser parameters and depth of the buried layer. Comparison of the data to different models of electron transport showed that they are consistent with a model of electron transport that predicts the bulk of the target heating is due to return currents.     

Fluid Model of a Sheath Formed in Front of an Electron Emitting Electrode Immersed in a Plasma with Two Electron Temperatures

The formation of a sheath in front of a negatively biased electrode (collector) that emits electrons is studied by a one‐dimensional fluid model. Electron and ion emission coefficients are introduced in the model. It is assumed that the electrode is immersed in a plasma that contains energetic electrons. The electron velocity distribution function is assumed to be a sum of two Maxwellian distributions with two different temperatures, while the ions and the emitted electrons are assumed to be monoenergetic. The condition for zero electric field at the collector is derived. Using this equation the dependence of electron and ion critical emission coefficients on various parameters ‐ like the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons ‐ is calculated for a floating collector. A modification of the Bohm criterion due to the presence of hot and emitted electrons is also given. The transition between space charge limited and temperature limited electron emission for a current‐carrying collector is also analyzed. The critical potential, where this transition occurs, is calculated as a function of several parameters like the Richardson emission current, the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Smith Purcell effect in GaAs/AlGaAs heterostructures

We report a new experimental technique to study the form of the hot electron distribution function in GaAs/AlGaAs heterostructures. A weak periodic surface potential induces Smith-Purcell-FIR-radiation of the electric field driven hot electrons in the 2-dimensional electron gas directly reflecting their velocity distribution. The FIR- radiation is detected by a magnetic field tuned InSb-detector. In samples with very low electron concentration and high mobility the emission spectra show a significant shift to higher energies and develop a steep high energy slope with increasing electric field when we use the geometry with grating vector q directed parallel to the electric field (q ∥ E). In the geometry q ⊥ E smooth decays are observed at lower energies. Comparison of the results with theory gives experimental evidence of a non-equilibrium shape of the distribution caused by the onset of LO-Phonon emission. In addition, the hot electron mean free path of the heated distribution is derived by investigating the experimental emission spectra as a function of the grating period length. The influence of a limited hot electron mean free path on the spectral width is described in terms of a Fourier-analysis of the interaction potential. In drift direction a mean free path of λ = 200 nm is obtained, whereas the mean free path is smaller in the direction perpendicular to the drift direction.     

二次发射微波电子枪的模拟计算及其特性分析

 采用URMEL-T程序进行了二次发射微波电子枪的腔内电磁计算和特性分析。同时利用URMEL-T得出的腔内轴向电场及自编程序,模拟电子在该高频场作用下的运动。计算表明二次发射微波电子枪确实具有相位选择性,进而探讨了腔形尺寸、射频电场强度对相位选择性和产生二次倍增的有关条件的影响,以及输出电子的能量稳定性。模拟结果表明,这类电子枪（MPG）可得到高电流密度（5303A/cm²）及短脉冲（3.15~10ps）的电子束。     

二次电子分布函数对绝缘壁面稳态鞘层特性的影响

由于缺乏详细的理论计算和实验结果,在研究绝缘壁面稳态流体鞘层特性时,通常假设壁面出射的总二次电子服从单能分布(0)、半Maxwellian分布等.在单能电子轰击壁面的详细二次电子发射模型基础上,采用Monte Carlo方法统计发现:当入射电子服从Maxwellian分布时,绝缘壁面发射的总二次电子服从三温Maxwellian分布.进而,采用一维稳态流体鞘层模型进行对比研究,结果表明:二次电子分布函数对鞘边离子能量、壁面电势、电势及电子/离子密度分布等均具有明显影响;总二次电子服从三温Maxwellian分布时,临界空间电荷饱和鞘层无解,表明随着壁面总二次电子发射系数的增加,鞘层直接从经典鞘层结构过渡到反鞘层结构.     

Measurements of energy transport patterns in solid density laser plasma interactions at intensities of 5x10(20) W cm-2

Kalpha x-ray emission, extreme ultraviolet emission, and plasma imaging techniques have been used to diagnose energy transport patterns in copper foils ranging in thickness from 5 to 75 microm for intensities up to 5x10(20) W cm-2. The Kalpha emission and shadowgrams both indicate a larger divergence angle than that reported in the literature at lower intensities [R. Stephens, Phys. Rev. E 69, 066414 (2004)]. Foils 5 microm thick show triple-humped plasma expansion patterns at the back and front surfaces. Hybrid code modeling shows that this can be attributed to an increase in the mean energy of the fast electrons emitted at large radii, which only have sufficient energy to form a plasma in such thin targets.     

二次发射微波电子枪的倍增特性

 利用3DRun程序通过数值模拟计算，对二次发射微波电子枪的束流倍增特性作了研究。用1维模型计算了束流倍增与腔两极间距离以及腔中场强的关系，并详细给出了腔两极间距离为10mm,场强为5-4MV/m时出射电子纵向聚束及能量聚焦过程；利用3DRun程序研究了在高频场及粒子束本身空间电荷场的共同作用下，束流在3维运动过程中的倍增特性，计算了出射束流的发射度。通过计算表明：二次发射微波电子枪可以提供低发射度、高流强的电子束。