Session CO7: Laser Wakefield Acceleration

Ultra-low-emittance electron bunches from a laser-plasma accelerator measured using single shot X-ray spectroscopy

Single-shot spectroscopic measurements of betatron X-rays are reported, and used to infer the transverse bunch size of both broadband sub-100 MeV and low-energy-spread 0.5 GeV electron beams produced by a laser-plasma accelerator. The measurements use semiconductor detector arrays, and spectra are obtained via single pixel absorption and cluster techniques. By matching the X-ray betatron spectra to analytical and numerical models of betatron radiation, the electron bunch radius inside the plasma is estimated to be $\sim$0.1$\mu$m. Combined with simultaneous electron spectrum and divergence measurements, the normalized transverse emittance is estimated to be as low as 0.1 mm mrad, consistent with three-dimensional particle-in cell simulations. This emittance is lower than previously measured, important for applications including gamma sources and colliders.   

Observation of long range coherent OTR from LPA electron beams

We report the observation of coherent optical transition radiation (COTR) from electron bunches that have propagated for up to 4 m from the exit of the laser plasma accelerator (LPA). This measurement indicates sub-percent-level slice energy spread of the LPA-produced electron beams. Transition radiation images, produced by electrons passing through two separate foils (located from the LPA at 2.3 m and 3.8 m) were recorded with a high resolution imaging system. Transition radiation in the visible wavelength regime was measured at signal levels of more than two orders of magnitude greater than expected from incoherent emission, indicating that femtosecond structure on the electron beams persisted over meter-scale propagation distances. This persistence implies an upper limit for the slice energy spread on the sub-percent level. Furthermore, for a selection of shots the coherent enhancement from the 3.8-m foil was higher than the closer 2.3-m one, consistent with dynamic changes of the bunch structure due to beam velocity bunching. Experimental results and modeling efforts will be presented. This work was supported by US DOE Contract No. DE-AC02-05CH11231.   

Tunable Laser Plasma Accelerator based on Longitudinal Density Tailoring

Laser plasma accelerators (LPAs) have produced high-quality electron beams with GeV energies from centimeter scale devices and are being investigated as drivers of hyperspectral femtosecond light sources and high-energy colliders. Such applications require a high degree of stability, beam quality, and tunability. Here we report on a technique to inject electrons into the accelerating field of a laser-driven plasma wave and coupling of this injector to a lower-density, separately-adjustable plasma for further acceleration. The technique relies on a single laser pulse powering a plasma structure with a tailored longitudinal density profile, to produce beams that can be tuned in the range of 100 to 400MeV with percent level stability, using 40TW laser pulses.   

Acceleration and radiation processes in controlled gradient gas jets

The controlled gradient gas jet was designed, constructed and tested at the Naval Research Laboratory. The gas jet is using a laser generated shock wave to control the density gradient between vacuum and neutral gas. The length scale of the density gradient is fully controlled by the strength of the shock wave and can be varied continuously from under 10 $\mu $m in case of strong shock to a 100 $\mu $m for a weak shock wave. To verify the experimental results a simulation was run to model the system using a three-dimensional hydrodynamic code, SPARC, developed at the Naval Research Laboratory. Controlling the gas density gradient is important for electrons and protons acceleration, as well as for optical transition radiation generation. Experiments on using the controlled gradient gas jet and preliminary results on electron acceleration will be presented.   

Electron Injection and Acceleration with Petawatt Laser Driven Wakefield Accelerator in 10$^{17}$cm$^{-3}$ Plasma

Here we report observation of self-injected laser-plasma acceleration of electrons up to $\sim $ 350 MeV in a tenuous plasma ($\sim $10$^{17}$ cm$^{-3})$ driven by the Texas Petawatt laser (TPL). The generated electron beam contains a charge of over 30 pC, and is well collimated ($<$ 10 mrad divergence). Electrons generated by TPL are not quasi monoenergetic and do not reach GeV level. This is likely due to imperfect PW laser quality. Simulations have shown that, driven by a high-Strehl-ratio TPL pulse, the laser plasma accelerator (LPA) can self-inject electrons and accelerate them to 3 GeV in such a tenuous plasma. The continuous electron energy spectrum also indicates that electron injection into plasma structure is continuous. Nevertheless, the observation of efficient self-injection at such low plasma density is a promising first step toward achieving multi-GeV LPAs in tenuous plasma. Simulations show that mild nonlinear plasma waves (a$_{0}\sim $1) are generated and a trapped electron can be accelerated up to $\sim $350MeV in this 5cm long tenuous plasma. Possible electron injection mechanisms are also discussed.   

Frequency-Domain Tomography for Single-shot, Ultrafast Imaging of Evolving Laser-Plasma Accelerators

Intense laser pulses propagating through plasma create plasma wakefields that often evolve significantly, e.g. by expanding and contracting. However, such dynamics are known in detail only through intensive simulations. Laboratory visualization of evolving plasma wakes in the ``bubble'' regime is important for optimizing and scaling laser-plasma accelerators. Recently snap-shots of quasi-static wakes were recorded using frequency-domain holography (FDH). To visualize the wake's evolution, we have generalized FDH to frequency-domain tomography (FDT), which uses multiple probes propagating at different angles with respect to the pump pulse. Each probe records a phase streak, imprinting a partial record of the evolution of pump-created structures. We then topographically reconstruct the full evolution from all phase streaks. To prove the concept, a prototype experiment visualizing nonlinear index evolution in glass is demonstrated. Four probes propagating at 0, 0.6, 2, 14 degrees to the index ``bubble'' are angularly and temporally multiplexed to a single spectrometer to achieve cost-effective FDT. From these four phase streaks, an FDT algorithm analogous to conventional CT yields a single-shot movie of the pump's self-focusing dynamics.   

Studies of Spectral Modification and Limitations of the Modified Paraxial Equation in Laser Wakefield Simulations

Laser pulses propagating through plasma undergo spectra broadening through local energy exchange with plasma waves. For propagation distances of a depletion length frequency shifts can be comparable to the laser central frequency. Over these distances, the electromagnetic dispersion predicted by the modified paraxial equation is no longer valid, due to its reliance on slow temporal variation. Here we examine the local frequency shift, energy depletion, and action conservation of nonlinear laser pulses using the modified paraxial simulation WAKE. Although action is theoretically conserved, we observe that for large red shifts, the numerical dispersion results in decay of the action. Numerical analysis of the propagation algorithm verified the observed behavior. While increased resolution improved action conservation, increased simulation times eliminated the strength of the modified paraxial solver--efficiency. We show that algorithms for the full second order wave equation are more conservative with the potential to be more efficient when propagation over several depletion lengths is important.   

Recent modifications to WAKE and effect of plasma temperature on energy gain and spread in plasma wake field acceleration

WAKE (previously 2D, now 2.5D) provides an efficient simulation platform for plasma wake field acceleration (PWFA) by utilizing the quasi-static approximation (QSA), in which the beam driver remains unchanged during the transit time of rest electrons. We implement 2.5D evolution and thermalization of the background plasma, driver beam evolution, and field ionization of plasma in WAKE. Simulations of PWFA are performed for a single bunch and two bunch (corresponding to experiments at FACET) electron beam driver. In two bunch experiments, the witness bunch behind the driver bunch flattens the axial wake field profile allowing for mono-energetic acceleration of the witness bunch. Finite plasma temperature modifies the wake field profile thereby affecting the energy gain and spread. Thermal modifications to plasma wake fields, and resulting energy gain and spread are examined for a range of temperatures relevant to experiments both for single and two bunch drivers.   

Trapping of Low Energy Electrons in Direct Laser Acceleration

Copropagation of a laser pulse and a relativistic electron beam in a corrugated plasma channel has been proposed for the direct laser acceleration. A laser pulse in a corrugated plasma channel consists of spatial harmonics whose phase velocities can be subluminal. The subluminal spatial harmonics can be phase matched to relativistic electrons resulting in linear energy gain over the interaction length. Scaling laws, supported by simulation, predict linear accelerating gradients of $\sim $100 MeV/cm for $\sim $2 TW of laser power and $\sim $0.6J of laser energy. However phase matching over extended lengths requires large initial electron energies. Here we examine ramped density profiles, tapered channel radii, and tapered modulation periods to lower the initial electron energy requirement for trapping. Each modification will be examined using the 2D cylindrical PIC simulation TurboWAVE, which at the same time will provide the first fully self-consistent PIC simulations of direct acceleration. Essential experimental components, a simple radial polarization converter and a ring grating to embed density modulation in a plasma channel, will be also discussed.   

Electron generation from a high repetition rate LWFA in the relativistic lambda-cubed regime

Ultrashort laser pulses of a few millijoules can provide focal intensities exceeding relativistic threshold. Intense laser pulses propagating in plasma drive nonlinear wakefield that can be used to accelerate electrons. Experiments were performed to investigate electron generation using the $\lambda^3$ laser at the University of Michigan - a table-top high-power laser system operating at 500 Hz repetition rate. The high repetition rate enables better data statistics and higher flux of particles, which are not accessible with typical sub-$0.1$Hz repetition rate systems. Experimental results from employing different laser condition (e.g. focal spot size, focus position, laser polarization state) and gas target condition (e.g. gas species, density profile) are presented. In addition, computational simulations using the 3D particle-in-cell code OSIRIS are performed to model the interaction.   

Laser Wakefield Acceleration of Quasimonoenergetic Electron Beams in Pure Nitrogen Gas Jets

Nitrogen has been used as an added gas at the few percent level in He gas jets to generate 100's of MeV electrons in the ionization induced seeding scheme for laser wakefield acceleration. Recent simulations (K.P. Singh~et al. Phys. Plasmas~16, 043113 (2009)) showed that quasimonoenergetic collimated GeV electrons could be also generated with pure N$_{2}$ using a chirped intense laser pulse. Here we report measurements of wakefield acceleration carried out in pure N$_{2}$ gas at the ALLS Laser facility at INRS, Varennes. Maximum energy higher than 0.5 GeV of quasimonoenergetic electron beam with a low divergence of 2.2 mrad was obtained with 80 TW, 30 fs laser pulses. Long-tail features were observed stretching from the quasimonoenergetic bunches due to continuous ionization injection. Measured peak electron energy decreased with the plasma density, which agrees with the predicted maximum electron energy gain scaling. Experiments showed a threshold density of 3x10$^{18}$ cm$^{-3}$ for self-trapping. Our experiments suggest that pure N$_{2}$ is a potential candidate gas to achieve GeV monoenergetic electrons in the ionization induced injection scheme for laser wakefield acceleration.   

Optical nonlinearity in Ar and N$_{2}$ near the ionization threshold

We directly measure the nonlinear optical response of argon and nitrogen in a thin gas target to laser intensities near the ionization threshold. These responses are fundamental to high intensity femtosecond filamentation. No instantaneous negative nonlinear refractive index is observed, nor is saturation, in contrast with a previous measurement [Loriot \textit{et al.}, Opt. Express \textbf{17}, 13429 (2009)] and calculations [Bree et al., Phys. Rev. Lett. \textbf{106}, 183902 (2011)]. In addition, we are able to cleanly separate the electronic and rotational components of the nonlinear response in nitrogen. In both Ar and N$_{2}$, we observe the peak instantaneous index response scale linearly with the laser intensity until the point of ionization, whereupon it turns abruptly negative and $\sim$ constant, consistent with plasma generation. In addition, we show that the results of Loriot \textit{et al.} are traceable to two-beam coupling via a plasma grating, and \textit{not} nonlinearity saturation or negative response.   

Direct measurement of the plasma density of a femtosecond laser induced-filament in air

The long-range filamentary propagation of an intense femtosecond laser pulse in atmosphere [1] results from the interplay between laser-induced plasma and nonlinear optical response of air molecules. To study the filamentation process in detail, it is crucial to determine the electron density in the transversely confined filament (typical diameter $<$ 100 $\mu $m), which is a challenging task due to the weak ionization (n$_{e}\sim $10$^{14}$-10$^{16}$ cm$^{-3})$. The previously developed techniques, for example, longitudinal spectral interferometry [2], shadowgraphy and optical diffraction [3], electron conductivity [4], and fluorescence spectroscopy [5] can only give crude estimations because they are either lack of spatial resolution or heavily model-dependent. Here we present the direct, radially and axially space-resolved measurement of electron density along an optical filament extending over $\sim $1 m in air. \\[4pt] [1] A. Braun et al., Opt. Lett. 20, 73(1995). \\[0pt] [2] B. La Fontaine et al., Phys. Plasmas 6, 1615 (1999). \\[0pt] [3] S. Tzortzakis et al., Opt. Commun. 181,123 (2000). \\[0pt] [4] R. P. Fischer et al., IEEE Trans. Plasma Sci. 35, 1430 (2007). \\[0pt] [5] J. Bernhardt et al., Opt. Commun. 281, 1268 (2008).   

High Power Laser Self-Guiding in Mixed Gases

Laser Wakefield Accelerators rely on self-guiding of short pulse high power lasers to drive relativistic plasma waves over centimeter-scale distances. The plasma is generally comprised of fully ionized Helium in order to reduce the effects of ionization-induced defocusing at the head of the laser pulse. However, the addition of higher-Z gases to the Helium background has been shown to aid in the trapping of electrons in the wakefield at low densities. We present experimental results of laser self-guiding in a gas cell over a distance of 4 mm using a nominally 100 TW Ti:Sapphire Laser in He/N2 mixtures over the range of 0-100{\%} N2 by partial pressure. Self-guiding is also observed in He/Ar mixtures with Argon concentrations as high as 10{\%} for electron densities of 5 - 10 (10$^{18}$ cm$^{-3 })$. Measurements of the relative laser pulse transmission, exit spot size and imaged spectrum will be presented. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.