Reverse battery model for anodic arc discharges near atmospheric pressure

Abstract

In this study, the macroscopic electrical parameters of anodic arc discharges performed with solid carbon and molybdenum anodes are described in terms of an equivalent circuit. The proposed 1D model shows that the plasma volume behaves as a dissipative medium combined with self-generated voltage from the plasma sheaths. This model is tested by fitting virtual circuits of resistors and batteries to the measured voltage-current (V-I) characteristics representative of atmospheric arc processes (300 and 500 Torr He), resulting in the identification of two phenomena relevant for arc technology. First, steady DC arc experiments show that a minimal arc voltage (V 0 ≈ 20-30 V) or threshold voltage is necessary to keep the plasma arc on. Second, we prove that pulsed arc discharges excited with rectangular current waveforms (1-5 Hz) are never extinguished but remain ignited during the inactive time of the pulse as long as the aforementioned threshold voltage is maintained. In summary, an atmospheric anodic arc discharge with solid electrodes consumes electrical power as a voltage source opposed to the external voltage supply together with a resistor that accounts for the plasma impedance. This approach permits us to estimate basic plasma parameters from simple electrical diagnostics, as well as to predict how power consumption is distributed within an anodic arc and to understand dynamics of arc plasmas excited by periodic signals.

Few-layer flakes of Molybdenum Disulphide produced by anodic arc discharge in pulsed mode

“…2c)shows the cyclic behaviour typical of pulsed arc discharges generated with the DC power supply mentioned above. Also, the fact that V-I curves float above the threshold voltage is a signature of arc plasma ignition, as investigated in a previous study [22]. Measured values of peak current (≈ 250-300 A) and of peak voltage (≈ 35 V) are contrasted with the lower current (225 A) and higher voltage (50 V) recorded in carbon pulsed discharge.…”

“…In our study, the probe is at a fixed location and pulse duration is 50 ms as defined by the duty cycle (10% of 500 ms period). Since arc current saturation is not established for such a short pulse time, [22,24] the effective exposure time of the probe to the pulsed discharge is less than 50 ms in each period. This fact, in combination with the safe distance to the arc core, leads to conclude that the W probe was not significantly damaged during the experiments.…”

“…These values suggest a plasma conductivity of a factor ≈ 2 compared to carbon arc experiments, as inferred from the respective ratios of current and voltage. With this comparison, one can estimate plasma parameters since electron density is proportional to plasma electrical conductivity [22,29]. From the signal values measured here, a lower estimate of plasma density should be roughly twice the plasma density of carbon discharge (≈ 3×10 22 m -3 ), resulting in around 5×10 22 m -3 .…”

“…The arc discharge processes were characterized by studying the V-I characteristics of the discharge. [22,28] The electrical circuit is schematized in Fig. 1a.…”

“…[21] This issue has been confirmed in a recent study reporting plasma parameters during Mo anode erosion by atmospheric arc discharge. [22] On the other hand, Sulphur-containing reactive gases like H 2 S are routinely used to deposit MoS 2 in reactive processes despite their toxicity. [20,23].…”