Search

We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 $\mathrm{\mu} \mathrm{m}$ to 2 mm and lengths of 9 to 40 cm. To the best of the authors’ knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for $\ge$ 10 GeV electron energy gain in a single laser-driven plasma wakefield acceleration stage. Evaluation of waveguide parameter variations showed that their focusing strength was stable and reproducible to $ % and their average on-axis plasma electron density to $ %. These variations explain only a small fraction of laser-driven plasma wakefield acceleration electron bunch variations observed in experiments to date. Measurements of laser pulse centroid oscillations revealed that the radial channel profile rises faster than parabolic and is in excellent agreement with magnetohydrodynamic simulation results. We show that the effects of non-parabolic contributions on Gaussian pulse propagation were negligible when the pulse was approximately matched to the channel. However, they affected pulse propagation for a non-matched configuration in which the waveguide was used as a plasma telescope to change the focused laser pulse spot size.

Physical review accelerators and beams 22(3), 032401 (2019). doi:10.1103/PhysRevAccelBeams.22.032401, A study of the optimization of iron dominated dipole magnets with pole face widths comparable or less than the gap size, i.e., low aspect ratio (AR), is conducted using both theoretical and computational approaches. This regime of magnet design is particularly relevant in the context of laser plasma accelerators (LPA) due to unique beam parameters and geometric constraints, namely large energy spreads and the requirement for large apertures to accommodate drive laser passage. The breakdown of commonly employed approximations and rules of thumb in typical AR≫1 magnet design is examined. A library of generalized, optimized pole face geometries is provided to expedite optimization of future magnets. Finally, this methodology is used to design an electromagnetic chicane which has been fabricated, validated, and is currently in use in an x-ray free electron laser driven LPA experiment at LBNL., Published by American Physical Society, College Park, MD

Precise measurements of the plasma density in ionized gas cells and discharged capillaries are critical to the design and operation of plasma-based accelerators, active plasma lenses, and plasma-based radiation sources. In this manuscript, a spectral-domain common-path second-harmonic interferometer is upgraded with the simultaneous measurement of the group and phase velocity, allowing for high-sensitivity density characterization (from the phase velocity advance) without the need for phase tracking from zero-density (enabled by the group velocity delay). The technique is applied to 1.5-cm-long plasma structures, without density ambiguity in parameter scans with >2π phase jumps. The single-shot sensitivity in phase retrieval is demonstrated at 63 mrad, equivalent to a density-length product of 1.8·1015 cm -2 . This is an improvement of ×45 compared to group velocity analysis alone.

Physical review accelerators and beams 21(12), 122801 (2018). doi:10.1103/PhysRevAccelBeams.21.122801, Active plasma lenses have the potential to enable broad-ranging applications of plasma-basedaccelerators owing to their compact design and radially symmetric kT/m-level focusing fields, facilitatingbeam-quality preservation and compact beam transport. We report on the direct measurement of magneticfield gradients in active plasma lenses and demonstrate their impact on the emittance of a charged particlebeam. This is made possible by the use of a well-characterized electron beam with 1.4 mm mrad normalizedemittance from a conventional accelerator. Field gradients of up to 823 T/m are investigated. The observedemittance evolution is supported by numerical simulations, which suggests the potential for conservationof the core beam emittance in such a plasma lens setup., Published by American Physical Society, College Park, MD

Author(s): Swanson, KK; Tsai, HE; Barber, SK; Lehe, R; Mao, HS; Steinke, S; Van Tilborg, J; Nakamura, K; Geddes, CGR; Schroeder, CB; Esarey, E; Leemans, WP | Abstract: Control of the properties of laser-plasma-accelerated electron beams that were injected along a shock-induced density downramp through precision tailoring of the density profile was demonstrated using a 1.8 J, 45 fs laser interacting with a mm-scale gas jet. The effects on the beam spatial profile, steering, and absolute energy spread of the density region before the shock and tilt of the shock were investigated experimentally and with particle-in-cell simulations. By adjusting these density parameters, the electron beam quality was controlled and improved while the energy (30-180 MeV) and energy spread (2-11 MeV) were independently tuned. Simple models that are in good agreement with the experimental results are proposed to explain these relationships, advancing the understanding of downramp injection. This technique allows for high-quality electron beams with percent-level energy spread to be tailored based on the application.

© 2015 American Physical Society. © 2015 American Physical Society. Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of 3000 T/m, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

An injection scheme for a laser wakefield accelerator that employs a counterpropagating laser (colliding with the drive laser pulse, used to generate a plasma wake) is discussed. The threshold laser intensity for electron injection into the wakefield was analyzed using a heuristic model based on phase-space island overlap. Analysis shows that the injection can be performed using modest counterpropagating laser intensity a_1 < 0.5 for a drive laser intensity of a_0 = 1.0. Preliminary experiments were preformed using a drive beam and colliding beam. Charge enhancement by the colliding pulse was observed. Increasing the signal-to-noise ratio by means of a preformed plasma channel is discussed.

The observation and modeling of coherent transition radiation from femtosecond laseraccelerated electron bunches is discussed. The coherent transition radiation, scaling quadratically with bunch charge, is generated as the electrons transit the plasma-vacuum boundary. Due to the limited transverse radius of the plasma boundary, diffraction effects will strongly modify the angular distribution and the total energy radiated is reduced compared to an infinite transverse boundary. The multi-nC electron bunches, concentrated in a length of a few plasma periods (several tens of microns), experience partial charge neutralization while propagating inside the plasma towards the boundary. This reduces the space-charge blowout of the beam, allowing for coherent radiation at relatively high frequencies (several THz). The charge distribution of the electron bunch at the plasma-vacuum boundary can be derived from Fourier analysis of the coherent part of the transition radiation spectrum. A Michelson interferometer was used to measure the coherent spectrum, and electron bunches with duration on the order of 50 fs (rms) were observed.

Electron beam based radiation sources provide electromagnetic radiation for countless applications. The properties of the radiation are primarily determined by the properties of the electron beam. Compact laser driven accelerators are being developed that can provide ultra-short electron bunches (femtosecond duration) with relativistic energies reaching towards a GeV. The electron bunches are produced when an intense laser interacts with a dense plasma and excites a large amplitude plasma density modulation (wake eld) that can trap background electrons and accelerate them to high energies. The short pulse nature of the accelerated bunches and high particle energy offer the possibility of generating radiation from one compact source that ranges from coherent terahertz to gamma rays. The intrinsic synchronization to a laser pulse and unique character of the radiation offers a wide range of possibilities for scientic applications. Two particular radiation source regimes are discussed: Coherent terahertz emission and x-ray emission based on betatron oscillations and Thomson scattering.

In typical chirped pulse amplification (CPA) laser systems, scanning the grating separation in the optical compressor causes the well known generation of linear chirp of frequency vs. time in a laser pulse, as well as a modification of all the higher order phase terms. By setting the compressor angle slightly different from the optimum value to generate the shortest pulse, a typical scan around this value will produce significant changes to the pulse shape. Such pulse shape changes can lead to significant differences in the interaction with plasmas such as used in laser wake-field accelerators. Strong electron yield dependence on laser pulse shape in laser plasma wake-field electron acceleration experiments have been observed in the L'OASIS Lab of LBNL [W.P. Leemans, P. Catravas, E. Esarey, C.G.R. Geddes, C. Toth, R. Trines, C.B. Schroeder, B.A. Shadwick, J. van Tilborg, and J. Faure, Phys. Rev. Lett. 89, 174802 (2002)]. These experiments show the importance of pulse skewness parameter, S, defined here on the basis of the ratio of the 'head-width-half-max' (HWHM) and the 'tail-width-half-max' (TWHM), respectively.

The effect of asymmetric laser pulses on plasma wave excitation in a self-modulated laser wakefield accelerator is examined. Laser pulse shape and frequency chirp asymmetries, controlled experimentally in the laser system through a grating pair compressor, are shown to strongly enhance measured electron yields for certain asymmetries. It is shown analytically that a positive (negative) frequency chirp enhances (suppresses) the growth rate of the Raman forward scattering and near-forward Raman sidescatter instabilities, but is of minimal importance for the experimental parameters. Temporal laser pulse shapes with fast rise times (≲ plasma period) are shown to generate larger wakes (compared to slow rise time pulses) which seed the growth of the plasma wave, resulting in enhanced electron yield.

The temporal shape of 50-100 femto second Ti: Saphire laser pulses has been controlled by appropriate choice of the higher order spectral phase coefficients, and used for optimization of a plasma wake-field electron accelerator.