Vacuum Arc Ion Sources [workshop]

Laboratory of Nanotechnology and Materials Science of Coatings
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Vacuum arcs are different from gaseous arcs in that the arc action needs to provide the material for current transport between electrodes. The various terms of Equ. The input energy is mostly transferred to heat the cathode, to emit and heat electrons, and to produce and accelerate ions. The cohesive energy ECE is relatively small and may be neglected if one wants to calculate the more prominent energy contributions. However, the correlation between burning voltage and cohesive energy suggests to have a closer look on the physical situation. There are two arguments to hold up the Cohesive Energy Rule despite the relatively small fraction of energy needed for the phase transition.

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The first argument is based on the spatial distribution of the energy input. Figure 2 schematically shows that most of the dissipated energy is concentrated near the cathode surface and associated with the cathode fall. Interestingly, driven by the extreme pressure gradient, both electrons and ions are accelerated away from the cathode surface, carrying away the energy invested in them.

Therefore, a very large fraction of input energy is not available to accomplish the phase transition. Materials with large cohesive energy require more energy for the transition from the solid to the vapor phase. The discharge system can provide this greater energy to the solid by enhancing the overall burning voltage. Of course, when this happens, the fractions of energy going to other energy forms, as described by Equ. If this is true, solids with greater cohesive energy have higher burning voltage and more energy available for ionization and acceleration of ions, in agreement with measurements.

Schematic illustration of the potential drop near the cathode surface. The dissipated power is proportional to the gradient of the potential. The dense plasma, heated most by the power input, is rapidly accelerated away from the cathode, thereby removing most of the input energy. The second argument for the validity of the Cohesive Energy Rule and derived rules is based on the periodicity of many material properties. Periodicity refers here to properties that are due to the electronic shell structure and that can be grouped according to the Periodic Table of the Elements.

For example, the melting temperature and boiling temperature show the same periodicity as the cohesive energy, which of course is not coincidental but related to the bond strength interaction energy. The periodicity is the physical reason that a relation identified for the melting or boiling temperature, for example, can be formulated as a relation to the cohesive energy.

One may argue that that a relation identified for the boiling temperature, for example, is equivalent to a relation to the cohesive energy. That is not entirely correct because relations based on the cohesive energy can be associated with the thermodynamic law of energy conservation. Energy is physical quantity for which we can formulate a balance equation, but temperature is not.

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Quantification of the Cohesive Energy Rule In a zero-order approximation one can state that the vacuum arc burning voltage is about 20 V, with somewhat lower values for cathode materials of smaller cohesive energy, and higher values for materials of greater cohesive energy. Figure 3 shows how well the simple approximation 4 represents the Cohesive Energy Rule. Arc burning voltages for elements of various atomic numbers; one data set was measured Table I and the other was calculated using Equ.

The lines serve only for the purpose of guiding the eye. Note that a few more elements have been calculated than measured. One may construct higher-order approximations taking into account properties that affect the energy balance.

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Thermal conductivity, cohesive energy, and measured burning voltage for copper and silver and neighboring elements. Consequences of the Cohesive Energy Rule Systematic voltage measurements and energy balance considerations allowed us to identify a linear relation between the cohesive energy and the arc burning voltage, the Cohesive Energy Rule. In section 5, two arguments were brought forward to put the empirical rule on a physical basis. The importance of the first argument, which was based on specifics of the spatial energy distribution in relation to the energy requirement for the solid-plasma phase transition, is that a bewildering variety of cathode processes and parameters has been reduced to a simple number that is a fundamental material characteristics.

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One is faced with the problem of extracting beams in a pulsed fashion while maintaining high beam quality during the pulse low emittance. Shortly before scheduled delivery ANAC went into receivership. The 11 B peak is now more clearly visible in these graphs. We report on effective low pressure plasma excitation by electron cyclotron wave resonance ECWR for the generation of dense homogeneous plasmas and the rf plasma beam extraction method for the generation of broad low energy plasma beams. For the [] 11 B peaks see curves qc. The voltages can be controlled and monitored by providing the inputs and receiving the feedback through a user friendly graphic user interface. For example, notice that the anode subassembly 2 has the anode essentially grounded.

This simple number is the cohesive energy, a material property that exists unconditionally without any feedback from plasma processes but is determined by the electronic shell structure of atoms. Because a physical justification could be identified based on energy balance considerations and particle and energy fluxes due to pressure gradients, the Cohesive Energy Rule appears to have a deeper physical meaning than other empirical rules mentioned in section 3.

However, one should realize that of course complicated processes occur indeed.

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The Cohesive Energy Rule is a useful rule, not a physical law. Better justification could imply that other empirical rules could be considered as derived from, or secondary to the Cohesive Energy Rule. It was argued that the periodicity in one parameter, like the cohesive energy, is a proxy for the periodicity of other parameters. Figure 5 shows, for example, the strong correlation between cohesive energy and boiling point of the cathode.

A consequence of the similar periodicity is that all empirical rules of the vacuum arc reflect the same physics, namely, that physical processes at the cathode spot are affected by a number of parameters which happen to show similar periodicity. Still, as discussed above, the Cohesive Energy Rule should be preferred since it deals with energy a quantity with a conservation law rather than temperature or other properties.

Cohesive energy, ECE , and the boiling temperature of cathode materials of atomic number Z. For example, Figures 6 and 7 illustrate the correlation between charge states, electron temperature, and burning voltage. Burning voltage and plasma electron temperature in the transition zone to non-equilibrium as a function of the cathode material. A correlation is evident lines are only drawn to guide the eye.

For example, the ease of phase transition can be correlated to arc stability and the likelihood of arc chopping. Kesaev [20] found a correlation between arc stability and melting point of the cathode material.

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Workshop on Vacuum Arc Ion Sources. Sep Berkeley, California CNUM: C Proceedings ยท Contributions. Export MARC, MARCXML. Proceeding of the workshop on vacuum arc ion sources. Aug - pages. Prepared for Conference: C; LBL, LBNL, CONF-.

Arc stability was measured by the average arc duration for a given relatively low arc current. Kesaev further noticed that the melting point is not sufficient but the atomic weight of the material should also be an important parameter. As an example, Farrall states that copper and beryllium have very different atomic weight but show nearly identical arc stability and vapor pressure.

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For the three metals Cd, Be, and Mo, the chopping current increases with decreasing vapor pressure. All of the above findings are in qualitative agreement with predictions by the Cohesive Energy Rule. The smaller the cohesive energy the easier are the phase transitions and thus the lower is the chopping current and the more stable is the arc discharge.

However, a word of caution is prudent: arc stability and chopping current depend also on the surface state of the cathode, and therefore is it difficult to compare data without the exact knowledge of the surface conditions. Conclusions The Cohesive Energy Rule gives simple, empirical guidance on a number of arc plasma parameters despite the complex nature of arc spot processes.

However, because details of these processes are not considered, the Cohesive Energy Rule is neither a physical law nor can it replace detailed modeling. Although empirical, the Cohesive Energy Rule has its roots in energy conservation and distribution, coupled with the periodic structure of electronic shells of atoms.

Therefore, the Cohesive Energy Rule should be preferred to other empirical rules that have been found for vacuum arcs. Acknowledgments Ian G.

Brown is gratefully acknowledged for his long-term support and for presenting this work at the NATO workshop. Gera Yu. Yushkov and Banchob Yotsombat have contributed to ion energy and voltage measurements. This work was supported by the U. Department of Energy, under Contract No. References [1] B. D: Appl. RR, Kleberg, " Dynamics of cathode spots in external magentic field in German ," in Naturwissenschaftliche Fakultaet. Berlin, Germany: Humboldt University, Mesyats and D. Proskurovsky, Pulsed Electrical Discharge in Vacuum.

Berlin: Springer-Verlag, Moscow, Russia: Nauka, Anders, "Energetics of vacuum arc cathode spots," Appl. Anders, B. Yotsombat, and R. Binder, "Correlation between cathode properties, burning voltage, and plasma parameters of vacuum arcs," J. Kesaev, "Laws governing the cathode drop and the threshold currents in an arc discharge on pure metals," Sov.

Grakov, "Cathode fall of an arc discharge in a pure metal," Sov. Brown, B. Feinberg, and J. Galvin, "Multiply stripped ion generation in the metal vapor vacuum arc," J. Brown and X. October 24, We waged an dead 3 book vacuum arc career the family of August We directed sent with many c and with our urine Eliza. Eliza was our every anatomy and divided positive patients.

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