Session BO5: Laser-Plasma Acceleration

Direct Laser Acceleration in Laser Wakefield Accelerators

The direct laser acceleration (DLA) of electrons in a laser wakefield accelerator (LWFA) has been investigated. We show that when there is a significant overlap between the drive laser and the trapped electrons in a LWFA cavity, the accelerating electrons can gain energy from the DLA mechanism in addition to LWFA. The properties of the electron beams produced in a LWFA, where the electrons are injected by ionization injection, have been investigated using particle-in-cell (PIC) code simulations. Particle tracking was used to demonstrate the presence of DLA in LWFA. Further PIC simulations comparing LWFA with and without DLA show that the presence of DLA can lead to electron beams that have maximum energies that exceed the estimates given by the theory for the ideal blowout regime. The magnitude of the contribution of DLA to the energy gained by the electron was found to be on the order of the LWFA contribution. The presence of DLA in a LWFA can also lead to enhanced betatron oscillation amplitudes and increased divergence in the direction of the laser polarization. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Influence of plasma density on the generation of 100’s MeV electrons via Direct Laser Acceleration

The role of plasma density and quasi-static fields in the acceleration of electrons to many times the ponderomotive energies (exceeding 400 MeV) by high-energy, picosecond duration laser pulses via Direct Laser Acceleration (DLA) from underdense CH plasma was investigated. Experiments using the OMEGA EP laser facility and two-dimensional particle-in-cell simulations using the EPOCH code were performed. The existence of an optimal plasma density for the generation of high-energy, low-divergence electron beams is demonstrated. The role of quasi-static channel fields on electron energy enhancement, beam pointing and divergence elucidate the mechanisms and action of DLA at different plasma densities.   

A detailed examination of the LWFA in the Self-Guided Nonlinear Blowout Regime for 15-100 Joule Lasers

We examine scaling laws for LWFA in the regime nonlinear, self-guided regime [Lu et al. Phys. Rev. Spec. Top. Accel. Beams 10, 061301 (2007)] in detail using the quasi-3D version of the particle-in-cell code OSIRIS. We find that the scaling laws continue to work well when we fix the normalized laser amplitude while reducing plasma density. It is further found that the energy gain for fixed laser energy can be improved by shortening the pulse length until self-guiding almost no longer occurs and that the energy gain can be optimized by using lasers with asymmetric longitudinal profiles. We find that when optimized, a 15 J laser may yield particle energies as high as 5.3 GeV without the need of any external guiding. Detailed studies for optimizing energy gains from 30 J and 100 J lasers will also presented which indicate that energies in excess of 10 GeV can be possible in the near term without the need for external guiding.   

Optimization of electron beam properties from intense laser pulses interacting with structured gas jets

Through precision tailoring of the plasma density profile, control of laser-plasma-accelerated electron beams injected along a shock-induced density downramp was demonstrated. The relationships between the electron beam spatial profile and steering, and the downramp slope, shock angle and the acceleration length were experimentally investigated using a 1.8 J, 45 fs laser interacting with a mm-scale gas jet. We demonstrate that injection is highly sensitive to these parameters, and by adjusting the density profile high-quality electron beams over a tunable range of energies were produced. Simple models were developed to explain these relationships and are in good agreement with the experimental results, advancing the understanding of downramp injection.   

Control of quasi-monoenergetic electron beams from laser-plasma accelerators by adjusting shock density profile

High-level control of a laser-plasma accelerator (LPA) using a shock injector was demonstrated by systematically varying the shock injector profile, including the shock angle, up-ramp width and shock position. Particle-in-cell (PIC) simulation explored how variations in the shock profile impacted the injection process and confirmed results obtained through acceleration experiments. These results establish that, by adjusting shock position, up-ramp, and angle, beam energy, energy spread, and pointing can be controlled. As a result, e-beam were highly tunable from 25 to 300 MeV with \textless 8{\%} energy spread, 1.5mrad divergence and \textless 1mrad pointing fluctuation. This highly controllable LPA represents an ideal and compact beam source for the ongoing MeV Thomson photon experiments. Set-up and initial experimental design on a newly constructed one hundred TW laser system will be presented.   

Measured emittance dependence on injection method in laser plasma accelerators

The success of many laser plasma accelerator (LPA) based applications relies on the ability to produce electron beams with excellent 6D brightness, where brightness is defined as the ratio of charge to the product of the three normalized emittances. As such, parametric studies of the emittance of LPA generated electron beams are essential. Profiting from a stable and tunable LPA setup, combined with a carefully designed single-shot transverse emittance diagnostic, we present a direct comparison of charge dependent emittance measurements of electron beams generated by two different injection mechanisms: ionization injection and shock induced density down-ramp injection. Notably, the measurements reveal that ionization injection results in significantly higher emittance. With the down-ramp injection configuration, emittances less than 1 micron at spectral charge densities up to \textasciitilde 2 pC/MeV were measured.   

Generating high brightness electron beams using density down ramp injection in nonlinear plasma wakefields

In the past few decades, there has been much progress in theory, simulation, and experiment towards using Plasma wakefield acceleration (PWFA) and Laser wakefield acceleration (LWFA) as the basis for designing and building compact x-ray free-electron-lasers (XFEL) as well as a next generation linear collider. Recently, ionization injection and density downramp injection have been proposed and demonstrated as controllable injection schemes for generating high quality relativistic electron beams. We present the concepts and full 3D simulation results using OSIRIS which show that downramp injection can generate electron beams with unprecedented brightnesses. However, full-3D simulations of plasma-based acceleration can be computationally intensive, sometimes taking millions of cpu-hours. Due to the near azimuthal symmetry in PWFA and LWFA, quasi-3D simulations using a cylindrical geometry are computationally more efficient than 3D Cartesian simulations since only the first few harmonics are needed in $\phi$ to capture the 3D physics of most problems. We also present results from the quasi-3D approach on downramp injection and compare the results against full 3D simulations.   

Pulse Front Tilt and Laser Plasma Acceleration

Pulse front tilt (PFT) is potentially present in any CPA laser system, but its effects may be overlooked because spatiotemporal pulse characterization is considerably more involved than measuring only spatial or temporal profile [1]. PFT is particularly important for laser plasma accelerators (LPA) because it influences electron beam injection [2] and steering [3]. In this work, experimental results from the BELLA Center will be presented that demonstrate the effect of optical grating misalignment and optical compression, resulting in PFT [4], on accelerator performance. Theoretical models of laser and electron beam steering will be introduced based on particle-in-cell simulations showing distortion of the plasma wake. Theoretical predictions will be compared with experiments and complimentary simulations, and tolerances on PFT and optical compressor alignment will be developed as a function of LPA performance requirements. [1] G. Pretzler, A. Kasper, and K.J. Witte, Appl Phys B 70 (2000), 1--9 [2] M. Schnell et al., Nature Comm 4 (2013), 2421 [3] A. Popp et al., Phys Rev Lett 105 (2010), 215001 [4] K. Nakamura et al., IEEE J Quant Electron 53 (2017) 1-21   

Relativistic Electron Acceleration with Ultrashort Mid-IR Laser Pulses

We report the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared laser pulses ($\lambda =$ 3.9 $\mu m$, pulsewidth 100 fs, energy \textless 20 mJ, peak power \textless 1 TW)), which enables near- and above-critical density interactions with moderate-density gas jets. We present thresholds for electron acceleration based on critical parameters for relativistic self-focusing and target width, as well as trends in the accelerated beam profiles, charge and energy spectra which are supported by 3D particle-in-cell simulations. These results extend earlier work with sub-TW self-modulated laser wakefield acceleration using near IR drivers [1] to the Mid-IR, and enable us to capture time-resolved images of relativistic self-focusing of the laser pulse. [1] 1. A.J. Goers \textit{et al}., Phys. Rev. Lett. \textbf{115}, 194802 (2015)   

Enhancement of Laser Wakefields via a Backward Raman Amplifier

The Backward Raman Amplifier (BRA) is proposed as a possible scheme for improving laser driven plasma wakefields. One- and two-dimensional particle-in-cell code simulations with SCPIC [1] and a 3-Wave coupling model are presented and compared to demonstrate how the BRA can be applied to the laser wakefield accelerator (LWFA) in the non-relativistic regime to counteract limitations such as pump depletion, diffraction, and dephasing [2]. Simulation results show that amplification of the driving pulse is strongest in the central high amplitude portion, causing the pulse to shorten both transversely and longitudinally. This results in a reduction or alleviation of the effects of diffraction, an increase in wake amplitude and sustainability, and provides direct insight into new methods of controlling plasma wakes in LWFA and other applications. [1] K. I. Popov et al, Physical Review Letters 105, 195002 (2010) [2] E. Esarey, C. B. Schroeder, and W. P. Leemans, Rev. Mod. Phys. 81, 1229 (2009)   

Strong Optical Shock excitation in the mismatched regime of bubble plasma-wave based LWFA

We present investigations into the excitation of a strong optical shock [1] through slicing of a high intensity laser pulse driving a bubble plasma wave in a regime of mis-match between the incident laser waist-size and the bubble size ($\simeq 2 ~ \sqrt{a_0} ~ c/\omega_{pe}$) [2]. In the matched regime, it is well-known that over long timescales, the laser continuously undergoes differential frequency-shifts in different bubble phases, forming an optical shock [2][3]. In the mis-matched regime, rapid laser waist and resulting bubble oscillations change the location of the peak laser ponderomotive force. This changes the location and the magnitude of the peak electron density interacting with the laser pulse. A sudden increase in the electron density during a laser radial squeeze event, slices the laser envelope longitudinally near its peak amplitude, exciting a strong optical shock state. This is shown to occur much earlier in laser evolution only over a narrow range of plasma densities where the imbalance between the longitudinal & radial ponderomotive forces excites elongated bubbles, injects ultra-low emittance electron beams and sustains ultra-high peak plasma fields [4]. [1] PRL 84, 3582 (2000) [2] PRSTAB 10, 061301 (2007) [3] NJP 12, 0450 (2010) [4] arXiv:1704.02913 (2017)   

Application of a laser-heater for advanced guiding of PetaWatt laser pulses in capillary plasmas

Laser-plasma accelerators (LPAs) form an attractive scheme for developing compact accelerators, due to their high acceleration gradients. In the push to higher electron beam energies, one of the main challenges is to increase the dephasing length $L_d$ over which energy can be transferred to the electrons, while keeping the laser confined to provide the required accelerating fields. Currently, the highest energy electron beams from an LPA have been achieved by using pre-formed density channels from capillary discharge plasmas.\textsuperscript{[1]} Confinement of laser pulses with higher order mode content required higher density than optimum for reaching higher energies. Improved laser confinement at lower density, extending $L_d$, has been proposed through use of a ns-scale heater pulse before the ultrashort, high-powered pulse arrives.\textsuperscript{[2]} Here, we present experimental results of applying this technique to channels of up to 20~cm in length to enhance guiding of PetaWatt pulses from the BELLA laser, including electron and laser properties from the accelerator.\newline [1] W.P.~Leemans \textit{et al.}, Physical Review Letters \textbf{113}, 245002 (2014).\newline [1] N.A.~Bobrova \textit{et al.}, Physics of Plasmas \textbf{20}, 020703 (2013)   

Group Velocity Measurements in Laser-Heated Capillary Discharge Waveguides for Laser-Plasma Accelerators

To date, the most energetic electron beams from laser-plasma accelerators have been produced using gas-filled capillary discharge waveguides, which increase the acceleration length by mitigating diffraction of the driving laser pulse. [1] To reach higher electron beam energies, lower plasma density is required to reduce bunch dephasing. However, confinement of the driver is reduced for lower plasma density, reducing the acceleration length. A laser-heated capillary discharge waveguide, where the discharge is heated by a coaxial laser pulse, was proposed to create a steeper density gradient at lower density. [2] Here the first measurements of group velocity in laser-heated capillary discharges, obtained via spectral interferometry, are presented. Increase of the driver group velocity and reduction in on-axis plasma density by laser-heating are shown. \\ \\$[1]$ W.P. Leemans, et al, Physical Review Letters 113, 245002 (2014). \\ $[2]$ N.A. Bobrova, et al., Physics of Plasmas 20, 020703 (2013).   

On the Asymmetric Focusing of Low-Emittance Electron Bunches via Active Lensing by Using Capillary Discharges

A novel method for asymmetric focusing of electron beams is proposed. The scheme is based on the active lensing technique, which takes advantage of the strong inhomogeneous magnetic field generated inside the capillary discharge plasma to focus the ultrarelativistic electrons. The plasma and magnetic field parameters inside a capillary discharge are described theoretically and modeled with dissipative MHD simulations to enable analysis of capillaries of oblong rectangle cross-sections implying that large aspect ratio rectangular capillaries can be used to form flat electron bunches. The effect of the capillary cross-section on the electron beam focusing properties were studied using the analytical methods and simulation- derived magnetic field map showing the range of the capillary discharge parameters required for producing the high quality flat electron beams.   

\textgreater 1 Hz Renewable Films for Plasma Mirrors for High Repetition Rate Petawatt Class Laser Systems

Improving the intensity contrast of \textgreater 1 Hz, high power lasers presents a unique challenge. Recently, we demonstrated a device capable of creating renewable plasma mirrors for intensity contrast enhancement based on variable thickness liquid crystal films. Tuning the thickness of these freely suspended films between 10 and 300 nm allows minimization of the weak-field reflectivity, where the films act as a conventional anti-reflection coating. The maximum possible intensity contrast enhancement from a single film exceeds a factor of 350 (Poole et al., Scientific Reports 6, 32041 (2016)). Films were formed on demand and in-situ, eliminating the need to raster or replace optics between shots. Here we describe a prototype device that can accommodate petawatt laser systems operating above 1 Hz. The prototype has shown sustained film production at 3 Hz for 20 hours, yielding \textgreater 200,000 plasma mirrors using \textless 10 $\mu $L of the liquid crystal 8CB. We also discuss measurements of film surface quality to diagnose prolonged plasma mirror reflection performance.