Ultrafast short wavelength generation from laser-accelerated electrons.

摘要

Previous research shows intense laser-plasma interactions in gases produce ultrafast (fs-ps) MeV electron bunches by the mechanism of wakefield acceleration. This research studies application of these electrons to production of narrow divergence, ultrafast, short-wavelength radiation. Three distinct laser-plasma mechanisms are experimentally studied: (1) non-linear Thomson scattering, (2) counter-propagating Thomson scattering, and (3) betatron motion. In the non-linear Thomson scattering study, interaction of laser intensity I > 1 x 1018 W/cm2 (400 fs pulse) with Helium gas generates VUV harmonic radiation along the laser direction with 3° divergence. Presence of harmonics with and without circular polarization as well as other experimental tests differentiate the mechanism from atomic harmonics. The harmonics are attributed to the non-linear Thomson scattering of the ultra-intense laser pulse on the electrons it accelerated. The experiment was modified in the second study such that a laser pulse directed head-on into the laser accelerated electron beam. This was intended to Doppler shift the Thomson scattering to higher photon energies. Imaging revealed interaction between the pulses within the plasma. The forward radiation spectrum showed up to 40% increase of the VUV harmonics. The signatures of the radiation increase exclude Thomson scattering as mechanism, but suggest enhancement of atomic harmonics in the highly ionized gas. Concurrent with the Thomson studies, manipulation of gas distribution delivered to the interaction volume by replacement of Mach 8 nozzle with Mach 3 nozzle increased the electron beam charge more than an order of magnitude. Interferometric studies characterizing the nozzles discount the role of the gas-vacuum interface and interaction length thus suggesting flow uniformity causes the improvement. In the third study, a 30 TW, 30 fs laser pulse in Helium gas creates > 108 photons in the several keV range with 4° divergence along the laser direction. The measurements indicate the radiation originates from oscillations, termed betatron motions, of the laser accelerated electrons in the plasma channel formed by the laser. The results of the research are significant to development of directional, broadband and compact sources of ultrafast short-wavelength light.