**3. Results**

### *3.1. Video Observation and Flow Reversal*

On the left side of Figure 3, a typical current waveform (red) is shown for the discharges in the model circuit breaker with a 5.3 kA peak value (setup (b)). The current does not have the exact shape of a perfect sine and has a slightly longer duration than expected for 50 Hz ( ∼10.6 ms). In this article the main focus is set on the time around CZ while the starting phase is of minor interest. Consequently, all points in time are given with respect to the current zero crossing for easier comparison. The voltage curve (blue) is characterized by a long period of values around 200 V flanked by two distinct peaks at the explosion of the ignition wire and at the arc extinction near CZ. During the high-current phase of arc discharge, the voltage increases from the local minimum of about 140 V at the end of the ignition peak (10 ms) to a maximum of about 240 V during the peak current (5 ms), and slightly decreases again with lower currents until about 200 V (3 ms before CZ).

**Figure 3. Left**: Current and voltage waveform—nearly a sine 50 Hz shape and duration ( ∼10.6 ms). **Middle**: Pressure simulation for two positions and measurement for heating volume. **Right**: Photo (grey) of window with ignition wire and example for observation of the general behavior of the discharge about 1 ms before current zero (CZ) (false color representation) showing an asymmetric position of a constricted arc.

The intense radiation emitted from the arc plasma is absorbed by the surface of a surrounding PTFE nozzle where it causes photo–ablation of the wall material [13]. Part of the ablated material is exhausted as vapor by the axial flow, the other part enters the arc and is heated to a plasma temperature by absorption of radiation coming from the arc interior, leading to a discharge that is dominated by the ablation [18]. Both the arc plasma and vapor create an overpressure which causes an axial expansion flow towards the ends of the nozzle. In the middle of Figure 3, three curves of the pressure are shown. The red and black curve were obtained for the slit position inside the PTFE nozzle and the heating volume, respectively, from CFD simulation (CFD-ACE+ software suite by the ESIGroup) of the discharge according to [17]. A comparison of simulation results for the heating volume (black) with our measurements (dashed blue) resulted in a good agreemen<sup>t</sup> concerning the shape of the curve. Small deviations of the absolute values were used to adjust the pressure in the model. Although no pressure sensors could be placed directly in the nozzle or heating channel in case of the MCB, values of the pressure at the OES position (slit) could be easily obtained from a comparison with the results from the CFD simulation (red curve). The maximum pressure of 3.5 times the filling pressure is built up close to the peak current. It should be noted that at CZ the pressure is still about two times the filling pressure, which results in a cooling of the arc.

On the right part of Figure 3, a grey scale image is shown to illustrate the area of HSC observation at the position of the nozzle slit. Note that illumination from the backside was applied and the ignition wire can be recognized in the lower part. Finally, an example of a HSC frame of the arc in false colors is given on the far right about 1 ms before CZ. Typically, the arc is constricted at this time to less than half of the nozzle diameter. Here, it is located in the upper part of the nozzle, showing a deviation from rotational symmetry.

The arc in the slit area was further investigated using the double frame optics (DFO). Filters were applied to reveal information on the plasma composition. Any emission from fluorine and oxygen can be clearly dedicated to the nozzle wall (PTFE, C2F4) and filling gas (CO2), respectively. Therefore, the left channel was equipped with a metal interference filter (MIF) with maximum transmission at 675 nm to become sensitive for several atomic fluorine lines F I and the right channel with a narrow filter at 777 nm for the atomic oxygen triplet O I. The emission from carbon lines could not be used since both materials contained carbon. On the upper part of Figure 4, six exemplary double frames are shown for significant points in time. Please note that the full video is available as supplementary material to this article. In the lower part of Figure 4 the plasma composition as obtained from the CFD simulation is shown as fractions of PTFE and CO2.

**Figure 4.** Visualization of the flow reversal before CZ for setup (b). **Top**: Selected frames of high-speed cameras (HSC) imaging using double frame optics; filters were applied for the atomic line emissions of F I 675 nm (left channel) and of O I 777 nm (right channel) to visualize emission from PTFE and CO2, respectively. **Bottom**: Plasma composition as obtained from the CFD simulation.

The first frame from the DFO mainly shows the atomic copper emission from ignition wire that is present on the left side (Cu I line at 674.1 nm). The wire was divided by the explosion into three pieces within the 2 mm slit area. Less than a millisecond later, the emission dominated in the right channel, indicating a discharge in the CO2 atmosphere (9.9 ms to CZ). Within few hundreds of microseconds, the brightness of the right channel faded while the left part became more intense. Here, the increasing wall ablation started to dominate the discharge, blowing the fill gas out of the nozzle. This is in good agreemen<sup>t</sup> with the simulation results. In the following, a long and stable period was observed that was dominated by ablation (cf. frame at 6.0 ms before CZ). Another reversal of flow was found about 2 ms before CZ: When the arc current and thus wall ablation were considerably decreased, the pressure in the nozzle also decreased to values below that of the heating volume, see Figure 3. As a consequence (relatively cold) gas from the heating volume with a high fraction of CO2 flowed back into the nozzle. There it was heated up by the arc current and its radiation could be seen in the right channel. Interestingly, different intensity distributions were found for the left and right channel at 2.0 ms before CZ. That indicated non-homogeneous gas mixture. Finally, only emission from O I could be seen in the last ms, indicating a plasma composition completely dominated by CO2.

### *3.2. OES of High-Current Phase Using High-Speed Camera*

Series of two-dimensional spectra were obtained by means of video spectroscopy, allowing to investigate the different phases of the discharge within one acquisition (setup (b) with same conditions as above). This is illustrated in Figure 5 where six selected frames of one shot are shown exemplarily. The vertical dimension of each frame is related to the spatial distribution of the arc cross section along the nozzle slit whereas the spectral distribution is represented by the horizontal dimension. Please note that the full video is available as supplementary material to this article. The major lines and the time in relation to current zero are labeled above and on the left, respectively. For line classification, energy levels, transition probabilities, and other information on spectral lines the databases from NIST [21] and Kurucz [22] were applied. The spectra are dominated by atomic and ionic line emission from Cu, C, O, and F; the highest intensities are found in the central part of the axis. Temperature profiles can be determined along most of the arc radius based on these lines. In cases of significant nozzle ablation, usually F I or C II lines were utilized [11,12,17].

**Figure 5.** 2D-optical emission spectroscopy (OES) frames acquired at selected time points during one discharge by video spectroscopy with setup (b). From top to bottom: Ignition phase dominated by atomic Cu I lines from wire, transition to ablation-dominated arc with ionic C II and atomic F I lines from PTFE and close to CZ only C II from CO2.

The first spectrum with detectable light emission acquired during the initial phase (10.7 ms before CZ) is characterized by the exploding ignition wire made of copper. Thus, only atomic Cu I lines are visible (labeled by green arrows). Obviously, the wire position was slightly off-axis for this shot. Although an influence of the ignition wire can be proved to last up to 1 ms, the bright peak is much shorter, typically below 100 μs [16]. Next, the filling gas CO2 causes an additional emission of carbon ionic C II lines (red) and of the oxygen lines (O I at 777 nm, not shown here). This emission was found to be visible in OES after 100–220 μs. Under extreme conditions, i.e., for the setup (a) with the tubular PTFE nozzle exposed to high peak currents of 11 kA at 100 Hz, the lines of Cu I, C II, and O I can have intensity peaks already at 300–400 μs after ignition, while the F I emission (from PTFE) starts at about 300–400 μs after ignition [16].

Further emission of Cu I lines can be observed due to erosion of the W–Cu electrode, cf. the second spectrum at 9.6 ms before CZ. Here, the Cu I emission is still intense and distributed over the whole nozzle diameter. Additionally, rather weak carbon ionic as well as much brighter atomic fluorine lines (F I at 624 nm only in this spectral range) are visible. Although the influence of CO2 is strongly decreased at 9.6 ms to CZ as it is known from the DFO investigation and plasma composition calculation, a ratio in particle density of about 1:2 of carbon to fluorine can be expected (C2F4 are the building blocks of PTFE). The transition probabilities of the line emission are roughly comparable, e.g., summarizing to about 10 ×10<sup>7</sup> s<sup>−</sup><sup>1</sup> for three C II lines at 609/610 nm and about 3 ×10<sup>7</sup> s<sup>−</sup><sup>1</sup> for the F I line at 624.0 nm. Considering the much stronger F I emission this means that excitation of F I with upper level at ∼14 eV is much more pronounced than an ionization of carbon atoms and excitation of C II with ∼24 eV.

The third spectrum at 8.6 ms before CZ is acquired in the phase of increasing PTFE nozzle ablation that causes a stronger emission of both atomic fluorine and carbon ionic lines. Furthermore, the C II and F I lines have comparable intensities, indicating more ionization and excitation of the C II levels than before and thus, considerable higher temperatures of the arc. The vanishing Cu I indicates that the metal vapor from electrode erosion is blown "backwards" by the new established flow of hot, ablated nozzle material.

Approaching the peak current (7.3 ms before CZ), continuum radiation can also be observed. On the one hand, there is a very broad continuum over the whole spectral range that is mainly emitting in the center of the arc. On the other hand, there is a pattern of many lines (often not resolved with this spectral resolution) of increasing intensities with increasing wavelengths until rather sharp edges, e.g., at 516 and 563 nm. This structure could be attributed to an emission of diatomic carbon molecules, i.e., the Swan band system arising from transitions between the d<sup>3</sup> <sup>Π</sup>*g* and the a3 Π*u* electronic states of C2 molecules. These Swan bands will be discussed later in detail. It should be noted that its spatial distribution ranges over the whole nozzle diameter, partially with peaks near to the nozzle walls.

With decreasing currents, the intensity of ionic C II lines decreases again, the atomic F I line at 624 nm has a much higher intensity at 2.8 ms before CZ whereas the C II lines are much weaker. Similar to the third spectrum at 8.6 ms to CZ, the excitation of ionic states is much less pronounced than that of fluorine atoms with upper levels around 14 eV due to a significantly lowered plasma temperature.

In the following phase of low current, the radiation emitted by the arc plasma no longer causes sufficient wall ablation to sustain the high pressure in the nozzle (cf. spectrum at 0.7 ms before CZ). The consequences are a reversal of the gas flow direction, disappearance of the fluorine line, and a spectrum dominated by C II line emission originating from the back-flowing CO2 from the heating channel (plus continuum). Furthermore it should be noted that the arc is constricted, i.e., only a rather small part of the radial profile carries a contribution to emission and thus, also to electrical conductance.

### *3.3. Optical Emission Spectroscopy near to Current Zero*

In the following the phase around current zero will be investigated for both nozzle geometries. Besides, the potential and limits of optical emission spectroscopy shall be discussed. The long, tubular nozzle of setup (a) is used to produce intense and short discharges (11 kA at 100 Hz) that will be

analyzed by OES with a high-speed camera (HSC). Investigations with a more realistic setup (b) in the model circuit chamber (5 kA at 50 Hz) will be carried out using OES with an intensified CCD (ICCD) camera to come as close as possible to CZ.

### 3.3.1. Oes with High–Speed Camera

At first, the case of a current zero phase after an intense arc is considered. A strong PTFE wall ablation and pressure built-up is realized with a higher current of 11 kA peak. Using setup (a), the material flow is directed towards the exhausts at both electrode sides since the tubular nozzle is not interrupted, e.g., by a heating channel. Thus, the plasma is expected to remain relatively hot and dense for a longer time near CZ, at least in the observation position in the middle of the tube. It should be noted that the setup does not perform switching, i.e., the current is continued after CZ with reversed polarity. Additionally, there is only a short period of low current, i.e., it takes only ∼2.5 ms from peak to CZ for 100 Hz. In other words, the best conditions have been chosen for the analysis of the fluorine lines close to CZ.

Concerning imaging spectroscopy with high-speed camera, both temporal and spatial resolution are of interest. However, a limit was given by the detector's readout rate. Hence, the highest temporal resolution was obtained by a drastic reduction of the number of vertical lines, practically giving up the spatial dimension (side-on information). In the following, only the central position is used to analyze the temporal evolution of the spectrum with a frame rate of 100 kfps (10 μs repetition rate). In the upper part of Figure 6, the spectral evolution around CZ is shown. It should be mentioned that neither the full spectral range nor all spectra acquired during the discharge are plotted in order to focus on distinguishable lines of the relevant species during the phase around current zero. Some major lines of atomic fluorine as well as oxygen and nitrogen are labeled. Although there were no lines from carbon observed in the diagrammed spectral range (738–783 nm), other lines were checked and confirmed the finding described below, e.g., the ionic carbon lines C II 658 nm and 723 nm.

Among the 30 successive spectra shown in Figure 6, 17 were taken before and 13 were taken after CZ (see timescale at Y-axis and arrow). As expected for an ablation-dominated arc, only the F I lines were found, cf. spectra from 170 μs to about 100 μs. The line intensities rapidly faded out due to the low decreasing energy input by the arc. Then a "dark" phase followed without detectable emission starting from about 100 μs before and lasting until about 100 μs after current zero. During this dark phase, other spectral techniques would be necessary for investigation of the residual plasma, e.g., intensified cameras for emission spectroscopy or absorption measurements. The first spectra with sufficient intensity after CZ shows a different behavior: Additional lines could be observed, namely the oxygen triplet O I at 777 nm and three N I nitrogen lines around 744 nm. They are marked by red to differ them from the fluorine lines and to prove that they were not detectable before CZ. This demonstrates the back-flow of ambient air containing oxygen and nitrogen into the nozzle after CZ. The temporal evolution of three exemplary lines is shown in the left lower part of Figure 6. It should be mentioned that for this plot first the line integral of each of the three lines was calculated, second the intensity at line edges (lower/higher wavelength) was taken as "line background signal" and subtracted, and finally the intensities of these line integrals were normalized for a better comparison. The two fluorine lines show a very similar behavior, decreasing before and increasing intensity after CZ with comparable falling/rise rates. In contrast, the oxygen lines (triplet 777 nm) have an intensity of background level during all times before and until 100 μs after CZ. Then they rise up very fast, i.e., from zero to a maximum level within about 100 μs. A slower decrease follows however, they can be detected until 500–600 μs. Thus, for both the ignition phase as well as the current zero crossing, it can be stated for the high current and setup (a) that the arc plasma needs about 0.5–1.0 ms to be completely dominated by the ablated wall material and the influence of the surrounding gas can be neglected—at least under ambient conditions of one bar air.

**Figure 6. Top**: Temporal evolution of spectra around current zero in the arc center for setup (b). Note the direction of the timescale from bottom to top. **Bottom left**: Temporal evolution of three selected line intensities. **Bottom right**: Radial temperature profile acquired 400 μs before CZ.

The next step is the determination of radial temperature profiles as close as possible to current zero with HSC. Hence, sufficient spatial resolution is required. Therefore, the number of lines used for the two-dimensional spectra was increased to 600. As a consequence, the repetition rate and thus, the temporal resolution had to be reduced to 133 μs and 7500 fps, respectively. Hence, the dark phase is reduced to one spectrum only. Furthermore, there are two spectra directly before CZ with an emission sufficient for F I line detection but not for plasma temperature determination due to restrictions in the signal-to-noise ratio. The third spectrum, i.e., 400 μs before CZ, could be applied. An absolute intensity calibration of the 2D spectrum was done by means of a tungsten ribbon lamp. Then Abel inversion was carried out to obtain radially resolved emission coefficients under the assumptions of an optically thin plasma and rotational symmetry of the arc. Finally, radial temperature profiles were determined assuming a plasma composition of 100% C2F4 at atmospheric pressure (no contribution from electrodes or air) and local thermodynamic equilibrium. Several fluorine lines were applied for comparison, yielding similar results. The temperature profile shown in the right part of Figure 6 has a maximum of 9400 K in the arc center and is rather broad: Even at a radial position of about 4 mm, i.e., 2 mm away from the nozzle wall, the temperature is still around 9000 K. In close vicinity to the wall, it decreases rapidly to values around 8000 K.

### 3.3.2. OES with Intensified Camera

The case of a current zero phase shall be investigated under the more realistic setup (b) in the model circuit chamber filled with 1 bar CO2 (5 kA at 50 Hz). The high–speed camera is replaced by an ICCD camera (Additionally, also a video file was added as supplementary material to this article (wavelength range 774.5–781.5 nm). It shows the change from oxygen to fluorine line emission and back, but lacks of temporal resolution and intensity close to CZ). Thus, an improvement in sensitivity can be demonstrated as a side effect. An example of a two-dimensional spectrum is shown in the upper part of Figure 7. It was acquired 300 μs before CZ with an exposure time of 200 ns. The vertical axis comprises the full nozzle cross section with a diameter of 12 mm.

**Figure 7.** Two-dimensional spectrum measured 300 μs before CZ by an intensified CCD (ICCD) camera to gain higher sensitivity in comparison to HSC (**top**). Vertical axis comprises of a nozzle diameter of 12 mm. After shifting the axis to the central position of the arc and symmetrization, side-on radiance was obtained by integration over the O I 777 nm triplet (**bottom left**). The deduced emission coefficient (**bottom right**) shows a distinct maximum at 1 mm caused by the higher ionization degree in the arc center.

Obviously, the arc was contracted to a few mm due to the low current (<500 A) and was slightly shifted upwards, i.e., not in the center of the nozzle. For further analysis, the symmetry axis had to be shifted by about 1.5 mm from the nozzle center to the central position of the arc. Thus, symmetrization of the two sides ("upper" and "lower") can be carried out, providing a side-on spectrum with spatial dependence from central to outer side-on positions. The spectral radiance for the position of the arc center is given as a 1D-spectrum in the middle of Figure 7. Due to the CO2 gas filling, atomic oxygen (O I 715 nm, 777 nm), and ionic carbon (C II 658 nm, 723 nm), lines are found but none from fluorine, e.g., F I at 775.5 nm and 780.0 nm. In general, F I lines could definitely be detected at 2 ms but never at 1 ms or less before CZ under these conditions. This will be illustrated later, cf. the collection of spectra at different times before CZ in Figure 9. This is a consequence of the different setup and lower arc current compared with the case described above (lower peak current of 5 kA instead of 11 kA, and longer duration of the half-wave of 10.7 ms instead of 5 ms), yielding less vapor from PTFE ablation. Additionally, atomic copper line emission Cu I 793.3 nm was observed.

Both the ionic carbon and atomic oxygen lines were used for the analysis of the radial temperature distribution. In the following, the procedure will be described for the oxygen triplet. At the first step, an absolute intensity calibration was carried out. Secondly, the total radiation flux of the O I 777 nm triplet was obtained by spectral integration over the line ( ∼775–780 nm with correction using left and right background) for each side-on position and the resulting line radiance in units of W m<sup>−</sup><sup>2</sup> sr<sup>−</sup><sup>1</sup> is plotted in the left bottom of Figure 7). This side-on profile was found to be rather flat in the first millimeter beginning from the central position and to decrease practically to zero within another millimeter. Although the arc center was not in the nozzle center, it could be assumed that the main path of current flow has sufficient rotational symmetry for the application of an inverse Abel transformation. Hence, in the third step the radial-dependent emission coefficient *ec* of the O I triplet could be determined as shown in the right bottom of Figure 7. A distinct maximum of *ec* was found at a radial position of *Rmax* ≈ 1 mm, towards the arc center *ec* decreased again though higher temperatures are to be expected there. Such a behavior gives hint that the "normal maximum" of the emission coefficient for a line transition is reached. This maximum results from ionization, which decreases the available number of atoms to be in an excited level with increasing temperature. For a better illustration, the temperature dependence of the emission coefficient is plotted in the left part of Figure 8. It was calculated on the base of NIST data for different pressures of pure CO2 assuming optically thin plasma. The normal maximum was found to be around 16,000 K with only weak dependence on the pressure, see the right part of Figure 8. It should be noted that the maximum emission coefficient has nearly a linear dependence on the CO2 pressure since it principally reflects the density of the radiating species.

**Figure 8. Left**: Calculated emission coefficient of the O I 777 nm triplet at different partial pressures of CO2 depending on temperature. Note that the normal maximum is strongly dependent on the pressure but its position is rather stable around 16,000 K. **Right**: Pressure dependence of emission coefficient and temperature at normal maximum.

As a fourth step to temperature profiles, plasma temperatures can be obtained from the experimentally determined emission coefficients by comparison, with the calculated values in Figure 8. Therefore, in the case of experimental *ec* from the "outer" part of the arc, i.e., values for radial

position *r* > *Rmax*, the left wing of the curve ("rising" = temperatures below normal maximum) has to be applied, while for the arc center *r* < *Rmax* the right wing is valid ("falling" = temperature above normal maximum). Knowing the pressure and plasma composition, an absolute value of the emission coefficient would not be mandatory. Hence, the absolute intensity calibration gives additional information and allows one to validate the experimental methods and assumptions. In general, the highest experimentally determined emission coefficients fitted best to the calculated normal maxima for a plasma composition of pure CO2 with 1.2 bar total pressure. The ionic carbon line C II at 658 nm shows the normal maximum at around 22,000 K. The experimentally obtained *ec* did show no indication for temperature above the normal maximum, which proves that double ionization was rather improbable in accordance with earlier studies. Hence, for C II only the left wing with "rising" *ec* were applied. However, the experimentally obtained emission coefficients are higher than the normal maximum calculated for a pressure of 1.2 bar in pure CO2 (the assumption used for the evaluation of the O I triplet). This is an indication that an additional amount of carbon is in the gas mixture, eventually originating from the delayed evaporation of carbon soot particles produced during nozzle ablation. Because the gas composition and pressure could not be determined accurately, pure CO2 but with an enhanced pressure of 2 bar (close to the results of CFD simulations) was assumed for the evaluation of the *ec* of the C II line.

Spectral radiance for different instants of time before current zero are shown in Figure 9. For a better visibility, only parts of the spectral range, comprising the most relevant lines and six selected spectra are plotted. A logarithmic intensity scale is used to show the enormous decrease of line intensities, e.g., by about 2 orders of magnitude for O I and Cu I. Note that the exposure time had to be increased from 0.2–0.5 μs to 2.0 μs for all spectra closer to CZ than 150 μs due to the decrease of intensity. Fluorine atomic lines were found only in the gray spectrum acquired 2 ms before CZ, proving that under the conditions of this setup and current waveform the influence of ablated wall material was negligible ≤ 1 ms before CZ. Carbon ionic lines could be detected until 80 μs before CZ whereas atomic oxygen and copper lines were visible in all spectra.

**Figure 9.** Selection of spectra acquired at different instants of time before the current zero.

The radial temperature profiles for five shots obtained from the emission coefficient of the C II at 658 nm considering 100% CO2 gas at 2 bar are plotted in the upper part of Figure 10. Only times of 1 ms or less before CZ were taken into the analysis, i.e., the spectrum at 2 ms was excluded due to the F I lines indicating still PTFE admixtures. The spectrum acquired 1 ms before current zero resulted in a broad and flat temperature profile. The temperature between the arc center and 1.5 mm was more or less constant around 18,000 K. Then it was continued with a slow decrease to about 12,000 K at a

radial position of nearly 4 mm, e.g., 2 mm away from the nozzle wall. This is typical behavior of a wall-stabilized arc at moderate current. For the following spectra, i.e., taken after 400 and 300 μs before CZ, two effects could be observed: (i) A considerable decrease of the arc diameter by nearly a factor of two that was accompanied by (ii) an increase of the core temperature to values above 20,000 K. Hence, a transition from the broad profile due to wall-stabilization to smaller, constricted profiles occurred. Since the arc current did not decrease as quickly as the square of the arc diameter, the core temperature had to be increased to provide a sufficient current density. In the following period, i.e., for times 130–150 μs before CZ, a further decrease of the arc diameter was found however, the maximum plasma temperature in the arc core could also be decreased due to further current decrease. Spectra acquired at less than 100 μs before CZ had too low intensities of the C II ionic line to allow for a determination of reliable temperature profiles.

**Figure 10.** Radial profiles of plasma temperature obtained from ionic carbon line C II at 658 nm (**top**) and from atomic oxygen triplet at 777 nm (**bottom**) with (130–1000 μs) and without (<100 μs before CZ, dashed curves) application of normal maximum.

In the lower part of Figure 10, radial temperature profiles for numerous shots obtained from the O I triplet at 777 nm are plotted . Here, an emission coefficient for 100% CO2 with a 1.2 bar total pressure has been considered for the temperature determination. In agreemen<sup>t</sup> with the results for the C II line, the broadest profile with the highest temperatures was obtained for 1 ms. Here, the position of the normal maximum connected with a fairly established temperature of 16,000 K was obtained at about 2.5 mm. Although the maximum temperature exceeded 20,000 K only values till 18,000 K were

plotted due to higher uncertainties for the central position. However, the plasma temperature was still about 10,000 K even at a radial position of 4 mm, i.e., the arc was spread over most of the nozzle area (12 mm diameter). For the following profiles, the effect of temperature increase in the core was not as clear as above for C II. Though some profiles do not perfectly fit into the generally smooth decreasing behavior, the general tendency was that both arc diameter and maximum temperature in the center decreased continuously with the decreasing current. The above described method based on the normal maximum of the oxygen triplet could be applied to all spectra taken ∼100–1000 μs before CZ. For the remaining spectra (<100 μs) the experimental emission coefficients were below the normal maximum. Hence, the common single-line method was applied using only the left ("rising") wing of the curve from Figure 8. These temperature profiles are plotted as dashed curves in Figure 10. The tendency to smaller profiles with lower peak temperatures can be followed until about 10 μs before current zero, i.e., at very low currents of few tens of amperes. Although spectra (of 2 μs exposure time) were taken at varying times of 13, 10, 8.5, and 6.5 μs before CZ and the exact acquisition times were measured by a comparison of current waveform and a monitor signal from the ICCD camera, nearly the same results were obtained for all four shots: A temperature maximum slightly above 11,000 K and a decrease to 8000 K within ≤1.0 mm. Here, the experimental limit is reached with possible changes in the temperature profile superimposed by shot-to-shot variation of the discharge itself, window transmission, and uncertainties in determination of the emission coefficient. As a control, another spectrum was acquired at current zero (from 1.0 μs before CZ to 1.0 μs after CZ) however, only the noise was recorded.
