  In this Letter, we report on the concept and analysis of a low-emittance
electron storage ring, in which the electron beams undergo an early-stage
self-amplified spontaneous emission lasing process on a turn-by-turn basis. The
lasing process for each pass through a long undulator in the ring is terminated
when the radiated power is still negligible compared to the total synchrotron
loss of each circulation, and the electron beams can be maintained in an
equilibrium state that supports sustainable lasing. A self-consistent model is
derived for evaluation of the properties of the electron beams, and a design
with numerical modeling is presented that demonstrates the feasibility of
generating short-wavelength radiation at the kW power level.
