Based on the accurate plasma position measurements and the various hardware system developments, the performance of the KAIST-Tokamak ohmic plasmas has significantly been improved. The details are as follows.
In order to have reproducible ohmic plasma discharges, a 2.45 GHz microwave source was developed using a magnetron taken from a conventional microwave oven. The system operation was checked by producing ECH plasmas inside the KAIST-Tokamak vacuum vessel with the appropriate toroidal magnetic fields. With this system, it was possible to obtain plasma startup with very low failure rate. The ohmic loop voltage was also reduced by about 3 V.
The development of the discharge automation system helped the efficiency of KAIST-Tokamak experiments. A 486 personal computer and a delay generator control the overall experimental activities from charging various capacitor banks to the collection of data acquired by many diagnostics. Thanks to the development of this system, a single operator can operate KAIST-Tokamak while others concentrated on their own experimental works.
KAIST-Tokamak had serious problems with the iron core transformer that prevented from obtaining the desired ohmic plasmas. The problems were first diagnosed by using several magnetic diagnostics and the numerical simulation using Poisson code. It was quantitatively found that the iron core showed up-down asymmetry with respect to the midplane of the tokamak and that the magnetic permeability of the core material was seriously reduced. In order to remedy the asymmetry, four more turns were added in series with the inside lower ohmic coil. Addition of more turns raised the location of the field null toward the mid-plane. The reduction of the magnetic permeability which made plasma position control difficult was solved by strengthening the vertical magnetic field provided by the external vertical field (VF) coils with the improved VF coil power system. These solutions made possible to produce the desired 40 kA, 100 ms ohmic discharges.
In getting long discharges, it was essential to have information on the plasma position during the ohmic discharge. For this, two independent methods were devised to measure the plasma position in a tokamak with an iron core such as KAIST-Tokamak. The first is the magnetic diagnostic method, which utilizes several magnetic pickup coils. So as to include the effects from toroidicity and iron core, Poisson code was run and the results were incorporated with the experimental measurements. A neural network used in the analysis reduced the calculation time significantly, which opens up the possibility of applying the method for real-time plasma position measurement. Another method for measuring the plasma position was an optical method using a CCD camera. The analysis with the image signal taken by the camera gave the position of the peak of the light emitted from the plasma. The results from the magnetic method and the optical method showed good agreement.
