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.     

Parity nonconservation in pα scattering

The elastic scattering of longitudinally polarized protons on ⁴He at energy E_p ? 15 MeV is considered. The difference of cross sections for protons of positive and negative helicities which is due to the weak interaction is calculated. The magnitude of the effect reaches 2–3 × 10^?7.     

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.     

Laser-triggered quasi-monoenergetic ion beams at a moderate intensity and pulse duration

Plasma produced by short laser pulses from thin homogeneous foils with light and heavy ions is capable of generating quasi-monoenergetic light ions. This happens for the tail of light ions near the front of heavy ions. It was found that this effect is well pronounced for a moderate laser intensity (～10¹⁸ W/cm²) and pulse duration (～1 ps) by using a 2D particle-in-cell simulation of the laser interaction with thin CD₂ foils. Quasi-monoenergetic deuterons form a jet from the rear side of the foil with the energy ～1 MeV. The conversion efficiency to these quasi-monoenergetic ions is 10^?3.     

强激光与高密度气体相互作用中电子和离子加速的数值模拟

在现有的一维粒子模拟程序的基础上发展了带光电离和碰撞电离及蒙特卡罗两体碰撞的模拟程序(1D PIC-MCC). 用此程序模拟研究了短脉冲激光与He气靶相互作用时电子和离子的加速过程. 研究表明当强激光与过临界密度的微米厚度的平面靶相互作用时，靶前表面物质将被激光脉冲前沿迅速离化；新生的电子被激光场有质动力加速成为高能电子，这些电子穿入到靶内，通过电子碰撞电离离化靶内物质；一部分高能电子穿透靶后，会在靶的后表面形成强的电荷分离场，该场迅速离化靶后表面物质，同时使得后表面离子得到加速. 部分穿透靶的超热电子将被电荷分离场重新拉回靶内，在靶的前后表面振荡. 一些振荡电子在此过程中得到电荷分离场加速，离开前表面，在前表面也形成电荷分离场，使前表面离子得到加速. 关键词： 激光等离子体 光电离和碰撞电离 电子加速 离子加速     

Energy calibration of a Δ<Emphasis Type="Italic">E</Emphasis>-<Emphasis Type="Italic">E</Emphasis> scintillation telescope in (α, p) reaction

A procedure for and the results from the energy calibration of a ΔE-E scintillation telescope used in experiments performed at the INR to study the nd-breakup reaction are described. The telescope was calibrated using a beam of α particles with an energy of 30 MeV of the U-120 cyclotron (INP). Secondary protons from the ^10,11B(α, p) reaction were recorded by the ΔE-E telescope at several recording angles and with the application of different absorbing foils. The calibration results from the ΔE-E telescope were obtained over the interval E _p = 10–30 MeV, allowing us to measure the energies of secondary protons in the ndbreakup reaction when the energy of primary neutrons is 20–60 MeV.     

The resonance-like structure observed in the <Superscript>40</Superscript>Ar(pγ)<Superscript>41</Superscript>K reaction

The excitation function of the ⁴⁰Ar(pγ)⁴¹K reaction in the energy range of accelerated protons E _p = 1.0–3.0 MeV was measured. Twelve new resonance states were revealed in the range E _p > 2.6 MeV. The strengths of all measured resonances were determined (for more than 200 states). A resonance-like structure with a center of gravity of 10.2 ± 0.5 MeV was identified; this agrees with the dependence of the center of gravity of the resonance-like structure found earlier on A in the nuclei of the sd shell.     

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.     

Structure and total strength of the magnetic dipole resonance in <Superscript>35</Superscript>Cl nuclei

The γ-decay of the resonance-like structure observed in the ³⁴S(pγ)³⁵Cl reaction in the energy range E _p = 1.0–3.0 MeV of accelerated protons was investigated. The M1 resonance on the ground and excited states of ³⁵Cl with E* = 1219 and 1763 keV was identified. The position of the center of gravity and total strength of the M1 resonance on ground state ³⁵Cl nucleus are determined. The position and total strength of the M1 resonance on the ground state in ³⁵Cl are explained taking into account pairing forces.     

Gamma-rays from the24Mg(p,γ)25Al reaction for proton energiesE p=800 keV toE p=1,830 keV

The yield curve and resonance strengths of resonances in the²⁴Mg(p, γ)²⁵Al reaction for the proton energy rangeE _p=800 keV toE _p=1,830 keV, have been determined and the existence of a resonance atE _p=1.184 MeV confirmed. Single energy spectra as well as decay schemes and branching ratios are given for these resonances. An upper level of 2.0×10^?15 sec for the lifetime of the 3.079 MeV level in²⁵Al results from a Doppler shift attenuation measurement carried out on the 2.624 MeV gamma line originating in the 3.070 MeV→0.455 MeV transition in²⁵Al.     

Instability-free ion acceleration by two laser pulses

We demonstrate the instability-free ion acceleration regime by introducing laser control with two parallel circularly polarized laser pulses at an intensity of I = 6.8 × 10²¹?W/cm², normally incident on a hydrogen foil. The special structure of the equivalent wave front of those two pulses, which contains Gaussian peaks in both sides and a concavity in the centre (2D), can suppress the transverse instabilities and hole boring effects to constrain a high density ion clump in the centre of the foil, leading to an acceleration over a long distance and gain above 1GeV/u for the ion bunches.     

Ion acceleration at the front and rear surfaces of thin foils with high-intensity 40-fs laser pulses

Ion acceleration from the front and rear sides of a foil target is observed by measurements of the ions’ spectral and spatial emission characteristics when irradiating the targets with ultrashort (40-fs) high-intensity laser pulses. The experimental results show that the origin of accelerated ions, from both the front and rear surfaces of the target, strongly depend on the laser energy absorption mechanism. In particular, laser pulse parameters such as pulse duration and contrast are crucial and determine the entire acceleration scenario. Thus, the experimental outcome can be controlled by selection of the irradiation conditions. The text was submitted by the authors in English.     

Spatial distribution of albedo particles on altitudes ∼500 km

Spatial distributions of energetic charged particles, neutrons and gamma rays at altitude 500 km (below the radiation belts of the Earth), obtained by the measurements of two apparatuses on board the Intercosmos-17 satellite, are presented. The latitudinal dependences, [i.e. the variation of flux with vertical cut-off rigidity of the measurement point], for neutrons (E _n = 1 –30 MeV), gamma rays (E =0·15–6 MeV), secondary electrons (E _e > 100 MeV) and for primary protons coming from the west and the east, respectively, are given. The main characteristic, the ratioN _p/N _e of the counting rate of the particles in the polar regionN _p(R_vert< <0·1 GeV/c) and on the equatorN _e(R_vert > 16 GeV/c), is obtained for the various types of particles. This value is 10 for neutrons, 3.7 for gamma rays, 1·8 for electrons, 11 for protons in westward direction, 10 for protons in eastward direction. The latitude profile of neutrons and gamma rays is in a good agreement with calculations assuming their production by nuclear reactions of primary cosmic rays with nuclei of the atmosphere. The weakening of rigidity dependence of protons coming from east in comparison with those coming from the west, is interpreted as the cause of additional proton albedo flux. The equality of albedo electron fluxes (E_e = 100–3500 MeV) from these directions is observed. With the use of the shadowing effect the obtained data on electron-positron component are consistent with the flux of albedo positrons (E_e + > 3·5 GeV) of the value (0·5±0·2) m^–2. s^–1. ster^–1. The possibility of abundance of albedo positrons above electrons at these altitudes for the energy intervalE=0·2÷0·3 GeV is indicated.     

Forward ion acceleration in thin films driven by a high-intensity laser

A collimated beam of fast protons, with energies as high as 1.5 MeV and total number of greater, similar10(9), confined in a cone angle of 40 degrees +/-10 degrees is observed when a high-intensity high-contrast subpicosecond laser pulse is focused onto a thin foil target. The protons, which appear to originate from impurities on the front side of the target, are accelerated over a region extending into the target and exit out the back side in a direction normal to the target surface. Acceleration field gradients approximately 10 GeV/cm are inferred. The maximum proton energy can be explained by the charge-separation electrostatic-field acceleration due to "vacuum heating."     

Energieverteilung und Wirkungsquerschnitt der Photoprotonen aus N15

The energy spectra of photo protons from highly enriched N¹⁵ were investigated with the bremsstrahlung from the Heidelberg betatron running at six different endpoint energies between 19 and 30.5 MeV. The protons were detected at 90° to theγ-beam by means of a CsJ-spectrometer with pulse shape discrimination. The energy spectra show pronounced maxima atE _p =3.2; 4.6; 9.5 and 13.3 MeV. Proton yields are given as a function of endpoint energy, the yield value atE ₀=30.5 MeV being (7.0±0.8) μb/MeV · ster. Because the first excited state in the daughter nucleus C¹⁴ lies 6.09 MeV above the groundstate, the cross section for groundstate transitions of the process N¹⁵(γ, p)C¹⁴ could be derived from the upper 6 MeV of the single proton spectra. The main contribution to the cross section comes from the region between 18 and 26 MeV excitation energy with maxima at 19.5; 20.4; 22.7 and 24.5 MeV. A “pygmy resonance” occurs at 15.2 MeV with further less pronounced structures at 13.6 and 17.0 MeV. The integrated cross section for groundstate transitions up to 30.5 MeV is (22±3) MeVmb assuming isotropic angular distribution. The ratios of protons from transitions to excited states and from the (γ, n p)-reaction to those of groundstate transitions rise from 0.45 atE ₀=24.5 MeV to 0.70 atE ₀=30.5 MeV.     

Additional production of deuterium and helium-3 by high-energy protons and antiprotons in the early Universe

The results of calculations of ³He, ³H, and D production as a result of the disintegration of primordial ⁴He by high-energy protons and anti-protons in the early Universe are reported. It is shown that for primary particle energies E _p , _p ^t- <0.2 GeV the main role in ³He, ³H, and D production is played by the secondary proton cascade that develops in the cosmological plasma and which destroys ⁴He most intensively at E~75 MeV. At the current state of the experimental data one can calculate the number of nuclei produced to within 10%, and investigation of the inelastic p ⁴He interaction channels at E~75 MeV will make it possible to improve substantially the accuracy of calculations of the additional production of light elements. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 609–613 (May 1999)     

Collective acceleration of ions in a system with an insulated anode

An investigation was made of the processes of collective acceleration of protons in vacuum in a system with an insulated anode and trans-anode electrodes, which were insulated or grounded, in high-current Tonus and Vera electron accelerators. The influence of external conditions and parameters of the electron beam on the efficiency of acceleration processes was investigated. Experiments were carried out in which protons were accelerated in a system with trans-anode electrodes. A study was made of the influence of a charge prepulse and of the number of trans-anode electrodes on the energy of the accelerated electrons. A system with a single anode produced N_p=10¹⁴ protons of 2E_e < E_p < 3E_e energy. Suppression of a charge prepulse increased the proton energy to (6–8)E_e and the yield was then 10¹³. The maximum proton energy of 14E_e was obtained in a system with three trans-anode electrodes. A possible mechanism of proton acceleration was analyzed. The results obtained were compared with those of other investigations. Ways of increasing the efficiency of this acceleration method were considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 53–58, November, 1980.The authors are grateful to A. I. Arbuzov and V. G. Tolmacheva for their great help in the measurements, and to Yu. G. Yushkov and Yu. P. Yusov for valuable comments and discussions of the results of the experiments.     

First experimental evidence of D(p, γ)3He reaction in titanium deuteride in ultralow collision energy region

The article is devoted to the investigation of a pd-reaction (p + d → ³He + γ(5.5 MeV)) under-going in titanium deuteride in astrophysical collision energy region of protons and deuterons ranging from 5.3 to 10.5 keV. The experiments have been performed using the Hall NR TPU (Tomsk, Russia) pulsed plasma accelerator. The number of accelerated protons in 10 μs pulse was 5 × 10¹⁴ at a repetition rate of 7 × 10^?2 Hz. Detection of 5.5 MeV gamma rays was carried out using eight detectors based on crystals of NaI(Tl) (100 × 100 × 400 mm) placed around the TiD target. For the first time, the dependencies between the astrophysical S-factor and the effective cross section of the pd-reaction from proton-deuteron collision energy, and the potential electronic screening of the interacting protons and deuterons in titanium deuteride have been measured.     

Introducing the fission–fusion reaction process: using a?laser-accelerated Th beam to produce neutron-rich nuclei towards the N=126 waiting point of the r-process

We propose to produce neutron-rich nuclei in the range of the astrophysical r-process (the rapid neutron-capture process) around the waiting point N=126 (Kratz et al. in Prog. Part. Nucl. Phys. 59:147, 2007; Arnould et al. in Phys. Rep. 450:97, 2007; Panov and Janka in Astron. Astrophys. 494:829, 2009) by fissioning a dense laser-accelerated thorium ion bunch in a thorium target (covered by a polyethylene layer, CH₂), where the light fission fragments of the beam fuse with the light fission fragments of the target. Using the ‘hole-boring’ (HB) mode of laser radiation pressure acceleration (RPA) (Robinson et al. in Plasma Phys. Control. Fusion 51:024004, 2009; Henig et al. in Phys. Rev. Lett. 103:245003, 2009; Tajima et al. in Rev. Accel. Sci. Technol. 2:221, 2009) using a high-intensity, short pulse laser, bunches of ²³²Th with solid-state density can be generated very efficiently from a Th layer (ca. 560 nm thick), placed beneath a deuterated polyethylene foil (CD₂ with ca. 520 nm), both forming the production target. Th ions laser-accelerated to about 7 MeV/u will pass through a thin CH₂ layer placed in front of a thicker second Th foil (both forming the reaction target) closely behind the production target and disintegrate into light and heavy fission fragments. In addition, light ions (d,C) from the CD₂ production target will be accelerated as well to about 7 MeV/u, also inducing the fission process of ²³²Th in the second Th layer. The laser-accelerated ion bunches with solid-state density, which are about 10¹⁴ times more dense than classically accelerated ion bunches, allow for a high probability that generated fission products can fuse again when the fragments from the thorium beam strike the Th layer of the reaction target.     

Generation of MeV photons and protons in laser picosecond plasmas

The results of experiments devoted to studying the generation of MeV photons and protons in picosecond laser plasmas at a laser beam intensity of 10¹⁸ W/cm² on Be, Ta, LiF and H⁷Li targets are presented. Nuclear reactions (γ, n) and (p, n) were used to detect MeV photons and protons. The number of MeV photons and protons generated in laser plasmas was found from the measured neutron yield. Possibilities of particle acceleration due to the formation of pinch structures in laser plasmas are discussed. Calculated and experimental results are compared.