Thin Solid Foils Research Articles

Monoenergetic electron bunches generated from thin solid foils irradiated by ultrashort, ultraintense circularly polarized lasers

Future ultrashort 10(22)-10(25) Wcm(-2) laser pulses offer the possibility of producing ultrashort mono-energetic electron beams in the GeV-TeV level by direct laser ponderomotive force acceleration (LPFA) in distances of millimeters to about 1 m. A scheme is proposed that a thin solid foil and a thick solid foil are placed on the laser axis, where the thin foil supplies the electron source for LPFA and the thick foil reflects the laser away, however allows the accelerated electrons to go through. By optimizing the distance between the foils, one can obtain the maximum electron beam energy. This scheme is demonstrated by particle-in-cell simulations.

Energetic-ion generation by the combination of laser pressure and Coulomb explosion

A scheme of generating energetic ions by the interaction of an ultrahigh-intensity laser pulse and a thin solid foil is studied. The combination of the effects of radiation pressure and Coulomb explosion makes the ion acceleration more effective. The maximum ion velocity variation with time is predicted theoretically while the temporal evolution of the electrostatic field due to the Coulomb explosion is taken into consideration. Two-dimensional particle-in-cell simulations are done to verify the theory.

Mechanisms of forward laser harmonic emission from thin overdense plasmas

As a high-intensity laser pulse impinges a thin solid foil, high-order harmonics of the incident frequency can be observed at the rear (non-illuminated) side of this foil. Using numerical simulations, we show that these harmonics can be generated either at the front or at the rear side of the target. We analyze the mechanisms responsible for these two types of emission, and discuss their connection with those involved in the generation of harmonics detected on the front side. The combined measurements of spectra on both sides of the target constitute a powerful, but often nontrivial, probe of the ultrafast plasma dynamics.

Laser Ion Acceleration and Its Medical Application

Approach to downsize an accelerator for particle-cancer therapy with the use of a very high acceleration-gradient by laser-plasma interaction has been performed combining laser-produced ions with a conventional RF acceleration technique. Protons produced by a high peak-power (∼a few tens TW) short-pulse (<50 fs) laser from a thin solid foil target several μm in thickness has been improved in its beam chracteristics by combination of a “Phase Rotation” and electron beam cooling in order to be accelerated with a short-rising-time (∼5 ms) synchrotron to the energy of ∼200 MeV needed for cancer-therapy. In the present paper, the scheme to improve the characteristics of the laser-produced protons in the longitudinal phase space by “Phase Rotation”, which has been demonstrated in its feasibility experimentally, is described together with the procedure of laser proton production.

Comparative spectra and efficiencies of ions laser-accelerated forward from the front and rear surfaces of thin solid foils

The maximum energy of protons that are accelerated forward by high-intensity, short-pulse lasers from either the front or rear surfaces of thin metal foils is compared for a large range of laser intensities and pulse durations. In the regime of moderately long laser pulse durations (300–850fs), and for high laser intensities [(1−6)×1019W∕cm2], rear-surface acceleration is shown experimentally to produce higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. For similar laser pulse durations but for lower laser intensities (2×1018Wcm−2), the same conclusion is reached from direct proton radiography of the electric fields associated with proton acceleration from the rear surface. For shorter (30–100fs) or longer (1–10ps) laser pulses, the same predominance of rear-surface acceleration in producing the highest energy protons is suggested by simulations and by comparison of analytical models with measured values. For this purpose, we have revised our previous analytical model of rear-surface acceleration [J. Fuchs et al., Nat. Phys. 2, 48 (2006)] to adapt it to the very short pulse durations. Finally, it appears, for the explored parameters, that rear-surface acceleration is the dominant mechanism.

Numerical modeling and applications of laser-accelerated ion beams

We use laser-plasma interaction and particle-transport codes to investigate ion acceleration when a short, intense laser pulse irradiates a thin solid foil, and to quantify the isotope yield that these ions can induce by nuclear reactions in secondary targets. Optimum acceleration, in terms of peak ion energy and secondary target activation, is obtained for ultra-low (sub-μm) target thickness.

Determination of the difference between the mean and the most probable energy loss of low-energy proton beams traversing thin solid foils

Precise determinations of the energy loss of ion beams in solid targets in the low-energy range (typically a few keV/u) using the transmission technique, require, among other things, a fine analysis of the energy distributions of the transmitted ion beams. In particular, it is known that possible asymmetries in the energy distributions – and related differences between the mean and the most probable energy loss – may be a source of discrepancies or uncertainties in stopping power determinations. We have performed detailed analyses of the energy-loss distributions of proton beams traversing various thin foils (12–24nm) of metallic elements (Al, Au and Bi) including mono and polycrystalline foils. The differences between the mean and the most probable energy losses in these conditions were analyzed. The analysis has been extended to the first three moments of the energy loss spectra and the results have been compared with theoretical predictions. The magnitude of the differences so obtained were in all cases very small and give good support to the use of the transmission method for precise energy loss determinations in the few-keV region.

In-flight emission of projectile Auger electrons from highly energetic heavy ions (20 MeV/u < E < 100 MeV/u) colliding with carbon foils

Highly energetic projectiles (20MeV/u<Ebeam<100MeV/u), which have been collisionally excited during penetration of thin solid foils, can deexcite by emission of Auger electrons. These in-flight emitted fast electrons are observed in the laboratory system with a velocity intermediate between vbeam (the ion beam velocity) and 2vbeam. Results obtained both at the Catania LNS superconducting cyclotron CS with 58Ni14+,19+ (23 and 45MeV/u) and at Ganil with 78Kr32+,34+ (64MeV/u) are presented. Absolute emission cross-sections and kinematic properties are discussed.

Electrostatic field distribution at the sharp interface between high density matter and vacuum

Ultrahigh intensity lasers are proven to be particularly suitable for ion acceleration to energies above hundreds of keV and even in the multi MeV range, due to their interaction with either planar thin solid foils, or spherically symmetric targets. With reference to these problems, a quasistationary model is developed, where the Poisson equation for the electrostatic potential distribution at the sharp solid target-vacuum interface is solved for a nonrelativistic Maxwellian distribution of trapped electrons. Analytical solutions are given and ion acceleration in the relevant electrostatic field configurations is discussed.

Dynamics of Electric Fields Driving the Laser Acceleration of Multi-MeV Protons

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense (approximately 10(18) W/cm2) and short (approximately 1.5 ps) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.

Harmonic emission from the rear side of thin overdense foils irradiated with intense ultrashort laser pulses.

The harmonic emission from thin solid carbon and aluminum foils, irradiated by 150 fs long frequency-doubled Ti:sapphire laser pulses at lambda=395 nm and peak intensities of a few 10(18) W/cm(2), has been studied. In addition to the harmonics emitted from the front side in the specular direction, we observe harmonics up to the 10th order, including the fundamental from the rear side in the direction of the incident beam, while the foil is still strongly overdense. The experimental observations are well reproduced by particle-in-cell simulations. They reveal that strong coupling between the laser-irradiated side and the rear side occurs via the nonlocal electron current driven by the laser light.

Vicinage forces between molecular and atomic fragments dissociated from small hydrogen clusters and their effects on energy distributions

In this paper we analyze the dynamic evolution of molecular and atomic fragments of small hydrogen clusters interacting with thin solid foils. We compare the vicinage forces, calculated within the dielectric formalism, for H 1 ,H 0 , and H2 1 fragments. Using a molecular dynamics numerical code we determine the energy distribution of the fragments after interacting with the target. This distribution is compared to experimental results for protons coming from the fragmentation of v52.02 a.u. H2 1 ions impinging on an aluminum foil; a fraction of neutral H 0 is needed to be included in the simulation to get a good agreement with the experimental results. The H2 1 energy spectra for v55.42 a.u. H3 1 interacting with amorphous carbon is also determined. The asymmetry in the Coulomb peaks appearing in the energy spectra both experimentally and in our calculation is opposite for H2 1 than in H 1 ; kinematic effects and differences in the electronic stopping are enough to reproduce the difference in the alignment of H 2 1 and H 1 fragments.

Compton scattering and photoluminescence for x-ray imaging

Experiments with high-intensity, submillimeter diameter, pulsed x-ray beams that will be generated by the planned Linac Coherent Light Source will require nonperturbing imaging of such beams. Two approaches to solving this problem are proposed: Compton scattering off a thin solid foil and photoluminescence induced in a thin gas jet. The first would be efficient for x-ray energies above a few keV, whereas the second can be used to detect lower-energy beams, below ∼1 keV. Spatial resolution of the time-integrated images would be ∼10 μm for the first technique and ∼60 μm for the second technique. The minimum number of x-ray quanta needed for reaching this spatial resolution is ∼10.11 The imaging does not introduce significant perturbations to the beam. A set of design equations and constraints is provided.

First observation of Moiré fringes in a proton beam generated by a 100 fs laser pulse

High contrast Moiré fringes have been observed when two gratings were inserted into a proton beam produced from the interaction of a 100 TW laser beam with a thin solid foil. Moiré fringes with modulation close to 20% were observed in protons with energies between 4 and 7 MeV. The fringes were rotated with respect to a collimated optical test beam, a finding consistent with the protons originating from a point source close to the original target surface. These important results indicate that proton Moiré can be used as a high precision diagnostic of proton beam deflections from electric and magnetic fields in plasmas.

Stochastic treatment of nonequilibrium ion stopping in solids

We study the energy loss of fast, hydrogenlike ions in thin solid foils, in the regime prior to the establishment of the ion-charge equilibrium. The projectile-charge evolution is described by a nonstationary, continuous-time Markov process, while the target response is described by a time-dependent dielectric-response formalism. We first derive the projectile self-energy in the presence of charge exchange, which is used to determine the bound-electron density in a self-consistent manner, by minimizing the total projectile energy in an adiabatic approximation. An expression for the ion energy-loss distribution is then used to derive the average value of the stopping power as a function of the traversal time in the foil, taking into account the projectile screening by the bound electron. The results of calculations for He ions in Al foils show significant coherence effects on the energy losses in the pre-equilibrium regime, which are interpreted by the overlap between the time delay in the target response and the characteristic time scale for the charge-changing collisions of the projectile with the target atoms.

Fluctuating charge-state effects on energy spectra of fast ions in solids

We describe the evolution of the charge state of a fast ion in a thin solid foil by a non-stationary continuous-time Markov process which takes two discrete values, appropriate for H and He projectiles. General expression is presented for the ion energy distribution after traversing the foil and is used to derive the expectation values for the ion stopping power and the energy-loss straggling. A careful analysis of the time auto-correlation function for the ion charge state in a non-stationary regime reveals that, in addition to the familiar exponential transient regime, the initial charge effects exhibit a long-time algebraic decay, inversely proportional to the penetration time through the foil.

Experimental study of electron ejection by heavy ion irradiation of solids: Observation of forward and backward emitted electron jets

Doubly differential cross sections for electron emission induced by the passage of swift heavy ions such as F q+ (1.5–2.0 MeV/u) through thin solid foil targets were measured at the Tandem accelerator of the JR Macdonald Laboratory at Kansas State University. The complete angular distribution of electron emission up to 4000 eV (beyond the maximum of the “binary encounter” electron peak) was determined as a function of the projectile charge state ( q=5 and 9) and the target material in a wide Z range: C ( Z=6), Al ( Z=13) and Au ( Z=79). Electrons emitted from the foils between 0 and ±180° with respect to the beam axis were energy and angle analysed by means of a toroidal electrostatic electron spectrometer equipped with a 2D position sensitive channelplate detector. In addition to low energy cascade electrons, electrons from collective excitation (plasmons), target Auger electrons, convoy electrons and binary encounter electrons, we also observe a new feature never before seen in electron angular distributions: narrow electron jets (“spikes”) emitted along the ion beam axis in forward and backward directions. This observation is made possible by the good angular resolution of our spectrometer and the possibility to record the entire angular distribution in a single run.

Energy straggling of protons through thin solid foils

The energy straggling of protons penetrating thin foils is theoretically and experimentally investigated for intermediate and high impact velocities. We calculate separately the contributions due to the interaction of the projectile with valence and core electrons. The energy straggling originated in the valence band is evaluated within the dielectric formalism, using the Mermin-Lindhard dielectric response function. The contribution coming from the core ionization is calculated with the continuum-distorted-wave--eikonal-initial-state (CDW-EIS) approximation. The predictions of the model are compared with recent measurements for Al, Zn, and Au targets. In addition, experimental results of straggling for Si are presented. The transmission method is used to measure the straggling, and experimental data are corrected to consider roughness effects. Theoretical values are in good agreement with the experiments in the whole range of energies considered. At low energies the binary interaction with valence electrons is the dominant mechanism, while the inner-shell contribution becomes the most important one as the energy increases.

Ion-charge distribution in fast, partially stripped clusters passing thin foils

We provide a self-consistent theoretical model describing the vicinage effect on charge states of partially stripped ions in a large cluster traversing a thin solid foil at a high speed. Starting from Brandt-Kitagawa variational theory for the effective charge on a fast heavy ion, supplemented by the dynamically screened interionic interactions within the cluster, we obtain a nonlinear integral equation for a function describing the spatial distribution of ion charges throughout the cluster. Numerical solutions of such an equation show that the ions in the cluster interior become increasingly neutralized as the cluster size grows.

Isotopic effects in the angular dependence of the energy loss and angular distribution of protons and deuterons in Al foils at low energies

Energy loss and straggling has been measured for hydrogen and deuterium ions in thin solid aluminum foils in the low-energy range $E&lt;10\mathrm{keV}$ and in the forward direction, both as a function of the energy, and at fixed energies as a function of the observation angle. Whereas no isotopic effect in the energy loss at the forward direction was observed, significant differences appeared when observing at nonzero angles. Monte Carlo simulations and model calculations of the energy loss as a function of the observation angle using a frictional-type energy loss, taking into account the path-length enlargement, the elastic energy loss, and the foil roughness, led to an understanding of the main physical features at these energies. The observed isotopic effect at nonzero angles can be fully accounted for by differences in the foil roughness influence and in the elastic energy loss.