Yasunori Mizuno

Feedback position control apparatus of tokamak plasma horizontal position with a digital drive

A plasma horizontal position feedback control system (PHPFCS) and a digital current drive (DCD) for controlling vertical magnetic field are made on an experimental base, and the horizontal position of a plasma in a small tokamak (HAMANA-T) is controlled by PID operations from the initial phase of plasma generation. The DCD consists of constant-current discharge units for multiple condensers and drives a digital current analogous as a whole to the control signal of PHPFCS in the range of ±90 A. The maximum value of the digital current increases in proportion to the number of units. This PHPFCS elongates plasma duration from 1.3 ms to 2.1 ms, smooths the waveform of plasma current, and reduces the plasma loop voltage from 25 V to 20 V. Furthermore, PHPFCS reduces the range of variation in plasma horizontal displacement from 3.5 cm to within 0.5 cm and makes it possible to set accurately the horizontal position from the initial phase in the inner region of a vacuum chamber.

Simulations of dusty plasmas using a special-purpose computer system designed for gravitational N-body problems

Simulations of dusty plasmas were performed using GRAPE-6, a special-purpose computer designed for gravitational N-body problems. The collective behavior of dust particles, which are injected into the plasma, was studied by means of three-dimensional computer simulations. As an example of a dusty plasma simulation, experiments on Coulomb crystals in plasmas are simulated. Formation of a quasi-two-dimensional Coulomb crystal has been observed under typical laboratory conditions. Another example was to simulate movement of dust particles in plasmas under microgravity conditions. Fully three-dimensional spherical structures of dust clouds have been observed. For the simulation of a dusty plasma in microgravity with 3×104 particles, GRAPE-6 can perform the whole operation 1000 times faster than by using a Pentium 4 1.6 GHz processor.

Drive to generate digital current for control vertical field in a small Tokamak

The horizontal position of a small Tokamak plasma has been controlled by a circuit to generate the control vertical field. The circuit must respond quickly to the position signal from a feedback circuit and feed a large current in a short time. We have assembled the circuit to generate a digital current called a drive. The drive includes several units, and each unit is made up of a capacitor, a transistor, a protective resistor and a comparator. We have installed the drive on a small Tokamak, the Hamana-T and obtained satisfactory results. The performance of the drive is described.

Evaluation of Error Poloidal Field in an Iron-Cored Tokamak

The error field (magnetic induction) conjectured to originate from the connecting conductors and the turnback coil of toroidal field coils and the poloidal field, which is applied to compensate the error field and the electron toroidal drift, are computed with semianalytic expressions. The vertical component of the computed error field agrees roughly with the error vertical field which is evaluated indirectly by determining the optimum value of the applied poloidal field from the standpoint of the tokamak gas breakdown. The computed value of the applied poloidal field produced by simple circular coils agrees satisfactorily with the directly measured one.