  We report on an experimental demonstration of laser wakefield electron
acceleration using a sub-TW power laser by tightly focusing 30-fs laser pulses
with only 8 mJ pulse energy on a 100 \mu m scale gas target. The experiments
are carried out at an unprecedented 0.5 kHz repetition rate, allowing "real
time" optimization of accelerator parameters. Well-collimated and stable
electron beams with a quasi-monoenergetic peak in excess of 100 keV are
measured. Particle-in-cell simulations show excellent agreement with the
experimental results and suggest an acceleration mechanism based on electron
trapping on the density downramp, due to the time varying phase velocity of the
plasma waves.
