Characteristics of surge noise generated by breakdowns at the protection spark gaps for the JT-60 negative ion source and countermeasure of malfunction due to the breakdown noise

Sasaki, Shunichi; Kojima, Atsushi; Shimizu, Tatsuo; Kawai, Mikito*; Hanada, Masaya

Breakdowns occur in the JT-60U 500 kV negative ion accelerator with a high probability during the high voltage conditioning. The breakdown in the accelerator is the same phenomena as a short circuit for the power supply. Surge noise generated by the short circuit causes malfunctions of components in the NBI system and other systems. To evaluate the surge noise characteristics, the breakdown noise has been measured using a test circuit with a spark gap which is the protection gap for the accelerator and it considered to be a source of noise. Further, reduction of the surge noise at the spark gap and countermeasure of the malfunction due to the noise have been studied quantitatively at the side of the system where the malfunction occurs. As a result, it has been observed that the noise propagates through space or grounding lines and gives malfunctions for other systems. To reduce the noise, resistors have been connected in the ground potential side of the gap. It was confirmed that the resistor is effective to reduce the noise and probability of the malfunction has been reduced from 100% to 15%. In addition, by placing the ferrite core in the grounded line and by using an isolation amplifier, the malfunction was further reduced to 10%.