Cohesive Energy Rule for Vacuum Arcs

Abstract

The Cohesive Energy Rule for vacuum arcs describes an empirical relationship between the cathode material and the arc burning voltage, namely, that the burning voltage depends approximately linearly on the cohesive energy. For-Berkeley’s vacuum arc ion source system it was quantified as V = V 0 +AE CE , with V 0 ≈ 14.3V and A = 1.69 V/(eV/atom). Two arguments are brought forward to identify physical justifications for the empirical rule. First, the self-adjusting burning voltage determines the power input for a given arc current, and therefore the Cohesive Energy Rule connects a material property with the energy conservation law. In order to accomplish the phase transition from the solid to the plasma, energy must be invested, and the cohesive energy represents the energy needed to reach the vapor phase. Only a small fraction of power is directly dissipated in the cathode, and the much larger fraction dissipated in the plasma moves away with the expanding plasma. A possible response of the discharge is to self-adjust the total burning voltage. Through this path, the cohesive energy would affect plasma parameters via the causal chain: cohesive energy — burning voltage — power dissipation — electron temperature — ion charge state and ion kinetic energy. The second reason to justify the rule is that many physical parameters show periodicity as expressed in the Periodic Table of the Elements. Therefore, the periodicity shown by the cohesive energy acts-as a proxy for the periodicity exhibited by other quantities, for example, melting and boiling temperatures or ionization energies.