Laboratory experiments on arc deflection and instability

This article describes experiments on arc deflection instability carried out during the past few years at the Princeton University Plasma Physics Laboratory (PPPL). Our approach has been that of plasma physicists interested in arcs, but we believe these results may be useful to engineers who are responsible for controlling arc behavior in large electric steel furnaces. Arcs are a type of plasma, - that is, a gas that is hot enough for some of the electrons to break free from the atoms (ionize) and conduct electricity. Arc temperatures are typically 10,000°C, which makes their electrical resistivity about 1,000 times higher than steel. This high resistance is desirable for the EAF application, in which the arc acts like a heating element in the EAF circuit. However, arcs (and most plasmas) are sensitive to deflection by magnetic fields and are generally unstable. The behavior of arcs has been studied for many years, 1-3 but the physical causes of arc deflection and instability in industrial scale electric arc furnaces (EAFs) are not yet well-understood. This is partly due to the difficulties of making arc measurements inside an operating furnace and the intrinsic complexity of arc behavior itself. PPPL does plasma physics research with the long-term goal of practical fusion energy production. Fusion furnaces, such as the Tokamak Fusion Test Reactor (TFTR) at PPPL 4 have plasma currents of 3 MA, diameters of 25 feet and heating powers of 40 MW, which produce a plasma temperature of up to 400,000,000°C. The horizontal and vertical location (i.e., deflection) of these plasmas is routinely controlled to well within 1 cm, and the large-scale plasma instabilities are successfully controlled during normal operation. Thus, we had reasons to believe that arc deflection and instability can be controlled, though not in exactly the same way as in fusion plasmas. The work described in the following sections was part of a Ph.D. thesis project to understand the physics of arc deflection and instability in laboratory-scale furnaces. 5 We have not had an opportunity to do systematic measurements or experiments on an industrial scale furnace.