Electric measurements:
Voltage measurements between the end contacts of both elekctrodes before and during electrode separation with a digital oscilliscope Yokogawa DLM2054.

High-speed imaging:
High-speed imaging of the arc side-on from a direction perpendicular to the arc axis with Photron Fast-Cam NOVA R3 with 1000 fps and minimum exposure time of 2 µs (without filters).
Spectrally selective high-speed imaging of the arc side on with Photron Fast-Cam NOVA R3 and a beam splitter (double image optics) with two narrow band metal interference filters with the centre wavelengths 514 nm and 777 nm corresponding to the CuI line at 515 nm and the OI triplet at 777 nm.

OES:
Optical emission spectroscopy (OES) with a 0.5 m imaging spectrograph with an entrance slit of 20 µm width and a 1200 grating coupled with a CCD camera PI-Max4:1024i and an optical mirror system to image part of the arc.

Fluid model:
An 1D unified non-equilibrium fluid model as described in detail in M. Baeva et al., J. Phys. D: Appl. Phys., 58(9):095204, 2024. doi:10.1088/1361-6463/ad9f79 is used to obtain the plasma parameters in air-Cu mixture. It considers the ground state atoms and singly charged ions of Cu along with the 11-species reaction scheme for air. The model solves the equations for conservation of species, energy of electrons and heavy particles (atoms, molecules, ions), the Poison equations for the electric potential, the heat transfer in the electrodes for a current level controlled by an external circuit. A collisional-radiative model for the population of the excited states is included.