Most ionic crystals have strong reflection bands in the far infrared. Many such crystals are also transmissive to infrared radiation. An unstable resonator that exploits these properties is described. Preliminary results from a device 134 cm long and of 6 cm diameter are presented. For a pump pulse of 10 J and of duration 75 ns output powers of 0.7 MW have been measured. A correlation between the mode beating of the pump wave and the FIR wave it generates is demonstrated.