  On the basis of the recently proposed {\it Thermal Wave Model (TWM) for
particle beams}, we give a description of the longitudinal charge particle
dynamics in circular accelerating machines by taking into account both
radiation damping and quantum excitation (stochastic effect), in presence of a
RF potential well. The longitudinal dynamics is governed by a 1-D
Schr\"{o}dinger-like equation for a complex wave function whose squared modulus
gives the longitudinal bunch density profile. In this framework, the
appropriate {\it r.m.s. emittance} scaling law, due to the damping effect, is
naturally recovered, and the asymptotic equilibrium condition for the bunch
length, due to the competition between quantum excitation (QE) and radiation
damping (RD), is found. This result opens the possibility to apply the TWM,
already tested for protons, to electrons, for which QE and RD are very
important.
