  Dust collisions in protoplanetary disks are one means to grow planetesimals,
but the destructive or constructive nature of high speed collisions is still
unsettled. In laboratory experiments, we study the self-consistent evolution of
a target upon continuous impacts of submm dust aggregates at collision
velocities of up to 71m/s. Earlier studies analyzed individual collisions,
which were more speculative for high velocities and low projectile masses.
Here, we confirm earlier findings that high speed collisions result in mass
gain of the target. We also quantify the accretion efficiency for the used SiO2
(quartz) dust sample. For two different average masses of dust aggregates
(0.29g and 2.67g) accretion efficiencies are decreasing with velocity from 58%
to 18% and from 25% to 7% at 27m/s to 71m/s, respectively. The accretion
efficiency decreases approximately as logarithmic with impact energy. At the
impact velocity of 49m/s the target acquires a volume filling factor of 38%.
These data extend earlier work that pointed to the filling factor leveling off
at 8m/s to a value of 33%. Our results imply that high speed collisions are an
important mode of particle evolution. It especially allows existing large
bodies to grow further by scavenging smaller aggregates with high efficiency.
