This PDF file contains the front matter associated with SPIE Proceedings Volume 9514, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

By employing a pair of partially overlapped supersonic gas jets, we separated injection and acceleration stages of laser wakefield acceleration to produce stable, monoenergetic, and tunable electron beams. The first jet (injector) utilized a He/N₂ mixture and resulted in electrons injected into the wake via ionization-assisted injection. These electrons were then accelerated in the second jet (accelerator) using pure He. By changing length and plasma density of the accelerator jet, we were able to tune electron energy in the 50 – 300 MeV range with energy spread of 10-30% and 20 pC charge. Simulations show that the injection region is limited within the overlap of the jets.

Ultra-low emittance (tens of nm) beams can be generated in a plasma accelerator using ionization injection of electrons into a wakefield. An all-optical method of beam generation uses two laser pulses of different colors. A long-wavelength drive laser pulse (with a large ponderomotive force and small peak electric field) is used to excite a large wakefield without fully ionizing a gas, and a short-wavelength injection laser pulse (with a small ponderomotive force and large peak electric field), co-propagating and delayed with respect to the pump laser, to ionize a fraction of the remaining bound electrons at a trapped wake phase, generating an electron beam that is accelerated in the wake. The trapping condition, the ionized electron distribution, and the trapped bunch dynamics are discussed. Expressions for the beam transverse emittance, parallel and orthogonal to the ionization laser polarization, are derived. An example is presented using a 10-µm CO₂ laser to drive the wake and a frequency-doubled Ti:Al₂O₃ laser for ionization injection.

An intense laser pulse propagating through a near-critical density plasma is capable of generating a high peak current electron beam (~100kA) by means of a laser wakefield operating in the bubble regime. This beam drives surface currents on the sheath of the cavitation that produces large azithumal magnetic fields (~100 MG) and induces an on-axis electron current. These effects lead to a Z-pinch of the ambient plasma ions. Eventually the field confining the pinch relaxes and the ions explode radially due to self-repulsive forces. If this process occurs at a sharp exit gradient, the ions acquire some forward momentum consistent with the magnetic vortex acceleration mechanism. Fully 3D simulations indicate that the highest energy ions are emitted conically with a secondary lower energy ion beam accelerated on the axis.

Recent theoretical and experimental studies suggest the possibility of enhancing the efficiency and ease of laser acceleration of protons and ions using underdense or near critical plasmas through electrostatic shocks. Very promising results were recently obtained in this regime. In these experiments, a first ns pulse was focused on a thin target to explode it and a second laser with a high intensity was focused on the exploded foil. The delay between two lasers allowed to control the density gradient seen by the second laser pulse. The transition between various laser ion acceleration regimes depending on the density gradient length was studied. With a laser energy of a few Joules, protons with energies close to the energies of TNSA accelerated protons were obtained for various exploded foils configurations. In the high energy regime (~180 J), protons with energies significantly higher than the ones of TNSA accelerated protons were obtained when exploding the foil while keeping a good beam quality. These results demonstrate that low-density targets are promising candidates for an efficient proton source that can be optimized by choosing appropriate plasma conditions. New experiments were also performed in this regime with gas jets. Scaling shock acceleration in the low density regime to ultra high intensities is a challenge as radiation losses and electron positron pair production change the optimization of the shock process. Using large-scale Particle-In-Cell simulations, the transition to this regime in which intense beams of relativistic ions can be produced is investigated.

We present our recent experimental results of monoenergetic protons accelerated from the interaction of an intense terawatt CO₂ laser pulse with a near-critical hydrogen gas target, with its density profile tailored by a hydrodynamic shock. A 5-ns Nd:YAG laser pulse is focused onto a piece of stainless steel foil mounted at the front edge of the gas jet nozzle orifice. The ablation launches a spherical shock into the near-critical gas column, which creates a sharp density gradient at the front edge of the target, with ~ 6X local density enhancement up to several times of critical density within ~<100 microns. With such density profile, we have obtained monoenergetic proton beams with good shot-to-shot reproducibility and energies up to 1.2 MeV.

The laser group velocity plays a crucial role in laser driven acceleration of electrons and ions. In particular, a highly efficient mechanism of laser driven ion acceleration, Radiation Pressure Acceleration, has a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. However there is another limiting factor that may shed the group velocity effects. It is due to the transverse expansion of the target, which happens in the course of a tightly focused laser pulse interaction with a thin foil. Transversely expanding targets become increasingly transparent for radiation thus terminating the acceleration. Utilization of an external guiding structure for the accelerating laser pulse may provide a way of compensating for the group velocity and transverse expansion effects.

We propose to use a combination of Hermite-Gaussian laser modes to generate a nonlinear ring bubble with a large longitudinal accelerating field and a transverse focusing field suitable for positron beam focusing and acceleration. The nonlinear bubble can provide higher accelerating gradients compared with a linear plasma wake. PIC simulations have been used to demonstrate control of the focusing force by changing the relative intensity ratio of the two laser modes, enabling matched positron beam propagation for emittance preservation. In addition, in order to improve phase space characteristics of laser wakefield produced electron beams, in particular, to reduce their transverse emittance, we propose to generate femtosecond electron beams with ultralow emittance using a two-color laser-ionization injection method. By controlling the amplitude and the duration of the injection pulse, the emittance can be controlled to the level of 10^-2 mm mrad.

At the Texas Petawatt laser facility we developed a novel ultra-short pulsed laser-driven neutron source generating an unprecedented output peak flux. Our results show a dramatic onset of high-energy electron generation from petawatt laser-irradiated plastic targets for targets thinner than a few microns. In this regime, the copious amounts of multi-MeV electrons emitted from the target are utilized to generate photo-neutrons from a metal converter. The neutrons are generated with a <50 ps pulse duration and a flux of 10¹⁸ n/cm²/s, exceeding any other pulsed or CW neutron source. In this paper, we will report on our measurement of the neutron yields produced from high atomic number converters.

The generation of energetic electrons by the interaction of a short laser pulse with solid “grating” targets, having a periodic groove on the irradiated surface, has been investigated in a regime of ultrahigh contrast (10¹²) and relativistically strong intensity (> 10¹⁹W/cm²). A strong enhancement of both the energy and number of electrons emitted from the target, with respect to at targets, has been observed for incidence angles close to the resonant condition for surface wave excitation. In particular we identified bunches of electrons with energies exceeding 10 MeV which are emitted in a direction close to the target surface. The experimental results are well reproduced by a three-dimensional particle-in-cell simulation, which confirms the dominant role of the surface wave in accelerating the electrons. These results are a step forward the development of high field plasmonics for a number of applications.

Compact γ-ray sources are of key importance not only for fundamental research but also for paramount practical applications such as cancer radiotherapy, active interrogation of materials, and high-energy radiography. Particular characteristics are required for meaningful implementation: multi-MeV energies per photon, a high degree of collimation, and a high peak brilliance. Laser-driven sources are theoretically expected to deliver such capabilities but experiments to date have reported either sub-MeV photon energies, or relatively low brilliance. By entering the non-linear regime of Thomson scattering, we report here on the first experimental realisation of a compact laser-driven γ-ray source that simultaneously ensures ultra-high brilliance (≈10¹⁹ photons s^-1 mm^-2 mrad^-2 0.1% BW), low divergence (≈ mrad), and high photon energy (up to 18 MeV). The reported brilliance exceeds by two orders of magnitudes those of alternative mechanisms and it is the highest ever achieved in the multi-MeV regime in a laboratory experiment.

We present recent experiments on the characterization of Betatron radiation in the blowout regime of laser-wakefield acceleration. We observed Betatron x-rays up to 80 keV, and the characterization of the angular dependence of the x-ray spectrum suggests anisotropic electron trajectories in the plasma. The characterization of the source opens up new possibilities for application experiments.

The dynamics of ionization-induced electron injection in the high density (~ 1:2 × 10¹⁹cm^-3) regime of Laser Wakefield Acceleration (LWFA) was investigated by analyzing betatron X-ray emission inside dielectric capillary tubes. A comparative study of the electron and betatron X-ray properties was performed for both self-injection and ionization-induced injection. Direct experimental evidence of early onset of ionization-induced injection into the plasma wave was obtained by mapping the X-ray emission zone inside the plasma. Particle-In-Cell (PIC) simulations showed that the early onset of ionization-induced injection, due to its lower trapping threshold, suppresses self-injection of electrons. An increase of X-ray fluence by at least a factor of two was observed in the case of ionization-induced injection due to an increased trapped charge compared to self-injection mechanism.

Evidence of magnetic reconnection (MR) events driven by relativistic electrons is observed between two high-intensity laser/plasma interaction sites. The two laser foci were on average 20um FWHM containing 50TW of power each, delivered with a split f/3 paraboloid onto copper foil targets at a focused intensity of 10¹⁹ W/cm² with the HERCULES laser system. Cu K-alpha emissions from the interactions were imaged with a spherically bent Quartz crystal, and by motorizing one half of the paraboloid vertically the focal separation was varied between 0- 400um.

Splitting the beam halves revealed an enhanced region between the foci with the highest a maximized K-alpha signal intensity at one inter-beam separation, evidencing inflow from relativistic electron driven MR. A filtered LANEX screen was imaged to search for outflow/jet electrons along the plane of the target surface and normal to the axis defined by the two spots, to calculate the electron temperature and to search for spatial profile nonuniformities potentially directly originating from reconnection events. Ongoing 2D and 3D PIC simulations are being conducted to better understand and model the measured electron outflow dynamics.

The development of very high power lasers opens up new horizons in various fields, such as laser plasma acceleration in Physics and innovative approaches for proton therapy in Medicine. Laser driven proton acceleration is commonly based on the so-called Target Normal Sheath Acceleration (TNSA) mechanisms: a high power laser is focused onto a solid target (thin metallic or plastic foil) and interact with matter at very high intensity, thus generating a plasma; as a consequence “hot” electrons are produced and move into the forward direction through the target. Protons are generated at the target rear side, electrons try to escape from the target and an ultra-strong quasi-electrostatic field (~1TV/m) is generated. Such a field can accelerate protons with a wide energy spectrum (1-200 MeV) in a few tens of micrometers. The proton beam characteristics depend on the laser parameters and on the target geometry and nature. This technique has been validated experimentally in several high power laser facilities by accelerating protons coming from hydrogenated contaminant (mainly water) at the rear of metallic target, however, several research groups are investigating the possibility to perform experiments by using “pure” hydrogen targets. In this context, the low temperature laboratory at CEA-Grenoble has developed a cryostat able to continuously produce a thin hydrogen ribbon (from 40 to 100 microns thick). A new extrusion concept, without any moving part has been carried out, using only the thermodynamic properties of the fluid. First results and perspectives are presented in this paper.

X-ray phase contrast imaging (X-PCi) is a very promising method of dramatically enhancing the contrast of X-ray images of microscopic weakly absorbing objects and soft tissue, which may lead to significant advancement in medical imaging with high-resolution and low-dose. The interest in X-PCi is giving rise to a demand for effective simulation methods. Monte Carlo codes have been proved a valuable tool for studying X-PCi including coherent effects. The laser-plasma wakefield accelerators (LWFA) is a very compact particle accelerator that uses plasma as an accelerating medium. Accelerating gradient in excess of 1 GV/cm can be obtained, which makes them over a thousand times more compact than conventional accelerators. LWFA are also sources of brilliant betatron radiation, which are promising for applications including medical imaging. We present a study that explores the potential of LWFA-based betatron sources for medical X-PCi and investigate its resolution limit using numerical simulations based on the FLUKA Monte Carlo code, and present preliminary experimental results.

This paper describes the extreme ultraviolet and soft x-ray emission recorded in the 2-12 nm region from Mo, Ru, Rh and Pd ions present in the laser produced plasmas. The spectra were found to be dominated by 3p-3d transitions in the 5-8 nm region, which shift slowly to shorter wavelengths with the increasing atomic number, and by 3d-4p and 3d-4f transitions at shorter wavelengths. These transitions, in a number of neighbouring ion stages, were distinguished by comparison with Cowan code calculations and previously reported data. The experimental results show that strong emission can be observed at the 6.X nm region for Ru, Rh and Pd plasmas.

The Spanish Pulsed Laser Centre (CLPU) is a new high-power laser facility for users. Its main system, VEGA, is a CPA Ti:Sapphire laser which, in its final phase, will be able to reach petawatt peak powers in pulses of 30 fs with a pulse contrast of 1 : 10¹⁰ at 1 ps. The extremely low level of pre-pulse intensity makes this system ideally suited for studying the laser interaction with ultrathin targets. We have used the particle-in-cell (PIC) code OSIRIS to carry out 2D simulations of the acceleration of ions from ultrathin solid targets under the unique conditions provided by VEGA, with laser intensities up to 10²²Wcm^-2 impinging normally on 5 - 40 nm thick overdense plasmas, with different polarizations and pre-plasma scale lengths. We show how signatures of the radiation pressure dominated regime, such as layer compression and bunch formation, are only present with circular polarization. By passively shaping the density gradient of the plasma, we demonstrate an enhancement in peak energy up to tens of MeV and monoenergetic features. On the contrary linear polarization at the same intensity level causes the target to blow up, resulting in much lower energies and broader spectra. One limiting factor of Radiation Pressure Acceleration is the development of Rayleigh-Taylor like instabilities at the interface of the plasma and photon fluid. This results in the formation of bubbles in the spatial profile of laser-accelerated proton beams. These structures were previously evidenced both experimentally and theoretically. We have performed 2D simulations to characterize this bubble-like structure and report on the dependency on laser and target parameters.

Laser wakefield acceleration (LWFA) is able to produce high quality electron bunches interesting for many applications ranging from coherent light sources to high energy physics. The blow-out regime of LWFA provides excellent accelerating structure able to maintain small transverse emittance and energy spread of the accelerating electron beam if combined with localised injection. A modulation of the back of a self-injected electron bunch in the blowout regime of Laser Wakefield Acceleration appears 3D Particle-in-Cell simulations with the code OSIRIS. The shape of the modulation is connected to the polarization of the driving laser pulse, although the wavelength of the modulation is longer than that of the pulse. Nevertheless a circularly polarized laser pulse leads to a corkscrew-like modulation, while in the case of linear polarization, the modulation lies in the polarization plane.

Relativistic electron beams have applications spanning materials science, medicine, and home- land security. Recent advances in short pulse laser technology have enabled the production of very high focused intensities at kHz rep rates. Consequently this has led to the generation of high ux sources of relativistic electrons- which is a necessary characteristic of these laser plasma sources for any potential application. In our experiments, through the generation of a plasma with the lambda cubed laser system at the University of Michigan (a 5 × 10¹⁸W=cm², 500 Hz, Ti:Sapphire laser), we have measured electrons ejected from the surface of fused silica nd Cu targets having energies in excess of an MeV. The spectrum of these electrons was measured with respect to incident laser angle, prepulse timing, and focusing conditions. While taken at a high repetition rate, the pulse energy of the lambda cubed system was consistently on the order of 10 mJ. In order to predict scaling of the electron energy with laser pulse energy, simulations are underway which compare the spectrum generated with the lambda cubed system to the predicted spectrum generated on the petawatt scale HERCULES laser system at the University of Michigan.