On the Gas Heating Mechanism for the Fast Anode Arc Reattachment in a Non-transferred Arc Plasma Torch Operating with Nitrogen Gas in the Restrike Mode

Significance Statement

Direct current (dc) non-transferred arc plasma torch is used in a number of applications like spray coatings, material synthesis, and waste treatment. Standard dc non–transferred plasma torch are characterized by a central rod–shape cathode and a concentric water–cooled copper anode. The arc connects the cathode tip to some point on the anode wall, crossing through the cold gas boundary layer that covers such wall. The anode attachment location is usually in motion, thus inducing arc voltage fluctuations. Three distinctive modes have been reported for the arc voltage fluctuations: the steady mode (rather detrimental for the anode life time because the anodic attached spot remains almost fixed in position), the takeover mode (obtained mainly with mono–atomic gases in which the anode attached spot presents a small amplitude oscillating motion); and the restrike mode, corresponding to diatomic gases or their mixtures, in which the spot expands along the anode moving away from the cathode (stretching the arc) until a new arc appears closer to the cathode and the original arc decays, giving place to an expanding movement of the new arc. The physical mechanism of the anode attachment restrike is well understood. The anode arc column which connects the main arc column and the anode wall is pushed forward by the gas flow thus elongating the arc length and increasing the arc voltage drop until the cold boundary layer “electrically breaks,” giving rise to a new anode attachment closer to the cathode (where the voltage difference between the arc and the anode is largest). As the new configuration of the arc has a lower voltage drop, the new attachment causes a decay of the old one, being again dragged away by the flow. However, the physical mechanism driving the fast anode arc reattachment process (time scale of the order of ms) is less clear.

This work presents a detailed kinetic analysis of the time–resolved dynamics of the gas heating during the arc reattachment in nitrogen gas in order to understand the main processes leading to such a fast reattachment. The model includes gas heating due to the relaxation of the energy stored in the vibrational mode (V–T and an–harmonic (V–V)–T relaxation mechanisms) as well as in the electronic mode of the molecules (the E–T relaxation mechanism). According to the findings, the arc reattachment is triggered by a vibrational instability lasting for a time of the order of 100 µs. For values of the reduced electric field (E/N) < 80–100 Td, most of the electron energy is transferred to gas heating through the mechanism of V–T relaxation. For larger values of E/N the (V–V)–T relaxation of nitrogen molecules becomes relevant together with the electronic E–T energy relaxation mechanism, thus producing a further intensification of the gas heating. The sharp increase of the gas heating rate during the last few µs of the vibrational instability give rises to a sudden transition from a diffuse (glow–like) discharge to a constricted arc with a high current density (~ 107 A/m2). This sudden increase in the current density gives rise to a new anode attachment closer to the cathode thus causing the decay of the old one.

Figure Legend 1: Non–transferred dc arc plasma torch.

 

Non-transferred Arc Plasma Torch Operating with Nitrogen Gas in the Restrike Mode. Advances in Engineering

Figure Legend 2: Typical arc voltage waveform corresponding to the restrike mode in a dc plasma torch.

Gas Heating Mechanism for Fast Anode Arc Reattachment in Non-transferred Arc Plasma Torch Operating with Nitrogen Gas in Restrike Mode. Advances in Engineering

Journal Reference

Plasma Chemistry and Plasma Processing, November 2015, Volume 35, Issue 6, pp 1057-1070. 

L. Prevosto 1, H. Kelly 1, 2, B. Mancinelli1, J. C. Chamorro1

[expand title=”Show Affiliations”]
  1. Grupo de Descargas Eléctricas, Departamento Ing. Electromecánica, Facultad Regional Venado Tuerto (UTN), Laprida 651, Venado Tuerto, 2600, Santa Fe, Argentina
  2. Instituto de Física del Plasma (CONICET), Facultad de Ciencias Exactas y Naturales (UBA), Ciudad Universitaria Pab. I, 1428, Buenos Aires, Argentina[/expand]

Abstract

The present work provides a detailed kinetic analysis of the time-resolved dynamics of the gas heating during the arc reattachment in nitrogen gas in order to understand the main processes leading to such a fast reattachment. The model includes gas heating due to the relaxation of the energy stored in the vibrational as well as the electronic modes of the molecules. The results show that the anode arc reattachment is essentiality a threshold process, corresponding to a reduced electric field value of E/N ~ 40 Td for the plasma discharge conditions considered in this work. The arc reattachment is triggered by a vibrational instability whose development requires a time of the order of 100 µs. For E/N < 80–100 Td, most of the electron energy is transferred to gas heating through the mechanism of vibrational–translational relaxation. For larger values of E/N the electronic–translational energy relaxation mechanism produces a further intensification of the gas heating. The sharp increase of the gas heating rate during the last few µs of the vibrational instability give rises to a sudden transition from a diffuse (glow-like) discharge to a constricted arc with a high current density (~107 A/m2). This sudden increase in the current density gives rise to a new anode attachment closer to the cathode (where the voltage drop between the original arc and the anode is the largest) thus causing the decay of the old arc spot.

Go To Plasma Chemistry and Plasma Processing

 

Check Also

Enhancing Mid-Infrared Er,Cr:YSGG Laser Pulse Performance via LiNbO₃ Acousto-Optic Q-Switching and Thermal Lensing Compensation - Advances in Engineering

Enhancing Mid-Infrared Er,Cr:YSGG Laser Pulse Performance via LiNbO₃ Acousto-Optic Q-Switching and Thermal Lensing Compensation