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.

Efficient laser-driven proton acceleration from a petawatt contrast-enhanced second harmonic mixed-glass laser system

Efficient laser-driven plasma acceleration of ion beams requires precision control of the target–plasma profile, which is crucial to optimize the laser energy transfer. Along the laser propagation direction, this can be achieved by tailoring the temporal structure of the laser pulse. We show for the first time that frequency-doubling of a short pulse (hundreds-femtosecond range) petawatt-class mixed-glass laser system, which results in temporal intensity contrast enhancement, enables surface and volumetric laser–energy coupling, and the acceleration of proton beams from few-nanometer-thick foil targets. Experimentally, maximum ion energies and laser-to-proton energy conversion efficiencies were found to be both maximized at optimum laser and target conditions manifested when the normalized target density nearly equalizes the normalized laser vector potential, which is in agreement with theory and simulations. These signatures are recognized as a unique indication of the interaction between ultra-intense laser pulses with high temporal intensity contrast and ultra-thin nanometer-scale targets. Transverse modulations of accelerated proton beams in the form of bubble- and ring-like structures measured in the thinnest targets provide additional evidence of volumetric laser-driven particle acceleration regimes and transitional features in ultra-thin foil targets specific to laser–plasma interactions characterized by a high temporal intensity contrast. These results open avenues in the generation of high contrast laser pulses from short-pulse-femtosecond petawatt mixed-glass laser systems and demonstrate the feasibility of this technique for applications requiring high laser intensity contrast with high efficiency.

Generation of Isolated Subfemtosecond Electron Bunches by the Diffraction of a Polarization-Tailored Intense Laser Beam.

We propose utilizing a polarization-tailored high-power laser pulse to extract and accelerate electrons from the edge of a solid foil target to produce isolated subfemtosecond electron bunches. The laser pulse consists of two orthogonally polarized components with a time delay comparable to the pulse duration, such that the polarization in the middle of the pulse rapidly rotates over 90° within few optical cycles. Three-dimensional particle-in-cell simulations show that when such a light pulse diffracts at the edge of a plasma foil, a series of isolated relativistic electron bunches are emitted into separated azimuthal angles determined by the varying polarization. In comparison with most other methods that require an ultrashort drive laser, we show the proposed scheme works well with typical multicycle (∼30 fs) pulses from high-power laser facilities. The generated electron bunches have typical durations of a few hundred attoseconds and charges of tens of picocoulombs.

Investigation on the intensity contrast of Kα line emission from laser–matter interactions

The intensity contrast and its angular distribution of Kα line emission originated from the difference of angular distributions of Kα and bremsstrahlung emissions from copper foil targets bombarded by electrons similar to the hot electrons generated in laser–matter interactions are investigated by Monte Carlo simulations. For mono-energetic electron incidences, a higher contrast Kα emission is generated at large detection angles relative to the incident electron direction and for higher electron energy. The Kα emission contrast is decreased with the increase in target thickness. When the areal density of targets is fixed, the contrast is almost unchanged with the change of target density and thickness. For incident electrons with a Boltzmann energy distribution, a higher contrast Kα emission can also be generated at large detection angles and for higher electron temperatures, but the contrast is lower compared to that for mono-energetic electron incidences, and it is changed only slightly with the increase in target thickness. These results help to understand the contrast of Kα emissions in previous experiments. Suggestions are proposed for future laser–matter interaction experiments for higher contrast Kα emissions.

Realizing laser-driven deuteron acceleration with low energy spread via in situ D2O-deposited target

Generation of quasi-monoenergetic ion pulse by laser-driven acceleration is one of the hot topics in laser plasma physics. In this study, we present a new method for the in situ deposition of an ultra-thin D2O layer on the surface of an aluminum foil target utilizing a spherical D2O capsule. Employing a 1019 W/cm2 laser, we achieve the acceleration of 10.8 MeV deuterons with an energy spread of ΔE/E = 4.6% in the most favorable shot. The energy spread depends on the exposure time of the D2O capsule in the vacuum chamber. This method has the potential to extend its applicability to other ion species.

High energy neutral atom emission from rear side of foils

Intense laser pulses focused onto a solid micron foil target are best known for generation of large electrostatic sheath fields at the target rear and acceleration of protons to MeV energies. The fact that such acceleration schemes also generate fast neutral atoms has received comparatively less attention. A quantitative assessment of the neutralization fraction reveals novel and often unexpected aspects of electron-ion interaction in regions far from dense plasmas. At the rear side of laser irradiated foil, a few hundred keV accelerated neutral Hydrogen atoms are observed. Neutralisation fraction is much larger than that anticipated from the collisional charge transfer and is about 300% larger for ions of ⩽ 100 keV energy. Corroborated by particle-in-cell simulations, the larger neutralisation fraction is attributed to electron-recombination as the protons co-propagate with the low energy electrons.

Optimised production of technetium-94m for PET imaging by proton-irradiation of phosphomolybdic acid in cyclotron liquid target

Natural-abundance phosphomolybdic acid (H3(Mo12PO40) ‧12H2O, 0.181–0.552 g Mo/mL) solutions were irradiated with 12.9 MeV protons on a GE PETtrace cyclotron using an adapted standard liquid target. Technetium-94m (94mTc) was produced through the 94Mo(p,n)94mTc nuclear reaction with saturation yields of up to 53 ± 6 MBq/μA. End of bombardment activities of 161 ± 17 MBq and 157 ± 7 MBq were achieved for the 0.552 g Mo/mL solution (10 μA for 30 min) and 0.181 g Mo/mL solution (15 μA for 60 min), respectively. No visible degradation of the niobium target body and foil were seen during the irradiations of up to 15 μA for 60 min. The produced 94mTc was separated from the target phosphomolybdic acid solution with >98% recovery using an aqueous biphasic extraction resin. Compared to previous reported liquid target methods for 94mTc production, the better production yield, in-target solution stability during irradiation and 94mTc separation recovery of phosphomolybdic acid makes it a very promising target material for routine clinical 94mTc production at medical facilities with liquid targets already installed for 18F production.

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.

Fabrication of 208Pb and 193Ir targets on Al-backing using high/ultra-high vacuum deposition technique for low-energy nuclear reaction experiments

Aiming to study the interplay of different nuclear reactions at low incident energies, thin targets of enriched stable isotopes, 208Pb and 193Ir, have been fabricated at the target development laboratory of the Inter-University Accelerator Centre (IUAC), New Delhi. As a means of preparing a large number of thin isotopically pure targets, physical vapour deposition techniques, (a) resistive heating, for fabrication of 208Pb targets of mass thickness 160-340μg/cm2, and (b) electron-gun evaporation technique, for fabrication of 193Ir targets of mass thickness 17-60μg/cm2, have been employed. The fabricated targets were characterized using Rutherford Backscattering Spectrometry (RBS) and Field Emission Scanning Electron Microscopy (FE–SEM). The thickness of targets was estimated based on the evaporation rate and measured by RBS. The RBS was employed to analyse the samples for trace- and heavy impurities, provided the measurements and analysis suggest that the targets prepared in the present work do not contain any impurities. FE-SEM was employed to examine the surface morphology and microstructure in high resolution. This technique enables the detection of any morphological changes induced by the irradiation process, offering insights into the effects of irradiation on the target films. The targets have been successfully deployed to measure complete and incomplete fusion excitation functions, mass distribution of fission fragments, and forward ranges of target-like recoils in 12C + 208Pb, 193Ir reactions from sub- to above barrier energies. Preliminary data analysis suggests the achievement of high-quality 208Pb and 193Ir target foils.

Laser-driven high-energy proton beams from cascaded acceleration regimes

Laser-driven ion accelerators can deliver high-energy, high-peak current beams and are thus attracting attention as a compact alternative to conventional accelerators. However, achieving sufficiently high energy levels suitable for applications such as radiation therapy remains a challenge for laser-driven ion accelerators. Here we generate proton beams with a spectrally separated high-energy component of up to 150 MeV by irradiating solid-density plastic foil targets with ultrashort laser pulses from a repetitive petawatt laser. The preceding laser light heats the target, leading to the onset of relativistically induced transparency upon main pulse arrival. The laser peak then penetrates the initially opaque target and triggers proton acceleration through a cascade of different mechanisms, as revealed by three-dimensional particle-in-cell simulations. The transparency of the target can be used to identify the high-performance domain, making it a suitable feedback parameter for automated laser and target optimization to enhance stability of plasma accelerators in the future.

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.

Joule-class THz pulses from microchannel targets.

Inference of joule-class THz radiation sources from microchannel targets driven with hundreds of joule, picosecond lasers is reported. THz sources of this magnitude are useful for nonlinear pumping of matter and for charged-particle acceleration and manipulation. Microchannel targets demonstrate increased laser-THz conversion efficiency compared to planar foil targets, with laser energy to THz energy conversion up to ∼0.9% in the best cases.

Spatially- and time-resolved measurements of HO2 radicals in a ns pulse atmospheric pressure plasma jet

The absolute, spatially-resolved, and time-resolved number density of the hydroperoxyl radical is measured in a quasi-two-dimensional, atmospheric pressure ‘curtain’ plasma jet powered by a train of ns discharge pulses. The spatial distribution of HO2 is measured across the shorter dimension of the jet. The measurements are made in two different configurations, (a) H2O–O2–He jet impinging on a copper foil target, and (b) O2–He jet incident on the liquid water surface. In the first configuration, the water vapor is added to the O2–He flow in a bubbler filled with distilled, deionized water. The measurements are made using the previously developed pulsed cavity ring down spectroscopy diagnostic near 1.5 μm. The ring-down cavity is formed between two high-reflectivity mirrors placed at the ends of the stainless steel ‘arms’ purged with dry air, with the plasma jet placed in the gap between the arms. The objectives of this work are to use the HO2 number density to assess the accuracy of the modeling predictions using a previously developed ‘global’ reaction mechanism, and to estimate the efficiency of hydrogen peroxide generation in the ns pulse discharge plasma. HO2 was detected only in the first configuration, most likely due to the rapid decay of the metastable He atoms and O atoms generated in the plasma, which prevents the generation of H atoms (dominant HO2 precursors) in the evaporation/mixing layer. Both the water vapor in the jet and HO2 generated in the plasma have been measured. The results exhibit a rapid accumulation of HO2 during the ns pulse discharge burst, followed by the decay in the afterglow on a ms time scale. The kinetic model overpredicts the quasi-steady-state HO2 number density, as well as the HO2 decay rate after the discharge is turned off. The relatively slow HO2 decay in the afterglow suggests that it may be affected by diffusion, along with the surface adsorption and desorption of radicals. The present approach demonstrates the utility of a 2D curtain plasma jet for the line-of-sight absorption spectroscopy measurements of radicals and excited species present in small concentrations in ambient plasma environments.

Quasi-monoenergetic heavy ion acceleration driven by sub-100 PW linearly polarized laser pulses in the radiation-dominated QED regime

Heavy ion acceleration from an ultrathin foil target irradiated by a p-polarized and spatially Gaussian laser pulse at intensity of 1023 W/cm2 is studied by using two-dimensional particle-in-cell simulations. We find that, in the extremely intense laser fields, the radiation reaction force from bright γ-rays radiated by radially oscillating electrons is large enough to match the Coulomb explosive force of foil electrons. The undesirable transverse expansion of the foil from the electron heating and inhomogeneous radial profile of the laser intensity is effectively suppressed. The foil maintains relatively good opacity in its central region stabilizing localized acceleration of heavy ions. With a laser of intensity 3.4 × 1023 W/cm2, duration of 33 fs, and power of 96 PW, a dense monoenergetic Au79+ ion bunch with a peak energy of ∼160 GeV can be obtained in the radiation-dominated QED regime. Such a high-quality heavy ion beam is useful for investigating nuclear matter equation of state and quantum chromodynamic phase transition in intermediate-energy heavy ion collisions.

Transverse emittance growth of proton sources from laser-irradiated sub-μm-thin planar targets.

Proton bunches with maximum energies between 12 and 22 MeV were emitted from submicrometer-thin plastic foils upon irradiation by laser pulses with peak intensity of 4×10^{20}W/cm^{2}. The images of the protons by a magnetic quadrupole doublet on a screen remained consistently larger by a factor of 10 compared to expectations drawn from the ultralow transverse emittance values reported for thick foil targets. Analytic estimates and particle-in-cell simulations attribute this drastically increased emittance to formerly excluded Coulomb collisions between charged particles. The presence of carbon ions and significant transparency likely play a decisive role. This observation is highly relevant because such thin, partially transparent foils are considered ideal for optimizing maximum proton energies.

100 MeV protons from nanostructured hemispherical target using PIC simulations

The improvement of laser-driven proton energy with the use of nano-structured hemispherical targets of 100 nm thickness over conventional flat foil has been reported in this work. The curvature of the target is found to result in focussed particle density at the center of the hemispherical target followed by emergence of energetic ions due to combined action of sheath electric field and ambipolar expansion. The presence of nano-rods on the curved hemispherical target further increases the laser energy absorption by the electrons, thus resulting in increase in the maximum proton energy. Use of hemispherical target embedded with nanorods is possibly reported here for the first time that may generate protons with energy 92 MeV by using linearly polarised laser of intensity 1021 W cm−2 and pulse duration of 30 fs. At this laser intensity, the energy gain by the protons is much higher compared to the conventional flat foil targets. The maximum proton energy can be increased further to 103 MeV by using truncated hemispherical target of similar parameter.

Multi frame radiography of supersonic water jets interacting with a foil target

Pulsed-power-driven underwater electrical explosion of cylindrical or conical wire arrays produces supersonic water jets that emerge from a bath, propagating through the air above it. Interaction of these jets with solid targets may represent a new platform for attaining materials at high pressure (>1010 Pa) conditions in a university-scale laboratory. However, measurements of the internal structure of such jets and how they interact with targets are difficult optically due to large densities and density contrasts involved. We utilized multi-frame x-ray radiographic imaging capabilities of the ID19 beamline at the European Synchrotron Radiation Facility to explore the water jet and its interaction with a 50 μm thick copper foil placed a few mm from the surface of water. The jet was generated with a ∼130 kA-amplitude current pulse of ∼450 ns rise time applied to a conical wire array. X-ray imaging revealed a droplet-type structure of the jet with an average density of <400 kg/m3 propagating with a velocity of ∼1400 m/s. Measurements of deformation and subsequent perforation of the target by the jet suggested pressures at the jet–target interface of ∼5 × 109 Pa. The results were compared to hydrodynamic simulations for better understanding of the jet parameters and their interaction with the foil target. These results can be used in future research to optimize the platform, and extend it to larger jet velocities in the case of higher driving currents supplied to the wire array.

Ion acceleration from aluminum foil coated with a gold nanolayer irradiated by ultrashort laser pulses

Enhancement of proton energy has always been a key aspect addressed via laser-driven proton acceleration. As the target normal sheath acceleration protons are driven by the electric field produced at the target rear surface, the presence of a gold nanolayer on the surface of the target foil will enhance the energy of accelerated ion beams. In our study, we used a 30 fs laser pulse with a wavelength of 800 nm and a peak intensity of 3×1020 W/cm2. The targets were 2 μm thick aluminum foils coated with a 10–20 nm layer of gold (Au). It was observed that the dynamics of proton acceleration from the foil target is a function of the position of the nanolayer (front or rear surface). 2D particle-in-cell simulation was also performed in support of the observed experimental results.

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