Thin Solid Targets Research Articles

Demonstration of bulk acceleration of ions in ultraintense laser interactions with low-density foams

Ion acceleration inside low-density foams irradiated by ultraintense laser pulses has been studied experimentally and theoretically. It is found that the ion generation is closely correlated with the suppressed hot electron transport inside the foams. Particle-in-cell simulations suggest that localized electrostatic fields with multi peaks around the surfaces of lamellar layers inside the foams are induced. These fields inhibit hot electron transport and meanwhile accelerate ions inside the foams, forming a bulk acceleration in contrast to the surface acceleration at the front and rear sides of a thin solid target.

Bulk acceleration of ions in intense laser interaction with foams

It is suggested that bulk acceleration of ions can occur in a target with density discontinuities when hot electrons are transported through the target. A foam target just belongs to such a kind of target, which is composed of irregular lamellar layers distributed randomly. To simplify the problem, we study the interaction of a high intensity laser pulse with a target consisting of regular micro-thin layers separated with a thickness of around a micrometre. Particle-in-cell simulations suggest that localized electrostatic fields with multi-peaks around the surfaces of the thin layers inside are induced when fast electrons produced are transported through such a target. These fields inhibit hot electron transport and simultaneously accelerate ions from the thin layers inside the target, forming a bulk acceleration in contrast to the surface acceleration at the front and rear sides of a thin solid target. Bulk acceleration can produce a large number of ions of moderate energy, which may be useful for applications such as fast ignition by fast protons. Experimental evidence of bulk acceleration is found with low-density foams irradiated by ultra-intense laser pulses.

Evidence of enhanced effective hot electron temperatures in ultraintense laser-solid interactions due to reflexing

It is shown that the effective hot electron temperature, Thot, associated with the energetic electrons produced during the interaction of an ultra-intense laser with thin solid targets is dependent on the thickness of the target. We report the first direct experimental observations of electron energy spectra obtained from laser-solid interactions that indicates the reflexing of electrons in thin targets results in higher electron temperatures than those obtained in thick target interactions. This can occur for targets whose thickness, xt, is less than about half the range of an electron at the energy associated with the initial effective electron temperature, provided the laser pulse length is at least cτp > 2xt. A simple theoretical model that demonstrates the physical mechanism behind this enhanced heating is presented and the results of computer simulations are used to verify the model.

High-intensity laser-plasma interaction studies employing laser-driven proton probes

Due to their particular properties (low emittance, short duration, and large number density), the beams of multi-MeV protons generated during the interaction of ultraintense (I > 1019 W/cm2) short pulses with thin solid targets are suited for use as a particle probe in laser-plasma experiments. When traversing a sample, the proton density distribution is, in general, affected by collisional stopping, scattering and deflections via electromagnetic fields, and each of these effects can be used for diagnostic purposes. In particular, in the limit of very thin targets, the proton beams represent a valuable diagnostic tool for the detection of quasi-static electromagnetic fields. The proton imaging and deflectometry techniques employ these beams, in a point-projection imaging scheme, as an easily synchronizable diagnostic tool in laser- plasma interactions, with high temporal and spatial resolution. By providing diagnostic access to electro-magnetic field distributions in dense plasmas, this novel diagnostics opens up to investigation a whole new range of unexplored phenomena. Several transient processes were investigated employing this technique, via the detection of the associated electric fields. Examples provided in this paper include the detection of pressure-gradient electric field in extended plasmas, and the study of the electrostatic fields associated to the emission of MeV proton beams in high-intensity laser-foil interactions.

L x-ray production in 57La, 58Ce, 60Nd and 62Sm by 35–60 MeV carbon and oxygen ions

L x-ray production cross sections have been measured for thin solid targets (∼50μg/cm2) of 57La, 58Ce, 60Nd and 62Sm for 35–60MeV C4+ and O5+ ions. There is an increase in these cross sections with the increasing projectile energy and the ratio of its atomic number to the atomic number of the target nucleus (Z1/Z2). The measured L x-ray production cross sections have been compared with the predicted values of the first Born approximation (FBA) and of the ECPSSR theory that accounts for the projectile energy loss and Coulomb deflection and the perturbed stationary-state and relativistic nature of the L-shell. For C4+ ions, there is a reasonable agreement with the ECPSSR predicted values for the higher Z elements (Nd and Sm), while for the lower Z elements (La and Ce), the FBA is in a better accord with the data. For O5+ ions, the FBA gives a better agreement for all the elements than the ECPSSR theory.

Fast electron production in collisions of swift heavy ions (20 MeV/ u < E < 100 MeV/ u) with foils of solids

In nuclear and atomic experiments at high incident ion energies, 20 MeV/ u < E < 100 MeV/ u, the impact of swift heavy ions on thin solid targets is a source of fast electrons. The knowledge of their spatial and kinematical distributions is very useful for experimental nuclear and radiobiological applications as well as testing atomic ionization theories. An overview on the main mechanisms underlying the production of the electrons is given. Some recent results obtained at the Catania LNS superconducting cyclotron, mainly with a 45 MeV/ u 58Ni 19+ beam are shown. In particular, the production and the properties of binary encounter-, convoy-, in-flight Auger- and backward emitted electrons are discussed.

Observations of the filamentation of high-intensity laser-produced electron beams

Filamented electron beams have been observed to be emitted from the rear of thin solid targets irradiated by a high-intensity short-pulse laser when there is low-density plasma present at the back of the target. These observations are consistent with a laser-generated beam of relativistic electrons propagating through the target, which is subsequently fragmented by a Weibel-like instability in the low-density plasma at the rear. These measurements are in agreement with particle-in-cell simulations and theory, since the filamentation instability is predicted to be dramatically enhanced when the electron beam density approaches that of the background plasma.

One-dimensional model of the electrostatic ion acceleration in the ultraintense laser–solid interaction

Effective ion acceleration of picosecond-duration well-collimated bunches in the strong relativistic interaction of a short laser pulse with a thin solid target has been experimentally demonstrated. In this work, with reference to the sharp rear solid–vacuum interface, where ion energization takes place, the one-dimensional Poisson–Boltzmann equation is analytically solved on a finite spatial interval whose extension is determined by requiring electron energy conservation, resulting in the consistent spatial distributions of the hot electrons created by the laser and of the corresponding electrostatic potential. Then, the equation of motions for an ensemble of test ions, initially distributed in a thin layer of the rear target surface, with different initial conditions, is solved and the energy spectrum corresponding to a given initial ion distribution is determined.

Dynamics and radiation of thin foil in the field of super-intense laser pulse

The formation of relativistic electron mirror produced via ionization of thin solid target by ultra-intense femtosecond laser pulse is considered. The reaction force of the intrinsic radiation of a bunch can play a significant role in the acceleration process because it gives rise to compression of the bunch in the longitudinal direction and stabilizes it. It is shown that the reflection of a weak counter-propagating wave from such a mirror can produce the coherent radiation in X-ray and gamma-ray bands. The efficiency of up-conversion process is estimated and the spectrum of up-shifted radiation is investigated.

Contribution of Coulomb excitation techniques in quantitative analysis of materials

This contribution is an attempt to analyze the elemental composition of materials by prompt gamma-ray spectrometry with protons and heavy ions. The productions of gamma rays by Coulomb excitation method consists in bombardments with charged particles with energies below the Coulombian barrier, therefore the excitation of the nucleus is an electromagnetic phenomenon. Thin solid target of different materials were bombarded with 30 MeV 16O, 55 MeV 35Cl, 5 MeV 1p at the Tandem accelerator of NIPNE – Bucharest. The experimental results allowed to set detection limits for quantitative analysis of various materials. These experiments showed that the detection limits strongly depend on the ion beam parameters (energy, intensity). Therefore an optimizing procedure of these parameters is essential for a successful output. Reliable results require the existence of standard samples.

Recent results on fast electron production induced by energetic heavy ions on thin solid targets

In order to study the emission of energetic electrons induced by the impact of swift heavy ions on thin solid targets, we carried out a series of experiments at the Superconducting Cyclotron of the Catania Laboratori Nazionali del Sud (LNS) in November and December 2001. We bombarded solid thin targets, ranging from carbon to bismuth, with different ion beams at fixed velocity, i.e. ∼23 MeV/nucleon 197Au 36+, 58Ni 14+ and 12C 3+. Absolute velocity spectra were measured in a wide laboratory angular range, from 1.5° to 175°. At forward angles, besides the well-known convoy and binary encounter components with the beam velocity and two times the beam velocity respectively, we observe also a high velocity tail and an intermediate velocity component. At backward laboratory angles, the spectra remain complex, still presenting an energetic tail. These electron velocity spectra strongly depend on the beam and target atomic numbers. We suggest a Fermi–Shuttle (or multiscattering) mechanism and an in-flight-emission of projectile Auger electrons to explain some of the observed features in the velocity spectra.

Proton imaging detection of transient electromagnetic fields in laser-plasma interactions (invited)

Due to their particular properties (small source size, low divergence, short duration, large number density), the beams of multi-MeV protons generated during the interaction of ultraintense (I&amp;gt;1019 W/cm2) short pulses with thin solid targets are most suited for use as a particle probe in laser–plasma experiments. In particular, the proton beams are a valuable diagnostic tool for the detection of electromagnetic fields. The recently developed proton imaging technique employs the beams, in a point-projection imaging scheme, as an easily synchronizable diagnostic tool in laser–plasma interactions, fields, with high temporal and spatial resolution. The broad energy spectrum of the beams coupled with an appropriate choice of detector (multiple layers of dosimetric film) allows temporal multiframe capability. By allowing, for the first time, diagnostic access to electric-field distributions in dense plasmas, this novel diagnostic opens up to investigation a whole new range of unexplored phenomena. Results obtained in experiments performed at the Rutherford Appleton Laboratory are discussed here. In particular, the article presents the measurement of highly transient electric fields related to the generation and dynamics of hot electron currents following ultraintense laser irradiation of targets. The experimental capabilities of the technique and the analysis procedure required are well exemplified by the data presented.

Recent results on fast intermediate velocity electron production induced by 19+ 45 A MeV 58Ni highly charged ions on thin solid targets

In order to study the emission of energetic electrons induced by the impact of swift heavy ions on thin solid targets, we carried out a series of experiments at the superconducting cyclotron of the Catania Laboratori Nazionali del Sud. We report results on a recent experiment where electron–electron coincidences were measured in a forward ring by bombarding a thin carbon target of 7.4 μg/cm2 with 19+ 45 AMeV 58Ni beam. The velocity1–velocity2 bidimensional plot is dominated by events in which the two detected electrons have a velocity close to the beam velocity 9.03 cm/ns (convoy electrons). The remaining small fraction of coincidences has still a convoy electron and a second electron having either a velocity almost twice the beam velocity 16.5 cm/ns (binary encounter, BE electrons) or a velocity of about 12.7 cm/ns intermediate between BE and convoy velocities (IV electrons). We interprete this last intermediate component as due to in-flight de-excitation of highly excited n+58Ni ions by Auger electrons. Although less distinct, we observe also an intense peak close to the convoy velocity peak, centered at ≈9.7 cm/ns, corresponding to electrons emitted with an energy of only about ≈170 eV in the projectile rest frame of reference.

Electric field detection in laser-plasma interaction experiments via the proton imaging technique

Due to their particular properties, the beams of the multi-MeV protons generated during the interaction of ultraintense (I&amp;gt;1019 W/cm2) short pulses with thin solid targets are most suited for use as a particle probe in laser-plasma experiments. The recently developed proton imaging technique employs the beams in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in laser-plasma interaction experiments. In recent investigations carried out at the Rutherford Appleton Laboratory (RAL, UK), a wide range of laser-plasma interaction conditions of relevance for inertial confinement fusion (ICF)/fast ignition has been explored. Among the results obtained will be discussed: the electric field distribution in laser-produced long-scale plasmas of ICF interest; the measurement of highly transient electric fields related to the generation and dynamics of hot electron currents following ultra-intense laser irradiation of targets; the observation in underdense plasmas, after the propagation of ultra-intense laser pulses, of structures identified as the remnants of solitons produced in the wake of the pulse.

Experimental studies of a Micromegas neutron detector

A Micromegas detector to be used as a neutron beam profiler has been tested in the CENBG neutron beam. Two thin solid targets ( 6 Li and 10 B) have been used as neutron converters. We report results on the detection efficiency and spatial resolution of the detector.

Electronic excitation effects on secondary ion emission from a foil of conducting material bombarded by high energy heavy ions

It is recognized that high electronic excitations play an important role in the damage process of solid targets by high-energy heavy ions. However, the conversion mechanism of the energy of the excited electrons into kinetic energy for target atom displacement is not well understood. In order to investigate the electronic excitation effects of high energy heavy ions in solids, we have studied the secondary ions mass spectra from thin conductive solid targets irradiated by swift heavy ion beams from the JAERI tandem accelerator in the energy region where the electronic stopping power is dominant. The results show the importance of the electronic excitation effect even in simple metallic (Cu) target. It is suggested that Coulomb explosion is the major mechanism of producing columnar defects along the incident ion path.

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.

Laser generation of proton beams for the production of short-lived positron emitting radioisotopes

Protons of energies up to 37 MeV have been generated when ultra-intense lasers (up to 10 20 W cm −2 ) interact with hydrogen containing solid targets. These protons can be used to induce nuclear reactions in secondary targets to produce β +-emitting nuclei of relevance to the nuclear medicine community, namely 11C and 13N via (p, n) and (p,α) reactions. Activities of the order of 200 kBq have been measured from a single laser pulse interacting with a thin solid target. The possibility of using ultra-intense lasers to produce commercial amounts of short-lived positron emitting sources for positron emission tomography (PET) is discussed.

Intense high-energy proton beams from Petawatt-laser irradiation of solids.

An intense collimated beam of high-energy protons is emitted normal to the rear surface of thin solid targets irradiated at 1 PW power and peak intensity 3x10(20) W cm(-2). Up to 48 J ( 12%) of the laser energy is transferred to 2x10(13) protons of energy >10 MeV. The energy spectrum exhibits a sharp high-energy cutoff as high as 58 MeV on the axis of the beam which decreases in energy with increasing off axis angle. Proton induced nuclear processes have been observed and used to characterize the beam.

Bremsstrahlung: an experimentalist’s personal perspective on the postmodern era

In this brief review I will discuss the recent experimental work on the doubly differential cross section, i.e. the photon energy and angular distribution, for electron bremsstrahlung from thin solid film and gas targets. Since the beginning of the modern era in the study of bremsstrahlung with the publication of the 1971 paper by Tseng and Pratt, Professor Pratt has been the dominant influence in bremsstrahlung research. Most, if not all, experimental research during the modern era has been motivated by the interest in comparing data with the theory of Pratt and his coworkers. As bremsstrahlung research has moved into its postmodern era, new experiments with increasing precision are concentrating on determining under what conditions ordinary bremsstrahlung theory needs to be supplemented by a contribution from polarization bremsstrahlung. Efforts to improve the comparison of thin-target experiment with theory have also led to new experimental and modeling work on bremsstrahlung from thick solid targets. Thick-target bremsstrahlung is interesting in its own right, but we also want to understand it better since it is the ever-present background in the thin-target experiments and the limiting factor in the effort to distinguish the polarization contribution to the total bremsstrahlung spectrum. Professor Pratt ushered in the modern era in bremsstrahlung research and has recently guided the transition into the postmodern era. It can be expected that he will continue to have a formative influence on the developments of bremsstrahlung research into the foreseeable future.