Numerical-Experimental Benchmarking of a Probabilistic Code for Prediction of Voltage Holding in High Vacuum

In the framework of the program for the construction of 1 MeV-16 MW negative neutral beam injector (NNBI) for ITER, a research and development activity on voltage holding in vacuum has been initiated since 2009, aimed at supporting the design, construction, and development of the NNBI accelerator. For this purpose, the voltage holding prediction model (VHPM) previously developed has been updated. In the VHPM, the effect of the anodic electric field and the cathodic electric field on the probability of breakdown is evaluated by means of two exponents: \alpha and \gamma . On the basis of the experimental results from different test stands and of detailed 3-D numerical simulation of the corresponding electric-field configurations, the predictions of the VHPM numerical code have been benchmarked. New exponents, \alpha and \gamma , have been proposed to obtain a more precise location of the weak point of the system and a better prediction of the maximum withstanding dc voltage in high vacuum.