Evaluation of Dark Current Profile for Prediction of Voltage Holding Capability on Multi-Aperture Multi-Grid Accelerator for ITER

Dark current analysis to estimate the current density of the field-emitted electrons have been developed for the prediction of the yoltage holding capability of multi-aperture multi-grid accelerators used in JT-60SA and ITER neutral beam injectors. From the experimental measurement of the field-emitted electrons in multi-aperture multi-grid accelerators, βEBD, which represent the critical current density to trigger the breakdown, were found to be constant with around 6 × 103 kV/mm regardless of the electric field profiles and the surface conditions in an accelerator with a surface area of 0.1 m2. In addition, the dark current was measured in the region with the electric field above ∼60% of the electric field at the breakdown, which leads to the determination of the emission region in the analytical estimation of the dark current. Furthermore, from the measurement of βEBD on electrodes with various surface area, βEBD was found to decreases with an increase of the surface area S (βEBD = 4 × 103S−0.3). From these results, the dark current profile at the critical current density can be estimated from the electric field analysis, which leads to the development of the breakdown model and the prediction of the voltage holding capability on the multi-aperture multi-grid accelerators.