T. Sokollik

Laser red shifting based characterization of wakefield excitation in a laser-plasma accelerator

Optical spectra of a drive laser exiting a channel guided laser-plasma accelerator (LPA) are analyzed through experiments and simulations to infer the magnitude of the excited wakefields. The experiments are performed at sufficiently low intensity levels and plasma densities to avoid electron beam generation via self-trapping. Spectral redshifting of the laser light is studied as an indicator of the efficiency of laser energy transfer into the plasma through the generation of coherent plasma wakefields. Influences of input laser energy, plasma density, temporal and spatial laser profiles, and laser focal location in a plasma channel are analyzed. Energy transfer is found to be sensitive to details of laser pulse shape and focal location. The experimental conditions for these critical parameters are modeled and included in particle-in-cell simulations. Simulations reproduce the redshift of the laser within uncertainties of the experiments and produce an estimate of the wake amplitudes in the experiments as...

High-peak-power surface high-harmonic generation at extreme ultra-violet wavelengths from a tape

Solid-based surface high-harmonic generation from a tape is experimentally studied. By operating at mildly relativistic normalized laser strengths a0≲0.2, harmonics up to the 17th order are efficiently produced in the coherent wake emission (CWE) regime. CWE pulse properties, such as divergence, energy, conversion efficiency, and spectrum, are investigated for various tape materials and drive laser conditions. A clear correlation between surface roughness and harmonic beam divergence is found. At the measured pulse properties for the 15th harmonic (conversion efficiency ∼6.5×10−7, divergence ∼7−15  mrad), the 100-mJ-level drive laser produces several MWs of extreme ultra-violet pulses. The spooling tape configuration enables multi-Hz operation over thousands of shots, making this source attractive as a seed to the few-Hz laser-plasma-accelerator-driven free-electron laser (FEL). Models indicate that these CWE pulses with MW level powers are sufficient for seed-induced bunching and FEL gain.

Spectral characterization of laser-driven solid-based high harmonics in the coherent wake emission regime

Laser-produced surface high-harmonic generation is an attractive source of extreme ultraviolet radiation due to its coherent properties and high peak power. By operating at subrelativistic laser intensities in the coherent wake emission regime, the harmonic spectrum was experimentally studied versus laser properties. At higher laser intensities (>10(17) W/cm(2)) a higher spectral cutoff was observed, with accompanying blueshifting and spectral broadening of the individual orders. A model based on an expanding critical surface provides qualitative agreement with the observations.

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

X-ray spectroscopy is used to obtain single-shot information on electron beam emittance in a low-energy-spread 0.5 GeV-class laser-plasma accelerator. Measurements of betatron radiation from 2 to 20 keV used a CCD and single-photon counting techniques. By matching x-ray spectra to betatron radiation models, the electron bunch radius inside the plasma is estimated to be ~0.1 μm. Combining this with simultaneous electron spectra, normalized transverse emittance is estimated to be as low as 0.1 mm mrad, consistent with three-dimensional particle-in-cell simulations. Correlations of the bunch radius with electron beam parameters are presented.

Undulator radiation from laser-plasma-accelerated electron beams

Recent experiments coupled electron beams from the LOASIS TREX laser plasma accelerator (LPA) [1, 2, 3] to the Tapered Hybrid Undulator (THUNDER). Using the 1.5m, 66 period undulator, followed by an XUV spectrometer, spontaneous radiation was observed at photon energies extending to 100 eV. Previous experiments have reported visible [4] and soft-x-ray [5] radiation. The purpose of our experiments is to do highly precise, single shot diagnostics of the energy spread and emittance for each electron beam. We present recent results including measurements of electron beam transport through the undulator with and without the use of permanent magnetic quadrapoles, and measurements of XUV spectra up to 100 eV from LPA produced e-beams.

Charge Diagnostics for Laser Plasma Accelerators

The electron energy dependence of a scintillating screen (Lanex Fast) was studied with sub-nanosecond electron beams ranging from 106 MeV to 1522 MeV at the Lawrence Berkeley National Laboratory Advanced Light Source (ALS) synchrotron booster accelerator. The sensitivity of the Lanex Fast decreased by 1percent per 100 MeV increase of the energy. The linear response of the screen against the charge was verified with charge density and intensity up to 160 pC/mm2 and 0.4 pC/ps/mm2, respectively. For electron beams from the laser plasma accelerator, a comprehensive study of charge diagnostics has been performed using a Lanex screen, an integrating current transformer, and an activation based measurement. The charge measured by each diagnostic was found to be within +/-10 percent.

Staged acceleration experiments

We present initial experiments on staging of two separately driven laser plasma accelerators (LPAs) towards high energy physics and beam deceleration experiments. A study establishing long term stability of electron beams accelerated by an LPA in density downramp configuration [1] is presented, demonstrating the appropriateness of these beams as an injector for staged acceleration. Subsequently, these injector beams are used to longitudinally probe the fully characterized wakefield [2] excited in a discharge-capillary-based second stage accelerator.

Megawatt level surface high-harmonic generation from thin, replenishing, solid tapes

High-harmonic generation (HHG) from replenishing tape surfaces allows for multi-Hz applications. Unlike conventional gas-based HHG, solid-surface HHG (SHHG) uses peak laser intensities upwards of 1018 - 1019 W/cm2. This allows a fs-class 100 mJ pulse to be focused down tighter, enabling a more compact harmonic generating setup. Operating in the sub-relativistic Coherent Wake Emission (CWE) regime, we have experimentally investigated MW-level XUV-domain photons produced from different tape materials. The tapes are thin and can be used as beam combining optics for LPA electron beams and HHG seed radiation.

High-harmonic generation from replenishing solid tapes

High-harmonic generation from a replenishing tape surface allows for multi-Hz applications. Operating in the sub-relativistic Coherent Wake Emission (CWE) regime, we have experimentally investigated the spectrum, conversion efficiency, and divergence for several different tape materials.

Staged laser plasma accelerators

We present current results on staged electron acceleration in the LOASIS program at the Lawrence Berkeley National Laboratory. The goal is to experimentally demonstrate laser driven electron acceleration in two stages, where each stage is driven by a separate laser pulse. This technology could provide the key to built compact laser driven accelerators which could potentially reach up to TeV in electron energy.