Thin Solid Foils Research Articles

Efficiency and resolution characterisation of a high-resolution bent crystal X-ray spectrometer using ray-tracing simulations

We report the characterisation of the efficiency and resolution of a high-resolution bent crystal spectrometer under Johannson geometry, using ray-tracing simulations. The spectral resolution is measured experimentally from electron-beam and fast ion-beam interactions with thin solid foils. The X-rays emitted by target atoms as well as projectile ions are studied. The results of the ray-tracing simulations are in very good agreement with the beam-foil spectroscopy experiments. The geometrical factors affecting the resolution are also discussed in detail. We also characterise the detection efficiency of the spectrometer by simultaneous measurements of X-rays using a semiconductor detector, i.e. Si(Li) detector, with known detection efficiency. These results are also compared with the ray-tracing simulation results.

Magnetic pinching of relativistic particle beams: a new approach to strong-field QED physics

Quantum electrodynamics (QED) is a foundation of modern physics, yet access to the strong-field QED regime in the laboratory remains a formidable challenge. Currently, high-power lasers at the multi-petawatt level and above are generally believed to be an important approach to test QED physics. Here, we present a different approach by use of an electron beam self-pinched to near-solid-density. The beam self-pinching is realized while it transports through a properly designed hollow cone target, where strong azimuthal magnetic fields are generated by the beam-induced plasma return currents at the inner surface of the cone target. In this way, the beam diameter can be reduced by more than an order of magnitude down to submicron and its density is increased by hundreds of times. The produced ultradense electron beams can unlock a new regime of QED-dominated beam–plasma interactions, for example, more than 60% of the beam energy can be converted into GeV gamma-rays with unprecedented brilliance when such a beam passes through a thin solid foil. Moreover, with proper parameter design, this beam-focusing scheme can also be applied to positron beams and thus may find applications in broad areas, such as particle colliders and strong-field physics.

Long-wavelength pulse generation via light-sail backscattering

The next generation of multipetawatt laser facilities may enable new opportunities in radiation-pressure driven ion-acceleration. Owing to the motion of the thin solid foil used as a target in such experiments, the laser light reflected from the surface will be Doppler downshifted. We show that the downshifted light can be correlated with the ion energies and may be used as a diagnostic of the acceleration mechanism. The possibility of using the back-scattered light in ion acceleration experiments to produce long-wavelength few-cycle pulses is also explored. The highest ion velocities are obtained in the light-sail regime, the theory of which is here extended to regimes with varying reflectivity due to a time-dependent intensity. The reflected pulse with a large Doppler redshift and approximately linear chirp may subsequently be compressed using a grating compressor to obtain a near single cycle pulse with long wavelength and relatively high intensity. Analytic estimates are supported by 1D3P relativistic particle-in-cell simulations.

Towards compact laser-driven accelerators: exploring the potential of advanced double-layer targets

The interest in compact, cost-effective, and versatile accelerators is increasing for many applications of great societal relevance, ranging from nuclear medicine to agriculture, pollution control, and cultural heritage conservation. For instance, Particle Induced X-ray Emission (PIXE) is a non-destructive material characterization technique applied to environmental analysis that requires MeV-energy ions. In this context, superintense laser-driven ion sources represent a promising alternative to conventional accelerators. In particular, the optimization of the laser-target coupling by acting on target properties results in an enhancement of ion current and energy with reduced requirements on the laser system. Among the advanced target concepts that have been explored, one appealing option is given by double-layer targets (DLTs), where a very low-density layer, which acts as an enhanced laser absorber, is grown to a thin solid foil. Here we present some of the most recent results concerning the production with deposition techniques of advanced DLTs for laser-driven particle acceleration. We assess the potential of these targets for laser-driven ion acceleration with particle-in-cell simulations, as well as their application to PIXE analysis of aerosol samples with Monte Carlo simulations. Our investigation reports that MeV protons, accelerated with a ∼20 TW compact laser and optimized DLTs, can allow performing PIXE with comparable performances to conventional sources. We conclude that compact DLT-based laser-driven accelerators can be relevant for environmental monitoring.

Vacuum laser acceleration of super-ponderomotive electrons using relativistic transparency injection

Intense lasers can accelerate electrons to very high energy over a short distance. Such compact accelerators have several potential applications including fast ignition, high energy physics, and radiography. Among the various schemes of laser-based electron acceleration, vacuum laser acceleration has the merits of super-high acceleration gradient and great simplicity. Yet its realization has been difficult because injecting free electrons into the fast-oscillating laser field is not trivial. Here we demonstrate free-electron injection and subsequent vacuum laser acceleration of electrons up to 20 MeV using the relativistic transparency effect. When a high-contrast intense laser drives a thin solid foil, electrons from the dense opaque plasma are first accelerated to near-light speed by the standing laser wave in front of the solid foil and subsequently injected into the transmitted laser field as the opaque plasma becomes relativistically transparent. It is possible to further optimize the electron injection/acceleration by manipulating the laser polarization, incident angle, and temporal pulse shaping. Our result also sheds light on the fundamental relativistic transparency process, crucial for producing secondary particle and light sources.

2D monochromatic x-ray imaging for beam monitoring of an x-ray free electron laser and a high-power femtosecond laser.

In pump-probe experiments with an X-ray Free Electron Laser (XFEL) and a high-power optical laser, spatial overlap of the two beams must be ensured to probe a pumped area with the x-ray beam. A beam monitoring diagnostic is particularly important in short-pulse laser experiments where a tightly focused beam is required to achieve a relativistic laser intensity for generation of energetic particles. Here, we report the demonstration of on-shot beam pointing measurements of an XFEL and a terawatt class femtosecond laser using 2D monochromatic Kα imaging at the Matter in Extreme Conditions end-station of the Linac Coherent Light Source. A thin solid titanium foil was irradiated by a 25-TW laser for fast electron isochoric heating, while a 7.0 keV XFEL beam was used to probe the laser-heated region. Using a spherical crystal imager (SCI), the beam overlap was examined by measuring 4.51 keV Kα x rays produced by laser-accelerated fast electrons and the x-ray beam. Measurements were made for XFEL-only at various focus lens positions, laser-only, and two-beam shots. Successful beam overlapping was observed on ∼58% of all two-beam shots for 10 μm thick samples. It is found that large spatial offsets of laser-induced Kα spots are attributed to imprecise target positioning rather than shot-to-shot laser pointing variations. By applying the Kα measurements to x-ray Thomson scattering measurements, we found an optimum x-ray beam spot size that maximizes scattering signals. Monochromatic x-ray imaging with the SCI could be used as an on-shot beam pointing monitor for XFEL-laser or multiple short-pulse laser experiments.

Two-stage γ ray emission via ultrahigh intensity laser pulse interaction with a laser wakefield accelerated electron beam

In this study, we investigate the generation of twin γ ray beams in the collision of an ultrahigh intensity laser pulse with a laser wakefield accelerated electron beam using a particle-in-cell simulation. We consider the composed target of a homogeneous underdense preplasma in front of an ultrathin solid foil. The electrons in the preplasma are trapped and accelerated by the wakefield. When the laser pulse is reflected by the thin solid foil, the wakefield accelerated electrons continue to move forward and pass through the foil almost without influence from the reflected laser pulse or foil. Consequently, two groups of γ ray flashes, with tunable time delay and energy, are generated by the wakefield accelerated electron beam interacting with the reflected laser pulse from the foil as well as another counter-propagating petawatt laser pulse behind the foil. Additionally, we study the dependence of the γ photon emission on the preplasma densities, driving laser polarization, and solid foil.

Progress with development of foil and slot thermal transfer technology

Foil And Slot Thermal Transfer (FASTT) devices have a unique set of characteristics and the technology may have an important role in future thermal engineering. The absence of freezing and boiling issues, together with the potential for very wide operating temperature range from below 100 K to above 1000 K, as well as their use of low cost, environmentally friendly materials are among those features which have already been mentioned in literature. The geometry of FASTT devices is currently limited to planar or extrudable sections, however, such formats are found in a number of important applications, such as high power, high flux cooling of semiconductors.The core of any FASTT device is the slotted region within a thermally conductive material through which solid foils move and, in a similar fashion to fluids, carries heat away to a larger, lower flux zone where the heat may be dissipated by convection. This allows very high input cooling flux levels to be achieved. There are a number of differences between FASTT and pumped fluid heat transfer. “Equivalent” heat transfer coefficients comparable to those in pumped liquid systems can be obtained but at much lower media velocities (0.05 cf 3.00 ms−1). Narrow channel flow for liquids can be limited by viscosity and allowable operating pressure level. The thin solid foils used in FASTT can run with very low friction at foil to slot gaps below 10 µm, and with channel widths below 100 µm. This ensures low circuit parasitic losses and low pressure operation. The foils can provide both primary heat transfer as well as acting as the extended output surfaces in unsealed variants of these devices. This can remove the need for fluid channels/pipework and thus, potentially, reduce system mass.Limiting flux levels for simple air blast cooled finned arrays are considered, as are theoretical flux limits for FASTT devices, using a selection of foil material and gap media. Results from an unsealed prototype device are given which show that flux values of over 250 Wcm−2 at 70C temperature differences are possible with this type of device operating at room temperature, even in prototype form. Future proposals include sealed devices operating at cryogenic temperatures where the technology is predicted to show significant advantages over existing cooling systems.

Inverse Compton scattering from solid targets irradiated by ultra-short laser pulses in the 1022–1023 W/cm2 regime

Emission of high energy gamma rays via the non-linear inverse Compton scattering process (ICS) in interactions of ultra-intense laser pulses with thin solid foils is studied using particle-in-cell simulations. It is shown that the angular distribution of the ICS photons has a forward-oriented two-directional structure centred at an angle ϑ =± 30°, a value which corresponds to a model based on a standing wave approximation to the electromagnetic field in front of the target, which only increases at the highest intensities due to faster hole boring, which renders the approximation invalid. The conversion efficiency is shown to exhibit a super-linear increase with the driving pulse intensity. In comparison to emission via electron-nucleus bremsstrahlung, it is shown that the higher absorption, further enhanced by faster hole boring, in the targets with lower atomic number strongly favours the ICS process.

Design and optimization of a laser-PIXE beamline for material science applications

AbstractMulti-MeV proton beams can be generated by irradiating thin solid foils with ultra-intense (>1018 W/cm2) short laser pulses. Several of their characteristics, such as high bunch charge and short pulse duration, make them a complementary alternative to conventional radio frequency-based accelerators. A potential material science application is the chemical analysis of cultural heritage (CH) artifacts. The complete chemistry of the bulk material (ceramics, metals) can be retrieved through sophisticated nuclear techniques such as particle-induced X-ray emission (PIXE). Recently, the use of laser-generated proton beams was introduced as diagnostics in material science (laser-PIXE or laser-driven PIXE): Coupling laser-generated proton sources to conventional beam steering devices successfully enhances the capture and transport of the laser-accelerated beam. This leads to a reduction of the high divergence and broad energy spread at the source. The design of our hybrid beamline is composed of an energy selector, followed by permanent quadrupole magnets aiming for better control and manipulation of the final proton beam parameters. This allows tailoring both, mean proton energy and spot sizes, yet keeping the system compact. We performed a theoretical study optimizing a beamline for laser-PIXE applications. Our design enables monochromatizing the beam and shaping its final spot size. We obtain spot sizes ranging between a fraction of mm up to cm scale at a fraction of nC proton charge per shot. These results pave the way for a versatile and tunable laser-PIXE at a multi-Hz repetition rate using modern commercially available laser systems.

Numerical studies on pair production in ultra-intense laser interaction with a thin solid-foil

A theoretical and numerical model of photon and electron–positron pair production in strong-field quantum electrodynamics (QED) is described. Two processes are contained in our QED theoretical model, one is photon emission in the interaction of ultra-intense laser with relativistic electron (or positron), and the other is pair production by a gamma-ray photon interacting with the laser field. This model has been included in a PIC/MCC simulation code named BUMBLEBEE 1D, which is used to simulate the laser plasma interaction. Using this code, the evolutions of electron–positron pair and gamma-ray photon production in ultra-intense laser interaction with aluminum foil target are simulated and analyzed. The simulation results revealed that more positrons are moved in the opposite direction to the incident direction of the laser under the charge separation field.

Generation of overdense and high-energy electron-positron-pair plasmas by irradiation of a thin foil with two ultraintense lasers.

A scheme for enhanced quantum electrodynamics (QED) production of electron-positron-pair plasmas is proposed that uses two ultraintense lasers irradiating a thin solid foil from opposite sides. In the scheme, under a proper matching condition, in addition to the skin-depth emission of γ-ray photons and Breit-Wheeler creation of pairs on each side of the foil, a large number of high-energy electrons and photons from one side can propagate through it and interact with the laser on the other side, leading to much enhanced γ-ray emission and pair production. More importantly, the created pairs can be collected later and confined to the center by opposite laser radiation pressures when the foil becomes transparent, resulting in the formation of unprecedentedly overdense and high-energy pair plasmas. Two-dimensional QED particle-in-cell simulations show that electron-positron-pair plasmas with overcritical density 10(22) cm(-3) and a high energy of 100s of MeV are obtained with 10 PW lasers at intensities 10(23) W/cm(2), which are of key significance for laboratory astrophysics studies.

Quasi-Monoenergetic Electron Beam Generation from Nanothickness Solid Foils Irradiated by Circular-Polarization Laser Pulses

Ultrashort 1022–1024 W/cm2 laser pulses will offer the possibility of producing monoenergetic electron beams in the GeV—TeV level in the future. A laser-electron acceleration scheme is proposed by the interaction between a thin solid foil and an ultra-intense laser pulse for α0 ≥ 800σ0, where α0 is the normalized laser field and σ0 is the normalized plasma surface density. The energy of the electrons as a function of time can be described by a simple model which indicates that an exponential relationship exists between the energy and the normalized time τ. A quasi-monoenergetic high density electron beam with 1.3 GeV energy and a 2.2% energy spread has been predicted for α0 = 223.5 by particle-in-cell simulations. Characteristics of the ultra-high density electron layer formed in the early period of the acceleration are discussed.

Increased efficiency of ion acceleration by using femtosecond laser pulses at higher harmonic frequency

The influence of laser frequency on laser-driven ion acceleration is investigated by means of two-dimensional particle-in-cell simulations. When ultrashort intense laser pulse at higher harmonic frequency irradiates a thin solid foil, the target may become re lativistically transparent for significantly lower laser pulse intensity compared with irradiation at fundamental laser frequency. The relativistically induced transparency results in an enhanced heating of hot electrons as well as increased maximum energies of accelerated ions and their numbers. Our simulation results have shown the increase in maximum proton energy and increase in the number of high-energy protons by a factor of 2 after the interaction of an ultrashort laser pulse of maximum intensity 7 × 1021 W/cm2 with a fully ionized plastic foil of realistic density and of optimal thickness between 100 nm and 200 nm when switching from the fundamental frequency to the third harmonics.

Three-dimensional thermal simulations of thin solid carbon foils for charge stripping of high current uranium ion beams at a proposed new heavy-ion linac at GSI

This paper presents an extensive numerical study of heating of thin solid carbon foils by $1.4\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ uranium ion beams to explore the possibility of using such a target as a charge stripper at the proposed new Gesellschaft f\"ur Schwerionenforschung high energy heavy--ion linac. These simulations have been carried out using a sophisticated 3D computer code that accounts for physical phenomena that are important in this problem. A variety of beam and target parameters have been considered. The results suggest that within the considered parameter range, the target will be severely damaged by the beam. Thus, a carbon foil stripper does not seem to be a reliable option for the future Gesellschaft f\"ur Schwerionenforschung high energy heavy--ion linac, in particular, at FAIR design beam intensities.

Resonances in the population of circular Rydberg states formed in beam-foil excitation

By directing fast moving 164MeV F2656e12+ ion beam on 90μg/cm2 thick carbon foil we have studied the unresolved decay of short-lived 2p (τ2p≈3.3fs) and long-lived 2s (τ2s≈350.6ps) states of H-like Fe in the time range 1.6×104–9.2×105τ2p, where, τ2p and τ2s are, respectively, radiative lifetimes of the 2p and 2s levels. At such large times four resonances have been observed as the humps riding on the decay curve of the beam-foil-excited 2s state. This unusual behavior is explained as consequence of the sequential cascading of circular Rydberg states (l=n−1) to 2p state, which modifies the time dependent photon intensity, I(t), of the 2p→1s transition from an exponential to hump-like structures for t»τ2p. From the detailed study of the cascading process the relative population of circular Rydberg states is determined. It is observed that certain circular Rydberg states are unexpectedly and profusely populated when fast H-like Fe ions emerge from the back surface of a thin solid foil.

Laser ion acceleration in the high laser energy and high laser intensity regimes

New laser facilities able to deliver either ultra high energy short pulses or ultra high intensity pulses are being constructed and will open new and exciting opportunities for laser ion acceleration. The interaction of a high intensity short pulse with underdense, near-critical and overdense targets has been studied using 2D Particle-In-Cell simulations in these regimes. In the ultra high energy regime, proton beams with maximum energies of hundreds of MeV and a high number of high energy protons could be accelerated using thin solid foils or low density targets. In the ultra high intensity regime, radiation losses will start affecting laser ion acceleration using thin overdense targets for intensities higher than 10 22 W/cm 2 , and produce very energetic ions.

Numerical study on coherent keV x ray generation by the interaction of ultra-short intense lasers with thin solid foils

The scheme of x ray generation by the interaction of ultra-short intense laser puls with two thin solid foils is re-investigated by one-dimensional numerical simulation. Attention is paid particularly to the effects of the thickness and the density distribution of the source target on the frequency spectrum and the conversion efficiency of the produced x ray emission, where the source target provides a relativistic electron layer. When the thickness of the source target is comparable to or smaller than the wavelength of x ray /4x2 ( is the wavelength of the incident laser), quasi-monochromatic x ray spectrum can be generated. Otherwise the spectrum will be broadened significantly and the maximum frequency will decrease rapidly. In addition, the presence of inhomogeneous preplasma in front of the foil will induce a similar change of the spectrum.

The interaction of H2D+ with thin solid foils and the detection of its structure

The Interaction between molecular ion H2D+ and ultra-thin solid film is reported for the first time. The importance of H2D+ in the areas such as astrophysics is stated and the study on H2D+ of recent years is outlined. The formation mechanism for the H2D+ under laboratory conditions is also analysed. The structural pattern of H2D+ and the mean internuclear separation are determined using Coulomb explosion technique. Once again, the existence of wake effects in the interaction of molecular ions with solid is confirmed and three-body wake effect module is used to determine the geometric structure of H2D+. Finally, the unusual structure of the wake of energy spectrum for the heavier ions in the H2D+ Coulomb explosion is discussed.

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