Brunel-dominated proton acceleration with a few-cycle laser pulse

Experimental measurements of backward accelerated protons are presented. The beam is produced when an ultrashort (5 fs) laser pulse, delivered by a kHz laser system, with a high temporal contrast (10(8)), interacts with a thick solid target. Under these conditions, proton cutoff energy dependence with laser parameters, such as pulse energy, polarization (from p to s), and pulse duration (from 5 to 500 fs), is studied. Theoretical model and two-dimensional particle-in-cell simulations, in good agreement with a large set of experimental results, indicate that proton acceleration is directly driven by Brunel electrons, in contrast to conventional target normal sheath acceleration that relies on electron thermal pressure.     

Intense laser-driven energetic proton beams from solid density targets

The effects of target density on proton acceleration driven by an intense sub-ps laser pulse are investigated using two-dimensional hybrid particle-in-cell simulations. Results show that at higher density the target-normal-sheath acceleration (TNSA) is more effective than shock acceleration for protons from a plastic target. Furthermore a lower-density target is favorable to higher energy of the TNSA protons. Moreover, the longitudinal electric fields at the target surfaces may reveal typical inhomogeneous structures for a long acceleration time. The conversion efficiency of laser energy into particle (electron, proton, and C(+) ion) energy is found to increase with decreasing target density.     

Laser ion-acceleration scaling laws seen in multiparametric particle-in-cell simulations

The ion acceleration driven by a laser pulse at intensity I= 10(20)-10(22) W/cm(2) x (microm/lambda)(2) from a double layer target is investigated with multiparametric particle-in-cell simulations. For targets with a wide range of thickness l and density n(e), at a given intensity, the highest ion energy gain occurs at certain electron areal density of the target sigma = n(e)l, which is proportional to the square root of intensity. In the case of thin targets and optimal laser pulse duration, the ion maximum energy scales as the square root of the laser pulse power. When the radiation pressure of the laser field becomes dominant, the ion maximum energy becomes proportional to the laser pulse energy.     

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。     

Laser-induced ion acceleration at ultra-high laser intensities

Ion acceleration by ultrashort laser pulses of very high intensities of the order 10²²?W/cm² is studied by two-dimensional Particle-In-Cell simulations. We show that laser normal incidence is preferred for such high intensities. For linearly polarized laser radiation, higher maximum proton/ion energies are achieved than for circular polarization. For linear polarization, the transition from the target normal sheath acceleration to the acceleration on the target front side by the radiation pressure is analyzed in detail. The transition intensity is increasing with the target thickness. The radiation pressure dominated regime leads to considerably higher number of accelerated protons and thus to a higher acceleration efficiency.     

Recent progress of laser driven particle acceleration at Peking University

Recently, radiation pressure acceleration （RPA） has been proposed and extensively studied, which shows that circularly polarized （CP） laser pulses can accelerate mono-energetic ion bunches in a phase-stable-acceleration （PSA） way from ultrathin foils. It is found that self-orgizing proton beam can be stably accelerated to GeV in the interaction of a CP laser with a planar target at 1022 W/cm2. A project called Compact LAser Plasma proton Accelerator （CLAPA） is approved by MOST in China recently. A prototype of laser driven proton accelerator （1 to 15 MeV/1 Hz） based on the PSA mechanism and plasma lens is going to be built at Peking University in the next five years. It will be upgraded to 200 MeV later for applications such as cancer therapy, plasma imaging and fast ignitiou for inertial confine fusion.     

Simultaneous generation of a proton beam and terahertz radiation in high-intensity laser and thin-foil interaction

We have observed simultaneously both the fast proton generation and terahertz (THz) radiation in the laser pulse interaction with a 5-μm thick titanium target. In order to control the proton acceleration and THz radiation, we have changed the duration of the amplified spontaneous emission (ASE) preceding the main pulse generated by the high-intensity Ti:sapphire laser. A fast proton beam with the maximal energy of ∼ 490 keV has been realized by reducing the duration of the ASE. Simultaneously, an intense emission of THz radiation is observed for various ASE durations. We propose the antenna mechanism for the THz radiation, according to which the fast electrons moving along the target surface emit the low-frequency electromagnetic wave. PACS 52.25.Os; 52.38.Kd; 52.50.Jm     

Effects of Ions Charge-Mass Ratio on Energy and Energy Spread of Accelerated Ions in Laser Driven Plasma

Effects of ions charge-mass ratio on energy and energy spread of accelerated ions in laser driven plasma are investigated in detail by proposing a simple double-layer model for a foil target driven by an ultrastrong laser. The radiation pressure acceleration mechanism plays an important role on the studied problem. For the ions near the plasma mirror, i.e. electrons layer, the dependence of ions energy on their charge-mass ratio is derived theoretically. It is found that the larger the charge-mass ratio is, the higher the accelerated ions energy gets. For those ions far away from the layer, the dependence of energy and energy spread on ions charge-mass ratio are also obtained by numerical performance. It exhibits that, as ions charge-mass ratio increases, not only the accelerated ions energy but also the energy spread will become large.     

超薄靶激光质子加速实验研究

在超短超强飞秒SILEX-Ⅰ激光装置上,开展了薄膜靶激光质子加速的实验研究。实验发现激光预脉冲、靶厚度对质子加速有很大的影响。在激光强度3×1018~3×1019W/cm2条件下,采用前表面厚度为3μm铜、后表面镀4μm厚CH靶,质子的最大能量达到3.15 MeV。而对190 nm厚CH膜靶,质子的最大能量为0.54 MeV。初步研究了激光偏振对质子加速的影响,相同激光功率条件下,圆偏振激光加速产生的质子最大能量略低于P偏振打靶。这些结果与靶后鞘层加速机制相一致。     

神光Ⅱ升级装置激光加速质子照相初步实验研究

在神光Ⅱ升级装置上开展了首轮激光加速质子对间接驱动快点火靶内爆过程的照相实验研究。通过激光与靶参数的优化,获得了能量高于18 MeV的质子束。通过静态客体的照相,获得了优于20 m的高空间分辨网格图像,为开展时间分辨的啁啾质子照相奠定了基础。开展了质子动态照相实验,获得了内爆压缩晚期的质子照相图像。实验发现内爆区域质子照相图像存在大量排空现象。内爆压缩区域不足以阻挡如此大范围质子束,证明了其中存在电磁场使得质子向外排开。动态照相的质子能量较低,分析是ns激光打靶过程产生的X射线及等离子体对质子加速存在影响。后续实验中需要进一步优化靶的屏蔽设计。     

采用超薄双层靶提高质子束的单能性

利用1维粒子模拟程序,研究了超短超强激光脉冲与超薄双层靶(基底层和加速层厚度均为nm量级)相互作用产生准单能质子束的过程。研究表明,基底层厚度及加速层厚度对质子能谱的影响至关重要。减小基底层厚度,靶后静电场增强,质子的最大能量显著增大;减小加速层厚度,靶后静电场分布变得更加均匀,质子能谱中心能量变化不大,单能性变好。通过优化参数,获得了能散度为7%的准单能质子束。     

Near‐3‐MeV protons from target‐normal‐sheath‐acceleration femtosecond laser irradiating advanced targets

Advanced targets based on graphene oxide and gold thin film were irradiated at high laser intensity (10¹⁸–10¹⁹ W/cm²) with 50‐fs laser pulses and high contrast (10⁸) to investigate ion acceleration in the target‐normal‐sheath‐acceleration regime. Time‐of‐flight technique was employed with SiC semiconductor detectors and ion collectors in order to measure the ion kinetic energy and to control the properties of the generated plasma. It was found that, at the optimized laser focus position with respect to the target, maximum proton acceleration up to about 3 MeV energy and low angular divergence could be generated. The high proton energy is explained as due to the high electrical and thermal conductivity of the reduced graphene oxide structure. Dependence of the maximum proton energy on the target focal position and thickness is presented and discussed.     

Proton Beam Generated by Multi-Lasers Interaction with Rear-Holed Target

Multi-lasers are proposed to enhance the proton acceleration in laser plasma interaction. A rear-holed target is illuminated by three lasers from different directions. The scheme is demonstrated by two-dimensional particlein-cell simulations. The electron cloud shape is controlled well and the electron density is improved significantly. The electrons accelerated by the three lasers induce an enhanced target normal sheath acceleration(TNSA) which suppresses the proton beam divergence and improves the maximum proton energy. The maximum proton energy is 22.9 Me V, which increased significantly than that of a single-laser target interaction. Meanwhile, the average divergence angle(22.3?) is reduced. The dependence of the proton beam on the length of sidewall is investigated in detail and the optimal length is obtained.     

基于带电粒子活化法开展的SGⅡ-U皮秒激光质子加速实验研究

基于带电粒子活化测谱方法在SGⅡ-U装置上开展了皮秒激光靶背鞘场机制质子加速实验研究,对靶参数进行了优化.利用带电粒子活化测谱方法测量了相同激光条件、不同Cu薄膜靶厚度情况下靶背鞘场加速质子的最高截止能量、角分布、总产额以及激光能量到质子的转化效率等关键参数.实验发现,SGⅡ-U皮秒激光靶背鞘场加速机制的最佳Cu薄膜靶厚度为10 μm,对应质子最高能量接近40 MeV,质子（>4 MeV）总产额约4×10¹²个,激光能量到质子的转化效率约2%.薄膜靶更厚或者更薄都会降低加速质子的最高截止能量;当靶厚减薄至1 μm时,皮秒激光的预脉冲开始对靶背鞘场产生显著影响,质子最高截止能量急剧下降,高能质子束斑呈现空心结构;而当靶厚增加至35 μm时,虽然质子束的能量有所降低,但是质子束斑的均匀性更好.     

Investigation of the effect of plasma waves excitation on target normal sheath ion acceleration using fs laser‐irradiating hydrogenated structures

Measurements of ion acceleration in polymethylmethacrylate foils covered by a thin copper film irradiated by fs laser in target normal sheath acceleration regime are presented. The ion acceleration depends on the laser parameters, such as the pulse energy; depends on the irradiation conditions, such as the focal point position of the laser with respect to the target surface; and depends on the target properties, such as the metallic film thickness. The proton acceleration increases in the presence of the metallic film enhancing the plasma electron density, reaching about 1.6 MeV energy for a focal position on the target surface. The plasma diagnostics uses SiC detectors, absorber foils, Faraday cups, and gafchromic films. Employing p‐polarized laser light and a suitable oblique incidence, it is possible to increase the proton acceleration up to about 2.0 MeV thanks to the effects of laser absorption resonance due to plasma waves excitation.     

Development of a laser-driven proton accelerator for cancer therapy

Recent advances in laser technology have made proton (light ion) acceleration possible using laser-induced plasmas. In this work, we report our work for the last few years on the investigation of a new proton therapy system for radiation oncology, which employs laser-accelerated protons. If successfully developed, the new system will be compact, cost-effective, and capable of delivering energy-and intensity-modulated proton therapy (EIMPT). We have focused our research on three major aspects: (1) target design for laser-proton acceleration, (2) system design for particle/energy selection and beam collimation, and (3) dosimetric studies on the use of laser-accelerated protons for cancer therapy. We have performed particle-in-cell (PIC) simulations to investigate optimal target configurations for proton/ion acceleration. We also performed Monte Carlo simulations to study the beam characteristics and the feasibility of using such beams for cancer treatment. Since laser-accelerated protons have broad energy and angular distributions, which are not suitable for radiotherapy applications directly, we have designed a compact particle selection and beam collimating system for EIMPT beam delivery. We also proposed a new gantry design to make the whole system compact to retrofit existing linac vaults. We have compared Monte Carlo calculated dose distributions using X-ray IMRT and laser-proton EIMPT. Our results show that EIMPT using laser protons produces superior target coverage and much reduced critical structure dose and integral dose compared to X-ray IMRT.     

Finite spot effects on radiation pressure acceleration from intense high-contrast laser interactions with thin targets

Short pulse laser interactions at intensities of 2×10(21) W cm(-2) with ultrahigh contrast (10(-15)) on submicrometer silicon nitride foils were studied experimentally by using linear and circular polarizations at normal incidence. It was observed that, as the target decreases in thickness, electron heating by the laser begins to occur for circular polarization leading to target normal sheath acceleration of contaminant ions, while at thicker targets no acceleration or electron heating is observed. For linear polarization, all targets showed exponential energy spreads with similar electron temperatures. Particle-in-cell simulations demonstrate that the heating is due to the rapid deformation of the target that occurs early in the interaction. These experiments demonstrate that finite spot size effects can severely restrict the regime suitable for radiation pressure acceleration.     

Generating high-current monoenergetic proton beams by a circularly polarized laser pulse in the phase-stable acceleration regime

A new ion acceleration method, namely, phase-stable acceleration, using circularly-polarized laser pulses is proposed. When the initial target density n(0) and thickness D satisfy a(L) approximately (n(0)/n(c))D/lambda(L) and D>l(s) with a(L), lambda(L), l(s), and n(c) the normalized laser amplitude, the laser wavelength in vacuum, the plasma skin depth, and the critical density of the incident laser pulse, respectively, a quasiequilibrium for the electrons is established by the light pressure and the space charge electrostatic field at the interacting front of the laser pulse. The ions within the skin depth of the laser pulse are synchronously accelerated and bunched by the electrostatic field, and thereby a high-intensity monoenergetic proton beam can be generated. The proton dynamics is investigated analytically and the results are verified by one- and two-dimensional particle-in-cell simulations.     

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。     

短脉冲强激光打靶产生的质子角分布实验研究

 采用短脉冲强激光辐照固体双层薄靶的方式对质子束的产生及质子束角分布开展了实验研究。在SILEX-Ⅰ短脉冲激光装置上利用脉宽为30 fs的强激光辐照背面镀有CH膜的金膜靶,在距离靶背3.3 cm处采用CR39记录靶背出射的质子角分布。通过分析靶背出射质子的角分布,研究了激光功率密度和对比度对质子加速机制的影响。研究结果表明:占主导地位的质子产生和加速机制对激光预脉冲比较敏感。激光预脉冲较弱时,靶背壳电场加速机制占主导地位;当激光预脉冲较强时,靶前加速机制占主导地位。此外,还对导致质子环形分布的磁场大小进行了估算。