High resolution energy-angle correlation measurement of hard x rays from laser-Thomson backscattering

Thomson backscattering of intense laser pulses from relativistic electrons not only allows for the generation of bright x-ray pulses but also for the investigation of the complex particle dynamics at the interaction point. For this purpose a complete spectral characterization of a Thomson source powered by a compact linear electron accelerator is performed with unprecedented angular and energy resolution. A rigorous statistical analysis comparing experimental data to 3D simulations enables, e.g., the extraction of the angular distribution of electrons with 1.5% accuracy and, in total, provides predictive capability for the future high brightness hard x-ray source PHOENIX (photon electron collider for narrow bandwidth intense x rays) and potential gamma-ray sources.

Production of large volume, strongly magnetized laser-produced plasmas by use of pulsed external magnetic fields

The production of strongly magnetized laser plasmas, of interest for laboratory astrophysics and inertial confinement fusion studies, is presented. This is achieved by coupling a 16 kV pulse-power system. This is achieved by coupling a 16 kV pulse-power system, which generates a magnetic field by means of a split coil, with the ELFIE laser facility at Ecole Polytechnique. In order to influence the plasma dynamics in a significant manner, the system can generate, repetitively and without debris, high amplitude magnetic fields (40 T) in a manner compatible with a high-energy laser environment. A description of the system and preliminary results demonstrating the possibility to magnetically collimate plasma jets are given.

Direct observation of prompt pre-thermal laser ion sheath acceleration

High-intensity laser plasma-based ion accelerators provide unsurpassed field gradients in the megavolt-per-micrometer range. They represent promising candidates for next-generation applications such as ion beam cancer therapy in compact facilities. The weak scaling of maximum ion energies with the square-root of the laser intensity, established for large sub-picosecond class laser systems, motivates the search for more efficient acceleration processes. Here we demonstrate that for ultrashort (pulse duration ~30 fs) highly relativistic (intensity ~10²¹ W cm⁻²) laser pulses, the intra-pulse phase of the proton acceleration process becomes relevant, yielding maximum energies of around 20 MeV. Prominent non-target-normal emission of energetic protons, reflecting an engineered asymmetry in the field distribution of promptly accelerated electrons, is used to identify this pre-thermal phase of the acceleration. The relevant timescale reveals the underlying physics leading to the near-linear intensity scaling observed for 100 TW class table-top laser systems.

High-intensity laser-plasma ion generation is promising as a compact proton source for applications like ion beam therapy. Using a femtosecond table-top laser system, Zeil et al. show that protons efficiently gain energy in the pre-thermal intra-pulse phase of the generation process.

Hot electrons transverse refluxing in ultraintense laser-solid interactions

We have analyzed the coupling of ultraintense lasers (at ∼2×10{19}  W/cm{2}) with solid foils of limited transverse extent (∼10  s of μm) by monitoring the electrons and ions emitted from the target. We observe that reducing the target surface area allows electrons at the target surface to be reflected from the target edges during or shortly after the laser pulse. This transverse refluxing can maintain a hotter, denser and more homogeneous electron sheath around the target for a longer time. Consequently, when transverse refluxing takes places within the acceleration time of associated ions, we observe increased maximum proton energies (up to threefold), increased laser-to-ion conversion efficiency (up to a factor 30), and reduced divergence which bodes well for a number of applications.

Efficient laser-ion acceleration from closely stacked ultrathin foils

A new scheme to efficiently accelerate protons by a single linear polarized high-intensity ultrashort laser pulse using multiple ultrathin foils is proposed. The foils are stacked at a spacing comparable to their thickness and subsequently irradiated by the same laser pulse. The foil thicknesses are chosen such that the laser light pressure can displace all electrons out of the foil. The authors present a simple, yet precise dynamical model of the acceleration process from which both optimum foil thickness and spacing can be derived. Extensive two-dimensional (2D) particle-in-cell simulations verify the model predictions and suggest an enhancement of the maximum proton kinetic energy by 30% for the two-foil case compared to a single foil.

Enhanced hot-electron localization and heating in high-contrast ultraintense laser irradiation of microcone targets

We report experiments demonstrating enhanced coupling efficiencies of high-contrast laser irradiation to nanofabricated conical targets. Peak temperatures near 200 eV are observed with modest laser energy (10 J), revealing similar hot-electron localization and material heating to reduced mass targets (RMTs), despite having a significantly larger mass. Collisional particle-in-cell simulations attribute the enhancement to self-generated resistive (approximately 10 MG) magnetic fields forming within the curvature of the cone wall, which confine energetic electrons to heat a reduced volume at the tip. This represents a different electron confinement mechanism (magnetic, as opposed to electrostatic sheath confinement in RMTs) controllable by target shape.

Emittance growth mechanisms for laser-accelerated proton beams

In recent experiments the transverse normalized rms emittance of laser-accelerated MeV ion beams was found to be < 0.002 mm mrad, which is at least 100 times smaller than the emittance of thermal ion sources used in accelerators [T. E. Cowan, Phys. Rev. Lett. 92, 204801 (2004)]. We investigate the origin for the low emittance of laser-accelerated proton beams by studying several candidates for emittance-growth mechanisms. As our main tools, we use analytical models and one- and two-dimensional particle-in-cell simulations that have been modified to include binary collisions between particles. We find that the dominant source of emittance is filamentation of the laser-generated hot electron jets that drive the ion acceleration. Cold electron-ion collisions that occur before ions are accelerated contribute less than ten percent of the final emittance. Our results are in qualitative agreement with the experiment, for which we present a refined analysis relating emittance to temperature, a better representative of the fundamental beam physics.

Study of saturation of CR39 nuclear track detectors at high ion fluence and of associated artifact patterns

The occurrence of saturation in CR39 solid state nuclear track detectors has been systematically studied as a function of the incident ion (alpha particles and laser-accelerated protons) fluence and the etching time. When overexposed (i.e., for fluences above approximately 10(8) particles/cm(2)) and/or overetched, the CR39 detectors enter a saturated regime where direct track counting is not possible anymore. In this regime, optical measurements of saturated CR39 detectors become unreliable as well, since the optical response of the saturated detectors with respect to the ion fluence is highly nonlinear. This nonlinear optical response is likely due to scattering from the surface of irregular clumping patterns which have a diameter approximately 20 microm, i.e., ten times larger than the diameter of individual tracks. These patterns, which aggregate many individual tracks, are observed to develop in highly saturated regimes. For fluences typical of high energy short pulse laser experiments, saturation occurs, inducing the appearance of artifact ringlike structures. By careful microscopic analysis, these artifact ring patterns can be distinguished from the genuine rings occurring below saturation and characteristic of low energy laser accelerated proton beams.

Dynamics of mass transport and magnetic fields in low-wire-number-array Z pinches

The dynamics of mass transport were observed in a wire array implosion with multiframe laser probing. Plasma bubbles arise at breaks in the wires. Interferometry shows that the leading edge of the bubbles brings material to the axis of the array. The speed of this material was measured to be > or =3 x 10(7) cm/s during the wire array implosion. A shock was observed during the collision of the bubbles with the precursor. The Faraday effect indicates current flowing in breaks on the wires. The current switches from the imploding mass to the on-axis plasma column at the beginning of the x-ray pulse.

Ultraintense laser-produced fast-electron propagation in gas jets

We study the propagation of fast electrons in a gas at different densities. A large relativistic electron current is produced by focusing a short-pulse ultrahigh-intensity laser on a metallic target. It then propagates in a gas jet placed behind the foil. Shadowgraphy in the gas shows an electron cloud moving at sub-relativistic average velocities. The experiment shows (i) the essential role of the density of background material for allowing propagation of fast electrons, (ii) the importance of the ionization phase which produces free electrons available for the return current, and (iii) the effect of electrostatic fields on fast-electron propagation.

Comparison of laser ion acceleration from the front and rear surfaces of thin foils

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers (approximately 1-6 x 10(19) W/cm2) from both the front and rear surfaces of thin metal foils have been measured under identical conditions. Using direct beam measurements and nuclear activation techniques, we find that rear-surface acceleration produces higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. Our observations are well reproduced by realistic particle-in-cell simulations, and we predict optimal criteria for future applications.

Plasma devices to guide and collimate a high density of MeV electrons

The development of ultra-intense lasers has facilitated new studies in laboratory astrophysics and high-density nuclear science, including laser fusion. Such research relies on the efficient generation of enormous numbers of high-energy charged particles. For example, laser-matter interactions at petawatt (10(15) W) power levels can create pulses of MeV electrons with current densities as large as 10(12) A cm(-2). However, the divergence of these particle beams usually reduces the current density to a few times 10(6) A cm(-2) at distances of the order of centimetres from the source. The invention of devices that can direct such intense, pulsed energetic beams will revolutionize their applications. Here we report high-conductivity devices consisting of transient plasmas that increase the energy density of MeV electrons generated in laser-matter interactions by more than one order of magnitude. A plasma fibre created on a hollow-cone target guides and collimates electrons in a manner akin to the control of light by an optical fibre and collimator. Such plasma devices hold promise for applications using high energy-density particles and should trigger growth in charged particle optics.

Ultralow emittance, multi-MeV proton beams from a laser virtual-cathode plasma accelerator

The laminarity of high-current multi-MeV proton beams produced by irradiating thin metallic foils with ultraintense lasers has been measured. For proton energies >10 MeV, the transverse and longitudinal emittance are, respectively, <0.004 mm mrad and <10(-4) eV s, i.e., at least 100-fold and may be as much as 10(4)-fold better than conventional accelerator beams. The fast acceleration being electrostatic from an initially cold surface, only collisions with the accelerating fast electrons appear to limit the beam laminarity. The ion beam source size is measured to be <15 microm (FWHM) for proton energies >10 MeV.

Spatial uniformity of laser-accelerated ultrahigh-current MeV electron propagation in metals and insulators

The evolution of laser-generated MeV, MA electron beams propagating through conductors and insulators has been studied by comparing measurement and modeling of the distribution of MeV protons that are sheath accelerated by the propagated electrons. We find that electron flow through metals is uniform and can be laser imprinted, whereas propagation through insulators induces spatial disruption of the fast electrons. Agreement is found with material dependent modeling.

Isochoric heating of solid-density matter with an ultrafast proton beam

A new technique is described for the isochoric heating (i.e., heating at constant volume) of matter to high energy-density plasma states (>10(5) J/g) on a picosecond time scale (10(-12)sec). An intense, collimated, ultrashort-pulse beam of protons--generated by a high-intensity laser pulse--is used to isochorically heat a solid density material to a temperature of several eV. The duration of heating is shorter than the time scale for significant hydrodynamic expansion to occur; hence the material is heated to a solid density warm dense plasma state. Using spherically shaped laser targets, a focused proton beam is produced and used to heat a smaller volume to over 20 eV. The technique described of ultrafast proton heating provides a unique method for creating isochorically heated high-energy density plasma states.

Evidence of ultrashort electron bunches in laser-plasma interactions at relativistic intensities

The second harmonic of the laser light (2omega(0)) is observed on the rear side of thick solid targets irradiated by a laser beam at relativistic intensities. This emission is explained by the acceleration by the laser pulse in front of the target of short bunches of electrons separated by the period (or half the period) of the laser light. When reaching the rear side of the target, these electron bunches emit coherent transition radiation at 2omega(0). The observations indicate that, in our conditions, the minimum fraction of the laser energy transferred to these electron bunches is of the order of 1%.

Effects of nonionizing prepulses in high-intensity laser-solid interactions

We present theoretical and experimental evidence that nonionizing prepulses with intensities as low as 10(8)-10(9) W/cm(2) can substantially alter high intensity laser-solid interactions. We show that prepulse-heating and vaporization of the target can lead to a preformed plasma once the vapor is ionized by the rising edge of the high-intensity pulse. Our results indicate that peak prepulse intensity is not the only important parameter to consider in determining preformed plasma thresholds, and that a more comprehensive analysis of the prepulse duration and the target material is required.

Doppler broadening of in-flight positron annihilation radiation due to electron momentum

We report the first observation of electron momentum contributions to the Doppler broadening of radiation produced by in-flight two-photon annihilation in solids. In these experiments an approximately 2.5 MeV positron beam impinged on thin polyethylene, aluminum, and gold targets. Since energetic positrons easily penetrate the nuclear Coulomb potential and do not cause a strong charge polarization, the experimental annihilation line shapes agree well with calculations based on a simple independent-particle model. Moreover, annihilations with the deepest core electrons are greatly enhanced.

Fast ignition by intense laser-accelerated proton beams

The concept of fast ignition with inertial confinement fusion (ICF) is a way to reduce the energy required for ignition and burn and to maximize the gain produced by a single implosion. Based on recent experimental findings at the PETAWATT laser at Lawrence Livermore National Laboratory, an intense proton beam to achieve fast ignition is proposed. It is produced by direct laser acceleration and focused onto the pellet from the rear side of an irradiated target and can be integrated into a hohlraum for indirect drive ICF.

Characterization of fusion burn time in exploding deuterium cluster plasmas

Exploiting the energetic interaction of intense femtosecond laser pulses with deuterium clusters, it is possible to create conditions in which nuclear fusion results from explosions of these clusters. We have conducted high-resolution neutron time-of-flight spectroscopy on these plasmas and show that they yield fast bursts of nearly monochromatic fusion neutrons with temporal duration as short as a few hundred picoseconds. Such a short, nearly pointlike source now opens up the unique possibility of using these bright neutron pulses, either as a pump or a probe, to conduct ultrafast studies with neutrons.