  Quasi-periodic Pedestal Burst Instabilities (PBIs), featuring alternative
turbulence suppression and bursts, have been clearly identified by various edge
diagnostics during I-mode to H-mode transition in the EAST Tokamak. The radial
distribution of the phase perturbation caused by PBI shows that PBI is
localized in the pedestal. Prior to each PBI, a significant increase of density
gradient close to the pedestal top can be clearly distinguished, then the
turbulence burst is generated, accompanied by the relaxation of the density
profile, and then induces an outward particle flux. The relative density
perturbation caused by PBIs is about $6 \sim 8\%$. Statistic analyses show that
the pedestal normalized density gradient triggering the first PBI has a
threshold value, mostly in the range of $22 \sim 24$, suggesting that a PBI
triggering instability could be driven by the density gradient. And the
pedestal normalized density gradient triggering the last PBI is about $30 \sim
40$ and seems to increase with the loss power and the chord-averaged density.
In addition, the frequency of PBI is likely to be inversely proportional to the
chord-averaged density and the loss power. These results suggest that PBIs and
the density gradient prompt increase prior to PBIs can be considered as the
precursor for controlling I-H transition.
