Research On The Frequency Stability Control System For The FIR Lasers

Controlled nuclear fusion has a great potential to be one of the main energy sources in the future. Magnetic confinement and inertial confinement are two main methods to achieve controlled nuclear fusion. Among all kinds of magnetic confinement devices, Tokamak which has been fully developed has great advantage to implement controlled nuclear fusion in the future. The gas temperature of the nuclear fusion plasma can reach 108 K, in which the spatial distributions of the electron number density and the magnetic field are the key factors to control the discharge modes of the Tokamak. The far-infrared polarimeter/interferometer system has been designed at the Institute of Plasma Physics in Chinese Academy of Sciences for the measurement of the electron number density and magnetic field distributions in the EAST Tokamak. In this experiment, the stability of the intermediate frequency(IF) of the lasers is one of the key factors for obtaining stable and reliable experimental data; while the present active frequency stability control methods for lasers are not suitable for applying in the far-infrared polarimeter/interferometer measurement system.In this thesis, based on the theory of the far-infrared polarimeter/interferometer measurement, the influences of the IF instability on the measured data of the electron number density and magnetic field, as well as the real-time control of discharge modes of East Tokamak, are analyzed; and then, a set of control scheme based on Advanced RISC Machines(ARM) is designed. This IF stability control system can calculate the value of IF signal, and compare it with the pre-set value, and then, produce a control voltage based on PID algorithm. The length of laser cavity changes since the length of PZT is driven by the control voltage. As a result, the frequency of one laser is changed so that the actual IF value becomes closer to the pre-set one. Based on this, a frequency controller is fabricated for improving the IF stability. The field experimental results show that a stable IF output can be obtained using this frequency stability control system.