Ralf Methling

Spectroscopic Study of a Single Vacuum-Arc Cathode Spot

The cathode-spot plasma in spark and arc stages of a vacuum discharge was studied spectroscopically. A single spot was generated with high reproducibility in a gap under ultrahigh-vacuum conditions with a liquid-metal cathode of GaIn alloy. The self-breakdown of the vacuum gap resulting in a short discharge of less than 100 ns as well as a discharge over and up to 3 mus has been considered. The combination of a 0.5-m spectrograph with a streak camera enabled observation of spot evolution with a time resolution in the nanosecond range. Applying the streak camera as an image converter, time-integrated spectra resolved in the direction along the arc axis have been obtained. Limits concerning wavelength and time resolution as well as the emission intensity are discussed. Spectral lines of Ga and In atoms and single- and double-charged ions have been observed simultaneously. At the beginning of the discharge, ionic lines of higher charge state and wide line broadening dominate the spectrum. With a delay of several hundreds of nanoseconds, atomic lines appear and fall down in intensity to a much lesser degree than the ionic lines. Hence, atomic lines finally dominate in the arc stage of the discharge.

Time- and Spectrum-Resolved Study of a Single Cathode Spot in Vacuum

Optical emission of a single cathode spot is studied to improve understanding of cathode spot initiation and dynamics. A single cathode spot was generated with high reproducibility in a gap under UHV conditions at a liquid Ga-In-alloy cathode. Discharges were initiated by rectangular voltage pulses of 100-ns length produced by a cable generator. Spot radiation was recorded in time with a high temporal and spectral resolution by a spectrograph coupled with a streak camera and a CCD camera. Cathode spot light radiation was found to be weak in the discharge beginning as the equipment set records nothing. Cathode spot radiation becomes bright when cathode spot plasma approaches closely to the anode. Unlike spot initiation in a pure vacuum gap, a cathode spot starts as a bright flash when the gap is filled with plasma.

Spectroscopic Investigation of a Cu—Cr Vacuum Arc

Experiments on high-current arcs carried out in an ultrahigh vacuum chamber are presented in this paper. For contact separation the lower electrode is moved by a mechanical-pneumatic device simulating the conditions of a vacuum circuit breaker. The arc behavior of the Cu-Cr test electrodes after the electrode separation without application of external magnetic field is observed by a high-speed video camera. Besides the usual electrical measurements, the emission in the gap along the electrode axis is investigated by spatially resolved optical emission spectroscopy. Differences are found in the intensity distribution of atomic and ionic lines along the gap.

An Improved Arc Model Based on the Arc Diameter

This paper introduces an improved electrical arc model based on the arc diameter variation. In the conventional arc models, the arc diameter is considered as an implicit parameter in arc model constants. In case of decreasing arc diameter along with the arc extinction, the arc voltage decreases as well. The arc diameter can be implemented as a function of current into the arc model. The results illustrate that by applying the arc diameter as a function of arc current in the arc model, the simulated arc voltage is much closer to the measured values. In particular, the arc voltage behavior at the zero crossing can be controlled more precisely. However, based on the model considered for the arc diameter, additional parameters of the arc model should be estimated using measured data. The results confirm that even applying a simple arc diameter model can improve the arc voltage simulation.

Spectroscopy of Single Vacuum Arc Cathode Spots With Improved Sensitivity

Recently, we reported on optical emission of vacuum arc discharges with a single cathode spot. The discharges were initiated by rectangular voltage pulses of 100 ns length produced by a cable generator. In combination with the application of a liquid Ga-In alloy cathode and UHV conditions a high reproducibility could be obtained. By replacing the streak camera the sensitivity and the signal-to-noise ratio could be improved. Thus, the accumulation of calibrated spectra with high temporal, spatial, and wavelength resolution has been possible. Distinct differences on the temporal development of the atomic and ionic line intensities have been found depending on the spatial position, i.e. comparing the spot core and the spot surrounding.

Video Spectroscopy of Vacuum Arcs During Transition Between Different High-Current Anode Modes

This paper presents spectroscopic results of high-current anode phenomena in a vacuum arc discharge between CuCr electrodes. AC (alternative current) 50-Hz and 10-ms pulsed dc (direct current) are applied as interrupting current. Time and space resolved optical emission spectroscopy (video spectroscopy) is used to examine the temporal and spatial distribution of different atomic and ionic copper lines. During the transition from low-current mode to different high-current modes, including footpoint, anode spot, and intense arc mode, the intensity of Cu I, Cu II, and Cu III line radiation is examined near the anode, the cathode, and in the interelectrode gap. The results show that during the formation of anode spot and intense mode the intensity and the distribution of all lines change noticeably in the different spectral regions. In fact, higher ionization states represent the arc dynamics behavior during transition to high-current anode modes. Significant differences have been found, for example, in the spatial structure of Cu II and Cu III lines in the anode spot mode. The results for Cu I lines indicate an active role of atoms together with the ions in different charge states in high-current anode modes. The impact of threshold current and transferred charge of the formation of high-current anode modes in case of ac and pulsed dc is also investigated regarding the intensity of copper lines near the anode.

Impact of Different Vacuum Interrupter Properties on High-Current Anode Phenomena

This paper presents the impact of current waveform and frequency on the formation of high-current anode phenomena in a vacuum interrupter experiment. Different waveforms including the alternative current pulses of 50, 180, and 260 Hz and direct current pulses of 5 and 10 ms are compared. The impact of different opening times and contact speeds on the high-current anode mode formation is investigated. The results show that both instantaneous current and total transferred charge are important in the formation of high-current anode modes. Therefore, the arcing time has a strong influence. Two types of anode spot modes with different electrical and optical characteristics are also observed. The transitions between different high-current modes are examined systematically, resulting in existence areas dependent on threshold current and gap length. The latter are determined for different contact materials including Cu, CuCr7525, and CuCr50 and different contact diameters.

Dynamics of cathode spot plasma parameters in spark and arc stages of vacuum discharge

Modern notations do not foresee principle differences in cathode spot mechanisms in the breakdown stage as compared with steady burning arc. There is no reason to have doubts in this statement. Really, cathode spots generate similar craters regardless of discharge burning time under the condition of a cold cathode. Cathode spot plasma has rather stable parameters while gradual changes in plasma parameters can be convincingly explained in terms of cathode heating effects. The only difference is that the beginning stage gives a chance to trace a separate cycle of cathode spot operation and confidence that the initial moment of a cycle is a proper one. Unlike stable long-lasting arc, the first spot cycle is not hazy with prehistory; and spot behaviour looks different as a result. Having utilized a first spot cycle for diagnostics, the authors have revealed dynamics of atomic and ionic spectral lines in single spot radiation with high temporal resolution. Higher charge state ions start to emit light first followed by lower charge state ions and neutrals being latest. All the spectral lines start to be highly broadened. These data was successfully used to revise conclusions drawn earlier concerning experiments on resonant laser diagnostics of cathode spot plasma.

Anode Surface Temperature Determination in High-Current Vacuum Arcs by Different Methods

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

Spatial distribution of charged particle emission in a copper-chromium high-current vacuum arc

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