Experimental investigation of the helical magnetic instability of an arc discharge in an axial magnetic field and comparison with theory

The helical magnetic instability of arcs is investigated experimentally at a long low current H2 arc in a tube of 20 mm diam. The measurements are compared with theoretical results given in a preceding paper [Phys. Fluids 3 0, XXXX (1987)] in which a first order homogeneous differential equation for the helix amplitude was derived from the basic differential equations. Confirmation is achieved for the critical external axial magnetic field which just destabilizes the arc. It amounts to only a few gauss. Agreement is also found for the growth rate of the instability in both cases of destabilization by an external magnetic field and by the self‐magnetic field. The measured growth rates have an order of magnitude of 1000 sec− 1.