High power CW THz DCN laser and FIR interferometer

Fusion plasma density can be measured by FIR laser and submillimeter wave interterometer. 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 mu m) and 1.58 THz (190 mu 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 N-2, 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 mu m), the EH11 mode, the wall temperature is 180 degrees C, and the current is 13A. With the N-2: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. It is inconvenient to open the laser often for cleaning the tube. Just like using H-2 to substitute He in HCN laser, D-2 was used to substitute He in DCN laser. After optimization of various parameters (such as the novel gas mixture rate, the pressure, the discharge current and the wall temperature), the laser output power is up to 0.2W on the 1.54 terahertz. The free space propagation of the EH11 mode of DCN laser is studied, and the beam profile is found to be qualitatively Gaussian in the far-field (Z > 4 m).