We present experimental results of vacuum laser acceleration (VLA) of electrons using radially polarized laser pulses interacting with a plasma mirror. Tightly focused, radially polarized laser pulses have been proposed for electron acceleration because of their strong longitudinal electric field, making them ideal for VLA. However, experimental results have been limited until now because injecting electrons into the laser field has remained a considerable challenge. Here, we demonstrate experimentally that using a plasma mirror as an injector solves this problem and permits us to inject electrons at the ideal phase of the laser, resulting in the acceleration of electrons along the laser propagation direction while reducing the electron beam divergence compared to the linear polarization case. We obtain electron bunches with few-MeV energies and a 200-pC charge, thus demonstrating, for the first time, electron acceleration to relativistic energies using a radially polarized laser. High-harmonic generation from the plasma surface is also measured, and it provides additional insight into the injection of electrons into the laser field upon its reflection on the plasma mirror. Detailed comparisons between experimental results and full 3D simulations unravel the complex physics of electron injection and acceleration in this new regime: We find that electrons are injected into the radially polarized pulse in the form of two spatially separated bunches emitted from the p-polarized regions of the focus. Finally, we leverage on the insight brought by this study to propose and validate a more optimal experimental configuration that can lead to extremely peaked electron angular distributions and higher energy beams.
The laser wake-field accelerator (LWFA) traditionally produces high brightness, quasi-monoenergetic electron beams with Gaussian-like spatial and angular distributions. In the present work we investigate the generation of ultra-relativistic beams with ring-like structures in the blowout regime of the LWFA using a dual stage accelerator. A density down-ramp triggers injection after the first stage and is used to produce ring-like electron spectra in the 300 - 600 MeV energy range. These well defined, annular beams are observed simultaneously with the on-axis, high energy electron beams, with a divergence of a few milliradians. The rings have quasi-monoenergetic energy spectra with an RMS spread estimated to be less than 5%. Particle-in-cell simulations confirm that off-axis injection provides the electrons with the initial transverse momentum necessary to undertake distinct betatron oscillations within the plasma bubble during their acceleration process.
The dynamics of ionization-induced electron injection in the high density (~ 1:2 × 1019cm-3) regime of Laser Wakefield Acceleration (LWFA) was investigated by analyzing betatron X-ray emission inside dielectric capillary tubes. A comparative study of the electron and betatron X-ray properties was performed for both self-injection and ionization-induced injection. Direct experimental evidence of early onset of ionization-induced injection into the plasma wave was obtained by mapping the X-ray emission zone inside the plasma. Particle-In-Cell (PIC) simulations showed that the early onset of ionization-induced injection, due to its lower trapping threshold, suppresses self-injection of electrons. An increase of X-ray fluence by at least a factor of two was observed in the case of ionization-induced injection due to an increased trapped charge compared to self-injection mechanism.
The innovating advent of powerful TW laser sources (~1019 W cm-z) and laser-plasma interactions providing ultra-short
relativistic particle beams (electron, proton) in the MeV domain open exciting opportunities for the simultaneous
development of high energy radiation femtochemistry (HERF) and ultrafast radiation biomedicine. Femtolysis
experiments (Femtosecondradiolysis) of aqueous targets performed with relativistic electron bunches of 2.5-15 MeV give
new insights on transient physicochemical events that take place in the prethermal regime of confined ionization tracks.
Femtolysis studies emphasize the pre-eminence of ultra-fast quantum effects in the temporal range 10-14 - 10-11 s. The
most promising advances of HERF concern the quantification of ultrafast sub-nanometric biomolecular damages (bond
weakening and bond breaking) in the radial direction of a relativistic particle beam. Combining ultra-short relativistic
particle beams and near-infrared spectroscopic configurations, laser-plasma accelerators based high energy radiation
femtochemistry foreshadows the development of real-time radiation chemistry in the prethermal regime of nascent
ionisation clusters. These physico-chemical advances would be very useful for future developments in biochemically
relevant environments (DNA, proteins) and in more complex biological systems such as living cells. The first
investigation of single and multiple irradiation shots performed at high energy level (90 MeV) and very high dose rate,
typically 1013 Gy s-1, demonstrates that measurable assessments of immediate and reversible DNA damage can be
explored at single cell level. Ultrafast in vivo irradiations would permit the development of bio-nanodosimetry on the
time scale of molecular motions, i.e. angstrom or sub-angstrom displacements and open new perspectives in the
emerging domain of ultrafast radiation biomedicine such as pulsed radiotherapy.
A wide-band spectral diagnostic system based on dispersion property of the Zinc Selenide prism, a crystalline
material highly dispersive in the near-to-far infrared spectral range, has been studied and developed for the laser
wakefield acceleration experiment at LOA for the measurement of few femto-seconds long electron beam. The
extensive PIC simulation studies of the colliding-beam LWFA have shown very short electron beam duration of
less than 10 femtoseconds. The prism spectrometer diagnostic with highly sensitive Mercury Cadmium Telluride
infrared detector and the diffraction-grating spectrometer with a high-resolution imaging visible camera together
have the spectral range coverage and resolution capable of detecting ultra-short Coherent Transition Radiation
(CTR) generated by interaction of bunch charges with a 100 microns thickness aluminum foil. The beam profile
of asymmetric shape then could be extracted from the CTR spectrum by inverse Fourier transformation with
Kramers-Kronig relation. The diagnostic system has been tested and calibrated for characterization of blackbody
source spectrum and spectral responsivity. The measurement of electron beam duration of few femtoseconds has
yet been convincingly shown with high resolution, and the measurements of this kind have important implication
in understanding and subsequent successful operation of the future FEL light source with a highly mono-energetic
LWFA beam source.
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