Evolution of the plasma bubble in a narrow gap.

We investigate the evolution of the plasma bubble in a narrow gap. According to the morphological changes, we further show that there are three phases during the evolution for spherical fluctuating, radial fingering, and dense branching plasma bubbles, which are similar to the radial fingering pattern in a Hele-Shaw cell. The dependences of the wavelength of the fingering boundary are experimentally discussed. The dense branching plasma bubble is found with a fractal dimension of D(f)=1.74. The reduced surface tension pressure from the local heatings due to the filamentary discharges is suspected of being responsible for the growth of the radial fingering and the dense branching plasma bubbles.