Arc channels tend to shrink mainly due to the fact that the plasma conductivity naturally increases with its temperature. In this letter, we report a method of generating a large area homogeneous arc plasma at atmospheric pressure. The plasma generator consists of an 80mm diameter graphite anode chamber and an 18mm diameter concentric graphite cathode. A solenoid coil is used to produce a magnetic field along the electrode axis. In the chamber, the arc plasma is observed in various configurations and temporally and spatially evolves from a contractive column to diffuse plasma cloud which fills the entire chamber cross section.

A theoretical model is presented to describe the electromagnetic, heat transfer and fluid flow phenomena within a magnetron plasma torch and in the resultant plume, by using a commercial computational fluid dynamics (CFD) code FLUENT. Specific calculations are presented for a pure argon system (i.e., an argon plasma discharging into an argon environment), operated in a turbulent mode. An important finding of this work is that the external axial magnetic field (AMF) may have a significant effect on the behavior of arc plasma and thus affects the resulting plume. The AMF impels the plasma to retract axially and expand radially. As a result, the plasma intensity distribution on the cross section of torch seems to be more uniform. Numerical results also show that with AMF, the highest plasma temperature decreases and the anode arc root moves upstream significantly, while the current density distribution at the anode is more concentrated with a higher peak value. In addition, the use of AMF then induces a strong backflow at the torch spout and its magnitude increases with the AMF strength but decreases with the inlet gas velocity.

A commercial CFD (computational fluid dynamics) code FLUENT was used and modified to model an atmospheric pressure argon arc in a low cross flow by solving the fully coupled conservation equations. Numerical experiments, with an arc current of 100 A to 200 A, an arcing distance of 3 mm to 6 mm, and a cross-flow velocity of 10 m/s to 30 m/s, were carried out. The modelling results show that the arc tends to take the shortest path to the anode when deflected by the cross flow; its anode attachment is farther downstream than the cathode one. Furthermore, due to the low input gas flow imposed in this study, the effect of electromagnetic force is important and it influences the crosscut shape of the arc significantly.

A mathematical model is presented to describe the heat transfer and fluid flow in a magnetron plasma torch, by means of a commercial computational fluid dynamics (CFD) code fluent. Specific calculations are presented for a gas-mixing system (i.e., an argon plasma discharging into an air environment), operating in a laminar mode. Numerical results show that an external axial magnetic field (AMF) may have a significant effect on the behavior of an arc plasma, i.e., the AMF will impel the plasma to retract axially and expand radially. In addition, the use of an AMF induces a strong air indraft at the torch spout, and the air mixing with the argon gas results in a marked increase in arc voltage. An increment in the amount of the oncoming argon gas restrains the quantity of the air indraft, and this should be responsible for a lower arc voltage in such an AMF torch when a larger gas inflow is used.

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