Collisional Damping of a Plasma Echo

It is shown that the collisional damping on the free streaming motion of plasma can be very important in certain circumstances even though the collisional frequency is small compared with the plasma frequency. Both spatial and temporal echoes are treated in this note. In the latter case, for example, we found that the collisional damping goes as $\exp [-\beta {{\omega }_p}^2{t}^3]$ (where $\beta$ is the collisional frequency).