Observation of Monoenergetic Electrons from Two-Pulse Ionization Injection in Quasilinear Laser Wakefields.

The generation of low emittance electron beams from laser-driven wakefields is crucial for the development of compact x-ray sources. Here, we show new results for the injection and acceleration of quasimonoenergetic electron beams in low amplitude wakefields experimentally and using simulations. This is achieved by using two laser pulses decoupling the wakefield generation from the electron trapping via ionization injection. The injection duration, which affects the beam charge and energy spread, is found to be tunable by adjusting the relative pulse delay. By changing the polarization of the injector pulse, reducing the ionization volume, the electron spectra of the accelerated electron bunches are improved.

Optimization of the electron beam dump for a GeV-class laser electron accelerator.

The advances of laser-driven electron acceleration offer the promise of great reductions in the size of high-energy electron accelerator facilities. Accordingly, it is desirable to design compact radiation shielding for such facilities. A key component of radiation shielding is the high-energy electron beam dump. In an effort to optimize the electron beam dump design, different material combinations have been simulated with the FLUKA Monte Carlo code in the range of 1-40 GeV. The studied beam dump configurations consist of alternating layers of high-Z material (lead or iron) and low-Z material (high-density concrete or borated polyethylene) in either three-layer or five-layer structures. The designs of various beam dump configuration have been compared and it has been found that the iron and concrete stacking in a three-layer structure with a thick iron layer results in the lowest dose at 1, 10, and 40 GeV. The performance of the beam dump exhibits a strong dependence on the selected materials, the stacking method, the beam dump thickness, as well as the electron energy. This parametric study provides general insights that can be used for compact shielding design of future electron accelerator facilities.

Scintillator detector characterization for laser-driven proton beam imaging.

The spatial resolution and imaging characteristics of plastic scintillators are characterized using laser-driven proton beams. Laser-driven proton beams typically have broad energy spectra and are accompanied by relativistic electrons and high-energy photons, both potentially contributing to background noise. Different types and thicknesses of Eljen Technology scintillators are compared to determine their intrinsic point spread function. Point-projection imaging of a mesh is used to compare the imaging resolution of the scintillator to the usual imaging detector, radiochromic film, and is found to be reasonably comparable and sufficient for many experimental applications.

Enhanced spatial resolution of Eljen-204 plastic scintillators for use in rep-rated proton diagnostics.

A pixelated scintillator has been designed, fabricated, and tested using a laser-accelerated proton source for use in proton diagnostics at rep-rated laser facilities. The work presented here demonstrates the enhanced spatial resolution of thin, organic scintillators through a novel pixelation technique. Experimental measurements using laser-generated protons incident onto 130 µm-thick scintillators indicate a >20% reduction in the scintillator point spread function (PSF) for the detectors tested. The best performing pixelated detector reduced the ∼200 µm PSF of the stock material to ∼150 µm. The fabrication technique may be tailored to reduce the pixel size and achieve higher spatial resolutions.

Polarization-Dependent Self-Injection by Above Threshold Ionization Heating in a Laser Wakefield Accelerator.

We report on the experimental observation of a decreased self-injection threshold by using laser pulses with circular polarization in laser wakefield acceleration experiments in a nonpreformed plasma, compared to the usually employed linear polarization. A significantly higher electron beam charge was also observed for circular polarization compared to linear polarization over a wide range of parameters. Theoretical analysis and quasi-3D particle-in-cell simulations reveal that the self-injection and hence the laser wakefield acceleration is polarization dependent and indicate a different injection mechanism for circularly polarized laser pulses, originating from larger momentum gain by electrons during above threshold ionization. This enables electrons to meet the trapping condition more easily, and the resulting higher plasma temperature was confirmed via spectroscopy of the XUV plasma emission.

Analysis of sinusoidally modulated chirped laser pulses by temporally encoded spectral shifting.

We present an analytical formalism elucidating how information is stored in chirped optical probes by describing the effects of sinusoidal temporal modulations on the electric field. We show that the modulations produce spectral sidebands which can be interpreted as temporal sidebands due to the time-wavelength mapping, an effect we call temporally encoded spectral shifting (TESS). A derivation is presented for the case of chirped-pulse spectral interferometry showing how to recover both the amplitude and the periodicity of the modulation from a Fourier transform of the interferogram. The TESS effect, which provides an intuitive picture for interpreting pump-probe experiments with chirped pulses, is illustrated for probing wakefields from a laser-plasma accelerator.

High-Flux Femtosecond X-Ray Emission from Controlled Generation of Annular Electron Beams in a Laser Wakefield Accelerator.

Annular quasimonoenergetic electron beams with a mean energy in the range 200-400 MeV and charge on the order of several picocoulombs were generated in a laser wakefield accelerator and subsequently accelerated using a plasma afterburner in a two-stage gas cell. Generation of these beams is associated with injection occurring on the density down ramp between the stages. This well-localized injection produces a bunch of electrons performing coherent betatron oscillations in the wakefield, resulting in a significant increase in the x-ray yield. Annular electron distributions are detected in 40% of shots under optimal conditions. Simultaneous control of the pulse duration and frequency chirp enables optimization of both the energy and the energy spread of the annular beam and boosts the radiant energy per unit charge by almost an order of magnitude. These well-defined annular distributions of electrons are a promising source of high-brightness laser plasma-based x rays.

Table-top laser-based source of femtosecond, collimated, ultrarelativistic positron beams.

The generation of ultrarelativistic positron beams with short duration (τ(e+) ≃ 30 fs), small divergence (θ(e+) ≃ 3 mrad), and high density (n(e+) ≃ 10(14)-10(15) cm(-3)) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

Scaling high-order harmonic generation from laser-solid interactions to ultrahigh intensity.

Coherent x-ray beams with a subfemtosecond (<10(-15) s) pulse duration will enable measurements of fundamental atomic processes in a completely new regime. High-order harmonic generation (HOHG) using short pulse (<100 fs) infrared lasers focused to intensities surpassing 10(18) W cm(-2) onto a solid density plasma is a promising means of generating such short pulses. Critical to the relativistic oscillating mirror mechanism is the steepness of the plasma density gradient at the reflection point, characterized by a scale length, which can strongly influence the harmonic generation mechanism. It is shown that for intensities in excess of 10(21) W cm(-2) an optimum density ramp scale length exists that balances an increase in efficiency with a growth of parametric plasma wave instabilities. We show that for these higher intensities the optimal scale length is c/ω0, for which a variety of HOHG properties are optimized, including total conversion efficiency, HOHG divergence, and their power law scaling. Particle-in-cell simulations show striking evidence of the HOHG loss mechanism through parametric instabilities and relativistic self-phase modulation, which affect the produced spectra and conversion efficiency.

Ultrafast electron radiography of magnetic fields in high-intensity laser-solid interactions.

Using electron bunches generated by laser wakefield acceleration as a probe, the temporal evolution of magnetic fields generated by a 4 × 10(19) W/cm(2) ultrashort (30 fs) laser pulse focused on solid density targets is studied experimentally. Magnetic field strengths of order B(0) ~ 10(4) T are observed expanding at close to the speed of light from the interaction point of a high-contrast laser pulse with a 10-µm-thick aluminum foil to a maximum diameter of ~1 mm. The field dynamics are shown to agree with particle-in-cell simulations.

Finite spot effects on radiation pressure acceleration from intense high-contrast laser interactions with thin targets.

Short pulse laser interactions at intensities of 2×10(21) W cm(-2) with ultrahigh contrast (10(-15)) on submicrometer silicon nitride foils were studied experimentally by using linear and circular polarizations at normal incidence. It was observed that, as the target decreases in thickness, electron heating by the laser begins to occur for circular polarization leading to target normal sheath acceleration of contaminant ions, while at thicker targets no acceleration or electron heating is observed. For linear polarization, all targets showed exponential energy spreads with similar electron temperatures. Particle-in-cell simulations demonstrate that the heating is due to the rapid deformation of the target that occurs early in the interaction. These experiments demonstrate that finite spot size effects can severely restrict the regime suitable for radiation pressure acceleration.

Control of energy spread and dark current in proton and ion beams generated in high-contrast laser solid interactions.

By using temporal pulse shaping of high-contrast, short pulse laser interactions with solid density targets at intensities of 2 × 10(21) W cm(-2) at a 45° incident angle, we show that it is possible to reproducibly generate quasimonoenergetic proton and ion energy spectra. The presence of a short pulse prepulse 33 ps prior to the main pulse produced proton spectra with an energy spread between 25% and 60% (ΔE/E) with energy of several MeV, with light ions becoming quasimonoenergetic for 50 nm targets. When the prepulse was removed, the energy spectra was broad. Numerical simulations suggest that expansion of the rear-side contaminant layer allowed for density conditions that prevented the protons from being screened from the sheath field, thus providing a low energy cutoff in the observed spectra normal to the target surface.

Current filamentation instability in laser wakefield accelerators.

Experiments using an electron beam produced by laser-wakefield acceleration have shown that varying the overall beam-plasma interaction length results in current filamentation at lengths that exceed the laser depletion length in the plasma. Three-dimensional simulations show this to be a combination of hosing, beam erosion, and filamentation of the decelerated beam. This work suggests the ability to perform scaled experiments of astrophysical instabilities. Additionally, understanding the processes involved with electron beam propagation is essential to the development of wakefield accelerator applications.

High-power, kilojoule class laser channeling in millimeter-scale underdense plasma.

Experiments were performed using the Omega EP laser, operating at 740 J of energy in 8 ps (90 TW), which provides extreme conditions relevant to fast ignition studies. A carbon and hydrogen plasma plume was used as the underdense target and the interaction of the laser pulse propagating and channeling through the plasma was imaged using proton radiography. The early time expansion, channel evolution, filamentation, and self-correction of the channel was measured on a single shot via this method. A channel wall modulation was observed and attributed to surface waves. After around 50 ps, the channel had evolved to show bubblelike structures, which may be due to postsoliton remnants.

Stimulated Raman side scattering in laser wakefield acceleration.

Stimulated Raman side scattering of an ultrashort high power laser pulse is studied in experiments on laser wakefield acceleration. Experiments and simulations reveal that stimulated Raman side scattering occurs at the beginning of the interaction, that it contributes to the evolution of the pulse prior to wakefield formation, and also that it affects the quality of electron beams generated. The relativistic shift of the plasma frequency is measured.

Formation of optical bullets in laser-driven plasma bubble accelerators.

Electron density bubbles--wake structures generated in plasma of density n(e) approximately 10(19) cm(-3) by the light pressure of intense ultrashort laser pulses--are shown to reshape weak copropagating probe pulses into optical "bullets." The bullets are reconstructed using frequency-domain interferometric techniques in order to visualize bubble formation. Bullets are confined in three dimensions to plasma-wavelength size, and exhibit higher intensity, broader spectrum and flatter temporal phase than surrounding probe light, evidence of their compression by the bubble. Bullets observed at 0.8 approximately < n(e) approximately < 1.2x10(19) cm(-3) provide the first observation of bubble formation below the electron capture threshold. At higher n(e), bullets appear with high shot-to-shot stability together with relativistic electrons that vary widely in spectrum, and help relate bubble formation to fast electron generation.

Ionization induced trapping in a laser wakefield accelerator.

Experimental studies of electrons produced in a laser wakefield accelerator indicate trapping initiated by ionization of target gas atoms. Targets composed of helium and controlled amounts of various gases were found to increase the beam charge by as much as an order of magnitude compared to pure helium at the same electron density and decrease the beam divergence from 5.1+/-1.0 to 2.9+/-0.8 mrad. The measurements are supported by particle-in-cell modeling including ionization. This mechanism should allow generation of electron beams with lower emittance and higher charge than in preionized gas.

Accelerating monoenergetic protons from ultrathin foils by flat-top laser pulses in the directed-Coulomb-explosion regime.

We consider the effect of laser beam shaping on proton acceleration in the interaction of a tightly focused pulse with ultrathin double-layer solid targets in the regime of directed Coulomb explosion. In this regime, the heavy ions of the front layer are forced by the laser to expand predominantly in the direction of the pulse propagation, forming a moving longitudinal charge separation electric field, thus increasing the effectiveness of acceleration of second-layer protons. The utilization of beam shaping, namely, the use of flat-top beams, leads to more efficient proton acceleration due to the increase of the longitudinal field.

Ultra-high intensity- 300-TW laser at 0.1 Hz repetition rate.

We demonstrate the highest intensity - 300 TW laser by developing booster amplifying stage to the 50-TW-Ti:sapphire laser (HERCULES). To our knowledge this is the first multi-100TW-scale laser at 0.1 Hz repetition rate.