dc voltage holding experiments of vacuum gap for high-energy ion sources

Abstract

dc voltage holding characteristics were investigated to obtain a data base for designing high-energy and high-power ion sources of neutral beam injectors. We confirmed that the voltage holding characteristics almost obey the clump theory in the experimental gap length of up to 50 mm. The magnetic field in the gap lowered the breakdown voltage in a gas discharge region higher than a pressure of 10−3 Torr. The breakdown voltage of 30% was reduced by seeding cesium on the electrode with one order higher density than that of actual ion source at the pressure region of lower than several mTorr.

1-MV Vacuum Insulation for the ITER Neutral Beam Injectors

“…1, which contains available data that have been taken as reference data for determining the design criteria. The square closed symbols denote BDVs of vacuum gaps between circular oxygen free copper electrodes without field concentration [4]. It is found that the BDV is proportional d 0.5 to 0.6 , where d is the gap length, which is similar to that 0093-3813/$26.00 © 2011 IEEE predicted by "clump theory" [3].…”

“…2 [9]. The data used in the plot are the data measured after BDVs were increased by repetition of conditioning and reached a plateau value after about 20 shots breakdowns [4]. The voltage corresponding to 0.1% breakdown probability in the Weibull plot was defined as withstand voltage 1, as shown as square open symbols in Fig.…”

“…Basic data on breakdown of vacuum gaps up to 50 mm between copper electrodes from [4] were used for the design of the accelerators of the JT-60U NBI, but recent studies have shown that increased vacuum gaps in the accelerator are necessary to obtain stable operation at 500 kV [6]. Test facilities were set up in JAEA, Japan, and the IRFM, Cadarache, France to study high-current-density H − /D − beam acceleration up to 1 MV for the ITER NBIs [7], [8].…”

Breakdown Phenomena in the Negative Ion Source of 500 keV Negative-Ion Based NBI System on JT-60U

“…The lengths of the ESG-A1G, A1G-A2G, and A2G-GRG gaps are 75mm, 65mm, and 55mm, respectively. The shortest gap length was mainly determined, based upon the R&D results on the voltage holding capability between the grids in vacuum [3]. The other gap lengths were set to converge the D-ion beams by forming electrostatic lenses.…”

Voltage holding prediction in multi-electrode multi-voltage systems insulated in vacuum

“…For the geometries typical of the NBI injectors [3] i.e. with gap length above some centimetres -there are experimental evidences of the voltage breakdown tendency to saturate with the gap distance (U proportional to gα, with 0.3<α <0.7), indicating that the breakdown is dominated by the "clump mechanism", proposed first by Cranberg in the fifties [4] or by electron bombardment of the anode when the electron energy exceeds 0.6-0.7 MeV [5].…”