  Strong terahertz radiation from mesa structure of
$\rm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ single crystal has been observed recently,
where the mesa intrinsically forms a cavity. For a thick mesa of large number
of junctions, there are many cavity modes with different wave vectors along the
c-axis corresponding to almost degenerate bias voltages. The mechanism
responsible for exciting the uniform mode which radiates coherent terahertz
waves in experiments is unknown. In this work, we show that the in-plane
dissipation selects the uniform mode. For perturbations with non-zero wave
numbers along the c-axis, the in-plane dissipations are significantly enhanced,
which prevent the excitation of corresponding cavity modes. Our analytical
results are confirmed by numerical simulations.
