Study of high DC voltage breakdown between stainless steel electrodes separated by long vacuum gaps

Abstract

High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER neutral beam injector (NBI) (Toigo et al 2017 New J. Phys. 19 085004). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER NBI (De Lorenzi et al 2011 Fusion Eng. Des. 86 742-5). A voltage up to 800kV DC was achieved in the HVPTF during the experimental campaigns with a sphere-plane configurations having variable gap length (from 30 to 150 mm) and pressure ranging from high vacuum (10 -7 mbar) to 10 -3 mbar in argon. Such an experimental campaign represents one of the few examples where voltages higher than 500-550 kV DC are sustained by a single vacuum gap between electrodes (Rohrbach 1971 CERN Report 71-5) and therefore constitutes additional experience to improve the knowledge of voltage holding across large vacuum gaps. The results in high vacuum indicate that at the beginning of voltage conditioning the breakdown events occur at the same cathodic electric field irrespective of the electrode geometry. However, after sufficient conditioning time, the breakdown voltage distribution seems to depend also on the electric field at the anode and on the total voltage between electrodes. A benchmark between a numerical tool previously developed to predict the voltage holding in high vacuum (voltage holding prediction model, VHPM (Pilan et al 2011 Fusion Eng. Des. 86 742-5)) and the experimental results is also reported and discussed.

Characterization of vacuum HV microdischarges at HVPTF through X-ray bremsstrahlung spectroscopy

“…The vacuum chamber is kept at ground level. More details are available in [5]. The current and voltage outputs of the two PS are sampled at a frequency of 100 Hz.…”

“…A set of scintillators is installed behind a glass window to detect X-ray radiation, approximately 1.5 meters away from the electrodes. Two scintillators, an inorganic NaI(Tl) and an organic EJ-228, were used in the past [4] but were proven unsuitable due to excessive pile-up [5]. To address the problem two other scintillators were installed: a 1-inch × 0.75-inch enhanced Lanthanium Bromide (LaBr) [7] scintillation decay times (25 ns for the LaBr [7] and 36 for the LYSO [8]), coupled with the fast electronics, makes these scintillators well suited for high flux scenarios [11,12].…”

“…In each campaign the configuration of the electrodes was modified by changing the shape and the distance of the electrodes, the gas composition, and/or the vacuum pressure. In each session the voltage was raised through an automated conditioning procedure to reduce the variability between the experiments [5]. The procedure applies voltage anti-symmetrically on both electrodes (V + = V − ) while keeping a current limit of I + max = I − max = 0.3 mA.…”

“…Discharge events are divided in two categories: the disruptive events, after which the voltage is brought to zero, referred as "breakdowns", and the events after which the voltage is restored to the previous value in less or about 100 μs, referred as "microdischarges". Since microdischarges are a frequent phenomenon when approaching the breakdown voltage [16], the study of these phenomena is of prime interest in the context of this research [5].…”

“…A different, pulse-mode measurement technique based on X-rays produced by bremsstrahlung is introduced and expected to deliver reliable results with a high time resolution. In past experiences a high X-ray flux in HVPTF was observed [4] causing an important amount of pile-up events on the detector electronic chains [5]. Recently, the installation of two new scintillators coupled with fast electronics opens new opportunities for improving X-ray spectroscopy on HVPTF [6].…”