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.     

Six MeV proton acceleration from plasma generated by high-intensity laser using advanced thin polyethylene targets

Proton acceleration can be induced by non-equilibrium plasma developed by high-intensity laser pulses, at 10¹⁶ W/cm², irradiating different types of thin polyethylene targets. The process of proton acceleration and directive yield emission was investigated, optimizing the laser parameters, the irradiation conditions, and the target properties. The use of 600 J pulse energy, a laser focalization inducing self-focusing effects and advanced targets with embedded nanoparticles and optimal thicknesses, has permitted to accelerate forward protons up to the energies of about 6 MeV and amount of the order of 10¹⁵ H⁺/pulse. High proton energy is obtained using thin foils enriched with gold nanoparticles, whereas high proton yield is obtained using targets with a thickness of about 10 μm. The plasma diagnostics using SiC semiconductor detectors in time-of-flight configuration was fundamental to monitor the optimal conditions to improve the plasma processes concerning the ion acceleration and the X-ray and relativistic electron emission.     

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。     

Protons and carbon ions acceleration in the target-normal-sheath-acceleration regime using low-contrast fs laser and metal-graphene targets

fs pulsed lasers at an intensity of the order of 10¹⁸ W/cm², with a contrast of 10⁻⁵, were employed to irradiate thin foils to study the target-normal-sheath-acceleration (TNSA) regime. The forward ion acceleration was investigated using 1/11 µm thickness foils composed of a metallic sheet on which a thin reduced graphene oxide film with 10 nm thickness was deposited by single or both faces. The forward-accelerated ions were detected using SiC semiconductors connected in time-of-flight configuration. The use of intense and long pre-pulse generating the low contrast does not permit to accelerate protons above 1 MeV because it produces a pre-plasma destroying the foil, and the successive main laser pulse interacts with the expanding plasma and not with the overdense solid surface. Experimental results demonstrated that the maximum proton energies of about 700 keV and of 4.2 MeV carbon ions and higher were obtained under the condition of the optimal acceleration procedure. The measurements of ion energy and charge states confirm that the acceleration per charge state is measurable from the proton energy, confirming the Coulomb–Boltzmann-shifted theoretical model. However, heavy ions cannot be accelerated due to their mass and low velocity, which does not permit them to be subjected to the fast and high developed electric field driving the light-ion acceleration. The ion acceleration can be optimized based on the laser focal positioning and on the foil thickness, composition, and structure, as it will be presented and discussed.     

Eight MeV per charge state from 300 ps laser ion acceleration by using micrometric foils

Proton acceleration using high-intensity laser pulses, at 10¹⁶ W/cm² was studied irradiating different types of thin metal and plastic targets having 1-micron thickness. The maximization of the proton energy process was investigated optimizing the laser parameters, the irradiation conditions and the target properties. Employing 600–700 J laser pulse energy, a focalization inducing self-focusing effects and using targets with optimized thickness, it was possible to accelerate protons up to energies of above 8 MeV. The time-of-flight diagnostics has allowed to monitor the plasma properties and to control the ion acceleration process.     

2.5-MeV neutron source controlled by high-intensity pulsed laser generating plasma

A laptop neutron source suited for the most demanding field or laboratory applications is presented. It is based on laser ablation of CD₂ primary targets, plasma acceleration of the D⁺ ions, and their irradiation of secondary CD₂ targets. The deuterium–deuterium (D-D) fusion reaction is induced in the secondary target, according to the values of fusion cross-section versus deuteron energy, which show a significant probability also at relatively low ion energies. The experiments were completed in the PALS laboratory, Prague, detecting monoenergetic neutrons at 2.45 MeV with an emission flux of about 10⁹ neutrons per laser shot. Other experiments demonstrating the possibility to induce D-D events were performed at IPPLM, Warsaw, and at INFN-LNS, Catania, where the deuterons were accelerated at about 4 MeV and 50 keV, respectively. In the last case, a low laser intensity and a post-ion acceleration system were employed. A special interaction chamber, under vacuum, is proposed to develop a new source of monochromatic neutrons or thermalized distribution of neutrons     

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.     

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.     

Ion acceleration from aluminium plasma generated by a femtosecond laser in different conditions

Non-equilibrium plasma was obtained by irradiating Al foils in vacuum with a femtosecond (fs) laser at intensities of the order of 10¹⁸ W/cm². Protons and other light ions were accelerated in the forward direction by using the target-normal-sheath acceleration regime. Time-of-flight technique was employed to measure the ions' kinetic energy using SiC detectors placed at known distances and angles. The ion acceleration was monitored under different conditions of laser focal position, laser pulse energy, and laser contrast. The target was irradiated using different thicknesses and anti-reflecting graphene films. By optimizing the laser parameters, irradiation conditions, and target properties, it was possible to accelerate up to 2.3 MeV per charge state, as will be presented and discussed.     

Generation of fast ions in femto-and picosecond laser plasmas at low intensities of the heating radiation

X-ray spectra from Teflon targets irradiated by laser pulses with a duration of 60 fs to 1 ps have been investigated experimentally. It is shown that, when the contrast of the laser pulse is sufficiently low, the effect of self-focusing of the main laser pulse in the plasma produced by the prepulse can significantly enhance the generation efficiency of fast particles. In this case, ions with energies as high as ～1 MeV are observed at relatively low laser intensities, q _las ≈ (4–6) × 10¹⁶ W/cm².     

Laser wiggler beat wave in a plasma

It is shown that by combining a laser wave and an electron beam propagating through a plasma inside a wiggler: (i) Electrons can be accelerated to high energies. For usual laser frequencies and wiggler wavelengths, plasma densities are in the range 10¹⁵–10¹⁶ cm^-3. The plasma density fluctuation in the longitudinal wave suffices to obtain electron energies of several hundred MeV over short distances. (ii) High frequency radiation can be amplified.     

Tantalum ion acceleration in laser‐generated plasma and dependence on the pulse duration

The laser irradiation of tantalum targets is presented for different pulsed laser intensities ranging from 10¹⁰ up to about 10¹⁸ W/cm² and pulse durations from 9 ns up to 40 fs. The results show that the produced non‐equilibrium plasma accelerates Ta ions in the backward direction from values of the order of keV up to values of about 5 MeV. In thin foils, the forward plasma, developed behind the target along the direction of incoming laser, at intensities of about 10¹⁶ W/cm² and 300 ps pulse duration, accelerates Ta ions at energies of the order of 4.6 MeV and produces charge states up to about 40+. For fs lasers at intensities of the order of 10¹⁸ W/cm², only proton acceleration occurs up to 2.1 MeV while no Ta ions are accelerated, due to the reduced duration of the electric field and to the too high inertial mass of the Ta ions.     

3 MeV质子辐照对AlGaN/GaN高电子迁移率晶体管的影响

研究了AlGaN/GaN 高电子迁移率晶体管(HEMT)的质子辐照效应. 在3 MeV质子辐照下, 当辐照剂量达到1× 10¹⁵ protons/cm²时, 漏极饱和电流下降了20%, 最大跨导降低了5%. 随着剂量增加, 阈值电压向正向漂移, 栅泄露电流增加. 在相同辐照剂量下, 1.8 MeV质子辐照要比3 MeV质子辐照退化严重. 从SRIM软件仿真中得到不同能量质子在AlGaN/GaN异质结中的辐射损伤区, 以及在一定深度形成的空位密度. 结合变频C-V测试结果进行分析, 表明了质子辐照引入空位缺陷可能是AlGaN/GaN HEMT器件电学特性退化的主要原因.     

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.     

MeV – proton emission from ultrafast laser-driven microparticles

Propagation of a high intensity (10¹⁹ W/cm²) ultrashort (35 fs) laser pulse through a cloud of water spheres (150 nm diameter) results in hot electron driven proton acceleration up to 1 MeV. It is suggested that during the propagation of the short pulse through the low density wing of the cloud, the leading pulse pedestal is reduced owing to absorption by the preplasma created. Then, the high-intensity peak of the pulse propagates through this underdense plasma and interacts with the high-density inner part of the cloud, which has not been transformed into an underdense plasma so that a sheath acceleration process at each individual microsphere can take place. The observed proton spectra show strong modulations, which are interpreted within the framework of a known fluid-expansion model incorporating two hot-electron populations with significantly different densities and temperatures. PACS 52.50.Jm; 52.38.Kd; 41.75.Jv     

Three-dimensional simulation of laser–plasma-based electron acceleration

A sequential three-dimensional (3D) particle-in-cell simulation code PICPSI-3D with a user friendly graphical user interface (GUI) has been developed and used to study the interaction of plasma with ultrahigh intensity laser radiation. A case study of laser–plasma-based electron acceleration has been carried out to assess the performance of this code. Simulations have been performed for a Gaussian laser beam of peak intensity 5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 1 × 10¹⁹ cm^− 3, and for a Gaussian laser beam of peak intensity 1.5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 3.5 × 10¹⁹ cm^− 3. The electron energy spectrum has been evaluated at different time-steps during the propagation of the laser beam. When the plasma density is 1 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~14 MeV, with an energy spread of ±7 MeV. On the other hand, when the plasma density is 3.5 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~23 MeV, with an energy spread of ±7.5 MeV.     

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.     

Laser-induced cavities and solitons in overcritical hydrogen plasma

A picosecond CO₂ laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼10²¹ cm⁻³, corresponding to laser wavelength λ = 1 μm. CO₂ laser offers a solution. The CO₂ laser’s 10 μm wavelength shifts the critical plasma density to 10¹⁹ cm⁻³, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO₂ laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.     

Particle-in-cell simulation for experimental ion acceleration by fs laser-generated plasma

ABSTRACT

Particle in cell simulation was applied to fit the measurements of protons and ions acceleration obtained using an fs laser pulse irradiating a thin foil in target-normal-sheath-acceleration regime. The simulation code calculates the maximum electrical field generated in the rear side of the target driving the forward ions acceleration. The electron density versus time and space, and the plasma temperatures are evaluated using a medium contrast laser at an intensity of about 10¹⁹?Wcm^?2. Proton acceleration above 10?MeV was experimentally measured using SiC detectors connected in time-of-flight configuration. A comparison between theoretical aspects and experimental data is reported and discussed.     

紧聚焦的飞秒激光脉冲在真空中对电子的加速

研究了紧聚焦的圆偏振飞秒相对论高斯激光脉冲与电子的相互作用，提出了一种激光加速电子的新机制.利用束腰小、强度大的激光脉冲上升沿加速电子，束腰大、强度小的脉冲下降沿减速电子，当光脉冲和电子分离时，电子获得了能量增益.研究发现，初始静止的电子与强度高于10¹⁹Wμm^{2/cm2的光脉冲作用以后，可以获得MeV量级的能量.初始位于焦点附近的电子被加速的效果较好，而远离焦点的电子几乎不能获得能量增益. 关键词： 电子加速 能量增益 高斯脉冲 束腰}