Session BO7: Laser Plasma Electron Acceleration

Two-Stage Laser Wakefield Acceleration Experiments in the Bubble Regime

We present experiments with the goal to produce high energy ($\sim $500 MeV), narrow energy spread ($<$10{\%}) electron bunches produced in a two-stage gas cell via Laser Wakefield Acceleration. The experiments are performed at the Jupiter Laser Facility, Lawrence Livermore National Laboratory, using the 800 nm, 200 TW, 60 fs Callisto laser focused onto the entance pinhole of the gas cell. The first stage is filled with He and $<$1{\%} of a high-Z dopant gas, while the second stage is filled with pure He to balance the electron densities between the stages. Rather than injecting charge throughout the experiment, electrons are injected into the wake over a limited distance in the first stage by ionization-induced trapping of the dopant K-shell electrons at densities that are too low to self-trap He electrons ($<$3x10$^{18}$ cm$^{-3})$. These electrons need to be coupled into the second stage, which continues to accelerate the bunch without contributing additional charge. Comparison to 3-D PIC simulations using OSIRIS are also presented. This work was performed under the auspices of the U.S. DoE by LLNL under Contract DE-AC52-07NA27344 and was partially funded by the Laboratory Directed Research and Development Program under project tracking code 06-ERD-056.   

Laser Wakefield Acceleration in Gas Cell Targets

Electron beams produced from laser wakefield acceleration (LWFA) may be a suitable injection source for TeV electron accelerators, having already demonstrated beam charge $>$ nC with GeV energy in just $\sim $cm length. Additionally, the LWFA process has produced x-ray beams with peak spectral brightness comparable to third generation light sources. One of the most promising LWFA methods to date has been acceleration within a preformed plasma to guide a high-power laser. However, nonlinear effects in the plasma, such as self focusing and self phase modulation, may allow the use of simpler quiescent gas cell targets. Channeling with cm scale has been observed in a gas cell using 100 TW laser power from the HERCULES laser (Ti:Sapphire, 30 fs, 0.1 Hz). Accelerated protons were observed in the radial direction, which may be produced when the electron beam passes through the plasma, causing a Coulomb explosion. Experimental results are presented showing the effect of gas mixing and focusing geometry on channeling, acceleration, and x-ray generation.   

Laser and electron beam propagation and stability in laser wakefield accelerators

Presented here are recent experimental and simulation results on laser and electron beam propagation and stability in laser wakefield accelerators (LWFAs) using the HERCULES laser. By using various gas jet nozzles with opening diameters ranging from 0.5 to 5 mm, electron injection, acceleration and laser propagation were studied. Electron beams produced in plasma channels significantly longer than the laser depletion length were observed to break into a number of filaments. This is likely due to a current filamentation instability as the electron beam propagates through unperturbed plasma after pump depletion. Experiments and simulations also 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 that it affects the quality of electron beams generated.   

Multi-GeV electron generation using Texas petawatt laser system

The parameters of the Texas Petawatt (PW) laser system presently make it the unique facility that can accomplish self-guided multi-GeV electron acceleration. Simulation results [1] show that the PW laser beam can be self-guided up to 10cm in the plasma bubble regime of LWFA, significantly increasing the electron acceleration length. It is also shown that electrons can be self-injected into the plasma bubble; small admixture of high Z gas may be required in some regimes to assist self-injection. $\sim $7GeV with less than 10{\%} energy spread and $\sim $1nC electron beams are expected to be generated with the above experimental conditions. Optical diagnostics include transverse Thomson scattering of the PW drive beam to observe laser self-guiding, and transverse shadowgraphy and interferometry to observe plasma morphology. Single-shot Frequency Domain Holography (FDH) [2] will also be employed in an off-axis geometry to visualize the formation and evolution of the plasma bubble. \\[4pt] [1] S. Y. Kalmykov et. al., Phys. Rev. Lett. 103, 135004 (2009) \\[0pt] [2] N. H. Matlis et. al., Nature Phys. 2, 749 (2006)   

Measurement of beam transverse emittance via measurement of the x-ray source size in a wakefield accelerator

We propose and use a new technique to measure the transverse emittance of a laser-wakefield accelerated beam of relativistic electrons. The technique is based on the simultaneous measurements of the electron beam divergence given by $p_{\perp}/p_{\parallel}$, the measured longitudinal spectrum $p_\parallel$ and the transverse electron bunch size in the bubble $r_{\perp}$. The latter is obtained via the measurement of the source size of the x-rays emitted by the accelerating electron bunch in the bubble. These so-called betatron x-rays $\left[ 1 \right]$ have also shown to be spatially coherent and as bright as currently existing 3rd generation Synchrotrons $\left[ 2 \right]$. We measure a normalized beam transverse emittance as small as 0.6~$\pi$~mm$\:$mrad for a monoenergetic electron beam with 400~MeV energy.\\ $\left[ 1 \right]$ A. Rousse, et. al. Phys. Rev. Lett. \textbf{93}, 135005 (2004)\\ $\left[ 2 \right]$ S. Kneip, et. al. Nature Physics, submitted (2010)   

Electro-Optic Detection of Ultrashort Electron Beams Produced in Laser Wakefield Accelerators

Electro-Optic (EO) detection is a non-invasive technique to measure the longitudinal profile of relativistic electron bunches. To extend this technique to ultrashort electron beams ($<$10 fs) produced in laser wakefield accelerators many of the assumptions used to describe EO detection are no longer valid. Current EO detection schemes avoid material resonances and assume that the effect of the electric field on the probe beam can be described purely as a Pockels effect. Unfortunately, material resonance cannot be avoided and the assumption of Pockels effect cannot be made when dealing with the ultrashort beams produced in a laser wakefield accelerator. Theoretical, simulation, and experimental work is being done at the U.S. Naval Research Laboratory to address these effects and extend EO detection to the measurement of ultrashort electron bunches. Current results will be discussed.   

Measurements of the Correlation between Plasma Bubble Dynamics and Electron Trapping in a Laser Wakefield Accelerator

Generation of conically emitted second harmonic radiation has recently been observed in a laser wakefield accelerator experiment at the Naval Research Laboratory. This second harmonic is the result of frequency mixing within the sheath surrounding a fully cavitated plasma region, ``plasma bubble,'' created by the pondermotive force of a laser. Using this second harmonic signature, we have indirectly studied the dynamics of a plasma bubble. It has been observed that the plasma bubble dynamics are strongly correlated to the generation of electrons. Specifically, the onset of the bubble is connected to the generation of off-axis electrons, while forward accelerated electrons have been observed when the conical distribution of second harmonic is broken, signifying the disruption of the plasma bubble. Further results on bubble dynamics and its connection to electron beam production will be presented.   

Dynamics of nonlinear laser-plasma accelerators (LPA) probed by frequency-domain holography(FDH)

We report three new results from the probing of nonlinear LPA structures by FDH. 1.The amplitude of sinusoidal wakes driven by mildly relativistic(a$_{0}\sim $1.5)laser pulses is observed to grow monotonically with increasing distance behind the drive pulse, before the wave breaks. The growth in amplitude correlates with growth in wavefront curvature. Both effects are explained by the coherent mixing of trajectories of plasma fluid elements possessing slightly different initial plasma frequencies as a result of radial variation of the relativistic gamma-factor across the drive pulse profile. 2.We reported that LPA structures (``bubbles'') reshape co-propagating probe pulses into optical ``bullets''. Here we report direct observation of the bubble by FDH. Correlation of the phase-modulated bubble image with the optical bullets reveals a temporal offset that is explained by beam loading of the plasma bubble accelerator. 3.Optical bullets are shown to possess a flat temporal phase, signifying efficient pulse compression, in contrast to the chirp of the rest of the probe pulse. The result suggests an application of bubbles as compressors for intense pulses.   

Injection and trapping of electrons into a LWFA via tunneling ionization

Results from experiments, PIC simulations and theory are presented on the injection of electrons into a laser wakefield accelerator via tunneling ionization. In this work, a Ti:Sapphire laser was focused to an a$_{o}$ of 1.5-2.5 onto a gas jet target comprised of mixture of 90:10{\%} He to N$_{2}$. There is a large step in the ionization potential (IP) between the L-and K-shell electrons of nitrogen. The step in IP can be matched to the laser intensity profile and plasma density, such that electrons from the K-shell of nitrogen that are created near the peak field of the laser, are injected into the electric field of the fully formed wake created from the He and L-shell electrons of N$_{2}$. It is shown that the injection of electrons via ionization reduces the wake amplitude and thus the laser power required to trap and accelerate electrons. The way in which ionization injection effects the electron energy gain, divergence and charge will be discussed. Work supported by DOE grants DE-FG02-92ER40727, DE-FC02-07ER41500, DE-FG52-09NA29552 NSF grants PHY-0936266, PHY-0904039.   

Controlled injection in plasma based acceleration using external magnetic fields

As the principles of plasma based acceleration are now firmly established, significant interest is being devoted to the control of the acceleration processes, critical in applications. In addition, the possibility to use plasma based accelerators technology to provide compact light sources is also being thoroughly investigated. In this work we present a novel controlled injection configuration which uses a transverse static magnetic field to trigger self-injection, and to taylor the final electron bunch properties. This novel scheme can be applied in both laser, and plasma wakefield accelerators, leading to off-axis injection and to coordinated betatron oscillations of the injected bunch. Key features of the emitted radiation will also be analysed by investigating the trajectories of the accelerated electrons. The results are supported by fully-kinetic relativistic particle-in-cell simulations in OSIRIS, complemented by an analytical Hamiltonean model.   

Transverse Electron Motion and Multiple Electron Injection in Blowout Bubble of Laser Wakefield Accelerator

An analytical formula for electron motion in a spherical bubble was compared with data from electron acceleration experiments using the HERCULES laser system showing reasonable agreement. This also provides evidence for continuous injection of electrons into the bubble with multiple bunches in the bubble separated both transversely and longitudinally. The only free parameter in the analytical model was radius of the bubble ($r_{b}$) which was found to be close to the matched spot size for self focusing. The RMS electron beam divergence is found to increase with bunch charge also suggesting tradeoff between beam divergence and photon number in applications for such beams as an x-ray source.   

Colliding Pulse Injection Control and X-Ray Sources

Reduced beam energy spread, fluctuation, and emittance are important to applications of high gradient laser-plasma wakefield accelerators including Thomson gamma sources and high energy colliders. Experiments and simulations will be presented on control of injection to improve beam quality using the beat between multiple laser pulses to via kick electrons in momentum and phase into the wake accelerating phase. Stable intersection and beam performance over hours are obtained using active pointing control. Dependence on laser and plasma parameters is characterized in coordination with simulations. Electron beam source size and position are measured using betatron X-ray emission produced when electrons oscillate in the focusing field of the wake to improve understanding of beam emittance and stability, also producing a broadband, synchronized fs source of keV X-rays.   

Predictive design and interpretation of colliding pulse injected laser wakefield experiments

The use of colliding laser pulses to control the injection of plasma electrons into the plasma wake of a laser plasma accelerator is a promising approach to obtaining stable, tunable electron bunches with reduced emittance and energy spread. Colliding Pulse Injection (CPI) experiments are being performed by groups around the world. We will present recent particle-in-cell simulations, using the parallel VORPAL framework, of CPI for physical parameters relevant to ongoing experiments of the LOASIS program at LBNL. We evaluate the effect of laser and plasma tuning, on the trapped electron bunch and perform parameter scans in order to optimize the quality of the bunch. Impact of non-ideal effects such as imperfect laser modes and laser self focusing are also evaluated. Simulation data are validated against current experimental results, and are used to design future experiments.   

Non-linear Pulse Propagation in Structured Plasma Channels

We consider two non-linear processes in structured plasma channels: forward Raman scattering, and phase locking of radial eigenmodes. In traditional analysis, Raman scattering is an interaction between an incident light wave, scattered light wave, and plasma density fluctuation that are \textit{linearly phase matched}. We find that the traditional analysis, including the damping due to phase mixing and the spatial localization and discreteness of modes, greatly overestimates the growth of the instability. Furthermore, we find that the presence of axial modulations reduce the growth of the Raman instability allowing for the stable guiding of long pulses. This effect is ideal for quasi-phase matched direct acceleration which relies on long pulse lengths for high energy gain [1]. The second effect we consider is \textit{non-linear phase-locking} of radial eigenmodes which \textit{cannot} be linearly phase matched. The eigenvalues of the radial channel modes are typically on the order of the relativistic shift to the plasma frequency. For modest vector potentials a subset of the channel modes become locked (degenerate) leading to a ``super-mode'' of the channel, which evolves with a single group velocity. This is an intermediate regime of guiding between channel guiding and non-linear self-guding. [1] J. P. Palastro, T. M. Antonsen, S. Morshed, A. G. York, and H. M. Milchberg, Phys. Rev. E 77, 036405 (2008).   

A Computational Investigation of the Impact of Non-Gaussian, ``Low-Quality'' Laser Pulses on Electron Beam Properties in Laser-Wakefield Acceleration Experiments

The impact of non-Gaussian, ``low-quality,'' laser pulse profiles on the performance of laser-wakefield acceleration (LWFA) experiments is investigated computationally using the particle-in-cell simulation code OSIRIS 2.0. A baseline simulation of a $TEM_{00}$-mode (Gaussian) pulse is performed, and the properties of the electron beam produced by this pulse are measured. This pulse is then ``mixed'' with a TEM 10- mode pulse across a range of mixing percentages (while maintaining a constant total pulse energy), as a preliminary investigation into the effects of non-Gaussian pulse profiles on LWFA performance. Scalings relating the divergence, emittance, and peak pulse energy of the LWFA-produced electron beam to the mode mixing percentage are established. Using this simplified parameter sweep as a reference, more complex simulations of LWFA experiments with optical aberrations including comaticities, astigmatisms, and spherical aberrations are performed and analyzed. Modifications to OSIRIS to enable the explicit inclusion of these aberrations are discussed.