Instabilities in the electron flow along magnetically insulated transmission lines

A magnetically insulated vacuum transmission line (MITL) is a means by which high currents can be carried from generators to loads. In addition to the current flowing in the negative conductor, the source high voltage propagating along the transmission line causes electron emission into the vacuum which then forms into an electron sheath restrained by magnetic forces to flow along the cathode surface. The dynamics of this high current sheath flow is complicated, in particular when the transmission line is non-uniform or when additional power is added along its length. In the present paper we present PIC calculations which show that the circumstances causing the electron sheath to become turbulent, forming persistent vortices flow along non-uniform transmission lines are the same as those for driven MITLs. Under-matched terminal loads or intermediate under-matched sections of a non-uniform MITL suppress sheath emission by a retrapping wave which propagates upstream. When the sheath along a uniform MITL section encounters an overmatched section it overflows downstream. We show that vortices form only when upstream advancing retrapping, downstream overflow and local emission compete.