Controlled injection and acceleration of electrons in plasma wakefields by colliding laser pulses

In laser-plasma-based accelerators, an intense laser pulse drives a large electric field (the wakefield) which accelerates particles to high energies in distances much shorter than in conventional accelerators. These high acceleration gradients, of a few hundreds of gigavolts per metre, hold the promise of compact high-energy particle accelerators. Recently, several experiments have shown that laser-plasma accelerators can produce high-quality electron beams, with quasi-monoenergetic energy distributions at the 100 MeV level. However, these beams do not have the stability and reproducibility that are required for applications. This is because the mechanism responsible for injecting electrons into the wakefield is based on highly nonlinear phenomena, and is therefore hard to control. Here we demonstrate that the injection and subsequent acceleration of electrons can be controlled by using a second laser pulse. The collision of the two laser pulses provides a pre-acceleration stage which provokes the injection of electrons into the wakefield. The experimental results show that the electron beams obtained in this manner are collimated (5 mrad divergence), monoenergetic (with energy spread <10 per cent), tuneable (between 15 and 250 MeV) and, most importantly, stable. In addition, the experimental observations are compatible with electron bunch durations shorter than 10 fs. We anticipate that this stable and compact electron source will have a strong impact on applications requiring short bunches, such as the femtolysis of water, or high stability, such as radiotherapy with high-energy electrons or radiography for materials science.     

Monoenergetic beams of relativistic electrons from intense laser-plasma interactions

High-power lasers that fit into a university-scale laboratory can now reach focused intensities of more than 10(19) W cm(-2) at high repetition rates. Such lasers are capable of producing beams of energetic electrons, protons and gamma-rays. Relativistic electrons are generated through the breaking of large-amplitude relativistic plasma waves created in the wake of the laser pulse as it propagates through a plasma, or through a direct interaction between the laser field and the electrons in the plasma. However, the electron beams produced from previous laser-plasma experiments have a large energy spread, limiting their use for potential applications. Here we report high-resolution energy measurements of the electron beams produced from intense laser-plasma interactions, showing that--under particular plasma conditions--it is possible to generate beams of relativistic electrons with low divergence and a small energy spread (less than three per cent). The monoenergetic features were observed in the electron energy spectrum for plasma densities just above a threshold required for breaking of the plasma wave. These features were observed consistently in the electron spectrum, although the energy of the beam was observed to vary from shot to shot. If the issue of energy reproducibility can be addressed, it should be possible to generate ultrashort monoenergetic electron bunches of tunable energy, holding great promise for the future development of 'table-top' particle accelerators.     

锥形飞秒强激光等离子体中Compton散射的能量转换

应用相对论性电子与多光子集团非弹性碰撞模型和经典相对论电动力学理论,分析、计算了锥形飞秒强激光等离子体中多光子非线性Compton散射的能量转换.发现等离子体中的耦合激光场会引起能量转换效率的振荡,而静电场会降低能量转换效率.当高能电子与光子发生双光子非线性Compton散射时,电子能获得最大的能量转换效率.     

超短超强激光与等离子体相互作用中质子发射一些研究进展

超短超强激光与等离子体相互作用中得到的高能质子在质子成像、粒子加速、诊断超短超强激光与等离子体相互作用的物理过程、“快点火”和治疗癌症等方面有一定的应用。使得对超短超强激光与等离子体相互作用得到的高能质子的研究成为目前的研究热点。文章综述了产生质子的两种主要加速机制以及在不同实验条件下超短超强激光与等离子体相互作用过程中得到质子的能量、角分布、产额以及相关的原理。     

在相对论性激光-等离子体系统中模拟激光参数对等离子弧柱形态的影响

基于相对论性激光-等离子体动力学理论和PIC方法建立了激光入射等离子弧柱的模型,该模型描述了激光入射等离子弧后粒子的运动,并模拟了弧柱形态的变化。通过改变激光的平均功率、脉冲宽度以及重复频率,模拟等离子弧柱形态变化,得到的激光参数对等离子弧柱形态的影响规律。通过控制弧柱的直径可以提高材料的熔积性能。计算结果表明增大激光平均功率、脉冲宽度,可以更深地压缩等离子弧;但是,重复频率的影响则会出现波动现象。激光等离子复合加工更适合于精细加工。     

High-quality electron beams from a laser wakefield accelerator using plasma-channel guiding

Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m(-1) (refs 1-3). These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators as compact next-generation sources of energetic electrons and radiation. To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread, which limits potential applications. Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 10(9) electrons above 80 MeV). Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance. The results open the way for compact and tunable high-brightness sources of electrons and radiation.     

Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio

Physical laws are believed to be invariant under the combined transformations of charge, parity and time reversal (CPT symmetry). This implies that an antimatter particle has exactly the same mass and absolute value of charge as its particle counterpart. Metastable antiprotonic helium (pHe(+)) is a three-body atom consisting of a normal helium nucleus, an electron in its ground state and an antiproton (p) occupying a Rydberg state with high principal and angular momentum quantum numbers, respectively n and l, such that n?≈?l?+?1?≈?38. These atoms are amenable to precision laser spectroscopy, the results of which can in principle be used to determine the antiproton-to-electron mass ratio and to constrain the equality between the antiproton and proton charges and masses. Here we report two-photon spectroscopy of antiprotonic helium, in which p(3)He(+) and p(4)He(+) isotopes are irradiated by two counter-propagating laser beams. This excites nonlinear, two-photon transitions of the antiproton of the type (n, l)?→?(n?-?2, l?-?2) at deep-ultraviolet wavelengths (λ = 139.8, 193.0 and 197.0?nm), which partly cancel the Doppler broadening of the laser resonance caused by the thermal motion of the atoms. The resulting narrow spectral lines allowed us to measure three transition frequencies with fractional precisions of 2.3-5 parts in 10(9). By comparing the results with three-body quantum electrodynamics calculations, we derived an antiproton-to-electron mass ratio of 1,836.1526736(23), where the parenthetical error represents one standard deviation. This agrees with the proton-to-electron value known to a similar precision.     

A laser-plasma accelerator producing monoenergetic electron beams

Particle accelerators are used in a wide variety of fields, ranging from medicine and biology to high-energy physics. The accelerating fields in conventional accelerators are limited to a few tens of MeV m(-1), owing to material breakdown at the walls of the structure. Thus, the production of energetic particle beams currently requires large-scale accelerators and expensive infrastructures. Laser-plasma accelerators have been proposed as a next generation of compact accelerators because of the huge electric fields they can sustain (>100 GeV m(-1)). However, it has been difficult to use them efficiently for applications because they have produced poor-quality particle beams with large energy spreads, owing to a randomization of electrons in phase space. Here we demonstrate that this randomization can be suppressed and that the quality of the electron beams can be dramatically enhanced. Within a length of 3 mm, the laser drives a plasma bubble that traps and accelerates plasma electrons. The resulting electron beam is extremely collimated and quasi-monoenergetic, with a high charge of 0.5 nC at 170 MeV.     

Fluid particle accelerations in fully developed turbulence

The motion of fluid particles as they are pushed along erratic trajectories by fluctuating pressure gradients is fundamental to transport and mixing in turbulence. It is essential in cloud formation and atmospheric transport, processes in stirred chemical reactors and combustion systems, and in the industrial production of nanoparticles. The concept of particle trajectories has been used successfully to describe mixing and transport in turbulence, but issues of fundamental importance remain unresolved. One such issue is the Heisenberg-Yaglom prediction of fluid particle accelerations, based on the 1941 scaling theory of Kolmogorov. Here we report acceleration measurements using a detector adapted from high-energy physics to track particles in a laboratory water flow at Reynolds numbers up to 63,000. We find that, within experimental errors, Kolmogorov scaling of the acceleration variance is attained at high Reynolds numbers. Our data indicate that the acceleration is an extremely intermittent variable--particles are observed with accelerations of up to 1,500 times the acceleration of gravity (equivalent to 40 times the root mean square acceleration). We find that the acceleration data reflect the anisotropy of the large-scale flow at all Reynolds numbers studied.     

Plasma devices to guide and collimate a high density of MeV electrons

The development of ultra-intense lasers has facilitated new studies in laboratory astrophysics and high-density nuclear science, including laser fusion. Such research relies on the efficient generation of enormous numbers of high-energy charged particles. For example, laser-matter interactions at petawatt (10(15) W) power levels can create pulses of MeV electrons with current densities as large as 10(12) A cm(-2). However, the divergence of these particle beams usually reduces the current density to a few times 10(6) A cm(-2) at distances of the order of centimetres from the source. The invention of devices that can direct such intense, pulsed energetic beams will revolutionize their applications. Here we report high-conductivity devices consisting of transient plasmas that increase the energy density of MeV electrons generated in laser-matter interactions by more than one order of magnitude. A plasma fibre created on a hollow-cone target guides and collimates electrons in a manner akin to the control of light by an optical fibre and collimator. Such plasma devices hold promise for applications using high energy-density particles and should trigger growth in charged particle optics.     

基于粒子系统实现船舶航迹仿真的加速方法

为解决大规模粒子系统的实时性问题,提出一种基于LOD技术和硬件加速技术的混合加速技术.根据视点与景物间的距离建立粒子系统的LOD模型,并将该模型融入基于GPU的渲染流程中.整个实现流程采用FBO技术,通过两遍渲染方式完成粒子系统的绘制.虚拟海洋场景的船舶航迹实时试验表明,该方法可显著提高粒子系统的运算效率和绘制速度,与未采用任何加速策略而单纯基于CPU的绘制方法相比,其绘制效率提高近12倍.     

kHz 脉冲分子束的产生及其在强场原子分子物理实验中的应用

采用悬臂压电脉冲阀, 制备出最高重复频率达到3 kHz 的脉冲分子束。通过测量氮气分子束在飞秒激光作用下隧道电离生成的N2 +(B-X)荧光光谱, 对脉冲阀的性能做了表征。这种kHz 脉冲分子束搭配kHz 飞秒激光器, 将推动飞秒强激光驱动的原子分子动力学实验研究。     

超高速微小碎片激光测速系统研制及应用

地面超高速模拟实验是研究微小空间碎片撞击效应经济有效的手段,其中等离子体加速器为微米量级碎片的主要地面模拟设备.本文研制了在等离子体驱动微小碎片加速器系统并应用于高速飞行微粒速度测量的激光测速系统.该激光测速系统工作原理是.利用主动激光照明,在颗粒飞行路径上形成光墙,通过检测颗粒通过光墙形成的散射激光,得到微粒到达光墙的时间,利用飞行时间法进行高速微粒速度测量.在激光测速系统原理测试实验中,采用信号响应上升时间小于10 ns,电子渡越时间小于20ns的高灵敏、快响应的光电倍增管,原理试验测得该探测系统的响应时间仅为约70 ns.该响应时间小于速度为15 km/s的颗粒通过3～5mm厚度的片状激光束的理论时间,并验证了该系统灵敏度高、响应时间快的特点,可以满足超高速微粒(8～20 km/s)通过3～5 mm激光墙的时间阈值(约0.1 μs)的需求.目前,激光测速系统已经应用于等离子体加速器发射超高速微粒的试验中,能有效测量等离子体加速器所发射的高速微粒的群速度,对15 km/s及以上速度的超高速颗粒亦能捕捉到有效信号,实现对微粒速度的测量,达到了良好的预期效果.在等离子体微小碎片加速器上开展的超高速撞击试验中,激光测速系统能够实现无损在线速度测量,对等离子体加速器上开展的超高速撞击试验提供了重要帮助.     

一种提高激光光束强度分布检测精度的算法研究

提出了一种新的提高测量激光光束横向强度分布的方法.利用取样窗口内被测量点光强度值是取样窗口的小区域强度积分值的特点和相邻取样窗口的小区域强度积分值的积分区域的相互交迭的性质,借助于线性方程组理论,可提高激光光束横向强度分布测量方法的精度;采用该算法对多种模式的高斯光束进行了模拟比较计算,在不改变取样窗口大小的情况下,光束强度分布的精度有了明显提高.     

屈曲式微加速度开关设计与分析

根据弹性梁的大挠度后屈曲理论，建立了多力耦合作用下加速度开关系统的动力学模型，由此设计了一种新型屈曲式微机械加速度开关．该开关采用倾斜微梁支撑敏感质量块，将两折叠梁固定于质量块的上下表面，从而保证了开关动作的方向性．利用弹性线方法计算了倾斜支撑梁结构大挠度后屈曲的非线性刚度，分析了气膜阻尼力和触点接触力对系统性能的影响，并运用数值方法对含有椭圆积分的强非线性系统进行了动态特性分析．仿真结果表明，所设计的开关响应时间低于6ms，不稳定接触时间小于0．02ms，在闭合状态下触点的接触力大于50mN，与其他类型开关相比，它具有良好的闽值特性和接触可靠性，因此在惯性控制系统中具有广泛的应用前景．     

Anti-planetward auroral electron beams at Saturn

Strong discrete aurorae on Earth are excited by electrons, which are accelerated along magnetic field lines towards the planet. Surprisingly, electrons accelerated in the opposite direction have been recently observed. The mechanisms and significance of this anti-earthward acceleration are highly uncertain because only earthward acceleration was traditionally considered, and observations remain limited. It is also unclear whether upward acceleration of the electrons is a necessary part of the auroral process or simply a special feature of Earth's complex space environment. Here we report anti-planetward acceleration of electron beams in Saturn's magnetosphere along field lines that statistically map into regions of aurora. The energy spectrum of these beams is qualitatively similar to the ones observed at Earth, and the energy fluxes in the observed beams are comparable with the energies required to excite Saturn's aurora. These beams, along with the observations at Earth and the barely understood electron beams in Jupiter's magnetosphere, demonstrate that anti-planetward acceleration is a universal feature of aurorae. The energy contained in the beams shows that upward acceleration is an essential part of the overall auroral process.     

在激光尾场加速中电子密度和初始动量对其自注入及加速的影响

利用哈密顿理论给出了等离子体电子在尾场中捕获及其加速与激光、等离子体参量的关系表达式.讨论了等离子体电子密度和初始动量对电子自注入和加速的影响机制.研究结果表明:静止电子不能被尾场捕获并加速,而具有一定初始动量的电子容易自注入至激光尾场中并得到加速.等离子体密度越小,激光尾场场强越强,电子将获得更大的能量.2维粒子模拟结果与理论结论一致.所得结果对超强超短脉冲激光尾场加速电子的方案具有理论指导意义.     

Basic and applied research at the TRIUMF meson factory

The TRIUMF 520 MeV H- cyclotron produces intense beams of protons, pions and muons supporting basic research in nuclear, particle and solid-state physics, nuclear chemistry and biomedicine, and applied research in electromagnetic breeding of nuclear fuel, proton radiography, radioisotope production and cancer treatment.     

自注入电子束的加速及其对激光尾场的影响

在激光等离子体相互作用中对自注入电子束的加速及其对尾场的影响进行了理论研究.自注入电子束在空泡底部削弱了激光尾波静电场.随着自注入电子束电荷密度的增加,尾波场结构明显改变和空泡纵向变长.随后的鞘层电子须以较高的初始纵向动量才能自注入到不断演化的空泡尾场中.当自注入电子束的库仑场大于空泡内静电场时,该电子束将驱动等离子体尾波场.     

Laser acceleration of quasi-monoenergetic MeV ion beams

Acceleration of particles by intense laser-plasma interactions represents a rapidly evolving field of interest, as highlighted by the recent demonstration of laser-driven relativistic beams of monoenergetic electrons. Ultrahigh-intensity lasers can produce accelerating fields of 10 TV m(-1) (1 TV = 10(12) V), surpassing those in conventional accelerators by six orders of magnitude. Laser-driven ions with energies of several MeV per nucleon have also been produced. Such ion beams exhibit unprecedented characteristics--short pulse lengths, high currents and low transverse emittance--but their exponential energy spectra have almost 100% energy spread. This large energy spread, which is a consequence of the experimental conditions used to date, remains the biggest impediment to the wider use of this technology. Here we report the production of quasi-monoenergetic laser-driven C5+ ions with a vastly reduced energy spread of 17%. The ions have a mean energy of 3 MeV per nucleon (full-width at half-maximum approximately 0.5 MeV per nucleon) and a longitudinal emittance of less than 2 x 10(-6) eV s for pulse durations shorter than 1 ps. Such laser-driven, high-current, quasi-monoenergetic ion sources may enable significant advances in the development of compact MeV ion accelerators, new diagnostics, medical physics, inertial confinement fusion and fast ignition.