Microwave Tokamak Experiment (MTX) ohmic heating system

The ohmic heating system for the Microwave Tokamak Experiment (MTX) at Lawrence Livermore National Laboratory (LLNL) provides both the voltage for the initial breakdown phase and the energy to drive the plasma current to a value of 400 kA or greater. Providing this voltage and flux swing requires a one-turn loop voltage of about 25 volts (11 kV across the coil) and a magnetic flux swing of 2 volt- seconds. This voltage and flux swing are accomplished by charging the ohmic heating coils to 20 kA, at which point the current is commutated off into a resistor generating the 11 kV across the coil. When the current passes through zero, another power supply drives the current in the opposite polarity to 20 kA, thus completing the full 2 volt-second flux swing. This paper describes the design features and performance of the ohmic heating circuit, with emphasis on the commutation circuit. In addition, the paper describes the use of the ohmic heating system for discharge cleaning and the changeover procedure. 3 refs., 4 figs., 1 tab