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.     

强激光与柱腔靶作用下准直高能电子束的产生

利用二维PIC粒子模拟程序研究了超短超强激光脉冲与柱腔靶相互作用产生的表面超热电子加速现象。采用光强为1021 W/cm2量级的超高斯激光脉冲掠入射进入柱腔靶,在靶的内壁上观察到了GeV量级的表面超热电子。超热电子束被准静态的电场和磁场约束在内壁表面附近,保证了电子束的准直性,发散角仅为1.6°;并且由于超热电子束在纵向激光电场中加速了mm级的距离,激光到高能电子(100 MeV)的转换效率达到了26.6%。另外,通过多参数模拟和理论解析讨论了激光的光强以及横向空间分布对这种表面超热电子加速的影响。     

利用射频腔对激光加速质子能谱的优化

基于靶背鞘层加速机制（TNSA）产生的质子束具有宽能谱的特性,限制了其应用范围。为了产生准单能质子束,研究了基于直线加速器射频腔结构的质子能谱优化方法。在给定射频腔电压和频率情况下,计算了腔间距随优化能量的变化关系,并针对不同优化能量设计了不同大小的腔间距和腔数。在给定腔数情况下,发现只在某个能量附近可以获得单能性最好的单能峰。对能量接受范围进行了分析,要实现最终2%的能散,进入射频腔的质子束能散不能大于15%,并分析了射频腔频率对能量接受范围的影响。最后对PIC模拟得到的半高全宽为15%的一个能谱进行优化,获得了谱宽小于2%的准单色质子能谱。 .     

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。     

高品质激光尾波场电子加速器

激光尾波场电子加速的加速梯度相比于传统直线加速器高了3—4个量级,对于小型化粒子加速器与辐射源的研制具有重要的意义,成为当今国内外的研究热点.台式化辐射源应用需求的提高,特别是自由电子激光装置的快速发展,对电子束流品质提出了更高的要求,激光尾波场电子加速的束流品质和稳定性是目前实现新型辐射源的首要障碍.本文归纳整理了中国科学院上海光学精密机械研究所电子加速研究团队十年来在研制台式化激光尾波场电子加速器过程中采取的方案和取得的进展.例如率先提出了注入级和加速级分离的级联加速方案,通过实验获得了GeV量级的电子束能量;基于级联加速方式利用能量啁啾控制,实验获得世界最高品质的电子束流;通过优化激光系统稳定性和特殊的气体喷流结构,获得稳定的高品质电子束流输出等.这一系列实验结果有利于进一步推进激光尾波场电子加速器的应用.     

Generation of high-quality electron beams from a laser-based advanced accelerator

At Shanghai Jiao Tong University (SJTU) we have established a research laboratory for advanced acceleration research based on high-power lasers and plasma technologies. In a primary experiment based on the laser wakefield acceleration (LWFA) scheme, multi-hundred MeV electron beams of reasonable quality are generated using 20-40 TW, 30 femtosecond laser pulses interacting independently with helium, neon, nitrogen and argon gas jet targets. The laser-plasma interaction conditions are optimized for stabilizing the electron beam generation from each type of gas. The electron beam pointing angle stability and divergence angle as well as the energy spectra from each gas jet are measured and compared.     

Using Target Ablation for Ion Beam Quality Improvement

During the laser foil interaction,the output ion beam quality including the energy spread and beam divergence can be improved by the target ablation,due to the direct laser acceleration(DLA) electrons generated in the ablation plasma.The acceleration field established at the target rear by these electrons,which is highly directional and triangle-envelope,is helpful for the beam quality.With the help of the target ablation,both the beam divergence and energy spread will be reduced.If the ablation is more sufficient,the impact of DLA-electron-caused field will be strengthened,and the beam quality will be better,confirmed by the particle-in-cell simulation.     

鞘场加速机理中质子束的特性与其初始尺寸的关系

为了研究激光鞘场中质子层的尺寸对质子束特性的影响,本文应用中国工程物理研究院 激光聚变研究中心的二维Particle-In-Cell (2D-PIC)数值模拟程序Flips2D进行了相关数值模拟研究. 研究了质子束总能量随时间的变化,得出了加速持续过程与激光脉冲持续时间的关系; 研究了质子层的宽度对加速后质子束发散角和能谱的影响;研究了质子层的厚与加速后质子束 发散角和能谱的关系;得出了质子层的初始尺寸对加速后质子特性的影响规律.     

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。     

质子初始位置对激光平面靶相互作用加速质子的影响(英)

为了更细致地理解鞘场质子加速机制,应用2维数值模拟程序Flips2D研究了质子的初始位置对加速以后质子束特性的影响。数值模拟结果表明:质子的初始位置对质子束特性的影响非常明显。质子的出射角与其在横向的初始位置有关,初始位置离激光轴越远,其出射角越大。     

Tunable high-energy ion source via oblique laser pulse incident on a double-layer target

The laser-driven acceleration of high quality proton beams from a double-layer target, comprised of a high-Z ion layer and a thin disk of hydrogen, is investigated with three-dimensional particle-in-cell simulations for an obliquely incident laser pulse. The proton beam energy reaches its maximum at a certain incidence angle, where it can be much greater than the energy at normal incidence. The proton beam propagates at some angle with respect to the target surface normal and with some tilt around the target surface, as determined by the proton energy and the incidence angle.     

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.     

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.     

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.     

Ballistic focusing of polyenergetic protons driven by petawatt laser pulses

By using a thick (250 μm) target with 350 μm radius of curvature, the intense proton beam driven by a petawatt laser is focused at a distance of ～1 mm from the target for all detectable energies up to ～25 MeV. The thickness of the foil facilitates beam focusing as it suppresses the dynamic evolution of the beam divergence caused by peaked electron flux distribution at the target rear side. In addition, reduction in inherent beam divergence due to the target thickness relaxes the curvature requirement for short-range focusing. Energy resolved mapping of the proton beam trajectories from mesh radiographs infers the focusing and the data agree with a simple geometrical modeling based on ballistic beam propagation.     

Design of a 10 MeV normal conducting CW proton linac based on equidistant multi-gap CH cavities

Continuous wave(CW) high current proton linacs have wide applications as the front end of high power proton machines. The low energy part of such a linac is the most difficult and there is currently no widely accepted solution. Based on the analysis of the focusing properties of the CW low energy proton linac, a 10 Me V low energy normal conducting proton linac based on equidistant seven-gap Cross-bar H-type(CH) cavities is proposed. The linac is composed of ten 7-gap CH cavities and the transverse focusing is maintained by quadrupole doublets located between the cavities. The total length of the linac is less than 6 meters and the average acceleration gradient is about1.2 Me V/m. The electromagnetic properties of the cavities are investigated by Microwave Studio. At the nominal acceleration gradient the maximum surface electric field in the cavities is less than 1.3 times the Kilpatrick limit,and the Ohmic loss of each cavity is less than 35 k W. Multi-particle beam dynamics simulations are performed with Tracewin code, and the results show that the beam dynamics of the linac are quite stable, the linac has the capability to accelerate up to 30 m A beam with acceptable dynamics behavior.     

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.     

Comparison of ion acceleration from ultra-short intense laser interactions with thin foil and small dense target

The comparative efficiency and beam characteristics of high-energy ions generated from the interaction of a petawatt laser pulse with thin foil target and a small solid-density plasma bunch target have been studied by particle-in-cell simulation under identical conditions. It is shown that thin foil and small solid dense target of micrometer size can be efficiently accelerated when irradiated by a laser pulse of intensity >10²¹?W/cm². Using direct beam measurements, we find that small solid dense target acceleration produces higher energy particles with smaller divergence and a higher efficiency compared to thin foil target acceleration. The merits of small solid target acceleration can be exploited for potential applications such as its role as ignitor for fast ignition in inertial confinement fusion.     

Comparison of laser ion acceleration from the front and rear surfaces of thin foils

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers (approximately 1-6 x 10(19) W/cm2) from both the front and rear surfaces of thin metal foils have been measured under identical conditions. Using direct beam measurements and nuclear activation techniques, we find that rear-surface acceleration produces higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. Our observations are well reproduced by realistic particle-in-cell simulations, and we predict optimal criteria for future applications.