No abstract

Desktop laser plasma acceleration has proven to be able to generate gigaelectronvolt-level quasi-monoenergetic electron beams. Moreover, such electron beams can oscillate transversely (wiggling motion) in the laser-produced plasma bubble/channel and emit collimated ultrashort X-ray flashes known as betatron radiation with photon energy ranging from kiloelectronvolts to megaelectronvolts. This implies that usually one cannot obtain bright betatron X-rays and high-quality electron beams with low emittance and small energy spread simultaneously in the same accelerating wave bucket. Here, we report the first (to our knowledge) experimental observation of two distinct electron bunches in a single laser shot, one featured with quasi-monoenergetic spectrum and another with continuous spectrum along with large emittance. The latter is able to generate high-flux betatron X-rays. Such is observed only when the laser self-guiding is extended over 4 mm at a fixed plasma density (4 × 10 18 cm −3 ). Numerical simulation reveals that two bunches of electrons are injected at different stages due to the bubble evolution. The first bunch is injected at the beginning to form a stable quasi-monoenergetic electron beam, whereas the second one is injected later due to the oscillation of the bubble size as a result of the change of the laser spot size during the propagation. Due to the inherent temporal synchronization, this unique electron-photon source can be ideal for pump-probe applications with femtosecond time resolution.S ynchrotron light sources are powerful in generating bright X-rays for a wide range of applications in basic science, medicine, and industry (1). However, these machines are usually large in size and expensive for construction and maintenance and are thus unaffordable to many would-be users. With the advent of tabletop ultrashort and ultraintense lasers, laser plasma acceleration (LPA) proposed by Tajima and Dawson (2) has demonstrated its great potential as a compact accelerator and X-ray source. Significant progress in LPA was made in the last decade (3-11): Well-collimated (approximately millirad) quasi-monoenergetic electron beams were first observed in 2004, and the electron energy above gigaelectronvolts over centimeter-scale acceleration lengths were demonstrated in several laboratories in the last few years.While accelerating longitudinally in the laser wakefield, the electron beams also oscillate transversally (wiggling motion) due to the transverse structure of the wakefield, which emits wellcollimated betatron X-rays (12-14). Among several mechanisms to generate X-ray radiation from laser-plasma interactions (15-20), betatron radiation is straightforward and able to deliver larger X-ray photon fluxes per shot [∼10 8 phs/shot (21)] and higher photon energies [up to gamma rays (22)]. The betatron oscillation frequency is given by ω β = ω p (2γ) −1/2 , where ω p is the plasma frequency and γ is the Lorentz factor of the accelerated electron beam. For large-amplitude betatron oscillations (i.e., a few mi...

Laboratory experiments have been carried out to model the magnetic reconnection process in a solar flare with powerful lasers. Relativistic electrons with energy up to MeV are detected along the magnetic separatrices bounding the reconnection outflow, which exhibit a kappa-like distribution with an effective temperature of ∼10 9 K. The acceleration of non-thermal electrons is found more efficient in the case with a guide magnetic field (a component of magnetic field along the reconnection-induced electric field) than that in the case without a guide field. Hardening of the spectrum at energies ≥ 500 keV is observed in both cases, which remarkably resembles the hardennings of hard X-ray and γ-ray spectra observed in many solar flares. This supports a recent proposal that the hardening in the hard X-ray and γ-ray emissions of solar flares is due to a hardening of the source-electron spectrum. We also performed numerical simulations that help examine behaviors of electrons in the reconnection process with the electromagnetic field configurations occurring in the experiments. Trajectories of non-thermal electrons observed in the experiments were well duplicated in the simulations. Our numerical simulations generally reproduce the electron energy spectrum as well, except the hardening of the electron spectrum. This suggests that other mechanisms such as shock and/or turbulence may play an important role in productions of the observed energetic electron.