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.     

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.     

Cold electrons acceleration in TNSA laser-generated plasma using a low-contrast fs laser

The fs laser facility in Bordeaux, delivering an intensity of 10¹⁸ W/cm² at normal incidence on thin foils, has been used to induce forward electron and ion acceleration in target-normal-sheath-acceleration (TNSA) regime. Micrometric thin foils with different composition, thickness, and electron density, were prepared to promote the charge particle acceleration in the forward direction. The plasma electron and ion emission monitoring were performed on-line using SiC semiconductor detectors in time-of-flight (TOF) configuration and gaf-chromics films both covered by thin absorber filters. The experiment has permitted to accelerate electrons and protons. A special attention was placed to detect relativistic hot electrons escaping from the plasma and cold electrons returning to the target position. The electron energies of the order of 100 keV and of about 1 keV were detected as representative of hot and cold electrons, respectively. A high cold electron contribution was measured using low-contrast fs laser, while it is less evident using high-contrast fs lasers. The charge particle acceleration depends on the laser parameters, irradiation conditions, and target properties, as will be presented and discussed.     

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.     

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.     

Carbon-plasma produced in vacuum by 532 nm-3 ns laser pulses ablation

A study of VIS laser ablation of graphite, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation gives an emission mass spectrum attributable to C_n neutral and charged particles. Mass quadrupole spectroscopy, associated to electrostatic ion deflection, allows estimation of the velocity distributions of several of these emitting species within the plume as a function of the incident laser fluence. Time gated plume imaging and microscopy measurements have been used to study the plasma composition and the deposition of thin carbon films. The multi-component structure of the plume emission is rationalized in terms of charge state, ions temperature and neutrals temperature. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions. The use of nanosecond laser pulses, at fluences below 10 J/cm², produces interesting C-atomic emission effects, as a high ablation yield, a high fractional ionization of the plasma and presence of nanostructures deposited on near substrates.     

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     

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.     

Energy analysis of protons emitted from Nd:YAg laser-generated plasmas

Hydrogenated targets have been irradiated in vacuum with the pulsed Nd:YAg laser at intensities of the order of 10¹⁰ W/cm². The laser-generated plasma, produced by the interaction with the solid, emits protons and other ions along the normal to the target surface. Ion collectors and ion energy analyzer were used to measure the current, the angular emission and the energy distributions of the emitted protons. Time-of-flight measurements, Coulomb–Boltzmann-distributions and the fits of experimental data were also used in order to evaluate the equivalent ion plasma temperature and the ion acceleration developed in the non-equilibrium-pulsed plasma.     

Ion and photon emission from laser-generated titanium-plasma

Titanium-plasma was obtained by nanosecond pulsed laser ablation technique. A Nd:Yag laser was employed to irradiate titanium in vacuum. The ion emission from plasma was on-line monitored by an electrostatic ion energy analyzer which permitted to measure the ion kinetic energy and charge state. The visible photon emission was monitored by an optical spectrometer. The ion energy, charge state and angular distributions, the temperature and density of the non-equilibrium plasma were investigated. The temperature gradient of the plasma plume was evaluated and discussed.     

Non-equilibrium plasma production by pulsed laser ablation of gold

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and a maximum power density of the order of 10¹⁰?W/cm². The laser–target interaction produces a strong gold etching with production of a plasma in front of the target. The plasma contains neutrals and ions having a high charge state. Time-of-flight (TOF) measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits measurement of the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6⁺ and 10⁺ at a laser fluence of 100?J/cm² and 160?J/cm², respectively. The maximum ion energy reaches about 2?keV and 8?keV at these low and high laser fluences, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the authors have corrected for the Coulomb interactions occurring inside the plasma.     

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.     

Tungsten ions produced by infrared pulsed laser irradiation

Energetic ions have been obtained irradiating a tungsten target with a Q-switched Nd:Yag laser, 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and power density of the order of 10¹⁰?W/cm². The laser-target interaction induces a strong metal etching with production of plasma in front of the target. The plasma contains neutrals and ions with high charge state. Time-of-flight measurements are presented for qualitative analysis of the ion production. A cylindrical electrostatic ion analyzer permits measuring of the yield of emitted ions, the charge state of detected ions and the ion energy distribution. Measurements indicate that, at a laser fluence of the order of 100?J/cm², the charge state may reach 9+ and the ion energy reaches about 5?keV. The ion energy distribution is given as a function of the charge state. Experimental results indicate that an electrical field is developed along the normal to the plane of the target surface, which accelerates the ions up to high velocity. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the author has corrected for the Coulomb interactions occurring inside the plasma.     

Protons emission from thin doped polymeric films using pulsed laser

Protons production and acceleration via laser-generated plasma from thin Fe₂O₃ and carbon nano-tubes doped polyethylene films are investigated at relatively low laser pulse intensity, of the order of 10¹⁰ W/cm². Time-of-flight technique is employed in order to measure the proton energy and the relative yield with respect to the carbon one. Two ion collectors are used in opposite directions to detect the proton energy and yield both in backward and forward directions, normally to the irradiated target surface, as a function of the thin target doping concentration. The comparison between the results obtained with thin films doped with two nano-particle species is presented and discussed, with a special regard to the high proton emission.     

X-Rays emission by high-intensity pulsed lasers irradiating thin foils at PALS laboratory

X-rays and forward ion emission from laser-generated plasma in the Target Normal Sheath Acceleration regime of different targets with 10-μm thickness, irradiated at Prague Asterix Laser System (PALS) laboratory at about 10¹⁶ W/cm² intensity, employing a 1,315 nm-wavelength laser with a 300-ps pulse duration, are investigated. The photon and ion emissions were mainly measured using Silicon Carbide (SiC) detectors in time-of-flight configuration and X-ray streak camera imaging. The results show that the maximum proton acceleration value and the X-ray emission yield growth are proportional to the atomic number of the irradiated targets. The X-ray emission is not isotropic, with energies increasing from 1 keV for light atomic targets to about 2.5 keV for heavy atomic targets. The laser focal position significantly influences the X-ray emission from light and heavy irradiated targets, indicating the possible induction of self-focusing effects when the laser beam is focalized in front of the light target surface and of electron density enhancement for focalization inside the target.     

Laser-generated ns plasma pulses characterized using SiC Schottky diode

The nonequilibrium plasma generated by nanosecond laser pulse is characterized using a SiC detector connected in time-of-flight configuration to measure the radiations emitted from the plasma. Different metallic targets were irradiated by the pulsed laser at an intensity of 10¹⁰ W/cm² and 200 mJ pulse energy. The SiC allows detecting ultraviolet radiations and soft X-rays, electrons, and ions. The obtained plasma has a temperature of the order of tens to hundreds eV depending on the atomic number of the irradiated target and ion accelerations of the order of 100 eV per charge state.     

Experimental study of the acceleration of protons emitted from thin foils irradiated by ultrahigh-contrast laser pulses

Experimental results are presented for proton acceleration from the back of a target irradiated by laser pulses with intensities up to 2 × 10¹⁹ W/cm² generated by the SOKOL-P facility. The proton acceleration efficiency increases with decreasing of the target thickness. However, thin targets are destroyed by the amplified spontaneous emission (ASE) prepulse before the main pulse arrival. An additional optical switch based on a Pockels cell has been used in the amplification section to carry out the experiments with ultrathin foils. As a result, the energy contrast with respect to the ASE prepulse has been increased up to 4 × 10⁶. Owing to high contrast, the experiments on studying proton acceleration from foils with thicknesses less than 100 nm have been carried out.     

Laser‐Triggered Proton Acceleration From Micro‐Structured thin Targets

By using relativistic massively parallel PIC code MANDOR, which features arbitrary target design including 3D micro‐structuring, a study of ion acceleration in short laser pulse interaction with different thin targets has been performed. Based on 3D simulation results it has been shown that micro‐structures on the front surface of thin plane targets increase a number and energy of hot electrons in comparison with that for the case of pure plain foils of optimal thickness. As a result, the energy of accelerated ions also increases up to 50%. However, the efficiency of ion acceleration from structured target reduces with laser pulse intensity increase, so that for laser pulses of ultra‐relativistic intensity a positive role of surface micro‐structuring diminishes. We have also studied to which extent a sub‐ps imperfection of the laser pulse shape, which smoothes the surface micro‐structures suppresses high‐energy ion generation. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Production of ion and electron streams by pulsed-laser ablation of Ta and Cu

A Nd:YAG laser with 10⁹ W/cm ² pulse intensity, operating at 532 nm wavelength, is used to ablate Ta and Cu targets placed in vacuum. The ablation process generates a plasma in front of the target surface, which expands along the normal to target surface. The ion and electron emissions from the plasma were measured by Faraday cups placed at different angles with respect to the normal to target surface. In the range of laser intensities from 10⁷ to 10⁹ W/cm², the fast electron yield is lower than the ion yield and it increases at higher laser intensities. The ablation threshold, the emission yield, the ion and electron average energies and the plasma ion and electron temperatures were measured for ion and fast electron streams.     

Source-size measurements and charge distributions of ions accelerated from thin foils irradiated by high-intensity laser pulses

We report on measurements of source sizes and charge state distributions of ions accelerated from thin foils irradiated by ultrashort (100–300 fs) high-intensity (1-6×10¹⁹ W/cm²) laser pulses. The source sizes of proton and carbon ion beams originating from hydrocarbon contaminants on the surfaces of 5 m thick aluminum foils were investigated using the knife-edge method. For low-energy protons and low-carbon charge states, the source area was found to exceed the focal spot area by a factor of 10⁴. For the determination of charge state distributions, sandwich targets consisting of a 25 m thick tungsten layer, a 2-nm thin beryllium layer, and again a tungsten layer whose thickness was varied were used. These targets were resistively heated to remove the light surface contaminants. Peaked energy spectra of oxygen and argon ions corresponding to the equilibrium distribution after propagation through matter were observed. PACS 41.75.Jv; 52.38.Kd; 52.25.Jm; 52.50.Jm; 52.70.Nc; 41.75.Ak