Validation progresses of the voltage holding prediction model at the high voltage Padova test facility HVPTF

In the framework of the program for the construction in Padova - Italy of the first prototype of the ITER 1 MeV-16 MW Negative Neutral Beam Injector a R&D program on voltage holding in vacuum has been initiated since 2009, aimed at supporting the design, construction and development of the NNBI accelerator. To this purpose, the testing facility HVPTF, designed to operate up to 800 kV, has been built, in parallel to the development of a Multi Voltage-Multi-Electrode-Voltage Holding Prediction Model-VHPM. The paper focuses the results of the campaign (up to 270 kV) carried out in the Facility on different electrode configurations, with the aim of validating the VHPM in well controlled condition. The prediction capability of the model has been partially confirmed. This result made worthy to make a critical review of the physical basis of the model-based on the Cranberg-Slivkov clump theory - taking into consideration also other breakdown initiation mechanism.