Laser-accelerated Proton Research Articles

Improve beam quality of laser proton acceleration with funnel-shaped-hole target

Improve beam quality of laser proton acceleration using a funnel-shaped-hole target is demonstrated through particle simulations. When an intense short pulse laser illuminates a thin foil target with a hole at the rear surface, the proton beam divergence is suppressed compared with that obtained in a traditional flat target. In this paper, a funnel-shaped-hole target is proposed to improve the proton beam quality. Using two-dimensional particle-in-cell (PIC) simulations, three different shapes of target (funnel-shaped-hole target, cylinder-shaped-hole target and flat target) are simulated and compared. The funnel-shaped hole in the rear surface of the target helps to focus the electron cloud significantly and improve the maximum proton energy and suppress the proton beam divergence. Different thicknesses of the new target are also simulated, and the effects of thickness on the divergence angle and proton spectra are investigated. The optimal size of the new target is obtained and the quality of the proton beam is improved significantly. The funnel-shaped-hole target serves as a new method to improve the proton beam quality in laser–plasma interactions.

Ion acceleration and plasma jet formation in ultra-thin foils undergoing expansion and relativistic transparency

At sufficiently high laser intensities, the rapid heating to relativistic velocities and resulting decompression of plasma electrons in an ultra-thin target foil can result in the target becoming relativistically transparent to the laser light during the interaction. Ion acceleration in this regime is strongly affected by the transition from an opaque to a relativistically transparent plasma. By spatially resolving the laser-accelerated proton beam at near-normal laser incidence and at an incidence angle of 30°, we identify characteristic features both experimentally and in particle-in-cell simulations which are consistent with the onset of three distinct ion acceleration mechanisms: sheath acceleration; radiation pressure acceleration; and transparency-enhanced acceleration. The latter mechanism occurs late in the interaction and is mediated by the formation of a plasma jet extending into the expanding ion population. The effect of laser incident angle on the plasma jet is explored.

Manipulation of the spatial distribution of laser-accelerated proton beams by varying the laser intensity distribution

We report on a study of the spatial profile of proton beams produced through target normal sheath acceleration using flat target foils and changing the laser intensity distribution on the target front surface. This is done by either defocusing a single laser pulse or by using a split-pulse setup and irradiating the target with two identical laser pulses with variable spatial separation. The resulting proton beam profile and the energy spectrum are recorded as functions of the focal spot size of the single laser pulse and of the separation between the two pulses. A shaping of the resulting proton beam profile, related to both an increase in flux of low-energy protons in the target normal direction and a decrease in their divergence, in one or two dimensions, is observed. The results are explained by simple modelling of rear surface sheath field expansion, ionization, and projection of the resulting proton beam.

The stopping power of relativistic electrons and laser-accelerated proton beams for fast ignition of DT and D3He and P11B fuels

The understanding of basic physics of processes associated with the beam in fast ignition is very important. In this paper energy deposition of 1 MeV relativistic electrons study by calculating stopping power in D3He plasma with uniform density 300 g cm−3 and compare with DT plasma in identical condition. The results show energy deposition in D3He plasma is more than DT plasma. Scattering due to of background electrons decreases the penetration depth from 0.38 to 0.24 g cm−2 in D3He plasma and from 0.51 to 0.33 g cm−2 in DT plasma. Nuclear reaction of protons with the boron-11 nuclei (p–11B) is the most promising of all the reactions that can be used for completely neutronless power generation in a fusion reactor. The level of radioactivity associated with concomitant and secondary reactions is negligible, while for D–T reactor 80% of the fusion power is released in neutrons and neutron yield of D–3He is about 5%. Note, D–3He reaction is neutronless, but neutrons are born as a result of D–D branch and the secondary reaction of tritium and deuterium. From a technical point of view the lack of neutrons in the reactor mixture p–11B is very attractive because it removes a major problem of the first wall. Therefore in this paper proton beam is proposed in fast ignition approach using P11B fuel at uniform density 300gcm−3 with considering Maxwellian energy distribution. The stopping power and total range of proton beam are calculated. The rate of energy transfer between ions and electrons and bremsstrahlung loss power with considering relativistic corrections in P11B fuel are exanimated. The results show that total stopping power of protons in P11B fuel at different temperatures is a function of proton energy and it decreases with rising temperature at constant EP. Bonus energy and total deposited energy is significant at smallerEP and higher Te.

Study of the production yields of 18F, 11C, 13N and 15O positron emitters from plasma-laser proton sources at ELI-Beamlines for labeling of PET radiopharmaceuticals

The development of novel compact PET radionuclide production systems is of great interest to promote the diffusion of PET diagnostics, especially in view of the continuous development of microfluidics labeling approaches. We studied the feasibility to produce clinically-relevant amounts of PET isotopes by means of laser-accelerated proton sources such that expected at the ELI-Beamlines facility. 18F, 11C, 13N and 15O production yields were calculated through the TALYS software, by taking into account the broad proton spectra expected. With the hypothesized proton fluencies, clinically-relevant amounts of radionuclides can be obtained, suitable to prepare single doses of 18F-, 11C- and 13N-labeled radiopharmaceuticals exploiting fast and efficient microfluidic labeling systems.

Calibration and Performance Tests of Detectors for Laser-Accelerated Protons

We present the calibration and performance tests carried out with two detectors for intense proton pulses accelerated by lasers. Most of the procedures were realized with proton beams of 0.46–5.60 MeV from a tandem accelerator. One approach made use of radiochromic films, for which we calibrated the relation between optical density and energy deposition over more than three orders of magnitude. The validity of these results and of our analysis algorithms has been confirmed by controlled irradiation of film stacks and reconstruction of the total beam charge for strongly non-uniform beam profiles. For the spectral analysis of protons from repeated laser shots, we have designed an online monitor based on a plastic scintillator. The resulting signal from a photomultiplier directly measured on a fast oscilloscope is especially useful for time-of-flight applications. Variable optical filters allow for suppression of saturation and an extension of the dynamic range. With pulsed proton beams we have tested the detector response to a wide range of beam intensities from single particles to $3 \times {10^5}$ protons per 100 ns time interval.

Dynamics and structure of self-generated magnetics fields on solids following high contrast, high intensity laser irradiation

The dynamics of self-generated magnetic B-fields produced following the interaction of a high contrast, high intensity (I > 1019 W cm−2) laser beam with thin (3 μm thick) solid (Al or Au) targets is investigated experimentally and numerically. Two main sources drive the growth of B-fields on the target surfaces. B-fields are first driven by laser-generated hot electron currents that relax over ∼10–20 ps. Over longer timescales, the hydrodynamic expansion of the bulk of the target into vacuum also generates B-field induced by non-collinear gradients of density and temperature. The laser irradiation of the target front side strongly localizes the energy deposition at the target front, in contrast to the target rear side, which is heated by fast electrons over a much larger area. This induces an asymmetry in the hydrodynamic expansion between the front and rear target surfaces, and consequently the associated B-fields are found strongly asymmetric. The sole long-lasting (>30 ps) B-fields are the ones growing on the target front surface, where they remain of extremely high strength (∼8–10 MG). These B-fields have been recently put by us in practical use for focusing laser-accelerated protons [B. Albertazzi et al., Rev. Sci. Instrum. 86, 043502 (2015)]; here we analyze in detail their dynamics and structure.

Simulations of radiation pressure ion acceleration with the VEGA Petawatt laser

The Spanish Pulsed Laser Centre (CLPU) is a new high-power laser facility for users. Its main system, VEGA, is a CPA Ti:Sapphire laser which, in its final phase, will be able to reach Petawatt peak powers in pulses of 30fs with a pulse contrast of 1:1010 at 1ps. The extremely low level of pre-pulse intensity makes this system ideally suited for studying the laser interaction with ultrathin targets. We have used the particle-in-cell (PIC) code OSIRIS to carry out 2D simulations of the acceleration of ions from ultrathin solid targets under the unique conditions provided by VEGA, with laser intensities up to 1022Wcm−2 impinging normally on 20–60nm thick overdense plasmas, with different polarizations and pre-plasma scale lengths. We show how signatures of the radiation pressure-dominated regime, such as layer compression and bunch formation, are only present with circular polarization. By passively shaping the density gradient of the plasma, we demonstrate an enhancement in peak energy up to tens of MeV and monoenergetic features. On the contrary linear polarization at the same intensity level causes the target to blow up, resulting in much lower energies and broader spectra. One limiting factor of Radiation Pressure Acceleration is the development of Rayleigh–Taylor like instabilities at the interface of the plasma and photon fluid. This results in the formation of bubbles in the spatial profile of laser-accelerated proton beams. These structures were previously evidenced both experimentally and theoretically. We have performed 2D simulations to characterize this bubble-like structure and report on the dependency on laser and target parameters.

Guiding and collimation of laser-accelerated proton beams using thin foils followed with a hollow plasma channel

It is proposed that guided and collimated proton acceleration by intense lasers can be achieved using an advanced target—a thin foil followed by a hollow plasma channel. For the advanced target, the laser-accelerated hot electrons can be confined in the hollow channel at the foil rear side, which leads to the formation of transversely localized, Gaussian-distributed sheath electric field and resultantly guiding of proton acceleration. Further, due to the hot electron flow along the channel wall, a strong focusing transverse electric field is induced, taking the place of the original defocusing one driven by hot electron pressure in the case of a purely thin foil target, which results in collimation of proton beams. Two-dimensional particle-in-cell simulations show that collimated proton beams with energy about 20 MeV and nearly half-reduced divergence of 26° are produced at laser intensities 1020 W/cm2 by using the advanced target.

CR-39 track detector calibration for H, He, and C ions from 0.1-0.5 MeV up to 5 MeV for laser-induced nuclear fusion product identification.

Laser-accelerated ion beams can be used in many applications and, especially, to initiate nuclear reactions out of thermal equilibrium. We have experimentally studied aneutronic fusion reactions induced by protons accelerated by the Target Normal Sheath Acceleration mechanism, colliding with a boron target. Such experiments require a rigorous method to identify the reaction products (alpha particles) collected in detectors among a few other ion species such as protons or carbon ions, for example. CR-39 track detectors are widely used because they are mostly sensitive to ions and their efficiency is near 100%. We present a complete calibration of CR-39 track detector for protons, alpha particles, and carbon ions. We give measurements of their track diameters for energy ranging from hundreds of keV to a few MeV and for etching times between 1 and 8 h. We used these results to identify alpha particles in our experiments on proton-boron fusion reactions initiated by laser-accelerated protons. We show that their number clearly increases when the boron fuel is preformed in a plasma state.

Temporal Narrowing of Neutrons Produced by High-Intensity Short-Pulse Lasers.

The production of neutron beams having short temporal duration is studied using ultraintense laser pulses. Laser-accelerated protons are spectrally filtered using a laser-triggered microlens to produce a short duration neutron pulse via nuclear reactions induced in a converter material (LiF). This produces a ∼3 ns duration neutron pulse with 10(4) n/MeV/sr/shot at 0.56 m from the laser-irradiated proton source. The large spatial separation between the neutron production and the proton source allows for shielding from the copious and undesirable radiation resulting from the laser-plasma interaction. This neutron pulse compares favorably to the duration of conventional accelerator sources and should scale up with, present and future, higher energy laser facilities to produce brighter and shorter neutron beams for ultrafast probing of dense materials.

Self-Consistent Simulation of Transport and Energy Deposition of Intense Laser-Accelerated Proton Beams in Solid-Density Matter.

The first self-consistent hybrid particle-in-cell (PIC) simulation of intense proton beam transport and energy deposition in solid-density matter is presented. Both the individual proton slowing-down and the collective beam-plasma interaction effects are taken into account with a new dynamic proton stopping power module that has been added to a hybrid PIC code. In this module, the target local stopping power can be updated at each time step based on its thermodynamic state. For intense proton beams, the reduction of target stopping power from the cold condition due to continuous proton heating eventually leads to broadening of the particle range and energy deposition far beyond the Bragg peak. For tightly focused beams, large magnetic field growth in collective interactions results in self-focusing of the beam and much stronger localized heating of the target.

Ion acceleration enhanced by target ablation

Laser proton acceleration can be enhanced by using target ablation, due to the energetic electrons generated in the ablation preplasma. When the ablation pulse matches main pulse, the enhancement gets optimized because the electrons' energy density is highest. A scaling law between the ablation pulse and main pulse is confirmed by the simulation, showing that for given CPA pulse and target, proton energy improvement can be achieved several times by adjusting the target ablation.

Feasibility study of using laser-generated neutron beam for BNCT

The feasibility of using a laser-accelerated proton beam to produce a neutron source, via (p,n) reaction, for Boron Neutron Capture Therapy (BNCT) applications has been studied by MCNPX Monte Carlo code. After optimization of the target material and its thickness, a Beam Shaping Assembly (BSA) has been designed and optimized to provide appropriate neutron beam according to the recommended criteria by International Atomic Energy Agency. It was found that the considered laser-accelerated proton beam can provide epithermal neutron flux of ∼2×106n/cm2 shot. To achieve an appropriate epithermal neutron flux for BNCT treatment, the laser must operate at repetition rates of 1kHz, which is rather ambitious at this moment. But it can be used in some BNCT researches field such as biological research.

On the feasibility of increasing the energy of laser-accelerated protons by using low-density targets

Optimal regimes of proton acceleration in the interaction of short high-power laser pulses with thin foils and low-density targets are determined by means of 3D numerical simulation. It is demonstrated that the maximum proton energy can be increased by using low-density targets in which ions from the front surface of the target are accelerated most efficiently. It is shown using a particular example that, for the same laser pulse, the energy of protons accelerated from a low-density target can be increased by one-third as compared to a solid-state target.

Optimization of laser acceleration of protons from mixed structure nanotarget

AbstractIn this study, ion acceleration from thin planar diamond-like carbon (DLC) and polystyrene (PS) foils irradiated by ultraintense (a0 = 200) and ultrashort (15 fs) laser pulses is investigated numerically. The effects of target composition and thickness on the acceleration of protons and carbon ions are reported by 1D3V particle-in-cell simulation code and compared with the analytical models of ion acceleration. In the analytical formalism, the acceleration criterion of ions with different charge-to-mass ratio (q/m) is obtained. This criterion is related to the potential difference through the electrostatic shock distortion and its velocity. According to this result, charged particles with large q/m ratio have a good chance to accelerate in front of the electrostatic shock field. It is shown that mono-energetic proton bunch with energies >1.5 GeV is produced by 20 nm DLC foil supported by 10 nm hydrogen layer. Finally nanometer PS foil is examined and 2.33 Gev protons with ~1.5% energy spread are obtained for 50 nm thickness.

Laser acceleration of protons using multi-ion plasma gaseous targets

We present a theoretical and numerical study of a novel acceleration scheme by applying a combination of laser radiation pressure and shielded Coulomb repulsion in laser acceleration of protons in multi-species gaseous targets. By using a circularly polarized CO2 laser pulse with a wavelength of 10 μm—much greater than that of a Ti: Sapphire laser—the critical density is significantly reduced, and a high-pressure gaseous target can be used to achieve an overdense plasma. This gives us a larger degree of freedom in selecting the target compounds or mixtures, as well as their density and thickness profiles. By impinging such a laser beam on a carbon–hydrogen target, the gaseous target is first compressed and accelerated by radiation pressure until the electron layer disrupts, after which the protons are further accelerated by the electron-shielded carbon ion layer. An 80 MeV quasi-monoenergetic proton beam can be generated using a half-sine shaped laser beam with a peak power of 70 TW and a pulse duration of 150 wave periods.

Effects of Laser Spot Sizes in Laser Driven Proton Therapy

We present a numerical study of the effect of the laser spot size of a circularly polarized laser beam on the energy of quasi-monoenergetic protons suitable for particle cancer therapy in laser proton acceleration using a thin carbon-hydrogen foil. The energy dosage distributions of impinging the generated proton beams into human tissues are also simulated, compared and analyzed. The used proton acceleration scheme is a combination of laser radiation pressure and shielded Coulomb repulsion due to the carbon ions. We observe that the spot size plays a crucial role in determining the net charge of the electron-shielded carbon ion foil and consequently the efficiency of proton acceleration. Using a laser pulse with fixed input energy and pulse length impinging on a carbon-hydrogen foil, a laser beam with smaller spot sizes can generate higher energy but fewer quasi-monoenergetic protons. We studied the scaling of the proton energy with respect to the laser spot size and obtained an optimal spot size for maximum proton energy flux. Using the optimal spot size, we can generate an 80 MeV quasi-monoenergetic proton beam containing more than a hundred million protons using a laser beam with power 250 TW and energy 10 J and a target made of 90% carbon and 10% hydrogen, capable of treating tumor cells with depth up to 5 cm in human bodies.

Characteristics and parameter optimization of electron beam radiography

The electron beam produced by an ultra-short, high-intensity laser pulse is of properties of small source size, short duration, and quasi-monoenergetic energy, and will play a unique role in radiographic diagnostics. By analyzing the scattering processes of electrons in materials and performing Monte-Carlo simulations, electron radiography for probing target surface non-uniformities or material interfaces is studied for electron energy ranging from 100 keV to several hundreds of MeV, and the results are compared with those of proton radiography and X-ray radiography, respectively. Features and parameter optimization of electron radiography are obtained, and some applications are suggested. By taking advantage of inelastic scattering or energy loss of charged particles, target surface nonuniformities could be diagnosed by a charged-particle beam whose range is close to the target thickness. Such a diagnosis would produce a higher detection contrast than that by absorption-type X-ray radiography. For a proton beam, a target thickness variation as small as 0.1% could be detected due to a more evident Bragg peak of the stopping power near its range. Nevertheless, the energy of laser-accelerated proton beams being up to 100 MeV would limit the applications. For an electron beam, since a thickness variation of 0.3% could be detected, its energy over 1 GeV has been realized by laser acceleration, the electron radiography could be extended to diagnose thicker targets. When using an electron beam to radiograph a thin or a foil target, for example, of thickness on the order of 100 μm, a spatial resolution of 11 μm or better could be achieved due to the reduced elastic scattering and angular deflection. By taking advantage of elastic scattering of electrons, an electron beam whose range is much greater than the target thickness could be used to diagnose a target interface composed of different materials or even a multilayered capsule, and a higher contrast of the electron fluence modulation at interfaces would be realized than that by absorption-type X-ray radiography, which is caused by stronger scattering of electrons as the electron scattering cross section is several orders of magnitude greater than that of X-ray scattering such as the Thomson scattering. As a laser-produced electron beam is prone to have an ultrafast pulse duration of 100’s of femtoseconds or less, it is anticipated that the electron radiography will produce an ultrasfast temporal resolution. These results and conclusions would be helpful to the applications and parameter optimization of electron radiography.

Tunable mega-ampere electron current propagation in solids by dynamic control of lattice melt.

The influence of lattice-melt-induced resistivity gradients on the transport of mega-ampere currents of fast electrons in solids is investigated numerically and experimentally using laser-accelerated protons to induce isochoric heating. Tailoring the heating profile enables the resistive magnetic fields which strongly influence the current propagation to be manipulated. This tunable laser-driven process enables important fast electron beam properties, including the beam divergence, profile, and symmetry to be actively tailored, and without recourse to complex target manufacture.