Affiliations: [a] National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba Central-2, Umezono, 1-1-1, Tsukuba, Ibaraki,
305-8568, Japan | [b] Science University of Tokyo, Yamazaki 2641,Noda,278-
0022, Japan
Abstract: Self-focusing of the intense laser pulses as well as production of
the well-collimated electron beams in the moderately underdense plasma of
n_{e}/n_{c} ≥ 10% 7 ×10^{17}
W/cm^{2}. Progresses of a relativistic self-focusing in the plasma
of a gaussian density profile were observed by an ultra-fast optical probing.
Even if the large gas-target of the quasi-uniform density was used, the
self-focusing was terminated after traveling approximately 1.2 mm by the abrupt
transition of the propagation mode from the smooth channeling to the complex
filamentation, which correlated with the emission of MeV-electron beams.
High-energy electron beams have the power-law spectrum of
E^{-3.5} reaching the maximum energy of 2 MeV. The maximum
number of the electrons was (1.3 ± 0.6) × 10^{11}
/sr/shot. Model calculations suggest that the most plausible mechanism of the
production of high-energy electron beam in the moderately underdense plasma is
that the phase of the electron oscillation in filaments of the laser beam
randomly changes many times.
