Energy Fluctuations in a Direct Current Plasma Torch with Inter-Electrode Inserts Operated at Reduced Pressure

Abstract

Direct current (dc) plasma torch with inter-electrode inserts has the merits of fixed arc length, relative high enthalpy and may show advantages in future plasma processes where stability and controllability are must-have. Energy fluctuations in the plasma may result from power supply ripple, arc length variation, and/or acoustic oscillation. Using an improved power supply with a flat waveform, the characteristics of an argon plasma energy instabilities under reduced pressure were studied by means of simultaneously monitoring the arc voltage and arc current spectrum. Dependence of the arc fluctuation behavior on the plasma generating parameters, such as the current intensity, the plasma gas flow rates and the vacuum chamber pressure were investigated and discussed. Results show that the plasma torch has a typical U-shaped voltage-ampere characteristic (VAC). The correlation between the VAC and the probability of energy distributions was studied. Through pressure measurements at the cathode cavity and the vacuum chamber, the existence of sonic flow in the inter-electrode insert channel was confirmed.

Fluctuation of arc plasma in arc plasma torch with multiple cathodes*

“…However, there is a numerical difference, about 100 Hz between the theoretical and measured values, which may be due to other factors in the multi-cathode arc device, such as energy fluctuations or thermal chocking in the anode nozzle. [26] These factors are coupled with the Helmholtz oscillation, which collectively cause the expanding and contracting of the plasma column. In our further work, these factors will be considered to study the fluctuation in the arc torch with multiple cathodes in more detail.…”

The distribution pattern of the electron density and the electron temperature in a weakly compressible argon plasma jet measured by a Langmuir triple probe

“…The main features of the torch are the relative steadiness due to the fixed length of the arc column and the relatively large and uniform plasma area of the plasma jet. Further information about the structures and characteristics of the torch is provided in [36].…”

“…Under this flow rate, the I -V curve of the torch is shown in figure 2 with an arc current varying from 70 A to 120 A, corresponding to the input power ranging from 5 kW to 8 kW. Fluctuations of the torch were highly suppressed [36], and the arc voltage variations, which were due to the Helmholtz oscillation, were approximately 4% of the mean voltage [37] in the range of the working conditions. Since the Helmholtz oscillation is high-frequency and has a small amplitude, the plasma jet is quasi-steady.…”

Fluctuation-Coupling of Cathode Cavity Pressure and Arc Voltage in a dc Plasma Torch with a Long Inter-Electrode Channel at Reduced Pressure

“…Moreover, the longer arc is also beneficial for obtaining relatively high enthalpy which allows effective convection of heat and momentum to injected particles. [2] Experimental works [3,4] have shown that in such a kind of torch, the velocity of plasma flow could reach the speed of sound at the inter-electrode channel exit and thermal choking occurs in the inter-electrode channel. The high flow velocity benefits the spraying velocity and the thermal choking makes the cathode cavity pressure abstain from the influence of vacuum chamber pressure.…”

“…The high flow velocity benefits the spraying velocity and the thermal choking makes the cathode cavity pressure abstain from the influence of vacuum chamber pressure. [3,4] However, even with such a kind of torch configuration, there are still small amplitude and high frequency oscillations in arc voltage, arc current and consequently leads to plasma energy fluctuations. Previous researches show that specific high frequency arc voltage fluctuations are closely related to the Helmholtz oscillation phenomenon.…”