Motion of Arc Cathode Spot in a Magnetic Field

Abstract

The motion of the cathode spot of a mercury arc was observed under various transverse magnetic fields, arc currents, and inert gas pressures. With no inert gas, the retrograde velocity increased as the magnetic field strength increased. Addition of an inert gas at low pressures reduced the retrograde velocity and at higher pressures caused a forward (direction of magnetic force) motion.The motion of the cathode spot in a magnetic field can be associated with electrostatic forces on positive ions produced by electron impact. A cathode spot assembly consisting of a group of electrons in the cathode, a positive ion sheath separated by a dark space from the cathode, and an electron cloud between the positive ion sheath and anode is postulated.Electrons after acceleration through the cathode dark space would travel a curved path due to the magnetic field and produce new positive ions on the forward side of the cathode spot. These new positive ions would be attracted by the cathode spot assembly with a resultant force that would cause them to overshoot the positive ion sheath before being pulled into the cathode to start a new spot in the retrograde direction.Addition of an inert gas reduces the mobility of positive ions to the order of that of the cathode electrons thus permitting forward motion of the cathode spot assembly to reduce the overshooting. Higher pressures would cause a forward spot motion.