Investigations of the ignition of cathode spots by arcs operated in atmospheric pressure noble gas atmosphere on electrodes made of Al, Cu, Ti and graphite are extended to electrodes made of Au, Pd and Pt. Rods with a diameter of 2 mm are inserted in a UHV tight stainless steel vessel filled with Ar 5.6 or Kr 4.8. The negatively biased electrodes are brought into interaction with arc plasma by a magnetic blast field. Their end faces are mostly polished with diamond grinding powder, some Pd electrodes are additionally glowed, and other Pd electrodes are pasted with Pd powder. The arc spot ignition is observed by short time photography, streak camera records and temporally highly resolved optical spectroscopy for a lower and higher voltage applied between the arc plasma and the commutation electrode. Measurements of the commutation time tc elapsing between a signal generated by the arc plasma in front of the commutation electrode and the start of the commutation current show that at low voltages tc is in parts drastically increased by oxide layers. The measurements of tc for different electrode materials demonstrate that tc decreases with decreasing boiling temperature of the material. It confirms that not only ions of the filling gas but also of vaporized electrode material in front of the commutation electrode initiate and promote the arc spot ignition. SEM records show that different electrode treatments cause primarily different arc traces on the electrode surface.

The study of the commutation of atmospheric pressure noble gas arcs on cold cathodes made of Al, Cu, Ti, graphite, Au, Pd, and Pt is extended to W cathodes. Rods with a diameter of 2 mm are inserted in a UHV tight stainless steel vessel filled with Ar 5.6 or Kr 4.8. The negatively biased electrodes are brought into interaction with arc plasma by a magnetic blast field. Their end faces are mostly polished with diamond grinding powder; some are electrolytic polished, others additionally covered with a thick oxide layer or pasted with W powder. Moreover, electrodes are investigated being doped with Al, K, and Si or doped with ThO2. The arc commutation is observed by short time photography, streak camera records, and temporally highly resolved optical spectroscopy for a lower and higher voltage applied between the arc plasma and the commutation electrode (CE). Varying the electrode properties revealed that basically ionized vapour of electrode material and less distinctly lowering of the work function by doping accelerate the arc commutation. It is initiated by plasma ions, which initially generate secondary electrons across the whole end face of the electrode. Since the electron emission varies locally, the multiplication of ions within the plasma layer by emitted electrons varies too. The positive feedback between both provokes a constriction of the commutation current and the power input into an arc spot. The electrode vapour promotes at first the development of an arc spot but finally quenches it by cooling the plasma layer in front of it.

The formation of arc spots on cold cathodes, called arcing, is an interference factor in all kinds of plasma technical devices. It is investigated by igniting an arc in a clean atmospheric pressure Ar or Kr filling of a stainless steel vessel. It is brought into interaction by a magnetic blast field with a negatively biased commutation electrode (CE), formed by a rod with a diameter of 2 mm consisting of pure Al, Cu, Ti, or graphite; its end face is polished with diamond grinding powder. The arc commutation is observed by short‐time photography, streak camera records, and temporally highly resolved optical spectroscopy for different applied voltages. The emission of ion lines of the filling gas and of vaporized electrode material by the plasma in front of the cathode is indicating the formation of a positive space charge layer in front of its surface. It provides the ions, which induce secondary electron emission of the cathode surface and with it, the arc commutation. The commutation time tc elapsing between a signal generated by the arc plasma in front of the CE and the beginning of a current flow through the electrode increases for low voltages with increasing permittivity of a surface layer formed by electrically insulating metal oxide and decreases with increasing electrical conductivity of the layer. It is lowest for graphite without an oxide layer. On scanning electron microscope pictures, taken of the end face of the electrode after arc commutation, craters with diameters of less than 1 μm are visible.