In this paper, we present results on a scalable high-energy electron source based on laser wakefield acceleration. The electron accelerator using 30–80 TW, 30 fs laser pulses, operates in the blowout regime, and produces high-quality, quasi-monoenergetic electron beams in the range 100–800 MeV. These beams have angular divergence of 1–4 mrad, and 5%–25% energy spread, with a resulting brightness 1011 electrons mm−2 MeV−1 mrad−2. The beam parameters can be tuned by varying the laser and plasma conditions. The use of a high-quality laser pulse and appropriate target conditions enables optimization of beam quality, concentrating a significant fraction of the accelerated charge into the quasi-monoenergetic component.
The radiation pressure of a multi-terawatt, sub-100 fs laser pulse propagating in an under-dense plasma causes complete electron cavitation. The resulting electron density "bubble" guides the pulse over many Rayleigh lengths, leaving the background ions unperturbed while maintaining GV/cm-scale accelerating and focusing gradients. The shape of the bubble, and, hence, the wakefield potentials, evolve slowly, in lockstep with the optical driver. This dynamic structure readily traps background electrons. 1 The electron injection process can thus be controlled by purely optical means. 2, 3 Sharp gradients in the nonlinear refractive index produce a large frequency red-shift (Δω ~ ω0), localized at the leading edge of the pulse. 2,3 Negative group velocity dispersion associated with the plasma response compresses the pulse into a relativistic optical shock (ROS). ROS formation slows the pulse (and the bubble), reducing the electron dephasing length and limiting energy gain. 4 Furthermore, the ponderomotive force due to the ROS causes the bubble to constantly expand, trapping copious unwanted electrons, polluting the electron spectrum with a high-charge, low-energy tail. 1,2 Here, we demonstrate a new, all-optical approach to compensating for the increase in pulse bandwidth, thereby delaying ROS formation and thus producing high quality, GeV-scale electron beams with 10-TW-class (rather than PW-class 4 ) lasers in mm-scale (rather than cm-scale4), highdensity plasmas (n e0 > 5 x 10 18 vs. 10 17 cm -3 ). We show that a negatively chirped drive pulse with an ultra-high (~ 400 nm) bandwidth: extends the electron dephasing length; prevents ROS formation through dephasing; and almost completely suppresses continuous injection. Precise compensation of the nonlinear frequency shift can be achieved using a higher-order chirp extracted from reduced simulation models. ROS formation can be further delayed by using a plasma channel to suppress diffraction of the pulse leading edge, minimizing longitudinal variations in the pulse. Plasma density tapering further delays dephasing, providing an additional boost in beam energy.
Suppression of a large-angle stimulated Raman scattering (LA-SRS) of a short modulated (two-frequency) laser pulse in a transparent plasma in the presence of a linear long- wavelength electron plasma wave (LW EPW) having relativistic phase velocity is considered under the conditions of weak and strong coupling. The laser spectrum includes two components with a frequency shift equal to the frequency of the LW EPW. The mutual influence of different spectral components of a laser on the SRS under a given angle in the presence of the LW EPW is examined.
We report holographic "snapshots" of laser-generated wakefields that capture transverse and longitudinal structure of multiple wake periods, detect structure variations as laser-plasma parameters change, and resolve wavefront curvature, features never previously observed.
