Session YO7: Laser Wakefield Acceleration

Stable and quality-preserving acceleration of electron beams in plasma-based accelerators

Stable and quality-preserving acceleration of electron beams is a prerequisite for the realization of plasma-based colliders. In this context, the hose instability is a crucial challenge. It is seeded by transverse beam or plasma phase space asymmetries. Beam centroid displacements are amplified along the beam and during the propagation in the plasma, thereby leading to beam breakup.
It was recently shown that the hose instability can be mitigated if the longitudinal or transverse wakefields vary along the beam, such that a head-to-tail decoherence of the betatron motion disrupts the resonant coupling between the beam slices via the plasma.
While these mitigation mechanisms can be effective for drive beams in the blowout regime or for trailing beams in the quasi-linear regime, they do not apply to mono-energetic trailing beams in the beam-loaded blowout regime.
In this contribution, we investigate theoretically and computationally the possibility to accelerate electron beams to multi-GeV energies in plasma-based accelerators in the blowout regime, while maintaining the energy spread and preserving a sub-micrometer emittance.   

Generating high quality relativistic electron beams using density downramp injection in plasma wakefields

In the past few decades plasma wakefield acceleration (PWFA) and laser wakefield acceleration (LWFA) have attracted a lot of interest in applications for compact next generation linear colliders and x-ray free-electron-lasers (XFEL). Recently, density downramp injection has been proposed and demonstrated as a controllable injection scheme for generating high quality relativistic electron beams. However, full-3D simulations of plasma-based acceleration schemes 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 azimuthal harmonics and are needed to capture the 3D physics of most problems. We present quasi-3D results from downramp injection simulations using OSIRIS to generate electron beams with ~ 10²⁰  A/m²/rad² peak brightness and low absolute slice energy spread of ~O(0.1) MeV. We also compare the accuracy of these results against 3D simulations and present results from parameter scans of downramp injection using the quasi-3D algorithm to optimize beam quality. Preliminary results of other injection schemes will also be presented   

Spectral measurements of mid-infrared radiation from a laser wakefield accelerator

The formation of a plasma “bubble” during Laser Wakefield Acceleration (LWFA) results in a co-moving refractive index gradient that produces time dependent frequency shifts in the driving laser pulse. The positive density gradient at the leading edge of the plasma bubble causes red-shifting of the front of the pulse, generating wavelengths extending into the mid-infrared. The mid-infrared spectral region contains the frequency range of molecular vibrations, and thus is of significant interest for a diversity of scientific and technological applications. High-resolution spectral measurements of mid-infrared radiation extending to 2.5 microns during LWFA in the bubble regime were obtained using the HERCULES laser system at the University of Michigan. The enhancement of this radiation using tailored density targets, as well as the conversion efficiency of this process as a function of plasma density, length and charge generation are presented.   

Self-Modulated Laser Wakefield Acceleration as an Electron Source for High Energy Density Science

Relativistic electron beams have many useful applications in high energy density science including x-ray generation, and positron generation. Laser wakefield accelerators offer a compact means of generating high energy low divergence electron beams to be used for these applications. In the self-modulated regime of laser wakefield acceleration, the laser pulse is many times longer than the plasma period, generating multiple bubbles to trap and accelerate electrons. The Titan laser at the Jupiter Laser Facility produces a Self-modulated laser wakefield accelerator (SM-LWFA) using a 120 J, 1 ps, 1 um laser pulse focused at an intensity of ~10¹⁹ W/cm² onto a He gas jet with an electron density of about 5*10¹⁸ cm^-3. This SMLWFA produces an electron beam of 10 nC with a maximum energy of 300 MeV and a divergence of 50x100 mrad. This work will present the electron beam properties as a function of laser and plasma parameters in experiments at the Titan laser.   

Laser Wakefield Acceleration Platform for OMEGA EP

Recently there has been interest in picosecond-scale, kilojoule-class lasers to drive a laser wakefield accelerator (LWFA) in the self-modulated regime to produce betatron x-ray sources.[1] Such a platform would enable betatron x-ray sources that could be coupled with large-scale lasers such as the OMEGA laser at the Laboratory for Laser Energetics (LLE) or the National Ignition Facility at Lawrence Livermore National Laboratory. Here, we report on recent efforts to develop a LWFA platform for the OMEGA EP laser at LLE. A new gas-jet system that can provide underdense plasma targets for OMEGA and OMEGA EP has been fielded and characterized. Efforts to extend the focal length of OMEGA EP to larger f#’s will be discussed. We will show preliminary results investigating LWFA driven by OMEGA EP.