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

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 ls. 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 electronictranslational energy relaxation mechanism produces a further intensification of the gas heating. The sharp increase of the gas heating rate during the last few ls 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 (*10 7 A/m 2 ). 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.

Power Transfer to Gas Heating in Pure N2 and in N2–O2 Plasmas

“…Moreover, another study of gas heating mechanisms dealing with an arc plasma torch at atmospheric pressure with nitrogen, shows that the gas heating processes (R1) and (R2) are also the most important ones, followed by the pooling processes (R3) and (R4) at times longer than 0.1 ms. That study shows a more important contribution to gas heating for earlier times than in our calculations, which is due to the fact that V–T relaxation time scales inversely proportional with gas pressure . Hence, for low degrees of ionization, of the order of 10 –6 , gas heating in nitrogen discharges is determined by V–V and V–T energy exchanges between vibrationally excited molecules and molecules and atoms.…”

“…Furthermore, recent simulations of the gas temperature in N 2 plasmas considered atmospheric pressure conditions involving the analysis of its radial profile 8 and its evolution in order to study the fast anode arc reattachment. 9 In addition, selfconsistent calculations of microwave discharges in N 2 at low pressures 10 have correctly simulated the value of the gas temperature, together with a discussion of the main gas heating mechanisms, but considering only a steady-state situation. Time-dependent studies of gas heating in pure N 2 discharges at intermediate values of pressure, 100 and 150 Torr, have also been carried out.…”

“…With respect to the first group, it is worth noting that the pooling reactions (R3) and (R4) are usually important for fast gas heating in pure N 2 , occurring earlier than vibrational excitation (R1) and (R2) . As in other papers ,, we have considered an available energy of 4 eV in process (R3) corresponding to N 2 (A) + N 2 (A) → N 2 (B) + N 2 (X, v = 2) instead of ... →N 2 (B) + N 2 (X, v = 8) with an energy release of 2 eV. , …”

“…That work showed an important contribution to gas heating from the pooling reactions N 2 (A) + N 2 (A) → N 2 (B) + N 2 and N 2 (A) + N 2 (A) → N 2 (C) + N 2 , followed, for longer times, by V–V collisions between vibrationally excited N 2 molecules. Furthermore, recent simulations of the gas temperature in N 2 plasmas considered atmospheric pressure conditions involving the analysis of its radial profile and its evolution in order to study the fast anode arc reattachment . In addition, self-consistent calculations of microwave discharges in N 2 at low pressures have correctly simulated the value of the gas temperature, together with a discussion of the main gas heating mechanisms, but considering only a steady-state situation.…”

Glow Discharge in a High-Velocity Air Flow: The Role of the Associative Ionization Reactions Involving Excited Atoms

“…The balance equations for the plasma particles under the local approximation, ∂∂t([Yi])=true∑jSij−[Yi]−[Yi0]τ where [ Y i ] is the number density and S i j is the rate of production of the Y i species in the j th reaction (negative if the species is destroyed), are solved for the species N 2 (A 3 ∑ u + ), N 2 (a ’ 1 ∑ u − ), NO, N( 4 S), N( 2 D), N( 2 P), O( 3 P), O( 1 D), O( 1 S), N 2 + , O 2 + , O + , O – , O 2 – , O 3 – , and electrons. The rate to produce the N 2 (A 3 ∑ u + ) state by cascading is assumed to be equal to the sum of the rates for the production by electron impact of the N 2 (B 3 Π g ) and N 2 (C 3 Π u ) states [71,72]. Estimates show [71] that this simplification causes a negligible error in the rate of production of the N 2 (A 3 ∑ u + ) state.…”