Ultra-thin Foil Targets Research Articles

Hybrid proton acceleration with an ultra-intense laser of low contrast

Proton acceleration from the interaction of an ultra-intense femtosecond-class laser pulse at varying picosecond contrast levels with an ultrathin-foil target is investigated numerically. It is found that lower contrast increases the optimal target thickness without reducing the proton cutoff energy at an ultrahigh laser intensity of I∼8×1021 W/cm2, contrasting with previous experimental results at low laser intensity I<1×1021 W/cm2. By employing particle tracking techniques, we show that due to the intense radiation pressure of the main pulse, the acceleration of protons in the pre-expanded target via the Hole-Boring mechanism under low contrast surpasses that in the steep-edge target under high contrast, which is driven by a hybrid regime of light-sail and sheath acceleration before the target is penetrated. After that, a thinner optimal target thickness under high contrast results in stronger electron heating, enabling the proton energy to catch up gradually in relativistic-induced transparency enhanced acceleration. Ultimately, a similar cutoff energy is obtained for both scenarios. Our work demonstrates that high laser intensity can offer some advantages to proton acceleration at the radiation-pressure-dominated stage when laser contrast control on the picosecond level is challenging, and a thicker target is necessary. This implies that the demanding requirement for laser contrast could potentially be relaxed for multi-petawatt laser facilities, simplifying experimental setups and enhancing proton energy.

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.

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.

High order modes of intense second harmonic light produced from a plasma aperture

Because of their ability to sustain extremely high-amplitude electromagnetic fields and transient density and field profiles, plasma optical components are being developed to amplify, compress, and condition high-power laser pulses. We recently demonstrated the potential to use a relativistic plasma aperture—produced during the interaction of a high-power laser pulse with an ultrathin foil target—to tailor the spatiotemporal properties of the intense fundamental and second-harmonic light generated [Duff et al., Sci. Rep. 10, 105 (2020)]. Herein, we explore numerically the interaction of an intense laser pulse with a preformed aperture target to generate second-harmonic laser light with higher-order spatial modes. The maximum generation efficiency is found for an aperture diameter close to the full width at half maximum of the laser focus and for a micrometer-scale target thickness. The spatial mode generated is shown to depend strongly on the polarization of the drive laser pulse, which enables changing between a linearly polarized TEM01 mode and a circularly polarized Laguerre–Gaussian LG01 mode. This demonstrates the use of a plasma aperture to generate intense higher-frequency light with selectable spatial mode structure.

Scaling laws for laser-driven ion acceleration from nanometer-scale ultrathin foils.

Laser-driven ion acceleration has attracted global interest for its potential towards the development of a new generation of compact, low-cost accelerators. Remarkable advances have been seen in recent years with a substantial proton energy increase in experiments, when nanometer-scale ultrathin foil targets and high-contrast intense lasers are applied. However, the exact acceleration dynamics and particularly the ion energy scaling laws in this novel regime are complex and still unclear. Here, we derive a scaling law for the attainable maximum ion energy from such laser-irradiated nanometer-scale foils based on analytical theory and multidimensional particle-in-cell simulations, and further show that this scaling law can be used to accurately describe experimental data over a large range of laser and target parameters on different facilities. This provides crucial references for parameter design and experimentation of the future laser devices towards various potential applications.

Production of intense isolated attosecond pulses with circular polarization by using counter-propagating relativistic lasers

We demonstrate theoretically and numerically that intense isolated circularly polarized (CP) attosecond pulses can be generated from ultrathin foil targets irradiated by two relativistic lasers from opposite sides, where their polarizations are orthogonal to each other. With a proper matching condition, the compressed oscillating plasma mirrors on both sides of the foil are pushed inside by laser radiation pressures, eventually merging together to form a dense electron nanobunch under the effect of orthogonal laser fields. This nanobunch reaches both high density and high energy in only half a laser cycle and smears out in others, resulting in coherent synchrotron emission of a single attosecond pulse with circular polarization. Two-dimensional particle-in-cell simulations show that an intense isolated CP attosecond XUV pulse with an intensity of 1.2 × 1019 W cm−2 and a duration of ∼75 as can be obtained by two lasers with the same intensity of 2.1 × 1020 W cm−2.

The need for speed

This paper reviews the challenges posed by the physics of the interaction of high-peak power femtosecond lasers with ultrathin foil targets. Initially designed to produce warm solid-density plasmas through the isochoric heating of solid matter, the interaction of an ultrashort pulse with ultrathin foils is becoming more and more complex as the laser intensity is increased. The dream of achieving very hot solid density matter with extreme specific energy density faces several bottlenecks discussed here as related to the laser technology, to the complexity of the physical processes, and to the limits of our current time-resolved instrumentations.

Self-Referencing Spectral Interferometric Probing of the Onset Time of Relativistic Transparency in Intense Laser-Foil Interactions

Irradiation of an ultrathin foil target by a high intensity laser pulse drives collective electron motion and the generation of strong electrostatic fields, resulting in ultrabright sources of high-order harmonics and energetic ions. The ion energies can be significantly enhanced if the foil undergoes relativistic self-induced transparency during the interaction, with the degree of enhancement depending in part on the onset time of transparency. We report on a simple and effective approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light, providing a route to optically controlling and optimizing ion acceleration and radiation generation. The scheme involves a self-referencing approach to spectral interferometry, in which coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil. The relative timing of the onset of transmission with respect to the transition radiation generation is determined from spectral fringe spacing and compared to simultaneous frequency-resolved optical gating measurements. The results are in excellent agreement, and are discussed with reference to particle-in-cell simulations of the interaction physics and an analytical model for the onset time of transparency in ultrathin foils.

Modeling the interaction of an ultra-high intensity laser pulse with an ultra-thin nanostructured foil target

New plastic nanostructured foil targets interacting with an ultra-high intensity laser pulse are investigated by performing particle-in-cell simulations. The thickness of the ultra-thin planar and nanostructured foil target is in the range of tens of nanometers. Complementary numerical simulations which use the finite-difference time-domain method without considering plasma generation have been performed. The laser pulse has the parameters of the two 10 PW lasers from ELI-NP. The circularly polarized laser pulse has the twice intensity of the linearly polarized (LP) laser pulse. We show a strong dependence of the maximum proton energy on the nanospheres diameter. But, the carbon ion maximum energy varies very weak with the nanospheres diameter. We prove that the acceleration regime of the protons is a combination of the radiation pressure acceleration and target normal sheath acceleration regimes. The carbon ions experience the target normal sheath acceleration. This study is a very useful tool for the preparation of laser-ion acceleration experiments with nanostructured foil targets at ELI-NP and other PW laser facilities.

Boosted acceleration of protons by tailored ultra-thin foil targets

We report on a detailed experimental and numerical study on the boosted acceleration of protons from ultra-thin hemispherical targets utilizing multi-Joule short-pulse laser-systems. For a laser intensity of 1 × 1020 W/cm2 and an on-target energy of only 1.3 J with this setup a proton cut-off energy of 8.5 MeV was achieved, which is a factor of 1.8 higher compared to a flat foil target of the same thickness. While a boost of the acceleration process by additionally injected electrons was observed for sophisticated targets at high-energy laser-systems before, our studies reveal that the process can be utilized over at least two orders of magnitude in intensity and is therefore suitable for a large number of nowadays existing laser-systems. We retrieved a cut-off energy of about 6.5 MeV of proton energy per Joule of incident laser energy, which is a noticeable enhancement with respect to previous results employing this mechanism. The approach presented here has the advantage of using structure-wise simple targets and being sustainable for numerous applications and high repetition rate demands at the same time.

Laser-driven acceleration of quasi-monoenergetic, near-collimated titanium ions via a transparency-enhanced acceleration scheme

Laser-driven ion acceleration has been an active research area in the past two decades with the prospects of designing novel and compact ion accelerators. Many potential applications in science and industry require high-quality, energetic ion beams with low divergence and narrow energy spread. Intense laser ion acceleration research strives to meet these challenges and may provide high charge state beams, with some successes for carbon and lighter ions. Here we demonstrate the generation of well collimated, quasi-monoenergetic titanium ions with energies ∼145 and 180 MeV in experiments using the high-contrast (<10−9) and high-intensity () Trident laser and ultra-thin (∼100 nm) titanium foil targets. Numerical simulations show that the foils become transparent to the laser pulses, undergoing relativistically induced transparency (RIT), resulting in a two-stage acceleration process which lasts until ∼2 ps after the onset of RIT. Such long acceleration time in the self-generated electric fields in the expanding plasma enables the formation of the quasi-monoenergetic peaks. This work contributes to the better understanding of the acceleration of heavier ions in the RIT regime, towards the development of next generation laser-based ion accelerators for various applications.

Absolute calibration of microchannel plate detector for carbon ions up to 250 MeV

A 375TW 40fs pulse was used at the ASTRA GEMINI facility located at the Rutherford Appleton Laboratory U.K. for investigating novel ion acceleration regimes employing ultrathin foil targets. An online detection system consisting of a Thomson Parabola Spectrometer (TPS)—Microchannel Plate (MCP) was used to determine maximum energies and spectra per species. The response of the MCP was calibrated for absolute particle (carbon) number per steradian using CR-39 up to 21 MeV/nucleon. This calibration provides a useful reference for a widely used diagnostic arrangement.

Role of magnetic field evolution on filamentary structure formation in intense laser–foil interactions

Filamentary structures can form within the beam of protons accelerated during the interaction of an intense laser pulse with an ultrathin foil target. Such behaviour is shown to be dependent upon the formation time of quasi-static magnetic field structures throughout the target volume and the extent of the rear surface proton expansion over the same period. This is observed via both numerical and experimental investigations. By controlling the intensity profile of the laser drive, via the use of two temporally separated pulses, both the initial rear surface proton expansion and magnetic field formation time can be varied, resulting in modification to the degree of filamentary structure present within the laser-driven proton beam.

Radiation Pressure-Driven Plasma Surface Dynamics in Ultra-Intense Laser Pulse Interactions with Ultra-Thin Foils

The dynamics of the plasma critical density surface in an ultra-thin foil target irradiated by an ultra-intense (∼6 × 10 20 Wcm − 2 ) laser pulse is investigated experimentally and via 2D particle-in-cell simulations. Changes to the surface motion are diagnosed as a function of foil thickness. The experimental and numerical results are compared with hole-boring and light-sail models of radiation pressure acceleration, to identify the foil thickness range for which each model accounts for the measured surface motion. Both the experimental and numerical results show that the onset of relativistic self-induced transparency, in the thinnest targets investigated, limits the velocity of the critical surface, and thus the effectiveness of radiation pressure acceleration.

Amplification of coherent synchrotron high harmonic emission from ultra-thin foils in relativistic light fields

We report on a remarkable enhancement of high harmonic (HH) radiation emitted from the interaction of an ultra-intense laser pulse with ultra-thin foils by a manipulation of foil pre-plasma conditions. With a strong counter-propagating pre-pulse, we introduce a concerted expansion of the ultrathin foil target, and this significantly raises the efficiency of the HH generation process. Our experimental results show how the emission efficiency can be easily controlled by the intensity and delay time of the pre-pulse. The results give an important insight into the high harmonic generation process from solid dense plasmas when spatially limited. 1D particles in cell simulations confirm our experimental findings and show a significant dependency of the HH emission efficiency on the plasma density. The simplicity of the ultra-thin foil target and interaction geometry hold promise for specifically compact realization of imaging experiments with ultra-short and bright extreme ultra violet-pulses.

Generation of high quality ion beams through the stable radiation pressure acceleration of the near critical density target**Project supported by the National Natural Science Foundation of China (Grant Nos. 11365020, 11475026, 11565022, and 11547304), the Science and Technology Program of Gansu Province of China (Grant No. 1606RJZA090), the Fundamental Research Funds for the Higher Education Institutions of Gansu Province of

In order to generate high quality ion beams through the stable radiation pressure acceleration (RPA) of the near critical density (NCD) target, we propose a new type of target where an ultra-thin high density (HD) layer is attached to the front surface of an NCD target, which has a preferable self-supporting property in the RPA experiments than the ultra-thin foil target. It is found that in one-dimensional particle-in-cell (PIC) simulation, by the block of the HD layer in the new target, there emerges the hole-boring process rather than propagation in the NCD layer when the intense laser pulse impinges on this target. As a result, a typical RPA structure that the compressed electron layer overlaps the ion layer as a whole is formed and a high quality ion beam is obtained, e.g., a circularly polarized laser pulse with normalized amplitude impinges on this new target and a 1.2 GeV monoenergetic ion beam is generated through the RPA of the NCD layer. Similar results are also found in the two-dimensional PIC simulation.

Angularly resolved characterization of ion beams from laser-ultrathin foil interactions

Methods and techniques used to capture and analyze beam profiles produced from the interaction of intense, ultrashort laser pulses and ultrathin foil targets using stacks of Radiochromic Film (RCF) and Columbia Resin #39 (CR-39) are presented. The identification of structure in the beam is particularly important in this regime, as it may be indicative of the dominance of specific acceleration mechanisms. Additionally, RCF can be used to deconvolve proton spectra with coarse energy resolution while mantaining angular information across the whole beam.

Influence of laser polarization on collective electron dynamics in ultraintense laser–foil interactions

The collective response of electrons in an ultrathin foil target irradiated by an ultraintense ( ${\sim}6\times 10^{20}~\text{W}~\text{cm}^{-2}$ ) laser pulse is investigated experimentally and via 3D particle-in-cell simulations. It is shown that if the target is sufficiently thin that the laser induces significant radiation pressure, but not thin enough to become relativistically transparent to the laser light, the resulting relativistic electron beam is elliptical, with the major axis of the ellipse directed along the laser polarization axis. When the target thickness is decreased such that it becomes relativistically transparent early in the interaction with the laser pulse, diffraction of the transmitted laser light occurs through a so called ‘relativistic plasma aperture’, inducing structure in the spatial-intensity profile of the beam of energetic electrons. It is shown that the electron beam profile can be modified by variation of the target thickness and degree of ellipticity in the laser polarization.

Proton acceleration enhanced by a plasma jet in expanding foils undergoing relativistic transparency

Ion acceleration driven by the interaction of an ultraintense (2 × 1020 W cm−2) laser pulse with an ultrathin ( nm) foil target is experimentally and numerically investigated. Protons accelerated by sheath fields and via laser radiation pressure are angularly separated and identified based on their directionality and signature features (e.g. transverse instabilities) in the measured spatial-intensity distribution. A low divergence, high energy proton component is also detected when the heated target electrons expand and the target becomes relativistically transparent during the interaction. 2D and 3D particle-in-cell simulations indicate that under these conditions a plasma jet is formed at the target rear, supported by a self-generated azimuthal magnetic field, which extends into the expanded layer of sheath-accelerated protons. Electrons trapped within this jet are directly accelerated to super-thermal energies by the portion of the laser pulse transmitted through the target. The resulting streaming of the electrons into the ion layers enhances the energy of protons in the vicinity of the jet. Through the addition of a controlled prepulse, the maximum energy of these protons is demonstrated experimentally and numerically to be sensitive to the picosecond rising edge profile of the laser pulse.

Modelling the effect of laser focal spot size on sheath- accelerated protons in intense laser–foil interactions

We present an approach to modelling the effect of the laser focal spot size on the acceleration of protons from ultra-thin foil targets irradiated by ultra-short laser pulses of intensity 1016–1018 W cm−2. An expression is introduced for the proton acceleration time, which takes account of the time taken for the laser-accelerated electrons, which expand laterally at the rear surface of the target, to escape the region of the sheath. When incorporated into an analytical model of plasma expansion, this approach is found to provide a good fit to measured scaling of the maximum proton energy as a function of intensity at large focal spot sizes.