Surface and sub-surface modifications of copper electrodes exposed to high-field conditioning at cryogenic temperatures
arxiv(2024)
摘要
In order to investigate the dependence of conditioning and field-holding on
temperature, three pairs of copper electrodes underwent high voltage
conditioning with direct current (DC) pulses while kept at a single
temperature, unique for each set (300 K, 30 K and 10 K), until saturation field
for each set was found. The sets conditioned at cold showed a significant
increase in the field holding capability, reaching fields up to 147 MV/m after
tens of millions of pulses and very few breakdowns (BDs). We interpret this as
an indication of the conditioning effect being due to high field pulsing rather
than exposure to BDs. The effect of the warm and cold conditioning was
investigated with high-resolution microscopy, characterizing the BD spots on
the anode and cathode according to their morphology and with scanning
transmission electron microscopy (STEM) analyzing the changes in the
sub-surface regions. Atypical BD spot features were found on the cryogenically
conditioned cathode surfaces, with very shallow craters of a star-like shape.
The number of atypical spots increased with decreased temperatures, reaching 26
and 53 percent of the total number of spots at 30 K and 10 K, respectively. A
hypothesis explaining the formation of these features is also presented. The
very different morphology of the anode and cathode BD spots is presented in
detail that suggesting an unknown shielding mechanism that prevents the center
of the anode spot from melting. These results provide important experimental
input for the development of quantitative theories and models for BD initiation
and inter-electrode plasma formation.
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