The phenomenon of secondary eiectron emission consists in the following. When electrons bombard a solid surface, the surface emits secondary electrons (and partially reflects the primary electrons, which Impinge on the surface). Secondary electron emission is characterized by the secondary emission coefficient `delta`, Which is the ratio of the secondary electron current to the primary current. The dependence of the secondary emission coefIicient on the primary electron energy `W_1` for a certain dielectric is depicted in the figure. At `delta=1` the surface of the dielectric does not change its potential under electron bornbardment, since the number of electrons leaving tho surface every second is equal in this case to the number of electrons bombarding the surface every second. The two points `a and b` on the `delta vs . W_1` curve correspond to `delta = 1`. At which point is the process stable and at which is it unstable?