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Unipolar arcing: Difference between revisions
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The electron emission at the cathode spot occurs in the form of discrete explosive electron emission splashes, so-called 'ectons'. These quanta of the explosive process represent the minimum actions required for the explosive events. The duration of one ecton is about <math>\sim</math>10 ns, the current <math>\sim</math>1 A, and the size of the emission centers is about <math>\sim1</math> <math>\mu</math>m. The explosion leaves a micro crater with a diameter of about <math>\sim1</math> <math>\mu</math>m. | The electron emission at the cathode spot occurs in the form of discrete explosive electron emission splashes, so-called 'ectons'. These quanta of the explosive process represent the minimum actions required for the explosive events. The duration of one ecton is about <math>\sim</math>10 ns, the current <math>\sim</math>1 A, and the size of the emission centers is about <math>\sim1</math> <math>\mu</math>m. The explosion leaves a micro crater with a diameter of about <math>\sim1</math> <math>\mu</math>m. | ||
According to the ecton model, arc operation is self-sustained, and occurs in stages <ref name="Barengolts">S.A. Barengolts (2010) ''The ecton mechanism of unipolar arcing in magnetic confinement fusion devices''</ref>. The first stage is the appearance of dense primary erosion plasma due to the external action, e.g. a laser pulse or ELM-plasma, onto the target. This dense plasma action results in a strong emission pulse (<math>10^8</math> A/cm<math>^2</math>) that leads to a thermal explosion of the emitting local area, the start of stage two. The created dense plasma produces two important effects: 1) the sheath thickness reduces, leading to an increase in the electric field at the surface, and 2) (due to the electric field) the ion bombardment heating increases. Now, if the local electric field is additionally enhanced by the fine structure of the surface, e.g. tungsten fuzz, this can all together intensify the local energy input, leading to a thermal run-away process. If the energy input rate exceeds the energy removing rate, this can lead to a micro-explosion. The micro-explosion creates another dense erosion plasma, and hence creates another emission site, so this causes repeating ignition of micro-explosions. The dense plasma provides the conditions for the ignition while 'choking' the already operating emission center by its limited conductivity. Ignition in this sense is not just the triggering of the arc discharge but the arc's perpetual mechanism to 'stay alive.' The probabilistic distribution of ignition of emission centers can be associated with a fractal spot model | According to the ecton model, arc operation is self-sustained, and occurs in stages <ref name="Barengolts">S.A. Barengolts (2010) ''The ecton mechanism of unipolar arcing in magnetic confinement fusion devices''</ref>. The first stage is the appearance of dense primary erosion plasma due to the external action, e.g. a laser pulse or ELM-plasma, onto the target. This dense plasma action results in a strong emission pulse (<math>10^8</math> A/cm<math>^2</math>) that leads to a thermal explosion of the emitting local area, the start of stage two. The created dense plasma produces two important effects: 1) the sheath thickness reduces, leading to an increase in the electric field at the surface, and 2) (due to the electric field) the ion bombardment heating increases. Now, if the local electric field is additionally enhanced by the fine structure of the surface, e.g. tungsten fuzz, this can all together intensify the local energy input, leading to a thermal run-away process. If the energy input rate exceeds the energy removing rate, this can lead to a micro-explosion. The micro-explosion creates another dense erosion plasma, and hence creates another emission site, so this causes repeating ignition of micro-explosions. The dense plasma provides the conditions for the ignition while 'choking' the already operating emission center by its limited conductivity. Ignition in this sense is not just the triggering of the arc discharge but the arc's perpetual mechanism to 'stay alive.' The probabilistic distribution of ignition of emission centers can be associated with a fractal spot model. | ||
Finally, the electron emission, and evaporation ceases, because the thermal conduction has led to an increase of the spot area, lowered the power density, and hence lowered the surface temperature. The explosively formed plasma has expanded, its density is lowered, therefore the cathode sheath thickness has increased, and therefore the electric field at the surface is reduced. | Finally, the electron emission, and evaporation ceases, because the thermal conduction has led to an increase of the spot area, lowered the power density, and hence lowered the surface temperature. The explosively formed plasma has expanded, its density is lowered, therefore the cathode sheath thickness has increased, and therefore the electric field at the surface is reduced. | ||
==Energy balance== | ==Energy balance== | ||
The transition from the cathode's solid phase to the plasma phase requires energy, which is supplied via the power dissipated by the arc, | The transition from the cathode's solid phase to the plasma phase requires energy, which is supplied via the power dissipated by the arc<ref name="Anders">A. Anders (2008) ''Cathodic Arcs: from Fractal Spots to Energetic Condenstation''</ref>, | ||
:<math>P_{arc} = V I_{arc}</math> | :<math>P_{arc} = V I_{arc}</math> | ||