Radiation reaction in a continuous focusing channel.

We show that the radiation damping rate of the transverse action of a particle in a straight, continuous focusing system is independent of the particle energy, and that no : quantum excitation is induced. This absolute damping effect leads to the existence of a transverse ground state which the particle inevitably decays to, and fields the minimum beam emit~ance that one can ever attain, ~emzn = h/2mc, limited only by the uncertainty principle. Due to adiabatic invariance, the particle can be accelerated along the focusing channel in its ground state without any radiation energy loss.