The instability of electric arcs burning axially in accelerated flow

Experiments with electric arcs in an expansion tube fitted with a plane-symmetric converging nozzle section have demonstrated that the arc in accelerating flow is inherently unstable. An axisymmetric disturbance is generated which tends to develop into a helical mode in downstream regions. The evidence suggests that the source of instability is the shear layer between the cold flow and the hot, more highly accelerated arc column. For flows at low Reynolds number (<100) the arc column may be regarded as stable and its measured properties then lie close to the theoretical steady-state characteristic.