Investigation of c-chamfering around aperture edge for high voltage holding performance in small electrodes with multi-apertures in vacuum

Summary

Neutral beam injector (NBI) with an electrostatic accelerator for nuclear fusion device is required to stably withstand high voltage application in vacuum to generate hydrogen-deuterium negative ion with high energy state. The accelerating parts of NBI device are composed of some grids of multi-aperture electrode which is quite large size (0.45m×1.1m). One of issues for developing the NBI is the occurrence of vacuum breakdowns in the multi-aperture electrode. Therefore, the design technique of the electrical vacuum insulation to restrict vacuum breakdown events is required. In this paper, to improve the high voltage performance of acceleration electrode. we investigate the breakdown characteristics for three types of small electrode with multi-apertures of which edge are different shapes to change electric field distribution around it. Additionally, we measured pre-breakdown current characteristics that flows before breakdown in the gap of multi-aperture small type electrodes. To predict the breakdown voltage for multi-aperture electrode, new shape parameter of the electric field integration “ES” around apertures on the electrode surface is introduced. This paper describes the dependence of electric field integration ES on the characteristics of dark current and breakdown voltage. As the result of breakdown tests for three types of multi aperture electrode, it is found that the electric field integration “ES” has a certain relation with breakdown voltage and dark current characteristic. Reducing the ES caused by the relaxation of electric fields leads the improvement of the breakdown performance. Moreover, the low value of ES induced the reductions of field enhance factor “ß” and emission area “A” of field electron emission from the electrode. Therefore, we assumed that reductions of pre-breakdown current that flows before the occurrence of breakdown by reducing the ES made the breakdown electric field improved.

Additional informations

Authors

Y. YAMANO, A. KOJIMA, R. NISHIKIORI, M. KASHIWAGI

Keywords

breakdown, vacuum, pre-current, field electron emission current, electric field integration