  Despite tremendous progress in x-ray free-electron laser (FEL) science over
the last decade, future applications still demand fully coherent, stable x-rays
that have not been demonstrated in existing X-ray FEL facilities. In this
Letter, we describe an active Q-switched x-ray regenerative amplifier FEL
scheme to produce fully coherent, high-brightness, hard x rays at a
high-repetition rate. By using simple electron-beam phase space manipulation,
we show this scheme is flexible in controlling the x-ray cavity quality factor
Q and hence the output radiation. We report both theoretical and numerical
studies on this scheme with a wide range of accelerator, x-ray cavity, and
undulator parameters.
