  A robust plasma gating to generate a single ultra-intense attosecond pulse is
developed. It is a manifestation of the hole-boring effect that limits the
strongest attosecond pulse emission within one laser cycle. The generated pulse
is characterized by a stabilized harmonic phase $\psi \approx \pm\pi/2$ and a
slowly decaying exponential spectrum bounded by $\gamma$-spike scaling and CSE
scaling. The phase oscillations in low-frequency region and fluctuations in
high-frequency region are discussed. We also show that the phase fluctuations
in high-frequency region can be reduced by including radiation reaction force.
