Cathodic arcs: Fractal voltage and cohesive energy rule

Abstract

The noise of the burning voltage of cathodic arcs in vacuum was analyzed for a range of cathode materials (C, Mg, Ti, V, Ni, Cu, Zr, Nb, Ag, Hf, Ta, W, Pt, Bi, and Si). Cathodic arcs were generated in a coaxial plasma source, and the voltage noise was measured with a broadband (250 MHz) measuring system. Each measurement of 50,000 points was analyzed by Fast Fourier Transform, revealing a power spectrum ~ 1/f 2 for all metals over several orders of magnitude of frequency f (brown noise). The absence of any characteristic time down to possible cutoffs at high frequencies supports a fractal model of cathode spots. Our preliminary research indicate that materials of high cohesive energy have a cutoff of self-similarity at 50 MHz while no cutoff could be found up to 100 MHz for non-refractory metals. The amplitude of colored noise scaled approximately linearly with the cathode's cohesive energy, which is another manifestation of the Cohesive Energy Rule. a) electronic address: aanders@lbl.gov LBNL-57025 1The plasma of cathodic vacuum arcs is produced at non-stationary cathode spots 1 . Much research, both theoretically 2-7 and experimentally [8][9][10][11][12][13][14] , has been done for decades to understand the spot phenomena that produce non-equilibrium plasmas containing multiply ion charge states. The plasma is mesosonic 15 ,i.e., subsonic for electrons and supersonic for ions, with respect to the local ion sound velocity. Difficulties in experimentation and modeling are associated with the spots' fast changes, small sizes, and extreme gradients of power density and particle densities. The formation of spot plasma is often described as a sequence of microexplosions, where cathode material transitions through various phases, from solid to liquid, non-ideal plasma and/or gas phases, finally turning into in a non-equilibrium, expanding, fully ion ionized plasma 6,16 .With improvement of experimental techniques, faster and smaller phenomena have been discovered. This becomes especially striking when the issue of current density is considered. As the apparent area of electron emission was found to be smaller than previously thought, the corresponding current density increased accordingly 17,18 . There were arguments what constitutes an electron-emitting site.In one extreme, electron emission should occur from areas much larger than the molten crater due to intense ion bombardment of the crater vicinity. On the other hand, the most relevant electron emission area might be smaller than the later visible crater because crater formation could be the result of the action of several spot fragments or cells.In order to make any useful statements on cathode processes and parameters of cathodic arc plasma, many researches decided to repeat measurements and average the measured data. For example, one can find data on average ion charge states 10,19 , average burning voltages 20,21 , average ion velocities 15 , or average ion velocity distribution functions 22 .All parameters fluctuate with large amplitude; in some cases up to...

Temporal development of ion beam mean charge state in pulsed vacuum arc ion sources

“…The ion beam mean charge state is determined by the fraction of differently charged ions in the vacuum arc plasma. The physical aspects of the reduction of ion charge states in vacuum arc plasma were subject of our detailed investigations [4,5], which have confirmed that the reduction of ion mean charge after arc initiation is consistent with the evolution of neutrals in the volume through which the metal plasma flows. This paper considers the reduction of the ion mean charge state throughout the vacuum arc pulse from the point of view of its influence on the parameters of a vacuum arc ion source.…”

Time-resolved ion energy and charge state distributions in pulsed cathodic arc plasmas of Nb−Al cathodes in high vacuum

“…This is generally referred to as the 'cohesive energy rule' [5]. It is related to the energy input per time and per eroded cathode material, which affects electron temperature and charge states in the plasma.…”

“…To give a few examples: the explosive electron emission or ecton model [3]; the fractal character of several arc properties, like the burning voltage and its noise [4]; the relationship of the latter and other arc properties to the cathode material's cohesive energy [5]; or the so-called 'freezing' of ion charge state and energy distributions in the process of plasma expansion [6]. Apart from general scientific curiosity, the economic need to improve applications such as thin film and ion implantation systems and processes, as well as high current switches, is a major driving force for the advancement of the physical understanding of these phenomena.…”

Material-dependent high-frequency current fluctuations of cathodic vacuum arcs: Evidence for the ecton cutoff of the fractal model

“…A short coaxial cathodic arc plasma source was flange-mounted to a cryogenically pumped vacuum chamber (it was similar to the one described in our previous work on fractal arc voltage 6 ). Most experiments were done with a planar anode (as shown in Fig.…”