  This paper studied the structures of debris discs, focusing on the conditions
that can form an axisymmetric-looking outer disc from systems with inner
clumps. The main conclusion was that as long as the dominated dust grains are
smaller than the blowout size, it is easy to form an axisymmetric-looking outer
debris disc, which is part of a quasi-steady state of the whole system. This
quasi-steady state is established through the balance between grain generations
and a continuous out-going grain flow. Assuming there is an event that starts
planetesimal collisions and the corresponding grain generations, this balance
can be approached in a few thousand years. This result suggested that a
quasi-steady-state picture could solve the possible mass budget problem of
Vega's outer debris disc.
