Instability of the arc from the cathode material

The purpose of this research was to analyze
instability phenomena in vacuum arc for the silver cathode,
characterized by noise on the current trace prior to the
actual current chopping. The analyzed instability
phenomena were derived from the cathode spot model,
cathode spot region is recognized as the collision-less space
charge sheath connected with the singly ionized collision
plasma. A cathode input voltage eff V You can find out by
using the principles of heat transfer. But the current arc I,
The temperature change of cathode to anode ΔTc, ΔTa
and the voltage input to the arc Va Is used to measure the
voltage drop across the cathode eff V Determinate
eff V =Va*( a ΔTc/ΔTa+ΔT ) The 8 dependent variables,
cathode spot radius, current density, cathode electric field,
electron current fraction, sheath voltage, plasma density,
the electron temperature of plasma and cathode surface
temperature is solved by using eight equations with two
unknown parameters. Two unknown parameters, cathode
input power and ion current fraction on the anode surface, are
eliminated by using experimental data. The instability
phenomena are proposed that a current level below that no
a real solution exists in unstable current and this current
corresponds to the point at which a vacuum arc turns into
unstable. Minimum arc current having real solutions is
restricted by the cathode electric field equation. It was found
that when the arc current is lower than 16.2 A, there is no
the real solution and that is defined the instability. The
the physical explanation of instability is considered the
electrons returning to the sheath region from the plasma
one dominates over positive ions. Consequently, the stable
ion sheath criterion does not satisfy.