强激光与细锥靶相互作用产生强流高能电子束的研究

本文提出采用了强激光与细锥形靶作用, 产生大量定向高能电子, 用于快点火激光聚变方案研究. 通过PIC 模拟, 研究了细锥靶和激光脉冲的各项参数, 对产生高能电子的影响. 模拟发现, 细锥靶开口10° 时能够产生较多的高能电子, 当开口角度逐渐增大时, 高能电子的能量和数目都有一定程度下降. 若为细锥靶加上预等离子体, 产生的高能电子的数目将大大提高, 而最高的电子能量将会下降. 中等能量的电子加速主要由于激光有质动力加速, 而高能量的电子加速主要由于电子感应加速. 随着激光脉宽的增加, 高能电子的数量直线上升. 关键词： 细锥形靶 电子加速 感应共振加速     

强激光与锥型结构靶相互作用准直电子束粒子模拟研究

利用PIC(particle-in-cell)方法模拟研究了超短强激光与锥型三明治结构靶相互作用快电子束的产生和传输,并与锥通道靶、锥丝靶和锥靶在相同激光参数下的作用结果进行了比较.研究发现强激光与锥三明治靶作用产生的快电子能被不同密度材料产生的准静态界面强磁场有效地准直传输.相对其他三种锥型结构靶,锥三明治靶能产生更多数目及更高能量的快电子,提高了激光到快电子的能量转换效率和快电子束的品质,这对快点火能量沉积是有利的.     

强激光与双锥靶作用加速准单能质子束

提出了一种新型的双锥靶结构用于准单能质子束加速。利用二维PIC粒子模拟程序研究了强激光与双锥靶作用加速产生质子束的物理过程以及质子束品质。双锥靶产生的质子束在峰值能量和发散角度等方面都明显优于相同激光条件下单锥靶和平面靶的结果。尤其与平面靶相比,双锥靶质子束的峰值能量提高了5倍以上,而且很好地保持准单能性。一方面双锥靶的内锥部分是临界密度材料,提高了激光的吸收效率;另一方面双锥靶内形成了更强的准静态磁场,可以约束引导更多的超热电子传输过锥尖,进而增强加速质子束的鞘层电场。     

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.     

Generation of a fast-ion beam upon the interaction of a multiterawatt picosecond laser pulse with a solid target

The parameters of fast particles generated upon the interaction of 10¹⁹ W/cm² laser pulses with solid targets are studied. The spatial and energy parameters of fast ions are investigated. It is found that approximately 1–3% of the laser energy is transformed to the energy of mega-and submegaelectronvolt ions at laser pulse intensities ≥10¹⁸ W/cm². It is shown experimentally that an ion beam is directed perpendicular to the target surface. The analytic and numerical simulations agree with experimental results and predict the propagation of fast electrons in the mirror direction with respect to the incident laser beam and of ions perpendicular to the target. The theoretical calculations are compared with the experimental output and spectra of fast electrons and ions.     

激光入射角对靶面方向超热电子发射的影响

实验研究了超短超强激光脉冲与薄膜靶相互作用中产生的超热电子角分布随激光入射角的变化.在靶面方向观测到一束方向性很好的高能超热电子.该高能超热电子束的电子数目随着激光入射角的增大而增大.对结果的分析表明，表面准静态磁场是导致表面电子产生的主要原因. 关键词： 超热电子 表面准静态磁场 超强激光脉冲与等离子体相互作用     

Generation of electron nanobunches and short-wavelength radiation upon reflection of a relativistic-intensity laser pulse from a finite-size target

We have proposed an efficient scheme of generation of short dense electron bunches during the interaction at large angles of incidence of a laser pulse with a thin transversally semibounded laser target. Streams of bunches can be used to simultaneously and independently generate pulsed X-ray radiation as fast electrons hit secondary targets. Dependences of bunch parameters (the number of particles in the bunch and the bunch energy and thickness) on the angle of incidence and laser intensity have been obtained. It has been shown that, upon reflection from the target, the relativistic-intensity laser wave is efficiently converted (the energy-conversion factor reaches ～20%) into a sequence of electromagnetic tens-of-nanometer-long atto pulses, which follow one after another in the period of the initial laser wave. We have investigated how the parameters of the atto pulse depend on the angle of incidence and the laser intensity. We have shown that atto pulses are generated most efficiently at large angles of incidence (≥50°) of the laser pulse on the target.     

Near QED regime of laser interaction with overdense plasmas

Interaction of laser plulses with intensities up to 10²⁵?W/cm² with overdense plasma targets is investigated via three-dimensional particle-in-cell simulations. At these intensities, radiation of electrons in the laser field becomes important. Electrons transfer a significant fraction of their energy to γ-photons and obtain strong feedbacks due to radiation reaction (RR) force. The RR effect on the distribution of laser energies among three main species: electrons, ions and photons is studied. The RR and electron-positron pair creation are implemented by a QED model. As the laser intensity inreases, the ratio of laser energy coupled to electrons drops while the one for γ-photons reaches up to 35%. Two distinctive plasma density regimes of the high-density carbon target and low-density solid hydrogen target are identified from the laser energy partitions and angular distributions of photons. The power-laws of absorption efficiency versus laser intensity and the transition of photon divergence are revealed. These show enhanced generation of γ-photon beams with improved collimation in the relativistically transparent regime. A new effect of transverse trapping of electrons inside the laser field caused by the RR force is observed: electrons can be unexpectedly confined by the intense laser field when the RR force is comparable to the Lorentz force. Finally, the RR effect and different regions of photon emission in laser-foil interactions are clarified.     

Surface acceleration of fast electrons with relativistic self-focusing in preformed plasma

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.     

Ion acceleration by superintense laser pulses in plasmas

Ion acceleration by petawatt laser radiation in underdense and overdense plasmas is studied with 2D3V-PIC (Particle in Cell) numerical simulations. These simulations show that the laser pulse drills a channel through the plasma slab, and electrons and ions expand in vacuum. Fast electrons escape first from the electron-ion cloud. Later, ions gain a high energy on account of the Coulomb explosion of the cloud and the inductive electric field which appears due to fast change of the magnetic field generated by the laser pulse. Similarly, when a superintense laser pulse interacts with a thin slab of overdense plasma, its ponderomotive pressure blows all the electrons away from a finite-diameter spot on the slab. Then, due to the Coulomb explosion, ions gain an energy as high as 1 GeV. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 80–86 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

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     

通道靶对超强激光加速质子束的聚焦效应

发散角过大是制约超强激光与固体靶相互作用加速产生高能质子束应用的一个重大物理难题.本文提出了一种结构化的通道靶型,与超强激光相互作用可提高质子束的发散特性,通道壁上产生的横向电荷分离静电场可对质子有效聚焦.采用二维particle-in-cell粒子模拟程序对激光通道靶相互作用过程进行了研究,分析了加速质子束的性能特点.模拟结果表明,与传统平面靶相比,通道靶可以在不过多损失能量的情况下产生具有更好准直性的质子束,尤其当通道靶的直径与激光焦斑尺寸和质子源尺寸相当时,横向静电场能够有效聚焦质子束,并且可保证相对较高的激光能量利用率.     

Particle acceleration in ultra-intense laser plasma interaction

The acceleration of electrons in laser plasma interaction has been observed since the seventies, when it was initially considered as a deleterious effect, as, in the inertial fusion context, the so-called suprathermal electrons preheat the target. However, it has been quickly observed that a large benefit could be taken from these electrons. Two main directions are now followed. In the first direction, one tries to accelerate electrons to high energy, presently in the GeV range. The electrons may originate from a pre-accelerated beam, or directly from a gas target instantaneously transformed in a plasma by the ultra-intense laser pulse. In the second direction, one tries to transfer the energy of the electrons to fast ions, especially protons, presently in the few tens of MeV range. Thin targets are used for this transformation, the electrons being accelerated at the front of the target, while the ions may originate from the front part or the back part of the target, or from inside the target, depending on the parameters of the experiment. While the maximum energy was the initial goal of the pioneer experiments, there are now strong experimental efforts to improve the quality of the beams, in terms of luminosity, emittance, and energy spectrum. In the recent years, quasi-monoenergetic beams were obtained both for electrons and for ions.     

Angular distributions of fast electrons, ions, and Bremsstrahlung x/gamma-rays in intense laser interaction with solid targets

We study the angular distributions of fast electrons, ions, and bremsstrahlung x/ gamma-rays generated during the interaction of an ultrashort intense laser pulse with solid targets. A relation is found on the angular directions for fast electrons and ions as a function of the particle's kinetic energy, experienced Coulomb potential changes, and the incident angle of the laser pulse. It is valid independent of the acceleration mechanisms and the polarization of the laser pulse, as confirmed by particle-in-cell simulations. The angular distribution of bremsstrahlung x/gamma-rays is presented to show explicitly its correlation with the corresponding angular distributions of electrons.     

Interaction of intense ultrashort laser pulse with shell target

Based on numerical PIC modeling and an analytical model, we analyze the interaction of a short high-intensity laser pulse with homo- and heterogeneous (two-layer) shell targets. We show that the shell target is capable of focusing ions in a narrow region. In this case, the ion energy exceeds the ion energy for a flat infinite target of the same thickness. We propose to use the effect of focusing and cumulation of ions to implement the collision of ionic beams that counterpropagate from opposite sides of the sphere and to increase the yield of ion-ion reactions. The constructed model makes it possible to calculate the energy of the ion and the number of ions in the region of focusing.     

Gd靶激光等离子体6.7nm光源的实验研究

本文对Gd靶激光等离子体极紫外光源进行了实验研究, 在 6.7 nm附近获得了较强的辐射, 并研究了6.7 nm 附近光辐射随打靶激光功率密度变化的规律以及收集角度对极紫外辐射的影响. 同时, 对平面Gd靶激光等离子光源的离子碎屑角分布进行了测量, 发现从靶面的法线到沿着靶面平行方向上Gd离子数量依次减少. 进一步研究结果表明采用0.9 T外加磁场的条件下可取得较好的Gd 离子碎屑阻挡效果.     

飞秒激光与固体靶相互作用的前向快电子束发散研究

为了研究前向快电子束的发散问题,采用飞秒激光辐照平面铜靶,以电子角分布仪和氟化锂(LiF)热释光探测器探测快电子的前向发散角。实验显示前向快电子束发散角均在95°(半高宽FWHM)以上。理论计算指出预等离子体中电子的初速度以及加速后的电子在靶内碰撞效应均不是产生大发散角的重要原因。根据激光-等离子体相互作用理论,等离子体中的电磁场才是导致前向快电子束产生较大发散角的主要原因。这个研究为获取较小发散角的前向快电子束提供了实验设置的依据。     

飞秒激光固体靶相互作用中超热电子的输运特性

 实验研究了在100 TW掺钛宝石超短超强脉冲激光装置上完成的飞秒激光-固体靶相互作用中超热电子的输运特性,获得了超热电子的能谱、产额、注量及超热电子在靶内输运能量沉积范围。测量结果表明:超热电子的注量和总能量随靶厚度的增加而减少,超热电子约80％的能量主要沉积在靶内的前约一个激光脉冲宽度的范围内,且能量沉积范围随激光脉冲宽度的增加而增加,这主要是静电场的影响所致。     

超热电子产生Kα辐射的参数影响

 超短超强激光与固体靶相互作用,可以产生大量的Kα特征谱线发射。这种Kα线光源在背光照相、医学成像和超快诊断等方面具有显著优势。为了寻求获得Kα线高产额的方法,利用ITS蒙卡模拟程序,对超短超强激光产生的超热电子在固体靶中产生的Kα线发射进行了模拟。系统研究了Kα线发射强度随出射角度、靶厚度和超热电子温度等参数的变化情况。研究发现在恒定的激光功率密度或超热电子温度下,存在最佳的靶厚度,使得Kα线光子产额最大,并用模拟结果对已有实验数据进行了解释。     

超热电子的阻止功率及其在慢化过程中引起的轫致谱

在高功率、长波长激光与等离子体耦合时将产生大量的超热电子，这些超热电子将通过轫致辐时，与热电子碰撞，以及晕区电场加速快离子损失其能量。在某些典型条件下，分析和估计了每种能耗机制损失能量的分额。给出了快电子在通过激光等离子体的慢化过程中发射轫致谱的解析表达式。用调整参数拟合实验结果的方法，能够得到超热电子的各种信息，诸如它们的温度、能量、数同以及能谱。计算谱与实验谱作了比较。