Generation of fast protons in moderate-intensity laser－plasma interaction from rear sheath

Forward fast protons are generated by the moderate-intensity laser--foil interaction. Protons with maximum energy 190~keV are measured by using magnetic spectrometer and CR-39 solid state track detectors along the direction normal to the rear surface. The experimental results are also modeled by the particle-in-cell method, investigating the time-varying electron temperature and the rear sheath field. The temporal and spatial structure of the sheath electrical field, revealed in the simulation, suggests that these protons are accelerated by target normal sheath acceleration (TNSA) mechanism.     

强激光与稠密等离子体作用引起的冲击波加速离子的研究

用二维PIC（Particle-in-Cell）程序模拟研究了强激光与稠密等离子体靶作用产生的无碰撞静电冲击波的结构和这种冲击波对离子的加速过程,研究发现由于冲击波前沿附近的双极电场的作用,具有一定初速度的离子能被该双极场俘获并获得加速,最终能够被加速到两倍冲击波速度.冲击波加速可以得到准单能的离子能谱,叠加在通过鞘层加速机理产生的宽度离子能谱上.还对不同激光强度和不同等离子体密度情况下形成的冲击波进行了比较.研究表明,强度相对较低的激光在高密度等离子体中可以产生以一定速度传播的静电孤波结构,后者只能加速 关键词： 强激光 稠密等离子体 无碰撞静电冲击波 离子加速     

激光加速质子束对电磁孤立子的照相模拟研究

激光在次稠密等离子中传输, 由于频率下移而被俘获, 从而产生电磁孤立子. 根据先前理论及PIC 模拟给出的孤立子的演化过程, 对不同阶段孤立子的电磁场分布进行了建模. 使用Geant4蒙特卡罗程序, 模拟研究了激光加速产生的能量为几个MeV的质子束对后孤立子的照相. 分析了质子能量, 质子源尺寸等因素对照相结果的影响, 并利用了TNSA加速产生质子束的分幅特性, 开展了时间分辨的孤立子照相模拟研究. 模拟给出的质子照相结果验证了文献中给出的孤立子静电场模型, 为以后在实验上探测孤立子提供了理论依据. 关键词： 超短激光 质子照相 孤立子 蒙特卡罗方法     

激光等离子体相互作用中的两级质子加速（英文）北大核心CSCD

研究了激光辐射压驱动的两级质子加速的相关问题。当超短超强激光脉冲与处在背景等离子体前方的薄固体平靶相互作用时,在固体靶后部形成一个电子层-离子层组成的双层结构。在激光的不断推进下,双层结构在背景等离子体里以一定速度传播,可以看成运动在背景等离子体中的电场。这样,在背景等离子体中的质子被这个运动电场捕获并能加速到很高的能量。通过二维PIC模拟方法和理论分析研究了质子加速的相关问题。研究结果表明,被加速质子的最大能量达到20GeV。     

Connecting shock velocities to electron-injection mechanisms

Electrons can be accelerated by their interaction with nonlinearly saturated electrostatic waves up to speeds with which they can undergo diffusive acceleration across supernova remnant shocks. Here, we model this wave-electron interaction by particle-in-cell and Vlasov simulations. We find that the lifetime of the saturated wave is considerably longer in the Vlasov simulation, due to differences in how these simulation methods approximate the plasma. Electron surfing acceleration which requires a stable saturated wave may thus be more important for electron acceleration at shocks than previously thought. For beam speeds above a critical value, which we estimate here, both simulation codes exclude surfing acceleration due to a rapid wave collapse.     

Experimental investigations of fast-proton production in a picosecond laser plasma

Results of experimental investigations of fast-proton production in a laser plasma are presented for the case where the intensity of laser radiation at the targets is 2 × 10¹⁸ W/cm². Three processes of fast-proton acceleration in laser plasma are investigated: (1) the acceleration of protons from the front surface toward the laser pulse, (ii) the acceleration of protons from the front surface of the target toward its interior, and (iii) the acceleration of protons from the rear foil surface in the outward direction. The activation procedure and CR-39 tracker detectors featuring a set of various-thickness aluminum filters were used to record fast protons. It turned out that the proton-acceleration process is the most efficient in the case of proton acceleration from the rear foil surface in the outward direction. Experimental results revealed that about N _p = 10⁷ protons of energy in the region E _p > 1.9 MeV that are accelerated from the target surface toward a laser ray, N _p = 4× 10⁷ protons of energy in the region E _p > 1.9 MeV that are accelerated fromthe front surface of the target toward its interior, and N _p = 4×10⁸ protons of energy in the region E _p > 1.9 MeV that are accelerated from the rear foil surface in the outward direction are generated at a laser-radiation intensity of 2 × 10¹⁸ W/cm² at the surface of aluminum, copper, and titanium targets. Experimental investigations aimed at optimizing the process of proton acceleration from the rear surface of aluminum foils were performed by varying the foil thickness over the range between 1 and 100 μm. The results of these experiments showed that there is an optimum foil thickness of 10 μm, in which case protons of maximum energy 5 MeV are generated.     

A source of solar protons: The temperature of flare plasma and injection moments

The relationship between injection moments of solar protons of ∼100 MeV in the heliosphere and the temperature of flare plasma in six long events of the 23d cycle is considered. Injection moments are evaluated by the arrival time of first protons to an observer in the ecliptic plane (GOES), and the temperature is assessed by the ratio of X-ray intensities in two energy channels of GOES. The temperature of the flare plasma depends on the efficiency of heating (the spectrum of accelerated electrons and properties of the target) and cooling. Time profiles of gamma radiation and moments of particle injection into the heliosphere in the ecliptic plane indicate that the acceleration of particles with the most rigid spectrum can occur under different physical conditions, i.e., during a temperature increase or decline. The phase of a temperature decline corresponds to the increase in the altitude of the acceleration region (for a coronal source) and a reduction in plasma density. At this time, the intensity of gamma radiation is usually below the threshold of modern detectors.     

Influence of the laser prepulse on proton acceleration in thin-foil experiments

We investigate the influence of the laser prepulse due to amplified spontaneous emission on the acceleration of protons in thin-foil experiments. We show that changing the prepulse duration has a profound effect on the maximum proton energy. We find an optimal value for the target thickness, which strongly depends on the prepulse duration. At this optimal thickness, the rear side acceleration process leads to the highest proton energies, while this mechanism is rendered ineffective for thinner targets due to a prepulse-induced plasma formation at the rear side. In this case, the protons are primarily accelerated by the front side mechanism leading to lower cutoff energies.     

Experimental study on high-energy surface arc plasma excitation control of cylindrical detached shock wave

In this study, an electrical parameter test system and a high-speed Schlieren system are used to study the control of a cylindrical detached shock wave through high-energy surface arc plasma excitation. The results show that, when plasma excitation is not applied, the bow shockwave angle around the cylinder is 52°. After the plasma excitation is applied, the arc discharge releases a large amount of heat within a short time, generating a shockwave and a control gas bulb (CGB). As a result, the bow shockwave angle first decreases and then increases, the pressure ratio before and after the shockwave decreases, and the intensity of the bow shockwave weakens. At t = 280 μs, the bow shockwave angle is reduced to a minimum of 46°. The effective interference time of high-energy surface arc plasma excitation on a cylindrical detached shockwave is 820 μs. A high temperature is used to control the heating effect of the bubbles, which will increase the local sound velocity near the wall, reduce the local Mach number, cause the sound velocity to move online, and eventually push the bow shockwave away from the cylinder. Concurrently, heating will accelerate the gas flow, reduce the pressure, and cause the mass flow in the unit flow area of the heated area to decrease, resulting in a strong compression effect, which deforms the bow shockwave. The high-energy surface arc plasma excitation will provide a potential technical means for high-speed aircraft detached shockwave control.     

Optical diagnostics for particle-cleaning process utilizing laser-induced shockwave

The dynamics of laser-induced plasma/shockwave and the interaction with a surface in the laser shock cleaning process are analyzed by optical diagnostics. Shockwaves are generated by a Q-switched Nd:YAG laser in air or with N₂, Ar, and He injection into the focal spot. The shock velocity is measured by monitoring the photoacoustic probe–beam deflection signal under different conditions. In addition, nanosecond time-resolved images of shockwave propagation and interaction with the substrate are obtained by the laser-flash shadowgraphy. The results reveal the effect of various operation parameters of the laser shock cleaning process on shockwave intensity, energy-conversion efficiency, and flow characteristics. Discussions are made on the cleaning mechanisms based on the experimental observations. PACS 81.65.Cf; 42.62.-b; 47.40.Nm     

Spin Effects in a Collision of Polarized Protons

The one- and two-spin asymmetries in a collision of polarized protons pp (^*,z) + X are studied in the present work within the framework of the quantum chromodynamics and the electroweak interaction. We use the method of helical amplitudes. Analytical expressions for one-spin A _L and two-spin A _LL asymmetries are derived and their dependences on the transverse momentum of the lepton pair are investigated for three values of invariant mass of the lepton pair. The results obtained allow the spin structure of the proton to be investigated.     

激光加速高能质子实验研究进展及新加速方案

利用超强激光与等离子体相互作用来加速高能离子是激光等离子体物理及加速器物理领域的研究热点.经过了近20年的发展,激光离子加速已取得丰硕成果,催生了一批新的应用.本文概述了国内外激光离子加速所取得的标志性实验研究进展,围绕高能质子的产生这一关键问题进行了深入的探讨,介绍了近几年来发展的有潜力的新加速方案.     

论反常作用、轻子结构和μ-e质量差

本文提出了反常作用的μ荷观点,μ荷不仅是一种反映μ-e禁戒性的量子数,而且是反常作用的承担者。在此观点下,讨论了轻子结构和μ-e质量差,得到了关于轻子结构的一些普遍定理。例如,证明了电子的反常作用和电磁作用形状因子相同等。在讨论轻子结构的基础上,得到了将μ-e质量差和轻子的电磁结构相联系的公式,证明了主要由于电磁作用干涉产生的μ荷-μ矩作用是反常作用自能的主要部分,它是负值,故是电子参与反常作用。利用电子的电磁结构数据能合理地解释μ-e质量差,而后者也提供了有关电子电磁结构的进一步知识。     

磁加速等离子体激励器激励特性

为了增强等离子体激励器的扰动能力、提升等离子体气动激励的控制效果,采用高压探针、烟流显示和PIV流场测试等多种研究手段,开展了磁场加速等离子体激励器特性研究,获得了激励器不同时刻的放电图像,分析了磁场强度对激励器电学特性与诱导流场特性的影响规律.结果表明,（1）放电等离子体的定向运动速度与磁场强度成正比,磁加速等离子体的最大移动速度达到了6 m/s;（2）通过对不同剖面的诱导流场进行研究发现,磁场加速等离子体激励器能够在近壁区产生一系列涡结构.此外,该诱导流场具有显著的三维特征与非定常特性.研究结果为开展基于磁加速等离子体气动激励的流动控制奠定了基础.      

Dynamic interaction of plasma flow with the hot boundary layer of a geomagnetic trap

The study of the interaction between collisionless plasma flow and stagnant plasma revealed the presence of an outer boundary layer at the border of a geomagnetic trap, where the super-Alfvén subsonic laminar flow changes over to the dynamic regime characterized by the formation of accelerated magnetosonic jets and decelerated Alfvén flows with characteristic relaxation times of 10–20 min. The nonlinear interaction of fluctuations in the initial flow with the waves reflected from an obstacle explains the observed flow chaotization. The Cherenkov resonance of the magnetosonic jet with the fluctuation beats between the boundary layer and the incoming flow is the possible mechanism of its formation. In the flow reference system, the incoming particles are accelerated by the electric fields at the border of boundary layer that arise self-consistently as a result of the preceding wave-particle interactions; the inertial drift of the incoming ions in a transverse electric field increasing toward the border explains quantitatively the observed ion acceleration. The magnetosonic jets may carry away downstream up to a half of the unperturbed flow momentum, and their dynamic pressure is an order of magnitude higher than the magnetic pressure at the obstacle border. The appearance of nonequilibrium jets and the boundary-layer fluctuations are synchronized by the magnetosonic oscillations of the incoming flow at frequencies of 1–2 mHz.     

Forward ion acceleration in thin films driven by a high-intensity laser

A collimated beam of fast protons, with energies as high as 1.5 MeV and total number of greater, similar10(9), confined in a cone angle of 40 degrees +/-10 degrees is observed when a high-intensity high-contrast subpicosecond laser pulse is focused onto a thin foil target. The protons, which appear to originate from impurities on the front side of the target, are accelerated over a region extending into the target and exit out the back side in a direction normal to the target surface. Acceleration field gradients approximately 10 GeV/cm are inferred. The maximum proton energy can be explained by the charge-separation electrostatic-field acceleration due to "vacuum heating."     

Particle saltation removal in laser-induced plasma shockwave cleaning

A new particle saltation removal model in laser-induced plasma shockwave cleaning is presented due to the drawbacks in other particle removal models. Considering the elastic energy stored in the adhered particle during shockwave-particle interaction, the threshold of the particle elastic deformation height, leading to the particle removal, is obtained. With taking the collision between gas molecules in plasma shockwave rear and adhered particle into account, the threshold of the plasma shockwave Mach number is also obtained which is a requirement to the shockwave strength. It is found that the smaller particle needs stronger shockwave than the larger particle. The gas pressure distribution characteristics in the plasma shockwave rear offer a saltation surrounding to the particle, and it is another requirement to the shockwave strength for a successful particle saltation. Finally, particle-cleaning experiment using laser-induced plasma shockwave proves the correctness of the particle saltation model.     

Transient electrostatic fields and related energetic proton generation with a plasma fiber

We observe a hollow structure and a fine ring in the proton images from a petawatt scale laser interaction with a "cone-fiber" target. The protons related to the hollow structure are accelerated from the cone-tip surface and deflected later by a radial electric field surrounding the fiber. Those associated with the fine ring are accelerated from the fiber surface by this radial electric field. This field is found to decay exponentially within 3 ps from about 5 x 10(12) V/m. Two-dimensional particle-in-cell simulations produce similar proton angular distributions.     

Energetic electron and ion beam generation in plasma openingswitches

In this paper, we present measurements of ion and electron flows in a nanosecond plasma opening switch (NPOS) and a microsecond plasma opening switch (MPOS), performed using charge collectors. In both experiments, an electron flow toward the anode, followed by an ion flow, were observed to propagate downstream toward the load side of the plasma during the plasma opening switch (POS) conduction. In the MPOS, ion acceleration was observed to propagate axially through the entire plasma. These results are in satisfactory agreement with the predictions of the electron magnetohydrodynamics (EHMD) theory and the results of fluid and particle-in-cell (PIC) code simulations. At the beginning of the POS opening, a high-current density (≈2 kA/cm²) short-duration (10-30 ns) axial ion flow downstream toward the load was observed in both experiments, with an electron beam in front of it. These ions are accelerated at the load side of the plasma and are accompanied by comoving electrons. In the NPOS, the ion energy reaches 1.35 MeV, whereas in the MPOS, the ion energy does not exceed 100 keV. We suggest that in the NPOS the dominant mechanism for the axial ion acceleration is collective acceleration by the space charge of the electron beam, while in the MPOS, axial ion acceleration is probably governed by the Hall field in the current carrying plasma     

超强激光与泡沫微结构靶相互作用提高强流电子束产额模拟研究

利用二维粒子模拟方法,本文研究了超强激光与泡沫微结构镀层靶相互作用产生强流电子束问题.研究发现泡沫区域产生了百兆高斯级准静态磁场,形成具有选能作用的"磁势垒",强流电子束中的低能端电子在"磁势垒"的作用下返回激光作用区域,在鞘场和激光场的共同作用下发生多次加速过程,从而显著提升高能电子产额.还应用单粒子模型,分析了电子在激光场作用下的运动行为,验证了多次加速的物理机理.