High Power CW THz DCN Laser and FIR Interferometer

Abstract

Fusion plasma density can be measured by FIR laser and submillimeter wave interferometer. The DCN laser has the highest specific output power among known submillimeter electricdischarge lasers. Its output characteristics fit the requirements for interferometric measurements on large tokamaks. So that, the continuous wave (CW) discharge-pumped 1.54 THz (195 ?m) and 1.58 THz (l90?m) deuterium cyanide (DCN) laser has been designed and developed as the source of FIR laser diagnostics in the EAST tokamak. The laser cavity is a 54mm inner diameter, 3m long of pyrex tube with plane reflectors (one of them is a nickel mesh) against its ends. A hot LaB6 cathode is essential in producing a stable discharge at high current density. The DCN laser oscillation is achieved by a glow DC-discharge in a gas mixture containing N2, CD4 and He in an optimization ratio of 3.78:4.9:37.5. The helium gas is added to the mixture for stabilization of the discharge. With the optimized working mixture, we also optimized other parameters, the DCN laser measured with an absolute calorimeter near the laser output window, was 220 mW for the 1.54 THz (195 ?m), the EH11, mode, the wall temperature is 180°C, and the current is 1.3A. With the N2:CD4:He gas mixture the DCN lasers produce brown polymer deposit on the lasers tube wall after decades hours operation, which could reduce the performance of DCN laser seriously. Typically, the laser power will decrease 90 percent after 100 hours operation.