No abstract

The X-ray free-electron laser has opened a new era for photon science, improving the X-ray brightness by ten orders of magnitude over previously available sources. Similar to an optical laser, the spectral and temporal structure of the radiation pulses can be tailored to the specific needs of many experiments by accurately manipulating the lasing medium, that is, the electron beam. Here we report the generation of mJ-level two-colour hard X-ray pulses of few femtoseconds duration with an XFEL driven by twin electron bunches at the Linac Coherent Light Source. This performance represents an improvement of over an order of magnitude in peak power over state-of-the-art two-colour XFELs. The unprecedented intensity and temporal coherence of this new two-colour X-ray free-electron laser enable an entirely new set of scientific applications, ranging from X-ray pump/X-ray probe experiments to the imaging of complex biological samples with multiple wavelength anomalous dispersion.

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV=m, in good agreement with an analytical model and PIC simulations.

The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al. Recent experiments have shown amazing progress for both plasma-wakefield acceleration (PWFA) and laser wakefield acceleration (LWFA) [1][2][3] in the electron blow-out regime. In this regime, plasma electrons are completely evacuated by the space charge force of an electron beam or the ponderomotive force of a laser pulse, forming an ion channel on the axis of the system with a laminar sheath at the channel boundary carrying large concentrations of relativistic electrons. However, the electron hosing instability [4 -7] of the drive and/or trailing beam remains a major concern for PWFA/LWFA concepts. The hosing instability results from the interaction between the electron sheath and the self-injected or externally injected electron beam. It leads to spatiotemporally growing oscillations of the beam centroid at each axial slice thus limiting the useful acceleration length and making it difficult to aim the beam. Existing standard theory [4,6] predicts rapid growth for this instability. However, recent experiments [1,2] have shown little evidence of hosing.In this Letter, we present a more general hosing theory based on a perturbation method to the zeroth order trajectory [8] for the ion-channel/electron-sheath boundary. The initial hosing growth predicted by the linearized coupling is found to be affected by the nonconstant channel radius, relativistic mass corrections, and the longitudinal velocity of electrons in the plasma sheath. We verify this theory using particle-in-cell (PIC) simulations and compare it to the standard theory.The existing work [4,6] focused on the hosing in a long ion channel with a radius near the charge neutralization radius, i.e., r c r neu n b R 2 b =n p q , where n b , n p are the beam and plasma density, respectively, and R b is the beam radius. Such a channel is either preformed or adiabatically formed. The electrons in the sheath layer are assumed to be at rest, i.e., the nonrelativistic limit; therefore, they do not generate or feel the magnetic fields. This adiabatic (referring to the channel formation), nonrelativistic (referring to the plasma sheath motion) limit is appropriate for a beam where, s is the propagation distance into the plasma, ct ÿ z is the location within the beam, and is the beam Lorentz factor. For a PWFA, the ''short-pulse'' limit of these equations is relevant, i.e., k s ! 0 , and the asymptotic solution for a linear tilt in x b is x b =x b0 0:341A ÿ3=2 e A cos k s ÿ A= 3 p =4 [6], where ...