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We study, by optical spectroscopy, the properties of high-current electric arcs that form between opening Cu contacts. The presence of a current that increases linearly with a rate of about 14 kA/ms leads to the separation of the contacts and to the formation of a bridge of molten metal that eventually ruptures. We show that after the molten bridge rupture, it takes about 40 $\mu \text{s}$ for the initial arc to be at local thermal equilibrium. This long thermalization time suggests that the plasma density during that time range is low, which we confirm using a simple modeling of the energy transfer between electrons and heavy particles. We explain these low densities by the fact that the molten bridge rupture leads to the formation of a shock wave. The initial dense plasma rapidly expands and a region with a pressure below the background pressure forms between the two contacts, leading to low plasma densities and long thermalization times. [ABSTRACT FROM AUTHOR]

We investigate by time-resolved optical emission spectroscopy the properties of electric arcs forming at the opening of Cu contacts and with a current of a few kA. The systematic observation of well-resolved transitions demonstrates that the dense Cu vapor that results from the rupture of the molten metal bridge between the contacts expands rapidly. Solving the radiative transfer equation through the line-of-sight to fit the lineshape of the detected spectra, we obtain that both the pressure and the temperature of the Cu-dominated plasma increase during the first 100 µs of arcing. At the same time, the peak wavelength and broadening of Cu I transitions exhibit a non-monotonic time-dependence. In particular, both the blueshift and narrowing of these transitions by Stark effect during the first 40 µs of arcing demonstrate a decrease in electron temperature. This result leads us to the conclusion that it takes at least 40 µs for the arc to be at local thermal equilibrium after the rupture of the molten metal bridge. [ABSTRACT FROM AUTHOR]