Steady state plasma is obtained in KAIST-Tokamak. The plasma parameters are as follows; plasma current 30 kA, duration time 80 ms, electron temperature 200 eV, edge safety factor 3.3. This is accomplished by controlling the plasma position. Cosine and sine coil are used to diagnose horizontal and vertical displacement of plasma current center. Based on this information, vertical and horizontal magnetic field coil currents are feedforward-controlled to maintain plasma at the vaccum vessel center. However, cosine coil gives incorrect result in toroidal geometry. So a new magnetic diagnostic is developed. We show that multipole moment of toroidal current is exactly measured by a half circle magnetic coil whose winding density is given as Legendre polynomial. Multipole moments determine external magnetic surface shape and plasma displacement.
Tokamak operation is accompanied by electrical engineering and computer engineering hard works because high voltage and current should be precisely controlled to generate complex forms of magnetic field and large number of data should be monitored and analyzed immediately after the experiments. Hence, detailed description is given for high voltage switching system and data acquisition system. We develop a new trigger circuit which stably operate 8 ignitrons in parallel. It is shown that trigger pulse shape is important in the operation of standard type ignitrons in a circuit where current rising-time is restricted by coil inductance. This system provides 0.5 T magnetic field.
A PC-based general purpose data acquisition system is developed and named as KDAS (KAIST-Tokamak Data Acquisition System). It collectively conrols the VXI, CAMAC, GPIB and RS232 instrument hybrids. Since it has a driver-based hierarchical structure, the system operates on any number of computers and can be applied to any hardware setup by simply editing a text file.
We also develop a soft x-ray spectrometer to diagnose steady state plasma. X-ray generates photo-electrons in Carbon target. The photo-electron energy distribution is analysed by using retarding potential grid. MCP is used as an electron detector. A preamplifier and a high voltage sweeping power supply are all home-made. Experiment results show that impurity line radiation is dominant.
