This paper presents differences between anode spot type 1 and type 2 and the discharge conditions for a transition from anode spot type 1 to type 2 are investigated by means of optical and electrical measurements. The transition from the diffuse mode to the high-current anode modes (footpoint, anode spot type 1, and anode spot type 2) is studied with respect to spatial and temporal distributions of different ionization levels of Cu lines, voltage-current waveforms, transferred charge, and aspect ratio. The driving current pulse was applied by a high-current generator which corresponds to a pulsed dc of 10-ms duration. Spectral lines of Cu I, Cu II, and Cu III are analyzed for various anode modes using a combination of an imaging spectrograph and a high-speed camera. The results show that during the formation of the anode spot type 2, the intensity of all lines change noticeably. Significant differences are found, for example, in the spatial structure of Cu III lines in the anode spot modes. A jump in the arc voltage corresponding to the time of intensity changes appears at the transition from anode spot type 1 to type 2. The impact of transferred charge, instantaneous current, and aspect ratio on the formation of the anode spot type 2 is also investigated. The transition from anode spot type 1 to type 2 is dominated by the ratio of electrode diameter to gap length. The formation of an anode plume is always observed after extinction of anode spot type 2 and before current zero.

The electrode surface temperatures of Cu–Cr butt electrodes exposed to vacuum arcs with sinusoidal currents of 10–20 kA and under external axial magnetic field were determined. Different experimental techniques were applied that can be distinguished by the used spectral wavelength range, their temporal, and spatial resolution. Near infrared spectroscopy was carried out by means of a fiber optic spectrometer working in the wavelength range from 900 to 1670 nm with a temporal resolution of 1–2 ms. Electrode surface temperatures after current zero were obtained from the relative shape of the spectrum using the Planck curve fitting procedure. Furthermore, electrode emissivities were derived after performing absolute calibration of the spectra. Pyrometric measurements were performed in the spectral range around 2 $\mu \text{m}$ with high temporal but without spatial resolution. 2-D distributions of the electrode temperature were obtained from high-speed video thermography using a camera with a metal interference filter at 890 nm.

This paper presents time and space resolved results of spectroscopic measurements in a vacuum circuit breaker experiment during high-current anode modes, i.e., anode spot type 1, anode spot type 2, and anode plume. Excited state densities for Cu I, Cu II, and Cu III transitions are determined during anode spot type 1 and type 2 as well as for the anode plume. Temporal evolution of excited state densities and Cu neutral gas densities are also determined during anode spot type 1 and type 2, which show that the Cu density in front of the anode during anode spot type 2 is about 6 times higher compared to anode spot type 1. Electron densities are also determined during both types of anode spot using Stark broadening. The electron densities during anode spot type 2 are remarkably higher than during anode spot type 1.

Results of broadband absorption spectroscopy in high-current vacuum arcs after current zero (CZ) are presented. Vapor density of atomic Cr is determined within 3 ms after CZ in cases with an anode plume which appears at about 400 $\mu \text{s}$ before CZ and after extinction of anode spot type 2. The anode plume is a cloud of metal vapor which develops on the anode after the extinction of one specific high-current anode mode, namely, anode spot type 2. An ac current pulse of about 7 kA from a high-current generator with operation frequency of 100 Hz was applied. Cr I resonance lines at 425.43, 427.78, and 428.97 nm were chosen for the analysis. The temporal evolution of the ground state density of Cr atoms within first 3 ms after CZ starts with about $2.5 \times 10^{18}~\text{m}^{-3}$ and decreases almost linearly with $0.65 \times 10^{18}~\text{m}^{-3}$ /ms. The Cr I densities in cases with and without anode plume corresponding to anode spot type 2 and 1, respectively, are compared at 100 $\mu \text{s}$ after CZ. In cases with anode plume, the metal vapor density is about 60% higher than in cases without plume, although the interrupting current is the same. The general behavior of the copper component is expected to be very similar to that of chromium.

This paper presents the results of different optical diagnostic methods to investigate an anode plume which forms after the extinction of anode spot type 2. The formation of the anode plume is investigated by means of three high-speed cameras. Two high-speed cameras, positioned perpendicular to each other, are used to observe the general anode plume evolution. The third camera is connected to a spectrograph enabling the video spectroscopy. The arc is driven by a damped 50-Hz ac between Cu/Cr 75/25 butt contacts with a diameter of 10 mm. The emission coefficients for atomic and ionic Cu lines during anode plume formation are calculated. The inner part of the anode plume is dominated by atomic line radiation, whereas the outer part is dominated by the emission of singly charged ionic lines. The radiator density of the Cu I 510.55 nm is about 3x 10¹⁷ m⁻³ compared with a radiator density of Cu II of 9x 10¹³ m⁻³ at the same position inside the anode plume. A broadband absorption spectroscopy is used to determine the ground-state densities of Cu and Cr after the extinction of anode plume at current zero (CZ). The density of Cu I is about 1.5 times higher than Cr I at the same time around CZ. Moreover, near-infrared spectroscopy together with thermography is used to determine the anode surface temperature around CZ after the extinction of an anode plume. The central anode surface temperature varies from 1530 to 950 K within 20 ms after CZ.

Anode activity in high current vacuum arcs leads to the formation of various high current modes and transitions between them. Intense material evaporation during the anode spot mode and formation of a neutral vapour cloud during the anode plume mode modify the arc plasma properties and, hence, can have a crucial impact in applications, like e.g. reduction of interruption performance of switching devices. The influence of anode mode appearance on arc plasma parameters and on anode surface temperature was studied in detail by a novel optical diagnostic technique—intensified video optical emission spectroscopy. Employing advanced diagnostic methods, the ground state density, excitation temperature and pressure profiles close to the anode surface have been determined. For the anode plume mode, higher copper vapour pressure was found in the plume shell compared to its core. The copper ion density distribution shows a maximum outside of the plume shell. Consequently, a higher electrical conductivity in the surrounding area of the plume might be expected, i.e. the arc current flows around the plume rather than through it. Analysis of the temporal evolution of electrical and optical signals reveals that voltage jumps and drops during the mode transitions are accompanied by noticeable changes in the anode surface temperature. Thus, the formation of the anode plume leads to temperature lowering while the transition to the anode spot mode is accompanied by a temperature increase. In general, a clear correlation between electrode surface temperature and arc voltage in the case of constricted anode attachment is found. The results of this study give new insights into anode plume properties and consequences of anode mode transitions. Reversible mode transitions and correlations between arc voltage and anode surface temperature, as well as changes in the current path during anode plume mode, have to be considered as factors for optimization of electrode design and choice of materials for switching applications.