Vacuum High Voltage Holding: DC Experiments and modelling

This report describes the activities carried out within the subtask T5.7 of the Work Programme WP2019 at the High Voltage Padova Test Facility HVPTF and on the new High Voltage Short Gap Test Facility HVSGTF. The activities mainly consisted in: 1) Test of two electrodes with double-polarity configurations (Common Mode Voltage tests). 2) Test of electropolished electrodes 3) Tests with cathode covered by a thick alumina cap 4) Removal of dust deposited inside the vacuum vessel before starting the vacuum system: 5) Tests with specific shaped electrodes designed to understand the effect of the mutual exchange of charged particles between electrodes are still in progress 6) Tests of Non Evaporable Getter (NEG) pumps at high voltage in high vacuum are still in progress 7) Ordinary Maintenance of the vacuum pump system 8) Tests in HVSGTF to measure the Fowler-Nordheim emission current from stainless steel electrodes. The activities mentioned in points #5 and #6 are still in progress and will be completed next year. Concerning the other experiments, the main outcomes can be reported as follows: 1) CMV tests showed that the main microdischarge activity of the electrodes appears to be related to phenomena involving only a single power supply unit and the vacuum chamber, rather than the two electrodes under test 2) Test of electropolished electrodes, tests with cathode covered by a thick alumina cap and Removal of dust deposited inside the vacuum vessel did not indicate significant improvements of voltage holding 3) The measurement of the Fowler-Nordheim emission current from stainless steel electrodes showed a behaviour much more complex than expected and their analysis is in progress.