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Abstract Photon engineering can be exploited to control the nonlinear evolution of the drive pulse in a laser–plasma accelerator (LPA), offering new avenues to tailor electron beam phase space on a femtosecond time scale. One promising option is to drive an LPA with an incoherent stack of two sub-Joule, multi-TW pulses of different colors. Slow self-compression of the bi-color optical driver delays electron dephasing, boosting electron beam energy without accumulation of a massive low-energy tail. The modest energy of the stack affords kHz-scale repetition rate at manageable laser average power. Propagating the stack in a pre-formed plasma channel induces periodic self-focusing in the trailing pulse, causing oscillations in the size of accelerating bucket. The resulting periodic injection generates, over a mm-scale distance, a train of GeV-scale electron bunches with 5D brightness exceeding 1 0 17 A ∕ m 2. This unconventional comb-like beam, with femtosecond synchronization and controllable energy spacing of components, emits, via Thomson scattering, a train of highly collimated gigawatt γ -ray pulses. Each pulse, corresponding to a distinct energy band between 2.5 and 25 MeV, contains over 1 0 6 photons. Highlights • Stacked-pulse-driven laser–plasma electron accelerator (SPD LPA) is introduced. • SPD LPA overcomes limitations of conventional single-color LPA energy scaling. • Joule-scale energy of stack permits kHz repetition rate with kW average laser power. • SPD LPA in plasma channel generates a train of bright, fs-scale GeV electron bunches. • Thomson scattering from e-bunch train produces multi-color train of MeV γ -ray pulses. [ABSTRACT FROM AUTHOR]

Laser wakefield acceleration of electrons in the blowout regime can be controlled by tailoring the laser pulse phase and the plasma target. The 100 nm-scale bandwidth and negative frequency chirp of the optical driver compensate for the nonlinear frequency red-shift imparted by wakefield excitation. This mitigates pulse self-steepening and suppresses continuous injection. The plasma channel suppresses diffraction of the pulse leading edge, further reducing self-steepening, making injection even quieter. Besides, the channel destabilizes the pulse tail confined within the accelerator cavity (the electron density “bubble”), causing oscillations in the bubble size. The resulting periodic injection generates background-free comb-like beams – sequences of synchronized, low phase-space volume bunches. Controlling the number of bunches, their energy, and energy spacing by varying the channel radius and the pulse length (as permitted by the large bandwidth) enables the design of a tunable, all-optical source of polychromatic, pulsed γ-rays using the mechanism of inverse Thomson scattering. Such source may radiate ~ 10 7 quasi-monochromatic 10 MeV-scale photons per shot into a microsteradian-scale observation angle. The photon energy is distributed among several distinct bands, each having sub-25% energy spread dictated by the mrad-scale divergence of electron beam. [ABSTRACT FROM AUTHOR]

Abstract: It is demonstrated that a laser pulse with an ultrahigh bandwidth ( nm) is an asset for future high-repetition-rate, quasimonoenergetic (QME), GeV-scale laser plasma electron accelerators. Manipulating the phase of the driver has a direct impact on evolution of the accelerating bucket (a cavity of electron density maintained by the pressure of the laser pulse radiation), making it possible to control electron self-injection and the final parameters of the QME beam by purely optical means. The large bandwidth makes it possible to compensate for the frequency red-shift accumulated at the pulse leading edge in transit through the plasma. Advancing higher frequencies in time (viz. introducing a negative frequency chirp of the incident pulse) reduces the red-shift, preventing self-compression of the pulse into a relativistic optical shock. This avoids constant expansion of the plasma bucket, suppressing continuous self-injection of copious unwanted electrons, keeping the beam almost free of a high-charge, poorly collimated low-energy tail. In addition, gradually increasing the plasma density in the forward direction locks electrons in the accelerating phase, delaying their dephasing and boosting their energy without increasing the tail. Advantages of this technique are demonstrated here for acceleration in sub-millimeter-length, dense plasmas ( ), using 100-mJ-scale energy laser pulses. Three-dimensional particle-in-cell simulations show that a negatively chirped, 15TW pulse with a 20fs length and a bandwidth corresponding to a transform-limited duration below two optical cycles, in combination with a modest density taper (corresponding to 25% linear increase of the density in the forward direction), transfers 3.5% of its energy to a 300MeV electron bunch with relative energy spread below 4% and flux a factor 4.5 higher than the average flux in the tail. The acceleration occurs over a 0.6mm plasma with the electron density at the center of the target . Chirp and taper increase the QME bunch energy by 50% and brightness more than twice, while reducing the average flux in the tail by more than a half. This optically controlled, miniature 100-MeV-scale accelerator naturally affords high-repetition-rate operation important for radiation physics applications. [Copyright &y& Elsevier]

Abstract: Optimal regimes and physical processes at work are identified for the first round of laser wakefield acceleration experiments proposed at a future CILEX facility. The Apollon-10P CILEX laser, delivering fully compressed, near-PW-power pulses of sub-25fs duration, is well suited for driving electron density wakes in the blowout regime in cm-length gas targets. Early destruction of the pulse (partly due to energy depletion) prevents electrons from reaching dephasing, limiting the energy gain to about 3GeV. However, the optimal operating regimes, found with reduced and full three-dimensional particle-in-cell simulations, show high energy efficiency, with about 10% of incident pulse energy transferred to 3GeV electron bunches with sub-5% energy spread, half-nC charge, and absolutely no low-energy background. This optimal acceleration occurs in 2cm length plasmas of electron density below 1018 cm−3. Due to their high charge and low phase space volume, these multi-GeV bunches are tailor-made for staged acceleration planned in the framework of the CILEX project. The hallmarks of the optimal regime are electron self-injection at the early stage of laser pulse propagation, stable self-guiding of the pulse through the entire acceleration process, and no need for an external plasma channel. With the initial focal spot closely matched for the nonlinear self-guiding, the laser pulse stabilizes transversely within two Rayleigh lengths, preventing subsequent evolution of the accelerating bucket. This dynamics prevents continuous self-injection of background electrons, preserving low phase space volume of the bunch through the plasma. Near the end of propagation, an optical shock builds up in the pulse tail. This neither disrupts pulse propagation nor produces any noticeable low-energy background in the electron spectra, which is in striking contrast with most of existing GeV-scale acceleration experiments. [Copyright &y& Elsevier]