Thin Foil Targets Research Articles

Contrasting effect of high-Z coating on proton acceleration from thin transparent low-Z foil target

Abstract In the present work, we investigate the impact of a thin metallic coating on proton acceleration from transparent dielectric foils. A substantial difference in the experimental results is observed based on the placement of the thin metal coating - either on the front surface (the side facing the laser) or on the rear surface of the foil. In the former case, we observed an enhancement in both the energy and flux of the accelerated protons/ions. However, in the latter case, the thin metal coating was found to have a highly detrimental effect on the acceleration process. Measurements conducted on transmitted laser pulse signals confirm the crucial role of the intensity-dependent target material transparency and the laser pre-pulse playing vital roles in determining the proton/ion acceleration dynamics. Numerical simulations involving radiation hydrodynamics followed by two-dimensional particle-in-cell simulations confirm the experimental observations.

Laser-plasma accelerated proton beam transport system using high-field pulsed solenoid magnet

An energetic proton beam with a high divergence cone angle of ∼10°, generated by the interaction of a 25 fs, 800 nm Ti: Sapphire laser pulse with a thin metal foil target, was transported and focused at a specified distance from the source. A pulsed high-voltage solenoid, which generates a magnetic field of 5.7 T at a current of 8 kA, was developed in-house for this purpose. The profile of the proton beam was monitored online at various positions using a combination of a fast thin plastic scintillator and a CCD camera. The effect of solenoid misalignment on the focused proton beam was also investigated experimentally. In addition, the proton beam was transported out of the vacuum chamber into the ambient air for radiography application. A proton flux of ∼2 × 108 protons (energy >4 MeV) was transported and focused at a distance of 1.27 m from the target with a transmission efficiency of 30 % in a single shot. Detailed simulations incorporating particle tracing and transfer matrix method were performed to highlight the role of beam chromaticity and the impact of spherical harmonics on the focusing characteristics of the solenoid. The simulation results qualitatively reproduce the experimental observations. The solenoid provides energy-selective proton focusing for a broad energy proton beam. The proton beam was used for the radiography of reference meshes and has the potential to be used for radiobiological irradiation studies.

Isofield plasma expansion in kJ petawatt laser-driven ion acceleration with a tailored fast electron temperature

Kilojoule-class relativistic intensity lasers, having multi-picosecond (ps) pulse durations, enable efficient ion acceleration in the interaction with thin foil targets. The foil plasma expands under the laser energy input over picoseconds where fast electrons keep increasing their effective temperature, while they convert a part of the energy into fast ions through generation of a sheath electric field. The temporal evolution of the sheath electric field is the key to understanding the efficient ion acceleration seen in kJ-class laser experiments. Here, we extend the non-isothermal plasma expansion model by introducing a temporal function of the effective temperature of fast electrons to obtain the sheath electric field in the expanding plasma. We theoretically derived that when the effective temperature of fast electrons increases in proportional to the square of the time, the strength of the sheath electric field is kept constant without depletion during the expansion. This ‘isofield’ expansion is confirmed by a quasi-one-dimensional particle-in-cell simulation. The isofield expansion results in a high energy ion acceleration with a small expansion length, which is favorable for realizing an efficient ion acceleration with less lateral energy loss in multi-dimensional situations.

Synergistic enhancement of laser-proton acceleration with integrated targets

In proton acceleration from laser-irradiated thin foil targets, adding foams on the front surface or connecting a helical coil on the rear surface of the foil has proven to be an effective scheme to enhance proton energy. In this paper, we make the first attempt to incorporate the above two enhancement schemes for laser-proton acceleration by simultaneously adding foams and connecting a helical coil to a thin foil target. By utilizing such integrated targets in the experiment, focused beams were generated. The maximum proton energy and the number of energetic protons are apparently enhanced. Moreover, quasi-monoenergetic peaks were formed at the high-energy end of the spectra. Particle-in-cell plasma simulations and electromagnetic beam dynamics simulations show that the double-layer target not only enhances the energy of protons but also leads to a multiple-fold increase in the number of escaped electrons, which results in an enhanced post-acceleration in helical coil subsequently.

Hot electron emission characteristics from thin metal foil targets irradiated by terawatt laser

Hot electron emission characteristics from thin metal foil targets irradiated by terawatt laser

Experimental 7Be production cross section from the [formula omitted] reaction at Ep = 3.5 − 13 MeV

The Li7(p,n)7Be reaction is widely used as neutron source for neutron induced reaction cross section measurements, and for 7Be radioactive source production. There are two prominent structures in the excitation function, a narrow resonance between Ep=2.2−2.3 MeV, and a broad peak, around Ep=5 MeV. There are tension between the experimental data sets both in the position and the width of this latter structure, as well as in the absolute scale of the data. In the present work the Li7(p,n)7Be reaction is investigated using the activation technique, with the aim of providing comprehensive cross section data covering the second structure and connecting prior literature data sets. The irradiations were performed with the Atomki cyclotron accelerator with pairs of thin foil targets, thus with precisely controlled reaction energy in the range of Ep=3.5−13 MeV. After the irradiations the activity of the samples was measured using a high-purity germanium detector. The energy uncertainty of the new data points is much smaller than in any of the previous works, while the cross section uncertainty is comparable with the most precise literature data. A consistent data set was obtained connecting the most recent and most precise literature data sets. With the new data the absolute magnitude of the Li7(p,n)7Be reaction cross section is constrained and became more precise.

Diagnosis of ultrafast surface dynamics of thin foil targets irradiated by intense laser pulses.

The temporal modulation of an electron bunch train accelerated from a foil target irradiated by an intense laser pulse is studied by measuring the coherent transition radiation (CTR) from the rear surface of a target. We experimentally obtained CTR spectra from a 1 µm thick foil target irradiated at a maximum intensity of 6.5 × 1019 W/cm2. Spectral redshifts of the emitted radiation corresponding to increases in laser intensity were observed. These measurements were compared with the theoretical calculation of CTR spectra considering ultrafast surface dynamics, such as plasma surface oscillation and relativistically induced transparency. Plasma surface oscillations induce a spectral redshift, while relativistic transparency causes a spectral blueshift. Both effects are required to find reasonable agreement with the experiment over the entire range of laser intensities.

Addressing key aspects of J × B driven MeV fast electron generation in ultra-short ultra-intense laser foil interaction

Applicability of J × B mechanism of MeV fast electron generation is clearly demonstrated through observation of directed electron beam along laser propagation direction (simultaneous emission also along laser polarization/transverse directions) in the interaction of ∼25 fs laser pulse with thin foil target at an intensity of 1–7 × 1019 W/cm2. Fast electron temperature is found to be lower (higher) than ponderomotive for shorter (longer) preplasma scale lengths. Role of pre-acceleration of electrons in the rising part of the laser pulse is suggested for observed efficient J × B acceleration for p-polarization case. 2D particle in cell simulation also supports the above-mentioned observations.

Angle-differential cross sections for Rayleigh scattering of highly linearly polarized hard x rays on Au atoms

We perform a study on Rayleigh scattering of highly linearly polarized hard x rays on a thin Au foil target. In the study the angular distribution of the scattered radiation is analyzed in a relativistic regime both within and out of the plane of polarization of the incident beam. Within this experiment we scatter a synchrotron beam with a photon energy of 175 keV on a high-$Z$ target foil, namely, gold. Our findings correlate well with state-of-the-art calculations of the scattering process performed in the framework of quantum electrodynamics and may have considerable impact on future experiments regarding a polarization-resolved analysis of Delbr\"uck scattering. Furthermore, we show that the angular distribution of Rayleigh scattering can be used for a highly sensitive determination of the degree and orientation of the linear polarization of the incident hard-x-ray beam, if we rely on the theoretical framework.

Efficient ion acceleration by multistaged intense short laser pulses

We present a computational study of laser-driven ion acceleration that optimizes a combination of target transparency and extended field acceleration at moderate relativistic intensity. Our scheme applies two sequential laser pulses irradiating a thin target foil along the same direction: The first pulse drives a rapid expansion of the target, while the second one drives a quasistatic electric field in the expanding target with increasing electron temperature. In our particle-in-cell simulations we observe proton peak energies and numbers enhanced by factors of up to 3 compared with regular target-normal sheath acceleration.Received 6 October 2020Accepted 4 May 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.L032003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh-energy-density plasmasLaser driven ion accelerationAccelerators & BeamsPlasma PhysicsParticles & Fields

Preparation of isotopically enriched 112,116,120,124Sn targets at VECC

Resistive heating and mechanical rolling methods have been employed to prepare isotopically enriched thin target foils of 116Sn (∼380 μg/cm2), 124Sn (∼400 μg/cm2) and thicker foils of 112Sn (∼1.7 mg/cm2), 120Sn (∼1.6 mg/cm2), respectively. Preparation of enriched targets with a small amount of material, selection of releasing agent for thin targets and separation of deposited material in solvent were among the several challenges during fabrication of the thin targets. The uniformity of the targets has been measured using 241Am α-source. NaCl has been used as releasing agent in preparation of the thin targets. These targets have been successfully used in nuclear physics experiments at VECC.

Dynamics of hot refluxing electrons in ultra-short relativistic laser foil interactions

We investigate the dynamics of hot refluxing electrons in the interaction of an ultra-short relativistic laser pulse with a thin foil target via particle-in-cell (PIC) simulations, which is governed by the multidimensional spatiotemporal evolution of a self-generated sheath field. The comparison of time-integrated energy spectra of refluxing and escaping electrons indicates the refluxing efficiency is higher than 95% in average for each bounce. The characteristics of wide transverse spatial distribution and energy-resolved angular distribution caused by the refluxing electrons show a direct correlation with the angular-dependent photon yield of bremsstrahlung emission, as verified by the hybrid simulations of coupling the PIC results with Monte Carlo particle transport code. We further clarify the energy dissipation mechanisms of refluxing electrons through the recirculation in the thin target under the electron-refluxing dominated regime and conclude that the self-generated sheath field plays a dominant role over the competing processes, such as the radiation loss, collisional stopping, and anomalous inhibition via the resistive field. The lifetime of recirculation is calculated to be a few hundred femtoseconds, that is, one order of magnitude shorter than the timescale of collisional dissipation, while it is one order of magnitude longer than the laser pulse duration. The results could provide useful insights to understand the hot electron transport and stopping, secondary radiation generation, and ion acceleration in the high energy density plasmas.

Chemical conversions in lead thin films induced by heavy-ion beams at Coulomb barrier energies

Superheavy elements are produced via fusion reactions, by bombarding thin target foils with intense beams of energetic ions with energies around the Coulomb barrier. Currently, production of targets of rare isotopes relies mainly on the molecular plating (MP) method, which yields thin films of the desired target material on a backing foil. Intense heavy-ion beams are known to modify the film structure, which leads to deterioration of the targets over irradiation time. To gain a better understanding of the processes at a microscopic scale, comparative tests were carried out using Coulomb barrier heavy-ion beams provided from the UNILAC accelerator at GSI Darmstadt, Germany. For this purpose, targets of lead were prepared using MP and Drop-on-Demand inkjet-printing. Lead targets were baked-in and analysed by digital optical microscopy, scanning electron microscopy and confocal Raman spectroscopy.

Investigation of Proton Beam-Driven Fusion Reactions Generated by an Ultra-Short Petawatt-Scale Laser Pulse

We present results from a pitcher-catcher experiment utilizing a proton beam generated with nanostructured targets at a petawatt-class, short-pulse laser facility to induce proton-boron fusion reactions in a secondary target. A 45-fs laser pulse with either 400 nm wavelength and 7 J energy, or 800 nm and 14 J, and an intensity of up to 5 × 1021 W/cm2 was used to irradiate either thin foil targets or near-solid density, nanostructured targets made of boron nitride (BN) nanotubes. In particular, for 800 nm wavelength irradiation, a BN nanotube target created a proton beam with about five times higher maximum energy and about ten times more protons than a foil target. This proton beam was used to irradiate a thick plate made of boron nitride placed in close proximity to trigger 11B (p, α) 2α fusion reactions. A suite of diagnostics consisting of Thomson parabola ion spectrometers, postshot nuclear activation measurements, neutron time-of-flight detectors, and differentially filtered solid-state nuclear track detectors were used to measure both the primary proton spectrum and the fusion products. From the primary proton spectrum, we calculated (p, n) and (α,n) reactions in the catcher and compare with our measurements. The nuclear activation results agree quantitatively and neutron signals agree qualitatively with the calculations, giving confidence that primary particle distributions can be obtained from such measurements. These results provide new insights for measuring the ion distributions inside of proton-boron fusion targets.

Numerical investigation of spallation neutrons generated from petawatt-scale laser-driven proton beams

Laser-driven neutron sources could offer a promising alternative to those based on conventional accelerator technologies in delivering compact beams of high brightness and short duration. We examine this through particle-in-cell and Monte Carlo simulations that model, respectively, the laser acceleration of protons from thin-foil targets and their subsequent conversion into neutrons in secondary lead targets. Laser parameters relevant to the 0.5 PW LMJ-PETAL and 0.6–6 PW Apollon systems are considered. Owing to its high intensity, the 20-fs-duration 0.6 PW Apollon laser is expected to accelerate protons up to above 100 MeV, thereby unlocking efficient neutron generation via spallation reactions. As a result, despite a 30-fold lower pulse energy than the LMJ-PETAL laser, the 0.6 PW Apollon laser should perform comparably well both in terms of neutron yield and flux. Notably, we predict that very compact neutron pulses, of ∼10 ps duration and ∼100 μm spot size, can be released provided the lead convertor target is thin enough (∼100 μm). These sources are characterized by extreme fluxes, of the order of 1023 n cm−2 s−1, and even ten times higher when using the 6 PW Apollon laser. Such values surpass those currently achievable at large-scale accelerator-based neutron sources (∼1016 n cm−2 s−1), or reported from previous laser experiments using low-Z converters (∼1018 n cm−2 s−1). By showing that such laser systems can produce neutron pulses significantly brighter than existing sources, our findings open a path toward attractive novel applications, such as flash neutron radiography and laboratory studies of heavy-ion nucleosynthesis.

Radiation pressure acceleration of protons from structured thin-foil targets

The process of radiation pressure acceleration (RPA) of ions is investigated with the aim of suppressing the Rayleigh–Taylor-like transverse instabilities in laser–foil interaction. This is achieved by imposing surface and density modulations on the target surface. We also study the efficacy of RPA of ions from density modulated and structured targets in the radiation dominated regime where the radiation reaction effects are important. We show that the use of density modulated and structured targets and the radiation reaction effects can help in achieving the twin goals of high ion energy (in GeV range) and lower energy spread.

Radiative double-electron capture by fully stripped and one-electron ions in gas and thin-foil targets

Radiative double-electron capture (RDEC), in which two-electron capture is accompanied by simultaneous emission of a single photon, was investigated for fully stripped and one-electron projectiles colliding with gaseous and thin-foil targets. RDEC can be considered the inverse of double photoionization by a single photon. For the gaseous targets, measurements were done for 2.11 MeV/u ${\mathrm{F}}^{9+}$ and ${\mathrm{F}}^{8+}$ ions interacting with ${\mathrm{N}}_{2}$ and Ne, while for the thin-foil target the measurements were done for 2.11 MeV/u ${\mathrm{F}}^{9+}$ and ${\mathrm{F}}^{8+}$ and 2.19 MeV/u ${\mathrm{O}}^{8+}$ and ${\mathrm{O}}^{7+}$ ions striking thin C targets. Reports on this work were already published separately in shorter accounts by La Mantia et al. [Phys. Rev. Lett. 124, 133401 (2020) for the gas targets and Phys. Rev. A 102, 060801(R) (2020) for the thin-foil targets]. The gas targets were studied under single-collision conditions, while the foil targets suffered unavoidable multiple collisions. The measurements were carried out by detecting x-ray emission from the target at ${90}^{\ensuremath{\circ}}$ to the beam direction in coincidence with outgoing ions undergoing double, single, and, in the case of the foil targets, no charge change inside the target. Striking differences between the gaseous and foil targets were found from these measurements, with RDEC for the gaseous targets occurring only in coincidence with q-2 outgoing projectiles as expected, while RDEC for the foil targets was seen in each of the outgoing q-2, q-1, and no charge-change states. The no charge-change result was totally unexpected. The cross sections for RDEC for the fully stripped ions on gas targets were found to be about six times larger than those for the one-electron projectiles. For the foil targets, the RDEC cross sections for the fully stripped and one-electron projectiles differ somewhat from one another but not to the the extent they did for the gas targets. In this work the cross sections for all of the projectiles for the foil targets were adjusted due to the target contaminant background from potassium and calcium atoms that existed in the spectra. Also, the cross sections for the incident one-electron projectiles were modified due to a correction for the fraction of these ions that becomes fully stripped in passage through the foil. These differences are attributed to the effects of the multiple collisions that occur for the foil targets. The differential cross sections at ${90}^{\ensuremath{\circ}}$ determined for each of the projectiles interacting with each of the targets are compared with each other and with the previous measurements. To the extent that the cross sections follow a ${sin}^{2}\ensuremath{\theta}$ dependence, the total cross sections are compared with theoretical calculations [E. A. Mistonova and O. Yu. Andreev, Phys. Rev. A 87, 034702 (2013)], for which the agreement is poor, with the measured cross section exceeding the predicted ones by about an order of magnitude. Possible reasons for this discrepancy will be discussed.

Gamma-ray flash generation in irradiating a thin foil target by a single-cycle tightly focused extreme power laser pulse.

We present a regime where an ultraintense laser pulse interacting with a foil target results in high γ-photon conversion efficiency, obtained via three-dimensional quantum-electrodynamics particle-in-cell simulations. A single-cycle laser pulse is used under the tight-focusing condition for obtaining the λ^{3} regime. The simulations employ a radially polarized laser as it results in higher γ-photon conversion efficiency compared to both azimuthal and linear polarizations. A significant fraction of the laser energy is transferred to positrons, while a part of the electromagnetic wave escapes the target as attosecond single-cycle pulses.

Spin and polarization effects on the nonlinear Breit–Wheeler pair production in laser-plasma interaction

The spin effect of electrons/positrons (e −/e +) and polarization effect of γ photons are investigated in the interaction of two counter-propagating linearly polarized laser pulses of peak intensity 8.9 × 1023 W cm−2 with a thin foil target. The processes of nonlinear Compton scattering and nonlinear Breit–Wheeler pair production based on the spin- and polarization-resolved probabilities are implemented into the particle-in-cell (PIC) algorithm by Monte Carlo methods. It is found from PIC simulations that the average degree of linear polarization of emitted γ photons can exceed 50%. This polarization effect leads to a reduced positron yield by about 10%. At some medium positron energies, the reduction can reach 20%. Furthermore, we also observe that the local spin polarization of e −/e + leads to a slight decrease of the positron yield about 2% and some anomalous phenomena about the positron spectrum and photon polarization at the high-energy range, due to spin-dependent photon emissions. Our results indicate that spin and polarization effects should be considered in calculating the pair production and laser-plasma interaction with the laser power of 10 PW to 100 PW classes.

Fabrication of ZnO-nanowire-coated thin-foil targets for ultra-high intensity laser interaction experiments

The coupling of ultra-intense, ultra-short laser pulses with solid targets is heavily dependent on the properties of the vacuum–solid interface and is usually quite low. However, laser absorption can be enhanced via micro or nanopatterning of the target surface. Depending on the laser features and target geometry, conditions can be optimized for the generation of hot dense matter, which can be used to produce high-brightness radiation sources or even to accelerate particles to relativistic energies. In this context, ZnO nanowires were grown on metallic, thin-foil targets. The use of a thin-foil substrate was dictated by the need to achieve proton acceleration via target normal sheath acceleration at the rear side. The chemical process parameters were studied in-depth to provide control over the nanowire size, shape, and distribution. Moreover, the manufacturing process was optimized to provide accurate reproducibility of key parameters in the widest possible range and good homogeneity across the entire foil area.